PSV Orifice

[Reneka1]

Greetings.

I need to replace a PSV with a different one. The operating conditions are the same and I will be able to adjust the set pressures when required. The only problem I have are the orifice sizes. The old PSV has an orifice designation of 30 mm2, but the replacement has an orifice designation of 40 mm2. Will there be a problem if I replace the old one with the new one?

I have another one where the old orifice is more than the replacement.

 

[IdanPV]

Did you compare the reliving capacity of the new PSV and the current PSV?

 

[Reneka1]

Did you compare the reliving capacity of the new PSV and the current PSV?

I did not. I used the data sheets to find the information. Those PSV data sheets don't have the relieving capacity.

 

[Latexman]

Possibly inlet pressure drop and outlet pressure drop will exceed Code, 3% and 10% respectively.

 

[shvet]

to add the above
assuming this is a PSV of the conventional design actual relief rate will be +33% - check if the relief utilization system has enough spare capacity

 

[LittleInch]

Will there be a problem if I replace the old one with the new one?

Quite possibly yes as its flow rate will be considerably higher. That might be ok or it might cause too much gas to escape when the PSV is lifted or it might overpressure the flare system. Orifices are there for a good reason - unless you know that you can increase the relief flow, then don't do it.

If it is smaller than the original then it is potentially a very bad move as the relief flow will be smaller and the pressure could continue to rise. That's a bad thing normally.

 

[The Obturator]

Without knowing the details such as manufacturer, model and in particular the application, it cannot be fully confirmed. If you have a quotation from a vendor for the new size, ask them to review the suitability of the new size for your application for the new selected valve. That is the only definitive approach.

You're also stating that you can adjust set pressure when required. Do you realise you may need to change the spring and that the relief capacity would change accordingly?

Are you actually discussing a Pressure-relief Valve or a Pressure Regulating Valve?

 

[Snickster]

As stated above the larger orifice will provide 33% more flow when it pops than the old orifice. Considering that the actual relief flow required is per the old orifice size then you will be relieving at 33% rate more than required so that the outflow from the protected vessel through the relief valve will be 33% greater than the inflow. This will cause fluctuation of pressure on the upstream side so the pressure on the upstream side will want to drop back below the relief set pressure as soon as the valve pops. This will cause chattering of the valve which is undesirable and could damage the valve seats.

I don't believe it will cause higher back pressure in the discharge header since the average flowrate in the discharge pipe can not be any greater than the actual flow (on average) into the vessel to be protected even though the valve is oversized.

There will be higher instantaneous inlet pressure drop though which may exceed the 3% allowable but this will just show up as lower inlet pressure on the relief valve during the low cycle pressure fluctuations due to the differences in flow in versus flow out, which will cause the chattering to be even greater.