Any Accident Types of Gas Risk in Vessel

The assessment of the hazards of a major fire event, requires a relationship between the thermal load (a function of the radiation intensity and exposure time) and the effects on people. The issue can be addressed from two perspectives.

1. How long can a worker continue to operate in an emergency situation whilst exposed to a given level of radiation?
2. What fraction of the population will die or sustain serious injury given exposure to a certain dose of radiation.

Gases are commonly stored in large pressurised vessels as liquefied gasses. If these vessels are subjected to engulfing pool or impinging jet fires significant amounts of heat may be transferred to the vessel. If the fire exposure lasts for sufficient time, the vessel may fail catastrophically, resulting in a Boiling Liquid Expanding Vapor Explosion (BLEVE). In these events, it is the temperature rise and subsequent loss of strength of the steel wall which determine the time to failure. Although vessels are usually protected with pressure relief valves, failure can occur in just a few minutes. The use of water deluge systems or passive fire protection (PFP) materials decrease heat flow to the vessel contents and can reduce or eliminate the risk of a BLEVE occurring36. In order to be able to assess this behavior and the hazards posed from fire-engulfment of liquefied gas storage vessels it is important to understand the mechanism of failure and to be able to predict the response of vessels under such conditions.

The HEATUP scenario has been developed in order to model the behavior of vessels containing liquefied gases exposed to fire and produce suitable data as input for hazard consequence analysis tools. HEATUP quantifies the thermodynamic properties in the vapor and liquid phase of the contents of vessels exposed to a range of fire scenarios. The code allows for fluid loss though a PRV whenever the set pressure of the valve is exceeded and it can also be set up to model vessels with PFP coatings. By calculating the thermodynamic properties of the fluid remaining inside the tank, at the point of catastrophic tank failure, HEATUP effectively determines the source terms essential to evaluating the hazards associated with the resulting BLEVE. The tank pressure, liquid fill level, fluid and wall temperatures and fluid enthalpy in the liquid and vapour zones are all predicted up to the point of vessel failure.

There are many different physical processes occurring when a flame interacts with a vessel containing liquefied gases due to the complex behavior of the flame, the vessel and the vessel contents. The important processes occurring during jet-fire impingement on vessels containing liquefied gas include:

1. Heat transfer between the fire and outer surface of the vessel, in the vapor and liquid 'zones', by radiation and convection.
2. Heat transfer through the vessel walls by conduction. The wall may comprise of an outer passive fire protection (PFP) coating plus the underlying steel wall.
3. Heat transfer into the vessel fluids by predominantly radiation in the vapor space, and by natural convection or nucleate boiling in the liquid phase. Mass transfer from the bulk liquid or vapour to the outside environment through any holes in the vessel.
4. Mass transfer out of the vessel through any open or partially open pressure relief valves (PRVs).
5. Mass transfer within the liquid phase by flow of heated fluid into a stratified 'hot' layer lying above the bulk liquid. The hot layer may or may not be stable.
6. Mass transfer between the liquid and vapor phases by evaporation.
7. Pressure, enthalpy and liquefied gas composition changes in the fluid during each of the above processes.
8. Catastrophic vessel failure resulting in a possible BLEVE.

Gas dispersion

Gas dispersion (or vapor dispersion) is used to determine the consequences of a release of a toxic or flammable material. Typically, the calculations provide an estimate of the area affected and the average vapor concentrations expected. In order to make this determination, one must know the release rate of the gas (or the total quantity released) and the atmospheric conditions (wind speed, time of day, cloud cover).

The steps required to utilize a gas dispersion model are:
1. Identify the scenario. What can go wrong to result in the loss of containment of the material?
2. Develop an appropriate source model to calculate the release rate or total quantity released based on the specified
3. Use an appropriate gas dispersion model to estimate the consequences.
4. Determine if the resulting consequence is acceptable. If not, then something must be changed to reduce the consequence

Calculations and experiments have demonstrated that even the release of a small quantity of toxic or flammable material can have a significant consequence. Thus, it is clear that the best procedure is to prevent the release in the first place. However, release mitigation must be a part of any process safety program. Release mitigation involves: (1) detecting the release as early as possible, (2) stopping the release as quickly as possible, and (3) invoking a mitigation/emergency response procedure to reduce the consequences of the release.

Parameters Affecting Gas Dispersion

A wide variety of parameters affect the dispersion of gases. These include: (1) wind speed, (2) atmospheric stability, (3) local terrain characteristics, (4) height of the release above the ground, (5) release geometry, i.e. from a point, line, or area source, (6) momentum of the material released, and (7) buoyancy of the material released.

As the wind speed is increased, the material is carried downwind faster, but the material is also diluted faster by a larger quantity of air. Atmospheric stability depends on the wind speed, the time of day, and the solar energy input. During the day, the air temperature is at a maximum at the ground surface as a result of radiative heating of the ground from the sun. At night, radiative cooling of the ground occurs, resulting in an air temperature which is low at ground level, increases with height until a maximum is reached, and then decreases with further height. Terrain characteristics affect the mechanical mixing of the air as it flows over the ground. Thus, the dispersion over a lake is different from the dispersion over a forest or a city of tall buildings.

Gaussian Dispersion

Gaussian dispersion is the most common method for estimating dispersion due to a release of vapor. The method applies only for neutrally buoyant clouds and provides an estimate of average downwind vapor concentrations. Since the concentrations predicted are time averages, it must be considered that local concentrations might be greater than this average; this result is important when estimating dispersion of highly toxic materials where local concentration fluctuations light have a significant impact on the consequences.

HAZARDS OF INERT GASES

The use of inert atmospheres should be considered to prevent fires and deflagrations when using flammable materials. However, inert atmospheres can be dangerous to personnel. One of the most important concerns in the use of an inert atmosphere is that it can kill if a person breathes it. The air we normally inhale contains about 21 percent O2, 79 percent N2, and small amounts of other components. Inhaling air containing less than about 16 percent oxygen causes dizziness, rapid heartbeat, and headache. One or two breaths of pure nitrogen and some other gases containing no oxygen can be lethal. Other gases of this type include methane, ethane, acetylene, carbon dioxide, nitrous oxide, hydrogen, argon, neon, helium, and some others. Oxygen in the lungs is washed out and replaced by gas containing no oxygen. Blood from the lungs receives insufficient oxygen and flows to the brain, where tissues rapidly become deficient. Within five seconds of inhaling only a few breaths of oxygen-free gas, there can be mental failure and coma. Symptoms or warnings are generally absent. Death follows in two to four minutes. However, a coma due to lack of oxygen is not always fatal. Cardiopulmonary resuscitation techniques should be used on persons who are not breathing due to lack of oxygen (Ventilation for Acceptable Indoor Air Quality, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, Ga.; Zabetakis, Flammability Characteristics of Combustible Gases and Vapors, Bulletin 627, Bureau of Mines, 1965).

Gases which act as simple asphyxiants, such as nitrogen and helium, merely displace oxygen in the atmosphere so that the concentration falls below that needed to maintain consciousness. There are also chemical asphyxiants, such as carbon monoxide, hydrogen sulfide, and hydrogen cyanide, which have a specific blocking action and prevent a sufficient supply of oxygen from reaching the body. Most deaths due to short-term gassing are caused by carbon monoxide (Lees, Loss Prevention in the Process Industries, Butterworths, London, 1980, p. 646).

Effects of Low Oxygen Levels
There are many factors which can affect the ability of human beings to adjust to lower oxygen levels. For example, two men were accidentally exposed to a low oxygen level in a vessel. One of them died, and one survived without permanent injury. The one who died had been in poorer general health and it is believed that this factor may have made the low oxygen level fatal for him, while the other person, who was in good health, survived. It is well known that people accustomed to living near sea level can take several days to adjust fully to the lower amount of oxygen available in mountainous regions such as Denver, Colorado. Anyone who has traveled to the top of Pike’s Peak knows how the altitude can make one tired, lethargic, and even sick. People react differently, however, and one cannot generalize as to exactly how a person will react to lower oxygen levels and higher altitudes.

Minimum Oxygen Limits
Oxygen limits are set at 19.5 percent minimum as recommended by OSHA and the American Standards Institute. Michigan has adopted these guidelines as well and has defined grade D air for O2 to be 19.5 percent to 23.5 percent as an obligation to the employee by their employer. The Ontario Ministry of Labour designates enclosures containing less than 18 percent O2 as hazardous.

