Why Pressure Reducing Valve doesn't prevent backflow

[Rob243]

I am scratching my head on this question. PRV is supposed to create a closed system requiring a thermal expansion tank. But at the same time PRV is not considered a backflow preventer. How can both be true?

 

[Q_Goest]

Hi Rob. Welcome to the board,
If you have a tied diaphragm, the pin holding the diaphragm to the valve will hold the valve shut when pressure goes up regardless of inlet pressure. The regulator in the picture below has the pin (STEM FEEDBACK MECHANISM) which ties the valve to the diaphragm.

But most regulators don't have a tied diaphragm. Instead of a solid rod capable of pulling the valve plug shut, they simply have a check valve arrangement that gets pushed open when the diaphragm stem moved down such as in the picture below. Note the picture below has the outlet on the left and inlet on the right.

So if the regulator is like the one you see in the second picture, and if the inlet pressure is lower than the discharge pressure, the fluid can open push the valve open and flow backwards through the regulator.

Most of the regulators I've seen are of the second type because the pin in the tied diaphragm type regulator is under significant tensile stress when discharge pressure is significantly higher than the set pressure and I've even seen instances where that pin breaks.

 

[Rob243]

Thanks for the detailed explanation! So to summarize, the increased inlet pressure can cause backflow, but usually it won't. Or is the the other way round?

 

[Q_Goest]

The other way around. If inlet pressure drops below discharge pressure, some regulators will allow flow in the reverse direction (from discharge to inlet) because essentially the poppet is a check valve pointing backwards. It checks in the forward direction (ie: shuts off flow) when inlet pressure is higher than discharge pressure but allows flow in the reverse direction (when discharge is higher than inlet pressure). It is only pushed open by the diaphragm which is how the downstream pressure is controlled.

Examine the second picture above. Note where the word "Poppet" is on the left side pointing to a small section of stem that sticks out of the poppet. Note the small section of stem doesn't touch the diaphragm. As the pressure on the diaphragm drops (outlet pressure or "set pressure" drops), the spring above the diaphragm pushes the diaphragm down which pushes on that small section of stem on the poppet, pushing the valve open. But if the pressure is too high for the diaphragm to be pushed down by the spring, it's held up as shown in the picture. And if the inlet pressure is lower than the discharge pressure, the pressure force on the poppet (pushing down) will overcome the spring force that's below the poppet (labeled "Valve Closing Spring") pushing up, and allow flow backwards through the regulator.

 

[Rob243]

Ah! Great explanation! This also explains why thermal expansion tank is needed. If both inlet and outlet pressure are high, the valve remains closed. I guess it would really depend upon the difference in outlet vs inlet pressure, but since these aren't fixed there is no way to guarantee that backflow will always happen when outlet pressure starts increasing. But if inlet pressure drops significantly, it is likely to backflow.

Thank You!