PIPING TWO 100% PSV's IN PARALLEL

[macabihaifa]

Hello,
when installing two 100% PSV's in parallel, one open and the second closed, what will be recommended
and good engineering practice from the mechanical point of view (reaction force in operation) and from
piping design as pressure drop (3% of set pressure in inlet piping):
to use 3-way valve between the 2 PSV's or each PSV with it's own valve (with an interlock) ?

regards and thanks

 

[The Obturator]

This is not a 'parallel installation' as such. Parallel installation is where you have more than one Pressure-relief Valve relieving at the same time to pass the minimum required capacity caused by an overpressure event. What you have here is a redundancy set up, sometimes referred to as a stand-by or isolation valve set-up.

Can you clarify your question? Are you expecting, or have calculated an inlet pressure drop greater than 3%? Have you considered the pressure drop through a changeover type valve? This would also be in addition to any pressure drop in the upstream piping.

Depending on the design (and manufacturer), the pressure drop through a changeover valve can vary considerably. There are solutions but it depends on the size and application of the whole thing.

It might be worth your while to study the issue and various solutions in API 520 Part II - Section 8 - PRD Isolation (Stop) Valves.

 

[georgeverghese]

3 way valves are no good from an isolation point of view - they are more like divertor valves.
Use full bore ball or gate valves in this application; pressure drop calcs to confirm pressure drop at rated flow should be done nevertheless.

 

[goutam_freelance]

As per ASME-1, isolation valves are used for power-actuated PRVs which are sized for not less than 10% of steaming capacity. However, no isolation valves were permitted between the main PRVs and the boiler.

Regarding reaction forces, for closed systems, the forces originate near the exit of PRV outlet vent pipe. For open systems, the forces on steam piping occur at or near the discharge elbow. The guidance on calculation for forces and moments is available in B31.1.

 

[The Obturator]

@goutam_freelance You are making incorrect statements (again, due to you referring to old editions of ASME) Per ASME I (2023) PG-71.3.1 you are allowed the use of changeover valves between boiler and pressure-relief valves provided certain conditions are met.

Since you also assumed the OP is discussing ASME I, for reference, changeover valves are allowed under ASME VIII (2023) Non Mandatory Appendix paragraph M-5, again provided certain conditions are met.

Regarding pressure-relief valve reaction forces, do you not think that reaction forces would act on the valves inlet and the piping leading to it?

 

[goutam_freelance]

You are making incorrect statements (again, due to you referring to old editions of ASME) Per ASME I (2023) PG-71.3.1 you are allowed the use of changeover valves between boiler and pressure-relief valves provided certain conditions are met.

I can not believe this as a problem in the change-over valve may lead to a catastrophic event.

Regarding pressure-relief valve reaction forces, do you not think that reaction forces would act on the valves inlet and the piping leading to it?

Yes. Dynamic forces at PSV elbow will be transferred to valve inlet piping.

 

[The Obturator]

That is why the code entry states "...under certain conditions". Maybe reading (having) the latest ASME Code editions will help.

 

[shvet]

good engineering practice from the mechanical point of view (reaction force in operation)

Do you mean these PSVs have open discharge nozzles? For info

https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/docs/El%20Paso-Mojave%20GT%202012-5-2%20Redacted.pdf

good engineering practice from ... piping design as pressure drop (3% of set pressure in inlet piping):

The good eng practice is to install a PSV directly on the PV it protects.
Most reputable manufacturers have a dedicated design of changeover valves to minimize the hydraulic losses, e.g.

Change-Over Valves for Industrial Applications | LESER

Find the right change-over valve for your industrial application: Products ✓ Basic knowledge ✓ Understand how they work ✓ ➤Find out more!

www.leser.com

Anderson Greenwood Safety Selector Valve | Emerson US

This Safety Selector Valve is a a dual pressure relief device system that provides an efficient method of switching from an active PRV to a standby, maintaining system overpressure protection.

www.emerson.com

also note that changing the PSVs over can be realized with conventional fulbore valves equipped with a mech linkage (e.g. chain) or a mech programmable lock

Protect your pressure safety valve system with interlocks

Alcatraz Interlock PSV system ensures maintenance procedures on safety relief systems operate in a safe sequence.

alcatrazinterlocks.com