Inerting Monomer Storage Tanks with Nitrogen
It is good practice to keep the vapor space of flammable liquids out of the flammable range. Monomers that can potentially polymerize require special consideration. The vapor space above some monomers, such as styrene and methyl acrylate, should be kept below about 10 percent oxygen in warm weather to be below the flammable range. For many of these monomers, a small amount of oxygen is required to maintain the activity of the inhibitor and to avoid polymerization in storage tanks, which could lead to overheating and explosions and fire. An oxygen concentration of 5 percent in the vapor space is recommended as a safety factor to stay out of the flammable range and maintain inhibitor activity. Maintaining an inert atmosphere for these applications can be difficult, since usually nitrogen is available as a high-purity gas, and it is necessary to add a small amount of oxygen (usually air) to the nitrogen to achieve the desired oxygen concentration. Mixing air and nitrogen has not proven to be a reliable method of maintaining the proper inert pad in the past. This is because instrument failure has caused high nitrogen concentration, which in turn has caused storage vessels to polymerize. One alternative to consider is the use of membrane systems, such as those sold by Generon Systems and other suppliers. This system can produce 95/5 percent nitrogen/oxygen for inerting, using plant compressed air available at 65 psig (449 kPa gauge). This system has an inherently stable output when operating at a specific pressure drop because the pressure drop across the membrane module sets the nitrogen purity.

Nitrogen is often the preferred gas for providing an inert atmosphere. In general, most organic combustible compounds will not propagate flame if oxygen in the mixtures of the
organic vapor, inert gas, and air is below about 10 percent and 13 percent, with nitrogen and carbon dioxide, respectively, as the inert gases. With carbon dioxide, the minimum oxygen concentration is higher than with nitrogen because carbon dioxide has a higher specific heat. Carbon dioxide is fairly soluble in many liquids and will react with alkaline
materials, so its use as an inerting material is limited. Heavy gases such as carbon dioxide provide superior inerting of vent stacks to prevent air entry. Water vapor is a good inerting gas if the temperature is high enough (above about 80 to 85°C [176 to 185°F]).

The use of an inert atmosphere can virtually eliminate the possibility of explosions and fire with flammable materials. However, inerting systems can be quite expensive and difficult to operate successfully and can be hazardous to personnel. Before using inert systems, alternatives should be explored, such as using nonflammable materials or operating below the flammable range.

Source: Perry 1999

Radiation Heat Flux from Pool Fires

The radiation from the hot gases and incandescent soot deep within the flame passes through the visible "surface" to external objects. The amount of heat emitted varies with the distance over which emission occurs and the concentration and type of emitting species within that path. As the thermal radiation passes through the atmosphere outside the flame it is attenuated by absorption of energy in the infrared wavelengths corresponding to the absorption bands of the atmospheric gases (principally carbon dioxide and water vapour). The attenuating effect on radiation is significant even over path lengths of a few tens of metres. Water sprays, mists and smoke can also strongly attenuate radiation.

The radiative heat emission process is modelled by assuming that the radiation comes from the visible surface. The surface emissive power, SEP, of a flame is the heat radiated outwards per unit surface area of the flame. Thus the use of SEPs is a two-dimensional simplification of a very complex three-dimensional heat radiation problem. These models can be used for reliable prediction of external radiative heat fluxes to within about a flame length of the fire. They cannot be used for near impingement conditions however.

The model average SEP depends on the fuel type and on the pool diameter. A uniform SEP is used over the whole of the sides of the model flame shape. This results in underprediction of the radiative heat flux near the base of large pool fires because the model fails to take account of the small bright, highly emissive region at the base of the flame. The following formulae are used in Shell FRED. For land-based LNG pool fires (Shell FRED 2004):

The physical effects modelled by this equation are the increase in SEP with increasing emitting path length, given by the first exponential term:

And the appearance and increase in the amount of dark obscuring smoke on the outside of the flame, given by the second exponential term:

For LPG and propane the SEP is given by:

The second term is only included if the pool diameter exceeds 18m. For butane, gasoline and kerosene, the SEP is given by:

Above figure shows the variation of SEP with pool diameter. There is a peak SEP for all three classes of fuels, representing a fire when the emission from bright areas of flame is at a maximum, but before significant amounts of smoke have appeared outside the flame. The peak occurs earlier for heavier hydrocarbons because the flames contain higher soot concentrations.

Above figure shows variation of model average SEP with pool diameter for different fuels!

Pool Fire

Gasoline was distributed by a transmission pipeline to the next transmission station and then carried away to the Public Gas Station by a tanker truck. Since the distribution process by a pipeline is very vulnerable, so a leak may happen and then the gasoline will spilled out, and the consequence is it will form a pool that have a potential to become a Pool fire.


Other Fire Modelling Packages

Several specialist fire packages have been recently developed:
.
1. CHIC - compartment fires
CHIC (Combustion Hazards In Compartments18) is a time dependent model which predicts the physical effects arising from the confinement of a jet or pool fire in a compartment, e.g. an offshore module. Internal heat fluxes, smoke layer thickness and temperature, CO and soot concentration and the extent, if any, of external flaming are predicted. The model has been validated by experimental work using propane, diesel and gas condensate as fuel.

2. SEAFIRE - fires on the sea
SEAFIRE predicts the behaviour of a subsea release of product at sea, from e.g. a broken pipeline. The release may occur above/on or below the sea surface and for the latter a new model predicts the behaviour of the rising bubble plume and subsequent surface spreading due to wind, waves and gravitational effects. Release products may be mixtures of gas and oil. The fire characteristics of the burning pool are based on experiment. Run time graphics show the development of fire with time.

3. CLOUDF - cloud or flash fires
Cloud or flash fires are transient in nature and are the product of the delayed ignition of a dispersing cloud in an unconfined environment. Two types of cloud fire are modelled19 with radiation dose predictions being supplied at user specified locations. The first is the delayed ignition of the cloud formed from a vertical release. This results in a fireball dying back to a steady state jet flame from the source. The second is the delayed ignition of the cloud formed from a horizontal negatively-buoyant release. Such a flame, when not accelerated by obstacles, will travel horizontally through the cloud at relatively low speeds (less than 20 m/s). Because of the short duration of the fire, radiation effects are generally not significant outside the burning cloud.

Unconfined Vapor Cloud Explosions

Article below is continuing of previous article:

Unconfined vapor cloud explosions (also known as vapor cloud explosions) in open air often result when accidental releases of vapors or gases to the atmosphere are ignited. In addition, some tests have been so-called meteorological area sources, while the dispersion equations are generally meteorological point sources. (Only concentrations relatively close to the location of discharge of the vapors will be affected by this difference in sources.) Also, the momentary concentration of a combustible gas or flammable vapor is the important duration of a concentration for UVCEs; not all dispersion models specify their averaging time of concentrations. Thus, predictions of concentrations must be treated as estimates.


Pressure Development
Overpressure in a UVCE results from turbulence that promotes a sudden release of energy. Tests in the open without obstacles or confining structures do not produce damaging overpressure. Nevertheless, combustion in a vapor cloud within a partially confined space or around turbulence-producing obstacles may generate damaging overpressure. Also, turbulence in a jet release, such as may occur with compressed natural gas discharged from a ruptured pipeline, may result in blast pressure.

Example
The combustion process in large vapor clouds is not known completely and studies are in progress to improve understanding of this important subject. Special study is usually needed to assess the hazard of a large vapor release or to investigate a UVCE. The TNT
equivalent method is used in this example; other methods have been proposed. Whatever the method used for dispersion and pressure development, a check should be made to determine if any governmental unit requires a specific type of analysis

Prevention and Protection
It is difficult to cope with a potential UVCE once an accidental release has occurred. Consequently, the best procedure to guard against a UVCE is to prevent the release in the first place. Safe piping is essential to protect against UVCEs. Forty percent of all major plant losses are due to piping failures, and corrosion is one of the largest single causes of plant and equipment breakdown. Moreover, mistakenly open valves that caused mammoth emissions of hydrocarbons have resulted in two major UVCEs with a total of 29 deaths in those two instances. Thus, close scrutiny regarding piping and valves is mandatory to help prevent UVCEs. Some other protection methods are summarized as follows:

1. Remotely Operated Shutoff Valves
These should be considered for supply lines and other vulnerable pipelines. Excess flow valves that close when flow exceeds a set amount are possible substitutes, but they are not acceptable to some operators.

2. Flammable Vapor Detectors
These should be installed to warn of leaks, although such devices do not effectively control UVCEs with sudden, massive releases.

3. Elevated or Remote Air Intakes
Elevated or remote air intakes for control rooms will help in reducing ingress of dense, flammable vapors into those rooms. Ordinarily, elevating the tip of the air intake duct 9 m (30 ft) above the ground is sufficient. Installing flammable vapor detectors in the air intake ducts provides additional protection. Controls that automatically stop air to control rooms if vapor concentrations reach 25 percent of their LFL should also be considered.

