Inlet pipe smaller than Relief valve inlet opening

[TylerM]

Here’s my situation. I have a relief valve with an inlet opening of ¾” but the line going to it is ½” [I know this configuration isn’t code compliant because of UG-135(b)(1)].

The problem I see with this configuration is, because the area increases at the relief valve inlet, the pressure in the line will drop at that point (the inlet of the RV) meaning the RV won’t open at the correct set pressure, it’ll open when the system is at a higher pressure and when that drop is pressure at the inlet is at the set pressure.
My solution to this problem is to calculate this drop in pressure from the smaller line and if it is under 3% of the set pressure (the set pressure tolerance) then this configuration would be acceptable.

I would like to get other’s opinions on this method because this isn’t “design by code” but more like “design by calculation”. Also, is the 3% set tolerance only applicable when the line in is at least the size of the inlet for the RV (like in the code)?

Thank you for all of your experience and wisdom.

 

[Ehzin]

Is this a thermal RV? It's the only scenario I would imagine where you may not run into inlet pressure drop issues during a relief event.

Thanks,
Ehzin

 

[mk3223]

A reference of the 3% pressure drop of the PSV inlet line per Thread1203-428995

 

[chicopee]

Rule is that the inlet and outlet of RV's have to be the same size as their inlets and outlets. Insurance co. and local state boiler and pressure vessel inspectors will require it.

 

[Ehzin]

As both you and Chicopee mentioned, multiple codes list requirements regarding minimum inlet size. For instance API-520 Part II, 6th Ed. Section 5.1 has a similar requirement. I'm going to avoid commenting on the code requirements but calculating the pressure drop as you intend should provide some confidence that the relief valve would behave as required.

If you can calculate the pressure drop and pressure at the inlet to the relief valve during the design relief scenario and so long as it doesn't exceed the 3% rule then you should at least satisfy that requirement, in my opinion. However, I believe you'll likely run into an issue if this is anything but a thermal relief. If there's any significant relief requirement I would expect problems.

Thanks,
Ehzin