Specifying Safety Relief Valves

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. 

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. 

The IPL must be capable of detecting and preventing or mitigating the consequences of specified, potentially hazardous event(s), such as a runaway reaction, loss of containment, or an explosion. A single IPL may address the multiple causes for that hazardous event and, therefore, multiple event scenarios may initiate action of one IPL. For example, high pressure in a vessel could be caused by either a pump dead-heading into it or by external fire. For both cases the vessel s pressure safety relief valve counts as a valid IPL. 

Flare systems are part of a plant s safety system. Most process units are aligned to safety relief valves that lift when specified pressures are exceeded. These safety valves discharge into the flare header. Unexpected process upsets are dumped to the flare system as a last course of action. A typical flare system includes ... 

For balanced bellows pressure relief valves, the maximum superimposed baek pressure should be specified. (For balanced bellows valves discharging to a safety valve header which is at atmospheric pressure under non-flowing conditions, the maximum back pressure is zero.)... 

How pressure vessels are designed and what determines the vessel wall thickness How the ASME Boiler and Pressure Vessel Code is used in pressure vessel design Why pressure safety valves are used and how to select, design and specify pressure relief systems... 

Although more nuanced than the OSHA rule to do with ladders, the above sentence is also prescriptive. It does, however, allow for some nonprescriptive judgment. For example, the standard does not specify the types of protection. Most designers will protect against overpressure using a combination of safety instrumentation and a mechanical device such as a pressure relief valve, but this approach is not actually a requirement of the standard. 

Alignment of the SCS relief valve to the RCS is specified by plant procedures to ensure RCS overpressure protection for all temperatures below the LTOP temperature, T p. The P-T Limit curves are shown in Figures 5.3-5a and 5.3-do For temperatures above the LTOP temperature, overpressure protection is provided by the pressurizer safety valves described in Section 5.2.2.4 or administrative procedures. 

Within this Workbook, the maximum pressure required to fully open the pressure relief device will be referred to as the "relief pressure". (Caution some papers on relief sizing refer to "set pressure" but mean "relief pressure"). For a bursting disc, the relief pressure will be the maximum specified bursting pressure and for a safety valve, it will be the set pressure plus 10% overpressure (or whatever percentage overpressure the valve has been certified at). 

Moreover, it can be observed that numerous mutual dependencies (e.g., between EquipmentSpecification and PID) as well as circular dependencies (e.g., between classes LineList, InstrumentSpecification, and SafetyValveSpecification) exist between the document classes. Usually, such dependencies arise if several versions or revisions of the documents are created during project execution. For instance, in the case of the mentioned circular dependencies, a first version of the LineList is created, based on which the InstrumentSpecification can be developed. The SafetyValveSpecification in turn requires some input from the InstrumentSpecification (namely the sizing information of the control valves). The SafetyValveSpecification, however, has an impact on the LineList, as all piping connected to a safety valve has to be sized according to the specified relief rates, such that a second version of the LineList needs to be established. 

The term set pressure is used to denote the pressure at which the relief device is open fully. This is not the definition used by some — for example, British Standards. For a safety valve the set pressure is often 10% above the set pressure as specified by the manufacturer or measured in the workshop, since this higher pressure is needed to open the valve fully. For a bursting disc, the set pressure is the nominal burst pressure plus any tolerance (or alternatively it is the maximum specified bursting pressure). The term overpressure is used here to mean the difference between the (redefined) set pressure and the maximum pressure reached during the venting process. 

---