4. Intentional Ignition
Intentional ignition to ignite a vapor cloud early before it spreads out to a large volume has been used or considered only rarely. Such ignition should not be employed for control of UVCEs solely without thorough study of the ramifications of its use. (In some infrequent cases when the gas is both flammable and particularly toxic, intentional ignition may be warranted.)

5. Large Fans
These could be used to dilute a vapor cloud below its LFL with ambient air (see, for example, Whiting and Shaffer, “Feasibility Study of Hazardous Vapor Amelioration Techniques,” Proc. 1978 Nat. Conf. on Control of Hazardous Material Spills, USEPA, Miami Beach, April 1978). But caution must be exercised because the turbulence produced by fans will likely promote rapid combustion and a resulting UVCE unless vapors are diluted below the LFL. Nevertheless, in new plants, strategic placement of air coolers may provide enough air flow to reduce the risk of a UVCE.

6. Water Sprays and Steam Curtains
These have been used and/or advocated to help protect against a UVCE. Such devices entrain air to dilute a vapor cloud. Also, some claim that water curtains form a physical barrier to stop the flow of the vapor cloud. As with large fans, vapors need to be reduced below LFLs to decrease the possibility of UVCEs from enhanced turbulence by the sprays or curtains. Moodie studied water spray barriers using carbon dioxide to approximate a heavy, flammable vapor cloud Release of a pressurized, liquefied gas to the atmosphere will cause the gas to cool and condense water vapor in ambient air, forming a visible vapor cloud. Firefighters and operators who attempt to move such a cloud away from furnaces and the like with fire hoses and water jet guns are at risk, because of the possibility of a UVCE near them. Plants and governmental agencies who recommend such practices need to reexamine their policies.

7. Structures
Structures that include partially confined spaces and turbulence-producing obstacles such as pipe bridges plus closely packed equipment, promote UVCE. This undesirable architecture-relative to UVCE, is often a product of congestion on a plant. Congestion is an enemy of safety. Thus, the probability of a UVCE, plus property losses and casualties, will likely be greater at a congested plant than at an uncontested site. Vapor cloud explosions also occur indoors when large amounts of flammable vapors are discharged accidentally into buildings.

8. Strong Buildings
Strong buildings may be prudent where people congregate, such as control rooms. For new plants, serious consideration must be given to stronger design of buildings that are vulnerable to a UVCE, compared to past designs.

GAS EXPLOSIONS

Flash Point and Flammable Limits

Flash points and flammable limits in percent by volume have been tabulated by the National Fire Protection Association (NFPA) (National Fire Protection Association, Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids, NFPA 325, Quincy, Mass.). Pressure particularly affects flash point and the upper flammable limit (UFL); see later section entitled “Effect of Temperature, Pressure, and Oxygen.” Mists of high-flash-point liquids may be flammable; the lower flammable limit (LFL) of fine mists and accompanying vapor is about 48 g/m3 of air, basis 0°C and 1 atm (0.048 oz/ft3). For practical purposes, LFL is the same as lower explosive limit (LEL). (Ignitability limits depend upon the strength of the ignition source; the ignitability range for relatively weak ignition sources is less than the flammable range.) LFLs in percent by volume generally decrease as molecular weight increases. The equilibrium vapor pressure of a flammable liquid at its closedcup flash point about equals its LFL in percent by volume. Thus, the vapor pressure of toluene at its closed-cup flash point (4.4°C or 40°F) of 1.2 percent (1.2 kPa) is close to its LFL of 1.1 percent.

Limiting Oxidant Concentration (LOC)

It is often prudent to base explosion prevention on inerting. The LOC is the concentration of oxidant—normally oxygen—below which a fuel-oxidant explosion cannot occur. (The LOC is also called MOC, the minimum oxygen for combustion.) With adequate depletion of oxygen, an explosion cannot occur whatever the concentration of fuel. Nevertheless, in these circumstances a fuel–air–inert gas mixture may become flammable if sufficient air is added. Many LOCs are given in NFPA 69. In general, organic flammable gases or vapors will not propagate flame in mixtures of the organic, added nitrogen, and air below about 10.5 percent by volume O2 at 1 atm and near normal room temperature. Hydrogen (LOC = 5 percent) and some other inorganic gases have lower LOCs. For LOCs of 5 percent and greater, the O2 concentration should not exceed 60 percent of the LOC, but with continuous monitoring the O2 may be kept 2 percent below the LOC (NFPA 69, 1992).
Neutronics, Inc., of Exton, Pennsylvania, supplies an inerting control system that has had wide application in many industries. Explosion prevention by inerting has several advantages over explosion protection techniques, such as explosion venting. For example, with successful inerting, fires or business interruptions cannot occur. Nevertheless, beware of the potential of asphyxiation with inerting; proper vessel entry procedures must be implemented and occasionally it may be prudent to monitor for oxygen in workplaces.

Detonation

A deflagration can develop into a gaseous detonation in vessels and piping under certain conditions with enhanced explosion effects. Many factors affect detonation formation and effects. Briefly, upon ignition, pressure waves in a closed tube move through unburned gas. Subsequent waves move faster through the unburned gas, because of heating from previous pressure waves. Adiabatic compression results in high enough temperature to ignite gas ahead of the original flame and a detonation develops. (This ignition by compression to form a detonation is sometimes also called pressure piling.) The peak pressure in a stable detonation is on the order of 30 times the initial absolute pressure, disregarding the usually nondamaging spike of still higher pressure; reflected pressure is much higher than this 30 multiplier. (Special review is necessary for overpressure developed in an unstable [overdriven] detonation.) Nevertheless, in usual plant vessels without large length/diameter ratios, detonation is unlikely at 1 atm and near normal room temperature. Strong equipment may be subject to damage in a detonation, and rupture disks alone cannot control a detonation. Flame arresters are now commonly used to help protect against detonations; see the article on flame arresters in this section. But the best procedure to guard against the destructive effects of detonations is to prevent the formation of flammable mixtures

Explosion Protection

Where prevention of flammable mixtures may not be feasible, protection facilities must be installed; sometimes, too, backup explosion protection facilities are used in conjunction with inerting systems. Containment, suppression, or venting are used for protection against internal deflagrations in fuel-air mixtures. Although these methods may protect against deformation or rupture of a vessel, damage to internal appurtenances may still occur. Containment and suppression prevent the discharge of environmentally unacceptable materials to the atmosphere.

Containment

The design pressure (maximum allowable working pressure) to prevent rupture of equipment for most gas-air mixtures initially at 1 atm should be 304 kPa gauge (44.1 psig), and to prevent permanent deformation, 608 kPa gauge (88.2 psig) (National Fire Protection Association, Explosion Prevention Systems, NFPA 69, Quincy, Mass., 1992, p. 11). NFPA 69 provides important additional design information on deflagration pressure containment.

Explosion Suppression

With explosion suppression, an incipient explosion is detected and—within a few milliseconds—a suppressant is discharged into the exploding medium to stop combustion. Pressure and optical detection systems are used; suppressors are pressurized
and release the suppressants when actuated by an electroexplosive device.

HAZARDOUS MATERIALS AND CONDITIONS

The main business of most chemical companies is to manufacture products through the control of reactive chemicals. The reactivity that makes chemicals useful can also make them hazardous. Therefore, it is essential that people who design or operate chemical processes understand the nature of the reactive chemicals involved. Usually reactions are carried out without mishaps, but sometimes chemical reactions get out of control because of problems such as using the wrong raw material, using raw materials containing trace impurities, changed operating conditions, unanticipated time delays, equipment failure, or wrong materials of construction.

Such mishaps can be worse if the chemistry is not fully understood. A chemical plant can be inherently safer if knowledge of the chemistry of the process and the reactive chemicals systems involved is used in its design.

