Single valve for both high and low pressure situations?

[chiggy]

I am working on a system where purchased potable water can be received at a high pressure of 143 psi or at a low of 2.16 psi through a 12” line.

The pressure is to be reduced to about 70psi, when it is above 70psi.

When the pressure is 2.16 psi the valve needs to have very low head loss.

My first reaction was to use a PRV and have a bypass (perhaps a ball valve) that opens under pressures below 70psi and short circuits the PRV. There will be a modulating valve and ultrasonic meter just downstream. I am trying to fit it all in a relativley small area and need to save space. So I was wondering, is there is a single valve that can do this job?

 

[bimr]

One valve should work. When the inlet pressure is low, the valve will be wide open which should allow the large flow that desire. Contact one of the PRV companies:

http://www.cla-val.com/pdfs/e-90-01.pdf

 

[Lan123]

What is the expected flow range?
When do you expect the pressure to drop down to 2.16 psi. As far as I know, to prevent contamination, residual pressure in a potable pipeline has to be maintained atleast at 20 psi.

 

[chiggy]

The pressure will be low if one of the treatment plants is shut down for any reason. I believe since it is not a distribution line the pressure can drop below 20psi.

I don't have numbers on expected flow ranges yet so i can't do any sizing. I was just concerned that a PRV would still have to much head loss even when wide open. With only 2 psi to work with i wanted to make sure.

 

[gerhardl]

The answer is yes,

Sizing and type of the PRV will however as always depend on pressure before and after the valve and required flow (all possible cases - from firewater to lowest during year and 24h).

If the parameters vary as wide as indicated you might be pushed towards high-quality, long- life valves with low cavitation risk, for instance needle type (motorized?) regulating valves, especially if you need precise regulation for all pressures and flow amounts.

You might end up with a 12inch valve, but could quite possibly, by lower flow at the lowest input pressure combined with low back pressure end up with a smaller one, depending on Cv for the selected type.

 

[jonr12]

You could put a pressure switch after the PRV. When the pressure is below 20 PSI, the pressure switch can close a solenoid on the incoming line to the pilot valve, and open a solenoid that dumps the water from above the diaphragm in the main valve. This will put the valve in a completely open position, as it will become a solenoid valve. When the pressure is above 20 PSI, the pressure switch changes state and makes the PRV start working again.