Pressure Relief Disposal Method – Atmospheric Discharge Safety Considerations

Non-hazardous Materials
Non-hazardous fluids may be safely discharged either to the atmosphere or to grade. Included in this class of materials are air, steam, and water, provided that they are discharged in a location and direction that will prevent contact of the relief stream with personnel.

Flammable and Combustible Materials
Flammable and combustible liquids generally cannot safely be discharged to the atmosphere. Their density virtually guarantees that they will fall to grade level, where they will present a hazard to equipment and personnel. They should be routed to separation and collection equipment for recycle or other ultimate disposition.

Flammable vapors are discharged from relief devices at very high velocities. The jet of material exiting the tailpipe therefore rapidly entrains large amounts of air, quickly reducing the concentration of flammable components in the jet below the levels at which combustion can be supported. In the absence of wind, the portion of the discharged “cloud” whose fuel concentration is between the lower and upper flammable limits forms a well-defined, narrow cylinder whose length and width depend on the exit diameter, exit velocity, and composition of the relief stream. In the highest wind speeds, this jet may be blown entirely horizontal. However, in the absence of downdrafts, even for vapors of high specific gravity (e.g., molecular weight two or three times that of air), the flammable portion of the vapor jet never extends below the level of discharge. As long as no ignition sources exist above the discharge elevation, therefore, there is very little chance of igniting flammable vapors discharged from a relief device to atmosphere.

This analysis is based on the discussion and methods outlined in API Recommended Practice 521, and backed up by vapor cloud dispersion calculations and by decades of operating experience. The analysis is also predicated on the absence of liquids in the discharged stream. When selecting atmospheric discharge for a relief device handling flammable substances, care should be used to ensure that liquid will not be present in the discharge. Possible mechanisms for liquid leaving a relief device that normally has only gases or vapors at its inlet include: overfilling of the equipment item with liquid, liquid carryover due to two-phase flow of viscous or foamy substances, and liquefaction of auto-refrigerating relief streams.

Toxic Materials
The considerations for relief streams that may contain toxic substances are similar to those for flammable streams. Streams with potentially toxic liquids should be contained in a closed disposal system for separation, possible treatment, and disposal. The discussion of the jet nature of a relief device discharge and the resultant dilution of the discharge with air holds equally well for gasphase streams containing toxic substances. However, the concentrations below which exposure to toxic materials can be tolerated are typically a great deal lower than those below which flammable substances will not support combustion. The lower flammable limit of most hydrocarbons, for example, is in the range of one to three percent by volume. The lower limits for acute (i.e., shortterm exposure) toxic effects vary considerably from one compound to another, but in some cases can be in the range of ten parts per million (ppm) by volume. The threshold concentrations at which longer exposures may produce undesirable effects can be even smaller, but because relief discharges are normally of short duration, these lower threshold concentrations are not usually of concern. Any decision to discharge potentially toxic gases or vapors to atmosphere should be based on modeling of the atmospheric dispersion of the discharge using carefully chosen, generally conservative modeling techniques and assumptions. The modeling results would be expected to indicate that at locations to which personnel access is possible, the predicted concentrations of toxic substances would not reach unacceptable levels.

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