The following steps outline some of the considerations to be given as part of the sizing procedure.

The aim of pressure relief valves is to prevent damage to equipment, prevent injury to personnel and to avoid potential risks to the environment.

It is therefore critical that proper relief-valve sizing is carried out to ensure the fluid has sufficient flow area to exit the process thus safeguarding the safe operation of the process plant and equipment.

As a safety pressure relief valve must be capable of operating at all times, in addition to proper sizing, it is also essential that due care has been given to the proper selection, manufacturing, assembly, testing, installation and maintenance of PRDs.

This is essential to ensure maximum protection in line with the design intent.

Each potential cause of overpressure is generally referred to as a scenario and all potential overpressure scenarios must be identified, characterised (as credible or non-credible) and documented prior to sizing a relief valve or relief system.

Two common Standards used for the sizing of pressure relief valves are ISO 4126 American Petroleum Institute API RP520 Part I.

The latter Standard is the most widely used for relief valve sizing.

These standards outline several equations for sizing relief valves for various flow conditions such as liquid flow, gas flow (critical and sub-critical), steam flow and two-phase flow.

The pressure is the least of the values for the internal or external pressure as determined by the vessel design rules for each element of the vessel using actual nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure.

The MAWP is the basis for the pressure setting of the pressure-relief devices that protect the vessel.

The MAWP is normally greater than the design pressure but can be equal to the design pressure when the design rules are used only to calculate the minimum thickness for each element and calculations are not made to determine the value of the MAWP.

Note, small amounts of leakage may occur at 92 to 95 of the set pressure for spring-operated relief valves.

When the relief valve is at the MAWP, the overpressure and accumulation are equal.

This includes the superimposed backpressure and built-up backpressure due to fluid discharge from the relief device through the downstream piping.

The ASME Boiler Pressure Vessel Code Section VIII outlines design requirements for pressure vessels and the relief valves protecting them as a percentage of the maximum allowable working pressure (MAWP).

The entire figure is relative to the MAWP which is assigned an arbitrary value of 100.

This relationship applies to vessels protected by a single device.

If the vessel is protected by multiple devices, then one relief valve must be set no higher than the MAWP however the others can be set as high as 105 of the MAWP.

The anticipated operating pressure should be sufficiently low to prevent activation of the relief device during normal operation.

The operating margin required depends on the type of relief device and the pressure control capability of the process.

The exception to this case is fire exposure scenarios where the allowable accumulation is 121 of the MAWP.

In scenarios where multiple relief devices are used for non-fire conditions, the allowable accumulation is 116.

Dresser Psv Sizing Software