design pressure
Why extra relif valve is provided with setting more than design pressure?
I came across oil and gas seperator vessel which is having 4- pressure relif valve.Vessel design pressure is 120 bar but out of 4 pressure relif valve one is having set pressure 126 bar which more than design pressure of vessel.What is pupose of this high setting of pressure relif valve?
The process that the vessel is in must be able to produce a wide range of gas volumes.
If you assume a large volume of gas is flowing when you size and choose the relief valve, and then, when the process actually overpressures with a small volume of gas, the relief valve will “chatter”, meaning it will open to relieve the pressure and because the valve has a high capacity flow rate the vessel pressure will drop very quickly so the valve will close. Then the pressure will build again and the valve will open again……if this continues too long the valve will destroy itself. So, you put a series of lower flow rate valves on the vessel so that they open in sequence as the process flow rate increases.
We used to size the relief valves on our steam lines in this manner because we had a very wide range of process flow rates. (and those babies would really scream when they opened).
Just out of curiosity check the flow rates on the valves and see if the flow rate on the higher pressure valve isnt much higher than on the other valves. It might be there for a an absolute worst condition and they set the pressure a little higher to assure that all the other valves would be open and fully relieving before this one could open and screw everthing up.