Reactive Hazard Review Reactive hazards should be evaluated using reviews on all new processes and on all existing processes on a periodic basis. There is no substitute for experience, good judgment, and good data in evaluating potential hazards. Reviews should include:
1. Review of process chemistry, including reactions, side reactions, heat of reaction, potential pressure buildup, and characteristics of intermediate streams
2. Review of reactive chemicals test data for evidence of flammability characteristics, exotherms, shock sensitivity, and other evidence of instability
3. Review of planned operation of process, especially the possibility of upsets, modes of failure, unexpected delays, redundancy of equipment and instrumentation, critical instruments and controls, and worst-credible-case scenarios

Worst-Case
Thinking At every point in the operation, the process designer should conceive of the worst possible combination of circumstances that could realistically exist, such as loss of cooling water, power failure, wrong combination or amount of reactants, wrong valve position, plugged lines, instrument failure, loss of compressed air, air leakage, loss of agitation, deadheaded pumps, and rawmaterial impurities. An engineering evaluation should then be made of the worst-case

the worst case occurs. A HAZOP study could be used to help accomplish worst-case thinking. When the process designers know what the worst-case conditions are, they should:
1. Try to avoid worst-case conditions.
2. Be sure adequate redundancy exists.
3. Identify and implement lines of defense.
a. Preventive measures
b. Corrective measures

Sometimes, as a last resort, it may be desirable to use a high degree of process containment or, possibly, abandon the process if the hazard is unacceptable. It is important to note that the worst case should be something that is realistic, not something that is conceivable but extremely unlikely. The Dow Chemical Company has adopted the following philosophy for design scenarios in terms of independent causative effects:
1. All single events that can actually and reasonably occur are credible scenarios.
2. Scenarios that require the coincident occurrence of two or more totally independent events are not credible design scenarios.
3. Scenarios that require the occurrence of more than two events in sequence are not credible.
4. A failure that occurs while an independent device is awaiting repair represents but one failure during the time frame of the initiation of the emergency and is therefore credible. The lack of availability of the unrepaired device is a preexisting condition

Reactive Chemicals Testing
Much reactive chemical information involves thermal stability and the determination of ;
(1) the temperature at which an exothermic reaction starts,
(2) the rate of reaction as a function of temperature,
(3) heat generated per unit mass of material.
The evaluation of thermal stability requires the determination of the temperature at which an exothermic reaction occurs, the rate of such a reaction as a function of temperature, and the heat generated per unit mass of material by the reaction. In many cases, data on the increase of pressure during a reaction are also required, especially for vent sizing. The term onset temperature Tonset is used in two contexts:
1. In a testing context, it refers to the first detection of exothermic activity on the thermogram. The differential scanning calorimeter (DSC) has a scan rate of 10°C/min, whereas the accelerating rate calorimeter (ARC)* has a sensitivity of 0.02°C/min. Consequently, the temperature at which thermal activity is detected by the DSC can be as much as 50°C different from ARC data.
2. The second context is the process reactor. There is a potential for a runaway if the net heat gain of the system exceeds its total heat loss capability. A self-heating rate of 3°C/day is not unusual for a monomer storage tank in the early stages of a runaway. This corresponds to 0.00208°C/min, 10 percent of the ARC’s detection limit. ARC data for the stored chemical would not show an exotherm until the self-heating rate was 0.02°C/min. Therefore, onset temperature information from ARC testing must be used with considerable caution!

Dust Explosions
Combustible, dusty materials, with particle sizes less than approximately 200 mesh, can explode if a sufficient concentration in air is present along with an ignition source. The standard test has been designed to determine rates of pressure rise during an explosion, the maximum pressure reached, and the minimum energy needed to ignite the material. These data are useful in the design of safe equipment to handle dusty combustible materials in a process. Combustible dusts need a minimum volume to develop their full reaction velocity. Bartknecht states that for determination of explosion data of combustible dusts, a minimum volume of 16 L (4.23 gal) would be required to ensure correlation with data from large test vessels (Bartknecht, 1981, p. 39). This has been confirmed by comprehensive testing with a 20-L (5.28-gal) sphere.

Source: Perry 1999

STORAGE AND HANDLING OF HAZARDOUS MATERIALS

Storage
Storage Facilities The Flixborough disaster (Lees, 1980) occurred on June 1, 1974, and involved a large, unconfined vapor cloud explosion (or explosions—there may have been two) and fire that killed 28 people and injured 36 at the plant and many more in the surrounding area. The entire chemical plant was demolished and 1821 houses and 167 shops were damaged. The results of the Flixborough investigation made it clear that the large inventory of flammable material in the process plant contributed to the scale of the disaster. It was concluded that “limitations of inventory should be taken as specific design objectives in major hazard installations.” It should be noted, however, that reduction of inventory may require more frequent and smaller shipments and improved

Minimal Use of Underground Tanks
The U.S. Environmental Protection Agency’s (USEPA) Office of Underground Storage Tanks
defines underground tanks as those with 10 percent of more of their volume, including piping, underground. An aboveground tank that does not have more than 10 percent of its volume (including piping) underground is excluded from the underground tank regulations. Note, however, that a 5000-gal tank sitting wholly atop the ground but having 1400 ft of 3-in buried pipe or 350 ft of 6-in buried pipe is considered an underground storage tank. At one time, burying tanks was recommended because it minimized the need for a fire protection system, dikes, and distance separation. At many companies this is no longer considered good practice. Mounding, or burying tanks above grade, has most of the same problems as burying tanks below ground and is usually not recommended. Problems with buried tanks include:
• Difficulty in monitoring interior and exterior corrosion (shell
thickness)
• Difficulty in detecting leaks
• Difficulty of repairing a tank if the surrounding earth is saturated
with chemicals from a leak
• Potential contamination of groundwater due to leakage.

Governmental regulations concerning buried tanks are becoming stricter. This is because of the large number of leaking tanks that have been identified as causing adverse environmental and human health problems. Consequences of Leaking Underground Tanks The following is a real possibility (Russell and Hart, 1987). A site where an underground tank has been used is found to have leaked. If the leak is not cleaned up to “background” levels by the time an environmental regulatory agency is involved, the agency may decide that a portion of the plant must be designated as a waste disposal site. The plant could then be required to provide a waste site closure plan, hold public hearings, place deed restrictions on the plant property, and, finally, provide a bond that would cover the cost of closing the site and also analyzing and sampling groundwater for up to 30 years. Product leaking from an underground storage tank will migrate
downward until it encounters the water table, where it will then flow with the groundwater, leaving a long trail of contaminated soil. Above the water table, some product will be absorbed on the soil particles and in the pore space between the soil particles. If the soil is later saturated by water, product stored in the pore spaces may be released, causing a reappearance of the free product and movement of the material into previously unaffected soil. The scope of the problem was revealed by the USEPA in 1983 management.

There may be more chances for errors in connecting and reconnecting with small shipments. Quantitative risk analysis of storage facilities has revealed solutions that may run counter to intuition (Schaller, Plant/Operations Progress, 9(1), 1990). For example, reducing inventories in tanks of hazardous materials does little to reduce risk in situations where most of the exposure arises from the number and extent of valves, nozzles, and lines connecting the tank. Removing tanks from service altogether, on the other hand, helps. A large pressure vessel may offer greater safety than several small pressure vessels of the same aggregate capacity because there are fewer associated nozzles and lines. Also, a large pressure vessel is inherently more robust, or it can economically be made more robust by deliberate overdesign than can a number of small vessels of the same design pressure. On the other hand, if the larger vessel has larger connecting
lines, the relative risk may be greater if release rates through the larger lines increase the risk more than the inherently greater strength of the vessel reduces it. In transporting hazardous materials, maintaining tank car integrity in a derailment is often the most important line of defense in transportation of hazardous materials.

Safer Storage Conditions
The hazards associated with storage facilities can often be reduced significantly by changing storage conditions. The primary objective is to reduce the driving force available to transport the hazardous material into the atmosphere in case of a leak (Hendershot, 1988). Some methods to accomplish this follow. Dilution Dilution of a low-boiling hazardous material reduces the hazard in two ways:
1. The vapor pressure is reduced. This has a significant effect on the rate of release of material boiling at less than ambient temperature. It may be possible to store an aqueous solution at atmospheric pressure, such as aqueous ammonium hydroxide instead of anhydrous ammonia.
2. In the event of a spill, the atmospheric concentration of the hazardous material will be reduced, resulting in a smaller hazard downwind of the spill.


Design of Liquid Storage So Leaks and Spills Do Not Accumulate Under Tanks or Equipment

Around storage and process equipment, it is a good idea to design dikes that will not allow toxic and flammable materials to accumulate around the bottom of tanks or equipment in case of a spill. If liquid is spilled and ignites inside a dike where there are storage tanks or process equipment, the fire may be continuously supplied with fuel and the consequences can be severe. It is usually much better to direct possible spills and leaks to an area away from the tank or equipment and provide a fire wall to shield the equipment from most of the flames if a fire occurs. The discussion on BLEVEs later in this section shows a design for diking for directing leaks and spills to an area away from tanks and equipment. The surface area of a spill should be minimized for materials that are highly toxic and have a significant vapor pressure at ambient conditions, such as acrylonitrile or chlorine.

This will make it easier and more practical to collect vapor from a spill or to suppress vapor release with foam. This may require a deeper nondrained dike area than normal or some other design that will minimize surface area, in order to contain the required volume. It is usually not desirable to cover a diked area to restrict loss of vapor if the spill consists of a flammable or combustible material. when it reported that, in the United States, 11 million gallons of gasoline seep into the soil each year. Just one gallon of gasoline can make one million gallons of water unsafe to drink; one ounce would pollute an Olympic-size swimming pool full of drinking water. Most of the contaminated sites the USEPA has documented involve corroded single-wall steel tanks and piping that have been in the ground for at least 16 years (Semonelli, “Secondary Containment of Underground Storage Tanks,” Chem. Eng. Prog., June 1990). A number of states
have enacted laws setting standards for underground storage tanks. The USEPA has issued regulations requiring notification to the appropriate regulatory agency about age, condition, and size for underground storage tanks containing commercial chemical products.

