Relief Valve Inlet Piping Size Criteria

[Engineer_M]

Appendix M-6 of the pressure vessel code states that the pipe connection to the relief valve should be at least as large as the relief valve inlet. What's the reason for this? Is it potentially dangerous to connect relief valves via expanders? Why is it okay to install it with a reducer?

Thanks!

 

[goutam_freelance]

The line up to the valve needs to have least possible resistance to flow as the valve capacity is sized based on full inlet pressure available to valve inlet. Too small a inlet pipe dia or too large pipe length will reduce the valve inlet pressure available during discharge thus reducing the actual flow from the certified capacity.

Engineers, think what we have done to the environment !

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[Engineer_M]

Okay, but then why is a reducer allowed and an expander is not?

 

[The Obturator]

Having anything smaller then the inlet nominal bore size of the PRV, will not only starve the valve of capacity but will give in a pressure drop resulting in valve chatter and ultimate malfunction. The valve will not be able to fully open and relieve the required capacity that is has been sized for, thereby putting the 'protected' vessel at risk.

The golden rule is never have anything smaller than the inlet size of the PRV. A reducer is totally OK but not an expander.

Note that there is a separate forum for safety relief valves.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[davefitz]

Most releif valves are designed to have acoustically choked flow ( mach no = 1.0) at their orifice, and the inlet nozzle will likely have a flow area at least 4 times that of the orifice , for a mach no < 0.25 . If the inlet pipe has a smaller area it will have a mach no > 0.25 and will have excessive pressure loss during valve operation, leading to chattering of the valve. Also, the piping reaction can lead to deformation or vibration of the piping ; the very high change of fluid momentum across the valve during lifting must be calculated by the piping designer and restrained by supports and included in the analysis of pipeline stresses.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[RVAmeche]

As others have said, piping smaller than the PSV inlet will result in excessive pressure drop due to the decrease in area. Here's a picture that shows why this is bad - the excessive pressure drop in the inlet piping results in the relief valve inlet "seeing" a lower pressure than the vessel once the relief valve opens, so it continually opens/closes (as Obturator mentioned) and destroys itself.

 

[davefitz]

it not only destorys itself, but the failure to remain open implies full releif capacity is not reached , and the pressure vessel itself could explode.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[goutam_freelance]

The following requirement from ASME VIII-1 Appendix M may be noted.

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