I have written often about how we should establish safe upper and lower limits for our covered process(s). (SAFTENG members can read Establishing Safe Upper and Lower Limits) One mistake we have discussed is related to safe upper pressures and its relationship to the MAWP of process vessels. We continue to see errors in such that a vessel with an MAWP of 300 psi, will have a Relief Device with a set point at 300 psi and therefore the facility sets their safe upper limit at 300 psi. This is just fundamentally wrong on several fronts, but I was so impressed with Parker Refrigerating Specialties Division spec sheet “advice” regarding this error…
Parker’s Bulletin 72-00D, dated 9/07, for the Installation, Service, and Parts Information for a model of their RV’s states the following:
SELECTION DATA
The Type SRH Safety Relief Valve is intended to prevent the pressure of the vessel from rising more than 10% above the Design Working Pressure (DWP) of the vessel or the pressure setting of the relief device, whichever is the lower pressure.
Whenever conditions permit, it is advisable to have the relief valve pressure setting (which must not exceed the design working pressure of the vessel) at least 25% higher than the normal operating pressure of the refrigerant used.
Pressure limiting devices, such as high-pressure cut-outs on positive displacement compressor systems, must stop the action of the pressure imposing element at no higher than 90% of the pressure setting for the pressure relief device.
I could not support this “advice” anymore! I am both amazed and so happy to see a company offering this advice to buyers of their equipment. It is SOLID advice that should be seriously considered when establishing the set points of our RV’s. Here is the link to Bulletin 72-00D:
http://www.parker.com/literature/Refrigerating%20Specialties%20Division/Safety%20Relief%20Valves/72-00D.pdf