Secondary Containment for Underground Storage Acceptable secondary containment systems for underground storage are described as barriers either integral to the tank system design (such as double-walled tanks or double-walled pipes) or located within the underground storage tank system that present a barrier between all parts of the underground storage tank system and the environment. Double-walled tanks and piping should be considered for above ground tanks and piping containing highly toxic liquids. Concrete and fiberglass vaults are often used, although they can be subject to environmentally induced cracks. Soil and clay liners are not allowed. Flexible liner systems have been developed that may be a cost-effective and environmentally sound alternative. State-of-the-art liner technology has overcome many of the previous problems with seams, low mechanical strength, and chemical resistance.

Piping Systems for Underground Service An important consideration is the USEPA’s concern over piping systems. For all underground storage tank systems, performance standards consistent with those for tanks were set for pipes and pipe systems. There is evidence that 84 percent of underground storage tank system test failures are due to loose tank fittings or faulty piping. Piping releases occur twice as often as tank releases. In particular, loose joints tend to occur. For hazardous substance underground storage tank systems, there are two options: trench liners and double-walled pipes. Double-walled pipes are difficult to assemble and are subject to failure caused by service conditions, such as frost heaves or pressure from above. Flexible trench liners (discussed previously) are becoming a popular solution to secondary containment of piping systems. Detecting Leaks Small leaks are difficult to detect. The USEPA and American Petroleum Institute standard for nonleaking underground tanks is 0.05 gal/h (3.15 cm3/min), above which a tank is considered to be leaking. Leak detection measurements can be influenced by many factors, making it difficult to detect small leaks.

Corrosion Problems Tanks subject to internal corrosion are not good choices for underground service because of the necessity of monitoring wall thickness. Underground tanks and piping of carbon steel should be considered for corrosion protection measures such as external tarlike coatings and magnesium anodes. Joints in underground piping should be minimized by welding. Pipes may use a combination of wrapping and sprayed-on coatings. When flanges are necessary, such as with valves, external coatings should be used. Summary of Use of Underground Tanks Because of more stringent regulatory requirements, potential future liabilities, and the cost of building and operating underground storage tank systems, it may be inherently safer to use aboveground storage with suitable spacing, diking, and fire protection facilities. With modern technology, if it is necessary, it is possible to design underground storage systems with a high degree of integrity and which will make leaks to the environment highly unlikely, but the cost may be high.

Design of Tanks, Piping, and Pumps Six basic tank designs are used for the storage of organic liquids: (1) fixed roof, (2) external floating roof, (3) internal floating roof, (4) variable vapor space, (5) lowpressure tanks, and (6) high-pressure tanks. The first four tank designs listed are not generally considered suitable for highly toxic hazardous materials.

Low-Pressure Tanks (below 15 psig) Low-pressure storage tanks for highly hazardous toxic materials should meet, as a minimum, the American Petroleum Institute (API) 620 Standard, “Recommended Rules for the Design and Construction of Large Welded, Low-Pressure Storage Tanks” (API Standards). This standard covers tanks designed for all pressures under 15 psig. There are no specific requirements in API 620 for highly hazardous toxic materials. API 650, “Welded Steel Tanks for Oil Storage” (API Standards) has limited applicability to storage of highly hazardous toxic materials because it prohibits refrigerated service and limits pressures to
2.5 psig and only if designed for certain conditions. Most API 650 tanks have a working pressure approaching atmospheric pressure and hence their pressure-relieving devices must vent directly to the atmosphere.

Its safety factors and welding controls are less stringent than required by API 620. Another reference for the design of lowpressure storage tanks may be found in ANSI/API-620-1986. Horizontal and vertical cylindrical tanks are used to store highly toxic liquids at atmospheric pressure. Horizontal, vertical, and spherical tanks are used for refrigerated liquefied gases that are stored at atmospheric pressure. The design pressure of tanks for atmospheric and low-pressure storage at ambient temperature should not be less than 100 percent of the vapor pressure of the material at the maximum design temperature. The maximum design metal temperature to be used takes into consideration the maximum temperature of material entering the tank and the maximum ambient temperature, including solar radiation effects. Since discharges of vapors from highly hazardous toxic materials cannot simply be released to the atmosphere, the use of a weak seam roof is not normally acceptable. It is best that tanks be designed and stamped for 15 psig to provide maximum safety, and pressure relief systems must be provided to vent to equipment that can collect, contain, and treat the effluent.

The minimum design temperature should be the lowest temperature to which the tank will be subjected, taking into consideration the minimum temperature of material entering the tank, the minimum temperature to which the material may be autorefrigerated by rapid evaporation of low-boiling liquids or mechanically refrigerated, and the minimum ambient temperature of the area where the tank is located. API 620 provides for installations in areas where the lowest recorded one-day mean temperature is -50°F. While either rupture disks or relief valves are allowed on storage tanks by Code, rupture disks by themselves should not be used on tanks for the storage of highly hazardous toxic materials since they do not close after opening and may lead to continuing release of toxic material to the atmosphere.

The API 620 Code requires a combined pneumatic hydrotest at 125 percent of design tank loading. In tanks designed for low-density liquid, the upper portion is not fully tested. For highly hazardous toxic materials, consideration should be given for hydrotesting at the maximum specified design liquid level. It may be required that the lower shell thickness be increased to withstand a full head of water and that the foundation be designed such that it can support a tank full of water or the density of the liquid if it is greater than water. Testing in this manner not only tests the containment capability of the tank, but it also provides an overload test for the tank and the foundation similar to the overload test for pressure vessels. API 620 also requires radiography. Proper preparation of the subgrade and grade is extremely important for tanks that are to rest directly on grade. Precautions should be taken to prevent ground freezing under refrigerated tanks, as this can cause the ground to heave and damage the foundation or the tank.
Designing for free air circulation under the tank is a method for passive protection from ground freezing. Steels lose their ductility at low temperatures and can become subject to brittle failure. There are specific requirements for metals to be used for refrigerated storage tanks in API 620, Appendices Q and R. Corrosive chemicals and external exposure can cause tank failure. Materials of construction should be chosen so that they are compatible with the chemicals and exposure involved. Welding reduces the corrosion resistance of many alloys, leading to localized attack at the heat-affected zones. This may be prevented by the use of the proper alloys and weld materials, in some cases combined with annealing heat treatment.

External corrosion can occur under insulation, especially if the weather barrier is not maintained or if the tank is operating at conditions at which condensation is likely. This form of attack is hidden and may be unnoticed for a long time. Inspection holes and plugs should be installed in the insulation to monitor possible corrosion under the insulation.

Unconfined Vapor Cloud Explosions (UVCEs)

Unconfined vapor cloud explosions (also known as vapor cloud explosions) in open air often result when accidental releases of vapors or gases to the atmosphere are ignited. Astonishingly high pressure can result from an unconfined vapor cloud explosion; 70 kPa (10 psi) or so may occur at the outer edge of the exploding cloud, with still higher pressures near the center of the blast. Numerous severe explosions of this nature have occurred in past years (Lenoir and Davenport, “A Survey of Vapor Cloud Explosions: Second Update,” Process Safety Prog., vol. 12, no. 1, January 1993, pp. 12–33).


In a survey of property damage losses in 100 large losses in the hydrocarbon-chemical industries, M & M Protection Consultants of Marsh & McLennan found that a vapor cloud was the initiating mechanism in 43 percent of the cases. Releases of liquefied dense gases have caused many of the reported UVCEs. Such heavy gases tend to hug the ground with limited dispersion in ambient air; this condition results in accumulation of these gases where they can cause maximum casualties to people and damage to property, if ignited. Notwithstanding, releases of mammoth amounts of compressed natural gas from ruptured pipelines have caused vapor cloud explosions. As an example, in 1969 a 356-mm (14-in) pipeline carrying natural gas at more than 5378 kPa gauge (780 psig) ruptured; about 8 to 10 min later the escaping gas exploded violently (National Transportation Safety Board, Pipeline Accident Report Mobil Oil Corporation, High-Pressure Natural Gas Pipeline Accident, Houston, Texas, September 9, 1969, NTBS-Par-71-1, Washington, D.C., 1971).


Elevated emergency unflared releases with vents of sufficient height normally do not cause damaging overpressure at the ground, if accidentally ignited (Bodurtha, “Vent Heights for Emergency Releases of Heavy Gases,” Plant/Operations Prog., vol. 7, no. 2, April 1988, pp. 122–126). Numerous tests on dispersion of heavy gases and on causes of UVCEs have been performed in recent years. Dispersion tests and computer models based on them may not be representative of all conditions at a plant, however, because of equipment plus heat sources that cause better spreading of a plume than is modeled in tests.


