Specifications Safety relief valves

Safety Relief Valves Conventional safety relier valves (Fig. 26-14) are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting or the valve is reduced by the amount of superimposed backpressure expecied. Higher built-up backpressures can result in a complete loss of continuous valve capacity. The designer must examine the effects of other relieving devices connected to a common header on the performance of each valve. Some mechanical considerations of conventional relief valves are presented in the ASME code however, the manufacturer should be consulted for specific details. 

Determine size and specifications for all safety relief valves and/or rupture disks for process safety relief (including run-a-way reactions) and relief in case of external fire. 

Note that in lieu of testing, Par (b) 2 and (b) 3 above allows the use of a capacity factor of 0.80 as a multiplier on the stamped capacity of the spring loaded safety relief valve (nozzle bpe). Some manufacturers test specific valve/rupture disk combinadons and determine the actual capacity factor for the combination, and then use this for the net capacity determination. See Figures 7-10, 7-11, 7-12, 7-13A and 7-13B. c. A rupture disk de ice may be installed on the outlet side of a spring loaded safety relief valve... 

Figure 7-33. Safety relief valve specification for process overpressure example.

Rupture disk, 455 Safety relief valves, 454, 467, 481 Specifications, centrifugal pumps, 209 Spray nozzle particle size, 225 Standards and Codes, 31, 32, 33 Static electricity, 536 Static mixing, 332 Applications, 336 Calculations, 337, 338 Materials of construction, 337 Principles of operation, 335 Type of equipment 334-338... 

More formal inspections also verily the operation of safety and other controls such as low water and fuel cutoffs, level controls, fusible plugs, pressure gauges, water glasses, gage cocks, stop valves, safety/relief valves, and BD valves and lines. Also, FW pumps, flue and damper arrangements, combustion safeguards, name plate specifications, set pressures, boiler connections, floats, mercury switches, bellows, and other components may be inspected. 

Codes and standards relevant to safety relief valves (SRVs) can vary quite considerably in format around the world, and many are sections within codes relevant to boilers or pressure-containing vessels. Some will only oudine performance requirements, tolerances and essential constructional detail, but give no guidance on dimensions, orifice sizes and so forth. Others will be related to installation and application. It is quite common within many markets to use several codes in conjunction with one another and it is not uncommon that specifications call for sections taken from several codes, which makes compliance by manufacturers complex and uneconomical. An overview of most common worldwide codes and standards is given in Appendix M. 

With the existing safety relief valve (SRV) technology, codes and regulations, NO specific type of SRV is suitable for all overpressure protection conditions. Therefore, there are different types and designs suitable for different applications and process conditions. 

These guidelines will provide some guidance in selecting a safety relief valve (SRV) for use in a specific application or process condition where the use of traditional spring-operated valves could be questionable. 

Individual safety relief valve specifications shall be communicated on API 526 specification sheets or equivalent substitute provided by the end user. When the relieving contingency is vapour and liquid, rates and properties for each at the allowable overpressure shall be provided. 

Occasionally, there may be business pressures or maintenance scheduling problems that would encourage the delay of prooftesting of safety critical alarms and shutdown systems. Such situations can also delay of vessel inspections and safety relief valve testing. Some type of variance procedure or review policy should be defined to handle this occasional need. Such a policy ought to require the review of all of the inspection and test records on the specific equipment involved as well as an approval of the superintendent of the area. 

Another widely-used predictive method is the use of Fault Tree Analysis. This is a reversethinking method. The analyst assumes an accident or specific undesirable event—the so-called TOP Event. This could be the release of a toxic gas from a reactor safety relief valve. 

These written procedures can be general for inspections of a specific grouping such as storage tanks, centrifugal pumps, heat exchangers, or safety relief valves. It can be stand-alone documents or embedded in work orders, or a separate manual. It can even be the original equipment manufactmer s manuals for the given model [6]. 

Comment A safety relief valve is an automatic pressure-actuated relieving device suitable for use either as a safety valve or a relief valve, depending on the application. Safety relief valves are designed in accordance with ASME Boiler and Pressure Vessel Code Section VIII or Section III for nuclear applications. These valves are general-purpose rehev-ing devices suitable for varied apphcations on compressible and noncompressible fluids. These valves are most commonly used in refineries, oil and gas production facilities, pipelines, paper mills, chemical plants, and other industrial facihties (Fig. 5.190). The following descriptions are for design variations of safety relief valves for specific apphcations. 

A specific inspection frequency shall be estabhshed for each safety relief valve. The inspection frequency will be a function of inspection history, relief system design, process conditions, operating experience, on-stream service capabilities, economics, and regulatory mandates. 

Usual practice is to use the terms safety valve or relief valve to indicate a relieving valve for system overpressure, and this will be generally followed here. ATien specific types of valves are significant, they will be emphasized. 

Commonly reported near misses include such events as exceeding operating limits, a release of a chemical or other hazardous substance that does not meet the threshold for a process safety incident metric, activation of relief valves, interlocks, or ruptured disks. Companies may establish near-miss metrics based on the specifics of their operation, based on their observations of frequent upsets or failures, or to track and correct observed unsafe practices or behaviors. 

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