Set pressure for safety relief valve is more than design pressure

[kwisatz_haderach]

Where i work there is this pressure vessel for hydrogenation reaction, on its name plate it is written that the design pressure is -1 +1 barg and HT pressure is 1.3 barg. It has a safety relief valve and it is written on it that the set pressure is 4.98 bar and also it has a rupture disc. It doesn't seem reasonable to have a set pressure of more than the pressure for which the vessel s designed. Also from readings i found out that the set pressure cannot exceed the MAWP of vessel. So is it possible that the MAWP is more than the design pressure. Can someone please explain this to me?

 

[LittleInch]

The way Pressure Vessel design goes the design pressure is essentially your start point for the design.

Once you've designed you vessel then the programs work out a MAWP which is equal to or higher than the DP. This is because some things are minimum thicknesses or material strength and hence the actual MAWP is greater than the DP. If there is no separate MAWP, then you use the DP.

Now there are few things which are not clear.

1) Is this in fact a Pressure Vessel? Many codes ad regulations limit PV design to 1 bar / 15 psig.
2) Most Pressure vessel have a plate welded to them which lists MAWP - Is this the nameplate you refer to? Can you post a picture of it?
3) Is the valve actually set to 4.98 bar? If so it seems very high
4) What pressure is the bursting disc set / designed for?
5) What is "HT pressure"?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[david339933]

10% or 3 psi above MAWP for Sec. VIII Div. 1

 

[kwisatz_haderach]

Thanks LittleInch for your kind reply.
1. I think it's a pressure vessel.A reaction called hydrogenation happens in that reactor. Inside, it has coils in which steam at 9 bar passes for heating the feed oil. Also on the name plate it is mentioned that it is designed according to ASME BPVC sec 8 Div 1.
2.I can't post a picture now. Will post tomorrow. But the name plate doesn't mention any MAWP, only shell design and HT pressures and Coil design and HT pressures
3. I don't know how to verify that. But going with name plate, it is written that the set pressure = 4.98 bar.
4. I will find out and let you know.
5. Hydrostatic test pressure.

 

[kwisatz_haderach]

david339933, so MAWP can be offset by design pressure by 3 bar? How can i know MAWP from design pressure?

 

[david339933]

3 psi not bar. MAWP is on the nameplate

 

[mk3223]

Can a P&ID be shared for reference.
It doesn’t make sense for a PSV setting higher than the vessel MAWP per design data as stated …

 

[georgeverghese]

No one would be more concerned about this than the Plant Manager, or his plant safety designate. Have you asked them ?

 

[kwisatz_haderach]

@mk3223 I don't have the P&ID for that only general arrangement drawing. Is it possible that the vessel is first designed for internal pressure of 1 barg and later they considered the vaccume condition and for a vessel of this size(2910mm dia and 5000mm height) it gave them more thickness than the one needed for internal pressure design. And based on that thickness they got more pressure taking capability?