Bourbon,

The ultimate goal behind the PSV inlet line max allowable pressure drop is to avoid chattering. Since the required relief capacity is way below the 25% rule of the rated capacity, Chattering is most likely to take place.

Regards,

First, with such a large difference between set pressure and vessel MAWP, and if this is a "safety" device, I hope you aren't using this relief valve as a pressure regulator.

From another perspective, is the source of liquid overpressure great enough to cause a safety concern for the referenced vessel?  If not, you may could make an arguement the relief valve isn't needed for that case.  However, you need to make sure there aren't other components in the protected system that require the low set point.

Otherwise, HDIMR gives good advice but as I understand the 25% rule relates more to valves in vapor service that tend to pop open to near full capacity upon reaching set point.  The pop action is related to the sudden expansion of the vapor inside the "huddling chamber" of the valve disc.

I have seen at least one company's practices that suggests that valves in liquid service are characterized more by a progressive increase in lift with increase in inlet pressure rather than pop action.  A similar discussion can be found in the following reference...
"Pressure Safety Design Practices for Refinery and Chemical Operations" by Cheremisinoff, N.P.

As an excerpt from the above reference...."Liquid service valves are, therefore, less liable to chattering at low relieving rates, and they will modulate down to about 10% of design flow."

The limitation on the inlet pressure drop is a direct function of the blowdown pressure of your relief valve.  You may want to discuss your issues with the relief valve manufacturer for possible options.

EGT01, I've recently read some recommendations indicating that safety valve chatter in liquid service, though less likely, is potentially more serious than in vapor service  because of the liquid hammer effect (In Ostrwski's presentation here: http://www.sache.org/workshop/2003Faculty/presentations.asp)

The previous valve was only sized for maximum R-123 pressures (< 10 psig according to operating temperatures and a P-H diagram).  I am  re-evaluating the valve for tube rupture which has been calculated to have a liquid (water) relief of ~78 gpm.  The valve is a Consolidated 1905N, 4" x 6".  I can only assume that the valve was fit minimally to the 4" evaporator outlet flange.

If the valve set pressure had been = to the MAWP (200 psig) then I would have no worries as the max water pressure is ~80 psig.  But since the set pressure is 15.1 psig, I can't get the inlet piping to pass (3% losses).  My question was in regards to whether code allows to accept inlet pipe losses using 3% of the MAWP if the valve set pressure is sig. lower.

I have also checked with the on-site mech. and he ensures that there is no other need for such a low set pressure.

Thanks for the help so far.

The requirement recommended by API RP520,PartII of a max inlet pressure loss to a relief valve of 3% of set pressure is mainly to avoid excessive pressure loss and then serious overpressure resulting from late response of PSV.
In your case with set pressure far far less than the MAWP of the vessel, the severe situation you can get is if your vessel has, due to any reason in the process, high pressure close to MAWP then the PSV must open to relief wherein the resulting high operating pressure will overcome the pressure loss in the suction piping to PSV.
In the worst case where suction pressure loss is too high and with no 3% rule, the PSV will open at all cases without reaching the vessel MAWP.
Then, I don't see any concern at all and the PSV can be taken out as I guess the vessel is protected somewhere in your system.
I hope above is clear and not confuse you

regards
SmartEngineer

The safety relieve valve is to protect the vessel. If the vessel is good to 200 psi, why wouldn't you set the PSV to 200 psi?