Pressure safety valves (PSVs) are one of the most commonly used devices in Process industry to protect a system from overpressure caused by any upset condition. They are one of the most reliable devices to reduce the risk arising out of over-pressurization. Typically, they can reduce the risk by 100 times. Therefore, it is extremely important to specify them correctly. Here are the step-by-step guides to specify a PSV:

i) Study the system to be protected against overpressure.

ii)  Identify various upset scenarios which can cause excessive pressure in the system i.e. which can cause a pressure which is more than design pressure of the system.

iii)  Such overpressure scenarios could be blocked outlet, control valve failure, external fire, instrument air failure, thermal expansion, heat exchanger tube rupture, cooling water failure, air-cooler fan failure, column reflux failure, etc.

iv) Define set pressure, over pressure and relieving pressure of the PSV depending upon maximum allowable working pressure and accumulation as per API Standard 520 Part I, and ASME Section VIII Division I or ASME Section I as the case may be. For this purpose, one must have clear understanding of difference between accumulation and overpressure. If the set pressure is less than maximum allowable working pressure (MAWP), the overpressure could be more than accumulation. However, if PSV set pressure is same as MAWP, the accumulation and overpressure cannot exceed the accumulation. The relieving pressure would be set pressure plus overpressure.

v)  Thus, if MAWP is 20 bar (g) and set pressure is 19 bar (g), for single operating PSV (1×100% or 2×100%) and for non-fire and non-steam case, the maximum accumulation would be 10% of MAWP or 2 bar, whereas the maximum over pressure would be 15.8% of set pressure or 3 bar. On the other hand, if set pressure is same as MAWP i.e. 20 bar (g), the maximum overpressure would be equal to accumulation i.e. 2 bar or 10% of set pressure. The maximum relieving pressure would be 22 bar (g). Design pressure can be considered in place of MAWP for this purpose as design pressure does not exceed MAWP.

vi)  While set pressure cannot exceed the maximum allowable working pressure (MAWP) for single operating PSV, it can exceed the MAWP for one or more PSVs in case of multiple operating PSVs (e.g. 2×50% or 3×50%). Thus, in above example, maximum set pressure can be 20 bar (g) for single operating PSV. Whereas for multiple operating PSVs, the maximum set pressure of the first one would be 20 bar (g) and of other operating PSVs 21 bar (g).

vii)  Overpressure should be taken differently for fire and non-fire cases and must be defined carefully for single and multiple operating PSVs. Thus, in above example, for single operating PSV (1×100% or 2×100%), with set pressure as 20 bar (g), the maximum overpressure would be 2 bar or 10% of set pressure for non-fire case and 4.2 bar or 21% of set pressure for fire case. In case of multiple operating PSVs, the maximum overpressure of the first PSV would be 16% of set pressure or 3.2 bar and of other operating PSVs, 10.5% of set pressure or 2.2 bar for non-fire case. For fire case, the maximum over pressure would be 4.2 bar or 21% of set pressure for first PSV, and 3.2 bar or 15.2% of set pressure for other operating PSVs.

viii)  The set pressure, accumulation, over pressure and relieving pressure for steam systems for single and multiple operating PSVs must be defined differently and as per ASME Section I and/or IBR (for India only).

ix)  Find out the relieving rate of the PSV corresponding to each of the applicable upset scenarios. Reliving rate calculation must be done at relieving conditions.

x)  Calculate the back pressure (superimposed plus built-up) depending upon the PSV discharge destination (flare or atmosphere in general).

xi)  Select the type of the PSV appropriately. One of the criteria for the selection is the built-up backpressure as proportion of the set pressure of the PSV. There could be a trade-off between cost of PSV (depending upon its type) and discharge pipe size, especially in the cases where PSV set pressure is low and optimum selection of PSV type should be done considering this trade-off.

xii)  Specify relieving rate, PSV design parameters and properties for all the applicable scenarios in Process datasheet of the PSV.

Date of Publication: 7 August, 2017

About Author: Satyendra Kumar Singh, B.Tech. (Chemical Technology) + M.B.A., is proprietor of Satsha Management Services-an award winning design engineering and management consulting company (www.satshamanagement.com). He possesses approximately 30 years’ experience in engineering consultancy in process and energy industries. Satyendra has authored several papers on energy, business and management, which have been published in some renowned journals/magazines such as ‘Chemical Engineering’, ‘Process Worldwide’, ‘Modern Manufacturing India’. He may be reached at satyendra.singh@satshamanagement.com, Ph. +919811293605.