OSV and line pressure

Miata

Too much time on my hands has resulted in the following question. So, an above ground oil tank is creating a certain amount of psi in the line to the burner. When the OSV valve is closed the psi between the tank and valve is X. The valve opens the pressure on both sides of valve is X. Hope I am correct so far. Now valve closes. If the pressure on the valve to burner side is still X, it brings me to my question. Many responses to various OSV questions have said that in addition to shutting off the oil flow in the event of a line leak, it also protects the pump seals. How does the valve protect the seal from "excess" pressure if it the same on both sides of valve whether open or closed or is my logic incorrect?

Jamie Hall

Nothing wrong with your basic logic, except that you neglected the elevation difference from the valve to the pump — but in most setups that's trivial.

The seal protection isn't a matter of excess pressure, though. It's a matter of running dry. Pump seals do not like to run dry, and if the stop valve is closed, the pump can't move the oil in the pipe (there's a very definite limit to the vacuum the pump can produce — and the oil is just going to sit there). So the seals still have some lube. If the stop valve is closed and the leak is somewhere upstream of the valve, you're in good shape. That said, if the leak is downstream the pump will be able to suck the remaining oil out of the pipe, and you hope that the burner — and pump — will shut off fast enough to prevent damage.

That works for single line systems. Two line systems are a bit different. The supply line is the same — but the return line isn't. There, if there is a stop valve on the line and it is closed, it is quite possible for the pump to generate enough pressure (they are positive displacement) to do all sorts of mischief.

HVACNUT

Miata

Thank you all for the information. If I read the Webster literature correctly it also acts as a pressure reducer which would explain my confusion.