Safety relief valve information

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. 

Investigators observed a device that appeared to be a combination safety relief valve and a vacuum breaker. The information on this chrome-topped instrument read, Press 26,700... 

This organization supplies practical information about block valves, safety relief valves, and control valves via their Valve Magazine. 

Investigators observed a device that appeared to be a combination safety relief valve and a vacuum breaker. The information on this chrome-topped instrument read, Press 26,700 CFM 32.5 psig, and VAC Relief 1,750 1 psig. Despite the nameplate, the trailer was not equipped to deal with vacuum [1],... 

On November 26, 1993, the USNRC issued Information Notice 93-89, "Potential Problems witii BWR Level Instrumentation Backfill Modifications," to alert licensees to potential problems that have been identified involving hardware modifications to the reactor vessel water level instrumentation system. This information involved the potential to pressurize the reference legs of the water level instrumentation if a backfill system is installed with the injection point on the instrumentation side of the manual isolation valve in the reference leg. If that valve is closed inadvertently during backfill system operation, the closure could result in a severe plant transient. At some plants, valve closure would cause all safety relief valves to open and potentially impact ECCS response. Licensees were advised to review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. 

Mr. Kletz provides a story about a packaged unit containing a reciprocating compressor. The compressor was started in error. The discharge valve was in the closed position, and the pressure increased until the packing around the cylinder rod blew out. This compressor was equipped with a relief valve that was merely a sentinel to warn the operator to take action. The sentinel was incapable of relieving the full output of the equipment. It is not an acceptable design standard today, but this example supports the need for available process-safety information on all equipment. [2]... 

The need for metrics is particularly important in process safety, in no small measure because the hazards may not be readily evident. Unlike some other safety risks where dangerous situations are more apparent—such as unsafe scaffolding, unsecured cables, trenches, and other excavations—information on the status and safety of hazardous containment systems (e.g., internal corrosion, an improperly sized relief valve) is not generally visible. Without a constant and reliable flow of information on process safety performance and management systems, leaders may, in essence, be flying blind. 

After the Three Mile Island Unit 2 (TNI) accident, the NRC determined that there was a need for direct indication of relief and safety valve position in the control room. This determination was made because the TMI accident demonstrated that, during an accident, these valves may not operate as expected and that the safety and relief valve instrumentation may fail to provide the operator with sufficient information concerning the status of these valves. Therefore, the NRC established new guidance in NUREG-0737 which addresses the installation of improved safety and relief valve indication in the control room to enhance the operator s ability to diagnose a safety and relief valve failure and/or incorrect position. 

Organisation Organisation Inadequate quality control Inadequate quality control procedures. The information that the isolation valve on the safety relief vent remained in the closed position and locked-out was limited to a handwritten note. Although quality control measures (lockout/tag-out procedures to manage the work on the heat exchanger rupture disk were applied), the progress and current state of the maintenance job was not clearly documented. Moreover, there was not a post-maintenance quality check to verify if the service was properly concluded. 

Pressure relief of a runaway reaction is likely to be via a bursting disc or a safety valve, or a combination of both these items. Further information about these is given in Chapter 9. For relief system sizing, it is important to know the pressure at which a relief device will open. 

If solids are allowed to accumulate on the underside of a bursting disc or safety valve, then it is likely that the relief device will not operate when required, at least not at. the required set pressure. Safety valves may be more vulnerable in this respect than bursting discs, and it is common practice to fit a bursting disc upstream of the safety valve to protect it. Further information is given in 10.2.1 and reference 1. 

Sizing formulae based on equations (A6.4) to (A6.6) are given in many safety valve manufacturers catalogues. Where possible, the method given by the manufacturer of the particular safety valve should be used to find its capacity because this will contain the correct values of flow area and discharge coefficient for the valve. (This is not usually true in the USA where National Board certification information should be used.) The relief lines upstream and downstream of the safety valve also need sizing. Further information is given in 9.7 and references 2 and 4. 

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. 

Detailed information regarding compressed gas containers, pressure regulators, pressure relief devices, and valve outlet connections is contained in Chapters 5 through 9. Additional safe handling procedures for compressed gases can be found in CGA P-1 [7], and in CGA AV-1, an audiovisual safety training program of the same title. 

Depending on the contents of a mixture, one or more pressure relief devices may be required on the container. Most pressure relief devices are an integral part of the valve. However, some devices are installed directly into the bottom or top of the container. For an in-depth discussion on pressure relief devices for mixtures, see CGA S-7, Method for Selecting Pressure Relief Devices for Compressed Gas Mixtures in Cylinders, [10] Also consult Chapter 7 of this Handbook for a presentation on pressure relief devices. The selection of the proper pressure relief device for a mixture involves a knowledge of the contents of the mixture, the final pressure, its physical state, and the type of container which will be used. To simplify the process of selecting the proper pressure relief device for a specific mixture, refer to the mixture safety selection algorithm in Fig. 2. Consult CGA S-7 for complete information on the use of this algorithm. [10]... 

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