Pressure Regulating Valve

[user]

Dear Doug
I am reading documents related to Pressure Regulating valve (PRV).

http://www.tyco-fire.com/TD_TFP/TFP/TFP1580_11_2007.pdf

I have one question regarding the working principle of Pressure Regulating Valve.
PRV maintains constant pressure at the outlet regardless of the variation in the inlet pressure.
It makes sense when the desired outlet pressure is less than the inlet pressure.
The documents says that the PRV can also maintain constant pressure if inlet pressure is low as compared to outlet pressure. (For me I think it as pressure boosting)
How is it possible to have higher outlet pressure as compared to inlet pressure? and I cannot figure out the scenarios in which we're having higher outlet pressure as compared to inlet pressure?

Regards

[Doug Hanson]

I would think that in the case of the ouput avhieving the desired pressure, at that point if the inlet pressure dropped for some reason, the set outlet pressure would hold steady ie. no leakage through back flow.
It would be impossible to achieve an oulet pressure higher than the imput unless there was some kind of pressure intensification mechanism built in.

[user]

Thanks..
Now the working principle of PRV is clear.