I am currently engineering a flush on a heat exchanger, shell-and-tube type.  I will be connecting the tube side (350 psi) to the shell side (50 psi).  The tube side is normally condensate, while the shell side is normally extraction steam.  This equipment has been out of service for some time, has had the tubes replaced, and is now ready to be commission tested and flushed.  Temperature for the flush will be ambient (around 70F or so).
The design pressure of this flush is based on the shell side of the heat exchanger, which is 50 psi.  I expect the flush to operate at 35 to 40 psi.
I would like to use the existing relief valves on the heat exchanger shell side for overpressure protection, if possible.  The relief valves (2 total) are set at 50 psi and were originally sized for steam service. 
In short, can I simply convert the steam capacity of the existing RVs to liquid water (based purely on the different densities) and use the existing RVs?  I plan on putting an orifice plate upstream of the heat exchanger to restrict flow to no higher than the capacity of whatever relief valve I use.
Check the design closely.  Steam valve trim is different from liquid trim.  I'd be a little leary of using a steam valve in liquid service.  It won't behave the same.  See API RP-520 for more details
As for capacity, use the same site and get the brocure.  It contains capacity charts.

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