Moreover, vapors flashed from release of a liquefied gas will be cold; such vapors flowing over warmer ground may promote atmospheric instability with accompanying turbulence and, thereby, cause more mixing with ambient air than in some tests. In addition, some tests have been so-called meteorological area sources, while the dispersion equations are generally meteorological point sources. (Only concentrations relatively close to the location of discharge of the vapors will be affected by this difference in sources.) Also, the momentary concentration of a combustible gas or flammable vapor is the important duration of a concentration for UVCEs; not all dispersion models specify their averaging time of concentrations. Thus, predictions of concentrations must be treated as estimates.

(Source: Handbook "Perry 1999")

BLEVE

A Boiling Liquid Expanding vapour Explosion (BLEVE) is caused by the catastrophic failure of a pressure vessel containing a liquid which is well above its boiling point at atmospheric pressure. Although rare events, the consequences of a BLEVE can be catastrophic, leading to the prominent position of the phenomenon in the safety analysis of LPG transport and storage.

Experimental observations show five distinct stages in the development of fireballs:

Stage 1: The vessel fails, missiles are generated and ejected and an initial overpressure wave, Ps1, is produced by the expanding vapour phase. The overpressure pulse is followed by a rarefaction.

Stage 2: The bursting vessel ejects a cloud of liquid droplets which flash adiabatically as the
pressure in the cloud drops. The mass, and therefore ultimately volume, of vapour flashed from the liquid droplets vastly exceeds that of the vapour initially released in Stage 1. There is little mixing with the surrounding air. The cloud pressure, eventually drops to the surrounding atmosphere and the volume of the cloud then becomes equal to the volume of flashed vapour at saturation temperature and pressure. If the outward radial velocity of the cloud exceeds the local speed of sound in the rarefaction following Ps1, then a blast wave can form as a result of the vapour expansion from the flashing liquid. As it separates from the outer cloud boundary, this blast wave, Ps2, leaves the cloud in a highly turbulent state at ambient pressure. It has been observed from experiments that the blast wave is only associated with the initial high fill ratio. At lower levels of fill the blast wave is unlikely to occur.

Stage 3: At this stage the overpressure peaks Ps1 and Ps2 leave the cloud. The cloud continues to expand due to outward radial momentum but the radial expansion velocity slows as the turbulent mixing entrains more and more air. Rapid expansion continues until the random velocities in the turbulent eddies overwhelm the radial expansion velocity, further expansion being due to the turbulence alone.

Stage 4: Ignition occurs near the centre of the cloud and an hemispherical fireball develops. The
expansion is suddenly arrested as the last visible part of the vapour cloud is consumed by flame. At this point the fireball is at its brightest. Since the cloud contains air, it is assumed that during this stage the flashed vapour is consumed, there being insufficient time for radiation to substantially heat and flash the cold liquid droplets. The expansion causes an overpressure pulse in the surrounding atmosphere which travels outwards from the cloud. The pulse is followed by a
rarefaction caused by the sudden arrest in cloud expansion. The expansion velocity of the fireball is equal to the flame propagation velocity through the turbulent vapour cloud. The heat radiation
pulse from the fireball peaks at the end of this stage.

Stage 5: The hemispherical fireball rises to become a sphere sitting on the ground. Combustion
continues but the fireball does not expand, indicating that the air required for combustion is already pre-mixed into the cloud. The fuel for combustion is supplied by the liquid droplets. The fireball then rises at approximately constant velocity and volume, and assumes a typical mushroom shape. The visible flame area decreases as the fireball becomes patched with sooty combustion products. Once combustion is substantially complete, the smoky torrid of hot combustion products rises, expands and dissipates with a flow pattern similar to a thermal. The heat radiation pulse decreases systematically to zero during this phase.

GAS EXPLOSIONS

Flash Point and Flammable Limits
Flash points and flammable limits in percent by volume have been tabulated by the National Fire Protection Association (NFPA) (National Fire Protection Association, Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids, NFPA 325, Quincy, Mass.). Pressure particularly affects flash point and the upper flammable limit (UFL); see later section entitled “Effect of Temperature, Pressure, and Oxygen.” Mists of high-flash-point liquids may be flammable; the lower flammable limit (LFL) of fine mists and accompanying vapor is about 48 g/m3 of air, basis 0°C and 1 atm (0.048 oz/ft3). For practical purposes, LFL is the same as lower explosive limit (LEL). (Ignitability limits depend upon the strength of the ignition source; the ignitability range for relatively weak ignition sources is less than the flammable range.) LFLs in percent by volume generally decrease as molecular weight increases.

Limiting Oxidant Concentration (LOC)
It is often prudent to base explosion prevention on inerting. The LOC is the concentration of oxidant—normally oxygen—below which a fuel-oxidant explosion cannot occur. (The LOC is also called MOC, the minimum oxygen for combustion.) With adequate depletion of oxygen, an explosion cannot occur whatever the concentration of fuel. Nevertheless, in these circumstances a fuel–air–inert gas mixture may become flammable if
sufficient air is added. Many LOCs are given in NFPA 69. In general, organic flammable gases or vapors will not propagate flame in mixtures of the organic, added nitrogen, and air below about 10.5 percent by volume O2 at 1 atm and near normal room temperature. Hydrogen (LOC = 5 percent) and some other inorganic gases have lower LOCs. For LOCs of 5 percent and greater, the O2 concentration should not exceed 60 percent of the LOC, but with continuous monitoring the O2 may be kept 2 percent below the LOC (NFPA 69, 1992). Neutronics, Inc., of Exton, Pennsylvania, supplies an inerting control system that has had wide application in many industries.Explosion prevention by inerting has several advantages over explosion protection techniques, such as explosion venting. For example, with successful inerting, fires or business interruptions cannot occur. Nevertheless, beware of the potential of asphyxiation with inerting; proper vessel entry procedures must be implemented and occasionally it may be prudent to monitor for oxygen in workplaces.

Explosion Pressure An explosion
Is the action of “going off ” with a loud noise under the influence of suddenly developed internal energy. Thus, an explosion is a result, not a cause. Deflagrations and detonations cause chemical explosions. A deflagration is a reaction that propagates to the unreacted material at a speed less than the speed of sound in the unreacted material. A detonation is a reaction that propagates to the unreacted material at a speed greater than the speed of sound in the unreacted material; it is accompanied by a shock wave and inordinately high pressure.

Explosion Protection
Where prevention of flammable mixtures may not be feasible, protection facilities must be installed; sometimes, too, backup explosion protection facilities are used in conjunction with inerting systems. Containment, suppression, or venting are used for protection against internal deflagrations in fuel-air mixtures.Although these methods may protect against deformation or rupture of a vessel, damage to internal appurtenances may still occur. Containment and suppression prevent the discharge of environmentally unacceptable materials to the atmosphere.

Pumps and Gaskets

Fugitive emissions often occur as a result of leakage of process materials through leak paths in rotating seals and susceptible gasketed joints such as are found in pipe flanges. When properly maintained, fugitive emissions from most conventional joints and sealing systems used in industry can be kept to a minimum. For volatile organic compounds (VOCs) this is usually significantly less than 500 ppm as measured at the leak path by a portable VOC analyzer specified in USEPA reference method 21 (40 CFR 60, Appendix A, Method 21). However, for some sealing systems such as packing glands on pump shafts in some services, the necessary maintenance frequency and potential risks of noncompliance have caused some companies to eliminate them from services where fugitive emissions are a concern and use tighter sealing systems such as mechanical seals instead. In services where entrained solids or fouling are not present to a significant extent and additional cost is justified, magnetic drive and canned-motor pumps, which have become more reliable and available in a wide variety of configurations and materials, are being used to virtually eliminate fugitive emissions from pumps. In services where fugitive emissions are a concern, valves such as quarter turn, diaphragm seal, or bellows seal valves, which are less susceptible to leakage, are sometimes being used in place of gate or globe valves with packed stem seals.

However, under many service conditions, high-cost equipment options are not necessary to comply with the provisions of fugitive emission regulations. Properly maintained packing glands or single mechanical seals on valves and pumps can often meet all emissions requirements. An informed choice should be made when specifying new valves and pumps, considering factors such as the type of service, accessibility for maintenance, cost, and the degree of emission reductions which may be achieved. The most common maintenance problem with centrifugal pumps is with the seals. Mechanical seal problems account for most of the pump repairs in a chemical plant, with bearing failures a distant second. The absence of an external motor (on canned pumps) and a seal is appealing to those experienced with mechanical seal pumps. Sealless pumps are very popular and are widely used in the chemical industry. Sealless pumps are manufactured in two basic types: cannedmotor and magnetic-drive.

