Specifying SRVs

Leak pressure The value of increasing inlet static pressure at which the first bubble occurs when an SRV is tested by means of air under the valve disc and a specified water seal on the outlet. 

An SRV is a safety device designed to protect a pressurized vessel or system during an overpressure event. An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP) (Section 3.6). 

The SRV must open at a predetermined set pressure, flow a rated capacity at a specified overpressure and close when the system pressure has returned to a... 

If the system is a corrosive environment, the rupture disc is specified with the more exotic and corrosion-resistant material to protect the SRV. 

The effective orifice size and effective coefficient of discharge specified in the API Standards are assumed values used for initial selection of an SRV size from configurations specified in API 526, independent of an individual valve manufacturer s design. In most cases, the actual area and the rated coefficient of discharge for an API-lettered orifice valve are designed so that the actual certified capacity meets or exceeds the capacity calculated using the methods presented in API 520. 

According to some end users, however, SRV vendors are only expected to provide data on the noise produced by their equipment and not on the noise radiated through associated pipework (not in the vendors supply) and the suggested means of reducing or controlling noise to an agreed level at a specified distance. 

We will consider some typical concerns when SRVs must be specified. 

What information does this test provide on the reliability and operability of the valve under the actual operating conditions Very little actually This test is specified as a copy from control valves or other isolation valves but in fact does not apply to SRVs. 

In this chapter, we will provide just one safe example of how end users can specify their requirements for safety relief valves (SRVs) so that the risk for having wrong valves installed is minimized. This example is, of course, not exhaustive but provides guidance on what sort of information could be provided to the SRV manufacturer in order to ensure a safe situation on the site. 

Undetected corrosion, fouling of SRVs, or fouling in the inlet or outlet piping of the SRV can adversely affect the setpoint and the relieving flow rate. Standby equipment that has been improperly specified, installed, or maintained is particularly susceptible to undetected corrosion and fouling. 

PPG Lake Charles was one of the first chemical plants to openly discuss problems that users experience by improperly specifying and testing safety relief valves. Expertise on new safety valves could be obtained from manufacturers sizing methods were documented by various designers and users, but no one published practical SRV testing and SRV corrosion problems. 

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