Proper design of inlet pipe and outlet (tail) pipe is key for smooth and stable operation of a pressure safety valve (PSV). Improper design of PSV inlet line may cause instable operation of PSV due to chattering. On the other hand, improper design of PSV outlet (tail) pipe may cause excessive backpressure resulting into performance reduction of the PSV. Therefore, it is vital to design PSV inlet pipe and tail pipe carefully.
PSV inlet pipe should be sized so as to limit non-recoverable pressure drop in the pipe to less than 3 % of set pressure of the PSV in line with API 520 part II. The pressure drop should be based on rated capacity of the PSV, not the required capacity as pipe will handle a flow corresponding to rated capacity of the PSV when it releases the fluid. If PSV is installed on a pipe, this pressure drop should be the sum of pressure drop in non-flowing line and incremental pressure drop in flowing line.
PSV outlet pipe should be designed to avoid excessive backpressure, erosional tendency and noise. Hence, design should meet following criteria: i) built-up backpressure should not exceed the overpressure of the PSV in case of conventional type PSV and approximately 50% of total backpressure (superimposed plus built-up) in case of balanced bellows type, ii) Mach No. should be limited to 0.7, and iii) density X velocity2 should not exceed 200000. Again, above criteria should be applied based on rated capacity of the PSV, not the required capacity. If superimposed backpressure is variable, the said criteria should be applied at minimum backpressure which would be the worst case due to maximum volumetric flow rate. Further it should be checked whether there is two phase flow in tail pipe as a result of flashing of saturated liquid or high-pressure gas/vapor flowing in inlet pipe. If so, the above criteria should be applied for two phase flow.
Another important aspect of PSV outlet pipe design is selection of material of construction (MOC). As there is expansion across the PSV when PSV releases the fluid, there could be significant drop in temperature at the outlet if inlet fluid is a gas at high pressure and/or low temperature. This could warrant a MOC of outlet pipe different from inlet pipe. For example, if inlet pipe is of carbon steel (CS) and outlet pipe temperature falls below -29 deg C as a result of expansion across the PSV, it would entail outlet pipe MOC to be low temperature carbon steel (LTCS). Heat transfer from ambience to flowing fluid inside the pipe should also be considered for right selection of MOC particularly if minimum ambient temperature is higher than flowing fluid temperature inside the outlet pipe, and right MOC should be selected based on pipe wall temperature instead of fluid temperature.
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.