Magnetic-drive pumps have thicker “cans,” which hold in the process fluid, and the clearances between the internal rotor and can are greater compared to canned-motor pumps. This permits more bearing wear before the rotor starts wearing through the can. Many magnetic-drive pump designs now have incorporated a safety clearance, which uses a rub ring or a wear ring to support the rotating member in the event of excessive bearing wear or failure. This design feature prevents the rotating member (outer magnet holder or internal rotating shaft assembly) from accidentally rupturing the can, as well as providing a temporary bearing surface until the problem bearings can be replaced. Because most magnetic-drive pumps use permanent magnets for both the internal and external rotors, there is less heat to the pumped fluid than with canned-motor pumps. Some canned-motor pumps have fully pressure-rated outer shells, which enclose the canned motor; others don’t. With magnetic-drive pumps, containment of leakage through the can to the outer shell can be a problem. Even though the shell may be thick and capable of holding high pressures, there is often an elastomeric lip seal on the outer magnetic rotor shaft with little pressure containment capability. Canned-motor pumps typically have a clearance between the rotor and the containment shell or can, which separates the fluid from the stator, of only 0.008 to 0.010 in (0.20 to 0.25 mm). The can has to be thin to allow magnetic flux to flow to the rotor. It is typically 0.010 to 0.015 in (0.25 to 0.38 mm) thick and made of Hastelloy.

The rotor can wear through the can very rapidly if the rotor bearing wears enough to cause the rotor to move slightly and begin to rub against the can. The can may rupture, causing uncontrollable loss of the fluid being pumped. It should not be assumed that just because there is no seal, sealless pumps are always safer than pumps with seals, even with the advanced technology now available in sealless pumps. Use sealless pumps with considerable caution when handling hazardous or flammable liquids. Sealless pumps rely on the process fluid to lubricate the bearings. If the wear rate of the bearings in the fluid being handled is not known, the bearings can wear unexpectedly, causing rupture of the can. Running a sealless pump dry can cause complete failure. If there is cavitation in the pump, hydraulic balancing in the pump no longer functions and excessive wear can occur, leading to failure of the can. The most common problem with sealless pumps is bearing failure, which occurs either by flashing the fluid in the magnet area because of a drop in flow below minimum flow or by flashing in the impeller eye as it leaves the magnet area. It is estimated that nine out of ten conventional canned-motor pump failures are the result of dry running. Canned pumps are available which, their manufacturer claims, can be operated dry for as long as 48 h. It is especially important to avoid deadheading a sealless pump. Deadheaded sealless pumps can cause overheating. The bearings may be damaged and the pump may be overpressured. The pump and piping systems should be designed to avoid dead spots when pumping monomers. Monomers in dead spots may polymerize and plug the pump. There are minimum flow requirements for sealless pumps. It is recommended that a recirculation system be used to provide internal pump flow whenever the pump operates. Inlet line filters are recommended, but care must be taken not to cause excessive pressure drop on the suction side. Typical inlet filters use sieve openings of 0.0059 in (0.149 mm).

For many plants handling monomers and other hazardous materials, sealless pumps are the first choice. They can practically eliminate the pump problems that can occur due to seal leaks, which can include product loss, flammability, waste disposal, and exposure of personnel to hazardous vapors. A number of liquids require special attention when applying canned-motor and magnetic-drive pumps. For example, a low-boiling liquid may flash and vapor-bind the pump. Solids in the liquid can also be bad for a sealless pump. Low-viscosity (in the range of 1 to 5 cP [1 ´ 10-3 to 5 ´ 10-3 Ns/m2]) fluids are normally poor lubricators and one should be concerned about selecting the right bearings. For viscosities less than 1 cP, it is even more important to choose the right bearing material. The Dow Chemical Company recommends canned-motor pumps or magnetic-drive pumps for phosgene service. Phosgene is an example of an extremely hazardous material. These pumps should have a secondary containment such that failure of the can does not create a phosgene release. The secondary containment should meet pipe specifications for pressure or relieve to the scrubber system in the plant. These pumps must have automated block valves on the suction and discharge. Operation of these valves should be managed such that the thermal expansion does not damage the pump. A mistreated sealless pump can rupture with potentially serious results. The can can fail if valves on both sides of the pump are closed and the fluid in the pump expands either due to heating up from a cold condition or if the pump is started up. If the pump is run dry, the bearings can be ruined. The pump can heat up and be damaged if there is insufficient flow to take away heat from the windings. Sealless pumps, especially canned-motor pumps, produce a significant amount of heat, since nearly all the electrical energy lost in the system is absorbed by the fluid being pumped. If this heat cannot be properly dissipated, the fluid will heat up with possibly severe consequences. Considerable care must be used when installing a sealless pump to be sure that misoperations cannot occur.

The instrumentation recommended for sealless pumps may seem somewhat excessive. However, sealless pumps are expensive and they can be made to last for a long time, compared to conventional centrifugal pumps where seals may need to be changed frequently. Most failures of sealless pumps are caused by running them dry and damaging the bearings. Close monitoring of temperature is necessary in sealless pumps. Three temperature sensors (resistance temperature devices, or RTDs) are recommended: (1) in the internal fluid circulation loop, (2) in the magnet, or shroud, area, and (3) in the pump case area. It is very important that sealless pumps be flooded with liquid before starting, to avoid damage to bearings from imbalance or overheating. Entrained gases in the suction can cause immediate imbalance problems and lead to internal bearing damage. Some type of liquid sensor is recommended. Sealless pumps must not be operated deadheaded (pump liquid full with inlet and/or outlet valves closed). Properly installed and maintained, sealless pumps, both canned and magnetic-drive, offer an economical and safe way to minimize hazards and leaks of hazardous liquids.

Combustion: Gas Characteristics and Sensitivity

Combustion thermodynamic calculations allow determination of peak deflagration and detonation pressures, plus stable detonation velocity. The peak pressure calculation may be used to determine combustion product venting requirements, although a conservative volume increase of 9:1 may be used for essentially closed systems. Other relevant gas characteristics are entirely experimental. The sensitivity to detonation depends on the detonatable range and fundamental burning velocity, although no specific correlations or measures of sensitivity exist based on fundamental properties. It is often considered that detonation sensitivity and the degree of difficulty in arresting flames increase with lower National Electrical Code (NEC) Groupings. Hence, Group A gases (acetylene) will be most sensitive and Group D gases (such as saturated hydrocarbons) will be least sensitive. This empirical method of characterizing gases is typically used in selecting deflagration arresters, where successful testing using one gas in an NEC electrical group is assumed to apply for other gases in that group. It is cautioned that, where the maximum experimental safe gaps (MESGs) of two gases within a single NEC group are significantly different, the assumption of equivalent sensitivity is dubious.

Regulations applying to detonation arresters in vapor control systems under the authority of the U.S. Coast Guard (USCG) provide that MESGs be solely used to characterize gases, under the assumption that mixtures with smaller MESGs are more difficult to stop. See “Deflagration and Detonation Flame Arresters,” (1993) for a discussion of MESG plus tabulated values.

Corrosion

Consideration should be given to possible corrosion of both the element material and the arrester housing, since corrosion may weaken the structure, increase the pressure drop, and decrease the effectiveness of the element. While the housing might be designed to have a corrosion allowance, corrosion of the element must be avoided by proper material specification. Common materials of construction include aluminum, carbon steel, ductile iron, and 316 stainless steel housings and aluminum or 316 stainless steel elements.

While special materials such as Hastelloy might be used for situations such as high HCl concentrations it may be more cost effective to use a hydraulic arrester in such applications.

Dinamic of Oil Price

In March, the Central Bank of Venezuela announced a new National Retail Price Index as the main economic indicators continue to 'update and revision process. On the other hand, the international oil price continues to increase and there is still hope that the economy produced by the United States in the first semester because of concerns about global recession is greater.

In March, the Central Bank of Venezuela presented a new National Retail Price Index. There is a difference between this and the index before the index: price index used is registered in the Caracas Metropolitan Area and Maracaibo, while using the new price listed on the country's ten most important areas such as the Metropolitan: Maracay, Valencia, Barquisimeto, Merida, San Cristobal, Ciudad Guayana, Barcelona-Puerto La Cruz and Maturin, in addition to 84 other small, medium and size of rural population.

This new index will complete the update indicator of the cycle, together with the change metodological basis for a better representation is a new basket of goods and services. Besides allowing a greater homogeneity in the calculation. This new index is expected to be available in April, together with the indicator is calculated for the Caracas Metropolitan Area.

