Pressure relief valves are used to protect the piping and equipment against excessive over-pressure. Before designing the relief device, each scenario must be considered carefully to determine the “worst case” condition which will dictate the relieving device capacity. Following overpressure scenarios should be considered.

Read More: Relieving Devices, types, and working

Blocked Discharge

Blocked discharge is one of the most common cases that can occur in a process industry. The outlet of any vessel, pump, compressor, heat exchanger or other equipment can be locked, either mechanically or due to human error. In this case, the relief load will be estimated based on the maximum flow capacity of the pump, compressor, or other linked flow sources at the relief condition.

Tube Rupture

A tube rapture case is considered when a large difference in design pressure exists between the shell and tube side of a heat exchanger. The pressure relief provision is provided on the low-pressure side. Generally, one tube failure is considered in the design of the relief device. The flashing effect due to the direct contact between hot and cold streams should be considered. Also, the possibility of a transient overpressure caused by the sudden release of vapor into an all-liquid system should be considered.

Control Valve Failure

The position failure of instruments and control valves requires careful consideration. Control valve failure can occur due to mechanical failure, control loop failure, and instrument air supply failure. Relief protection for these factors must be provided. Relief valve sizing requirements for these conditions should be based on flow coefficients provided by the manufacturer and pressure differentials for the specific control valves and the facility involved.

Thermal Expansion

If isolation of a process line on the cold side of a heat exchanger can result in excess pressure due to the continuous heat transfer from the hot side fluid, then the line or cold side of the exchanger should be protected by a relief valve. If any equipment item or line can be isolated while full of liquid, a relief valve should be provided for thermal expansion of the contained liquid. Low process temperatures, solar radiation, or changes in atmospheric temperature can necessitate thermal protection. Flashing across the relief valve also needs to be considered.

Utility Failure

General plant utilities include cooling water, electric power, and instrument air. Failure of any of these utilities may create an over pressure scenario.

For instance, loss of cooling water may cause over pressure scenario in heat exchangers, distillation columns or any other equipment utilizing it for cooling purpose. The cooling water failure is considered as the governing case in the sizing of flare system.

Process units contains electric pumps, coolers, and motor driven compressor which can be affected by the electric power failure. The instrument air system failure can affect all control loops of the units. The failure of instrument loops should be considered to estimate the relief load.

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