Controlling A Dual Zone Water Distibution System

[lottolearn]

We have a system which, due to terrain variations, operates as a dual pressure zone system. The high pressure zone (HPZ) is pressurized by the filtration plant high head pumps, while the low pressure zone (LPZ) is pressurized by an elevated storage tank.
The desired operation is to allow the tank to fill at which point both PRV's close. After a reasonble tank drawdown, both PRV's open allowing the tank to refill (through both PRV's to provide good circulation).
Question:
Assuming I have 2 PRV's separating the 2 zones, how do I control them in such a way as to have both open simultaneously when the elevated tank calls for water? If they don't open at the same time, the first PRV to open will satisfy the downstream pressure setting of the second, and circulation will be one-sided.

 

[BobPE]

lottolearn:

In essence what you are trying to accomplish is a parallel PRV system. Setting up both PRV's to operate identically will only allow one PRV to open when the LPZ gradient calls for it. For the second PRV to open, the system gradient will have to collapse again. Look at the H/Q curves for the PRV's you are using. They have a high initial head loss then fall off to lower loss as they flow then rise as the valve approaches its max flow. It's at this rise that the second PRV (if set identical to the first) will come into service. They will go out of service in a similar fashion, one valve then the next. Its hard to say which valve will open first. Being that you probably stratigically located them, the heavier demand valve will probably see the collapse in the LPZ gradient first and will operate first.

This is where the art of operation comes in. Most PRV stations I get the chance to model are operated by people who have adjusted them over the years to operate perfectly, which is what I thin you will have to do in your case.

How did you size the valves? This is probably more important than how they operate.

BobPE

 

[lottolearn]

Only 1 PRV has been installed at this time and it was sized at 8-inches to match the distribution main size in that section of the system. The system is currently operated in a partially manual mode until the second PRV is installed and it's size has yet to determined, although, we will most likely use a 10-inch PRV, again simply because that is the size main it will control. The H/Q characteristics during opening and operation may be the answer, however, I don't believe the modeling software we use (Pipe2000) typically incorporates those chacteristics when simulating PRV operation. As is usually the case, though, there is probably a way to work it into the model.

 

[BobPE]

lottolearn:

Of course you know that sizing a PRV has absolutely nothing to do with the size of the main you are putting it in. Pipe2000 will model those valves for you without any problem. I would hope that that 8 inch is not too big as the transients it will produce during its operation at low flows will be pretty incredible.

Model the flow att he points whre you want to put the PRV's and see what size PRV you will need. Set the PRV's from the model information, then field adjust.

Good luck,

BobPE