In regard to the financial system and according to reports from Sudeban, the people's deposits is 176 985 million at the end of February, which is an additional 1.39% variation compared to the previous month. In the savings account and deposit down at 1.37% and 0.26% respectively, while in the long-term deposits increased by 8.90%.

On credit, in February they signed a 104.4 million with the increase of 59.5% (27.3% in real terms) compared with the same month the previous year. Economy with the destination of many variations of credit and consumption, with increased 113.4% and 80.2%, respectively. Credit on commercial activities (sectors that represent nearly 50% of the total credit) increased by only 22.5%.

So..... Is it good or bad? What do u think?

Source Article: www.conapri.org

A Generator from CO2 (Carbon Diokside)

Carbon Dioxide (CO2) is one of the easiest ways to accelerate plant growth. Plants grown with additional CO2 can produce up to 40% more flowers or fruit. A propane or natural gas CO2 generator is the most cost effective way to add CO2 to the environment. Many greenhouses use CO2 generators to boost carbon dioxide levels economically.CAP safe and has developed several safe and most reliable CO2 generator. They can produce between 3 and 26 cubic feet of carbon dioxide per hour. C.A.P. manufactures 2 models, the GEN-1 and GEN-2.

GEN-1 is recommended for small to medium size areas up to 15 feet 15 feet. Features including "two-stage" safety pilot valve, which will not allow fuel flow to the burner unless the pilot is lit, a tip-over "switch that will close the main fuel burner in the event the unit falls or tips over, and powder coated construction steel enclosure that resists moisture, rust and discoloration, ensuring years problem free operation.

Available in both natural gas and propane versions, the GEN-1 comes complete with low pressure, in-line natural gas regulator or the new Type 1-style "no-tools-required" propane regulator that makes tank replacement easy and fast, depending on the configuration purchased. C.A.P. GEN-1 CO2 Generators
C.A.P. GEN-1 CO2 Generator Interior

GEN-1 comes standard with one (1) burner. The purchase of up to 3 additional Burners below.

GEN-1 is capable of producing between 3 and 13 cubic feet of carbon dioxide per hour. This unit comes standard with one burner, producing approximately 3 cubic feet per hour. You can add up to three additional Burners, increase the unit output capacity to about 13 cubic feet CO2 per hour. Burners are available for additional purchase from homeharvest.com

Sharing About Hidrocarbon

Natural Gas Information

Natural gas is gaseous fossil fuel consisting primarily of methane but including significant quantities of ethane, butane, propane, carbon dioxide, nitrogen, helium and hydrogen sulfide.It is found in oil fields and natural gas fields, and in coal beds (as coalbed methane). When methane-rich gases are produced by the anaerobic decay of non-fossil organic material, these are referred to as biogas. Sources of biogas include swamps, marshes, and landfills (see landfill gas), as well as sewage sludge and manure by way of anaerobic digesters, in addition to enteric fermentation particularly in cattle. Natural gas is often informally referred to as simply gas, especially when compared to other energy sources such as electricity. Before natural gas can be used as a fuel, it must undergo extensive processing to remove almost all materials other than methane. The by-products of that processing include ethane, propane, butanes, pentanes and higher molecular weight hydrocarbons, elemental sulfur, and sometimes helium and nitrogen.

Chemical Composition

The primary component of natural gas is methane (CH₄), the shortest and lightest hydrocarbon molecule. It also contains heavier gaseous hydrocarbons such as ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀), as well as other sulphur containing gases, in varying amounts, see also natural gas condensate. Natural gas also contains and is the primary market source of helium.

Nitrogen, helium, carbon dioxide and trace amounts of hydrogen sulfide, water and odorants can also be present . Mercury is also present in small amounts in natural gas extracted from some fields[3]. The exact composition of natural gas varies between gas fields.

Organosulfur compounds and hydrogen sulfide are common contaminants which must be removed prior to most uses. Gas with a significant amount of sulfur impurities, such as hydrogen sulfide, is termed sour gas; gas with sulfur or carbon dioxide impurities is acid gas. Processed natural gas that is available to end-users is tasteless and odorless, however, before gas is distributed to end-users, it is odorized by adding small amounts of thiols (usually ethanethiol) or tetrahydrothiophene, to assist in leak detection. Processed natural gas is, in itself, harmless to the human body, however, natural gas is a simple asphyxiant and can kill if it displaces air to the point where the oxygen content will not support life.

Natural gas can also be hazardous to life and property through an explosion. Natural gas is lighter than air, and so tends to dissipate into the atmosphere. But when natural gas is confined, such as within a house, gas concentrations can reach explosive mixtures and, if ignited, result in blasts that could destroy buildings. Methane has a lower explosive limit of 5% in air, and an upper explosive limit of 15%.

Explosive concerns with compressed natural gas used in vehicles are almost non-existent, due to the escaping nature of the gas, and the need to maintain concentrations between 5% and 15% to trigger explosions.

Energy Content, Statistics and Pricing

Quantities of natural gas are measured in normal cubic meters (corresponding to 0° C at 1 atm) or in standard cubic feet (corresponding to 60° F and 30 inHg). The gross heat of combustion of one normal cubic meter of commercial quality natural gas is around 39 megajoules (˜10.8 kWh), but this can vary by several percent. In US units, one standard cubic foot of natural gas produces around 1000 British Thermal Units (BTUs). The actual heating value when the water formed does not condense is the net heat of combustion and can be as much as 10% less.

The price of natural gas varies greatly depending on location and type of consumer, but as of 2006 a price of $10 per 1000 cubic feet is typical in the United States. This corresponds to around $10 per million BTU's, or around $10 per gigajoule. Natural gas in the United States is traded as a futures contract on the New York Mercantile Exchange. Each contract is for 10,000 MMBTU (gigajoules), or 10 billion BTU's. Thus, if the price of gas is $10 per million BTU's on the NYMEX, the contract is worth $100,000. In the United States, at retail, natural gas is often sold in units of therms (th); 1 therm = 100,000 BTU. Gas meters measure the volume of gas used, and this is converted to therms by multiplying the volume by the energy content of the gas used during that period, which varies slightly over time. Wholesale transactions are generally done in decatherms (Dth), or in thousand decatherms (MDth), or in million decatherms (MMDth). A million decatherms is roughly a billion cubic feet of natural gas.

In the rest of the world, LNG and LPG is traded in metric tons or mmBTU as spot deliveries. Long term contracts are signed in metric tons - and to convert from one system to the other requires should better be described here, than a very isolated market. A cubic foot is a volumetric measure, MT is weight. The LNG and LPG is transported by special ships/containers, as the gas is liquified - LPG cryonic. The specification of each LNG/LPG cargo will usually contain the energy content, but this information is in general not available to the public.

Natural Gas Processing

The image below is a schematic block flow diagram of a typical natural gas processing plant. It shows the various unit processes used to convert raw natural gas into sales gas pipelined to the end user markets.

The block flow diagram also shows how processing of the raw natural gas yields byproduct sulfur, byproduct ethane, and natural gas liquids (NGL) propane, butanes and natural gasoline (denoted as pentanes +).

Schematic flow diagram of a typical natural gas processing plant

Storage and Transport

The major difficulty in the use of natural gas is transportation and storage because of its low density. Natural gas pipelines are economical, but are impractical across oceans. Many existing pipelines in North America are close to reaching their capacity, prompting some politicians in colder climates to speak publicly of potential shortages.

LNG carriers can be used to transport liquefied natural gas (LNG) across oceans, while tank trucks can carry liquefied or compressed natural gas (CNG) over shorter distances. They may transport natural gas directly to end-users, or to distribution points such as pipelines for further transport. These may have a higher cost, requiring additional facilities for liquefaction or compression at the production point, and then gasification or decompression at end-use facilities or into a pipeline.

In the past, the natural gas which was recovered in the course of recovering petroleum could not be profitably sold, and was simply burned at the oil field (known as flaring). This wasteful practice is now illegal in many countries, especially since it adds greenhouse gas pollution to the earth's atmosphere. Additionally, companies now recognize that value for the gas may be achieved with LNG, CNG, or other transportation methods to end-users in the future. The gas is now re-injected back into the formation for later recovery. This also assists oil pumping by keeping underground pressures higher. In Saudi Arabia, in the late 1970s, a "Master Gas System" was created, ending the need for flaring. The natural gas is used to generate electricity and heat for desalinization. Similarly, some landfills that also discharge methane gases have been set up to capture the methane and generate electricity.

Natural gas is often stored in underground caverns formed inside depleted gas reservoirs from previous gas wells, salt domes, or in tanks as liquefied natural gas. The gas is injected during periods of low demand and extracted during periods of higher demand. Storage near the ultimate end-users helps to best meet volatile demands, but this may not always be practicable.

I Hope this article helpful for us......... hmmmmmm...........

Source Article: www.naturalgasbank.com