Pressure reducing valves

[scheideler]

Hello. I'm on a project that needs to transfer clean water from a high-service to low-service network via a connection with a pressure reducing valve. The goal is to improve reliability and add redundancy to a municipal distribution system. The inlet pressure is 90, and the outlet is 77 psia. The second network will experience breaks with any less pressure drop (such as adding a flow at 80 psi). Many PRV manufacturers have stated that the minimum step-down is 15 psi.

If I were to set the PRV to drop down to 75, what affect would this have on the low service network? Would the higher pressures in the network prevent flow from crossing over? Would the flow from my cross-connection immediately reach the same pressure as the rest of the low-service network?

The owner requested PRVs and doesn't want to use other valve types to throttle.

Thanks.

 

[bimr]

For your application, the PRV would probably be acceptable at less than 15 psi difference for your application. With less than 15 psi, the pressure control will be less precise. For you application, there is no need for very precise control. Nobody is going to notice a pressure difference of 15 psi.

It is common for the water level in towers to change 30 feet over the course of a day which is approximately 15 psi. The pressure change is not observable.

If you reduced the pressure setpoint from 77 psi to 75 psi, nobody will notice it as the pressure difference is so slight.

Municipal systems generally have a minimum pressure of 20-30 psig. The pressures in the system will depend on elevation differences, building heights, and the piping restrictions.

 

[LittleInch]

This depends on how you are controlling this valve and flow. You can't separate pressure and flow. If your valve is simply controlling on pressure then if the pressure of the downstream system is below your 77psia, then the valve will open and allow more flow until the back pressure rises to your set point when it will start to close and reduce more pressure and hence reduce flow. Immediately downstream of your valve, the pressure will be the same as the airstream system.

I can't see 2spsi making a big difference, but it's rare for water distribution systems to stay at a fixed pressure.

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[scheideler]

Thanks for the responses. I think that's what I needed to proceed, but i'll come back if I hit a wall.

 

[danw2]

What about the absolute value?

I'll assume a value of 14.5 psia as sea level atmospheric pressure.

The 77 psia value is the same as 77 psia minus 14.5 psia = 62.5 psig. If you're in Denver Colorado at a 5,000 ft elevation, you can take off another 2.5 psi.

So for an inlet pressure of 90 psig and an outlet pressure of ~62 psig, the difference is ~28 psi, well above the minimum step-down of 15 psi.

 

[LittleInch]

Edited my previous post. Got flow and pressure mixed up...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.