Reducer/Expander effect on PSV? - Safety Relief Valve engineering (PSV) (2024)

My current D orifice PSV has an inlet size of 20mm. If I were to attach a 8mm reducer to the inlet, what effects does it have on the PSV in terms of its set pressure, flow capacity etc?

Thanks in advance!

You still haven't read API 520 and 521 have you?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

The PSV has a defined flow rate. Reducing the area of the piping will either result in a large pressure drop between the vessel/system and device at the flow capacity, or the device will not be able to reach it's flow capacity. API 520 and ASME both spell this requirement out.

3% losses is the general standard to prevent chatter. Basically the when closed the system pressure rises; once the valve begins to open the pressure seen at the valve drops below its set pressure due to the high losses of the piping and the valve re-closes. Since the system pressure is still high, the valve keeps opening and closing and destroys itself.

Just going through api 520 clause 7.3.2 and I have another question.

Since the excessive pressure loss will cause chattering and hence, lower the capacity of the PSV and quite possible cause damage to the seating surface, will the PSV still be able to relieve excess pressure in the event of an over pressurisation?

Seat damage is a trivial matter, and a risk that's generally tolerable. Note that every metal seated PSV has a high risk of leakage after activation, due to seat damage. Chatter is a concern because it risks: (1) insufficient relieving capacity, and (2) loss-of-containment due to mechanical failure of the installation. The flowrate through a chattering valve is only a fraction of the valve's rated capacity, and this will cause the system pressure to rise. In most cases an equilibrium will be reached, at which point the PSV operates in a stable manner, albeit at an pressure that's probably greater than the allowable accumulation. The risk of loss of containment, due to mechanical failure, is much higher for PSVs in liquid service. That's because the force of impact, which occurs during each open-close cycle, is much greater in a incomprssible fluid service. Loss of containment can occur with a chattering PSV in incompressible fluid service, but the risk isn't as great.

To minimize the risk of chatter, most design codes require the inlet pipe (x-sec area) to be at least as large as the PSV inlet connection. In this particular case (8mm pipe leading to a 20mm PSV inlet), the restriction is severe. That's not a good design. It's a code noncompliance, and most likely it's an significant safety risk.