It is because a PSV provides little bit of pressure relief then it closes. A PRV implies pressure reduction of a continuous flow. What you absolutely don't want is continuous outflow of lube oil!
The "R" usually stands for Regulation (sometimes "reduction") and regulation implies a type of control over a continuous process. You are not technically "regulating" anything if a flow is basically on or off, as PSVs tend to function more in that fashion. At least in my mind it is so.
API requires relief valves on oil systems be designed as full-flow type. These valves do not "pop" open at set-point. They crack open at some pressure and then open more as the pressure increases. Many of them have cut seats designed to leak by even when closed to keep the lines and the valve warm. Some oil systems use this relief valve as the pressure regulating valve. Others have independent pressure controls and only use the relief valve for overpressure protection.
A standard PSV that pops fully open at set-point would tend to cause a sudden drop in oil pressure that could result in an immediate trip of the major machine that is being lubricated. So, full-flow relief valves are used instead. This allows the machine to continue running, in many cases even if an overpressure event occurs. In fact, it is standard practice to block in the running oil pump (usually a positive displacement screw pump) and force the flow to relieve through the full-flow relief valve before shutting the pump down. This allows oil pumps to be switched with little or no pressure change in the oil system and little or no risk of tripping off the big machine.
We only used PSV (“pop”) valves in oil systems where the seal oil pressure is too high to allow for the use of a full-flow relief valve.
I can understand why you might want to regulate lube oil flow, say through a filter, but why would you ever want to have a PRV as a pressure relief where relief volumes are generally small and not continuous?
Although the terminology is very unclear - PRV being able to be both Pressure RELIEF valve and Pressure REGULATING valve, ultimately the key is not what acronym you decide to call it, but what its function is.
PSV, as implied by the "Safety" bit, are normally seen as valves which are not part of normal operation, but are there to protect or make safe an abnormal condition (overpressure, fire, maloperation) and are sized for full flow. They tend to be more of a snap on snap off design rather than modulating.
Pressure relief and pressure regulating are normally seen as valves which relieve pressure at some pre-defined set point <= Design pressure and are commonly more about controlling and regulating pressure as part of normal operation.
your lube oil system may have both - a PRV (relief or regulating) as part of the normal circuit, and a PSV in some section where overpressure or perhaps contagion from a higher pressure source could occur due to say failure of a compressor or pump seal.
Who knows? Without seeing the particular flow diagram and understanding it's purpose, all we're in danger of doing is talking about which set of initials to write on the flow diagram / P&ID....
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
