![]() ![]() Therefore, a designer can develop a “typical” rated Cv list for globe valves which in turn can be used for the bypass in the preliminary sizing of the PSV. Note that the "typical" globe valve Cv is selected such that it is conservative.Īs indicated in Table 1, the rated Cv of 300# globe valves are comparable between suppliers. Note this table only focuses on ASME #300 cast steel globe valves but could be developed for other ASME flange rating globe valves. Table 1 shows the rated Cv of various ASME 300# cast steel globe valves, with Fisher ED equal percent full-sized trim control valve for comparison. Option 1 – Specify a “typical” globe valve rated Cv for the bypass when sizing PSV. The remainder of the paper addresses this concern and provides options that can be used to design the control valve bypass to minimize the risks of unexpected late changes. This can result in in unexpected changes late into the project (either to the bypass or to the PSV and associated piping). If the 8” Fisher control valve rated Cv was assumed for the bypass valve when sizing the PSV, yet the Pacific globe valve was installed on the bypass, then the PSV could be undersized. This is particularly evident for an 8” Fisher ED equal percent full-sized trim control valve has a rated Cv of 567 CITATION Eme19 \l 4105 (Emerson, 2019) and an 8” Pacific ASME 300# cast steel globe valve has a rated Cv of 873 CITATION Pac \l 4105 (Pacific®CSV). This assumption may not be valid if a larger globe valve (>4” body as observed per Table 1, this threshold may vary based on specific vendors) is used on the bypass. This may result in the designer assuming the bypass rated Cv equals that of the control valve for the same body size. Despite the clear acknowledgement in API 521 Section 4.4.8.1 CITATION Ame14 \l 4105 (American Petroleum Institute, 2014) that bypasses around a control valve needs to be considered in the failure of automatic controls, designers may not give the globe valve on the bypass the same attention regarding its rated Cv. The bypass globe valve rated Cv is typically not known when the preliminary sizing of the PSV is conducted. Typically in preliminary design a control valve is assumed to be one line size smaller than the piping.įrequently, control valves have a bypass to allow operation when the control valve is isolated for inspection or maintenance. ![]() This relief flow calculation is based in part on the rated Cv of the control valve, which can be assumed from typical values based on valve body size and specified to the supplier as a maximum allowable rated Cv. When a control valve is the principle restriction to flow between the high pressure and lower pressure rated systems, it can be used to determine the required relief flow and size the PSV orifice. A common means of overpressure protection is installing a pressure relief valve (PSV), which will open when exposed to high pressures and relieve flow to a safe location (flare, atmosphere, etc.) to prevent overpressure. ![]() Psv Relief Sizing code#This can result in both a safety issue, and a code noncompliance. Without a means of overpressure protection, the high pressure upstream of the control valve can overpressure the downstream system. It is not always possible to design the system downstream of a control valve to the design pressures upstream of the control valve. This paper discusses various design options for the control valve bypass to reduce the risk of late changes to PSV sizing, and their respective advantages and disadvantages. This assumption can lead to the PSV being undersized if the certified bypass rated Cv is greater than what was initially assumed. Despite this requirement, the bypass Cv might incorrectly be assumed to match the associated control valve’s maximum flow coefficient (rated Cv) during preliminary sizing of the PSV when final supplier data is not available. API 521 Section 4.4.8.1 CITATION Ame14\l 4105 (American Petroleum Institute, 2014) recommends consideration of the bypass around a control valve in the sizing of a downstream pressure relief valve (PSV). ![]()
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