Valves

Question

Hi, 
 I have a pump station that has 5 constant speed pumps (11,700 gpm each) I would like to install a valve that maintains a certain pressure in the system. 
 This valve shall remain close as long as the set pressure is maintained, once the pressure gets higher due to a lower demand in the system, it shall open to discharge to the atmosphere. As per the attached, I am thinking to install this valve on the header. 
 
 Would you please explain how to do so? and which valve I need to use? I tried using the Discharge to Atmosphere and PRV valves and they didn't work. 
 
 Thanks

Tom Walski · Accepted Answer

The first three solutions should have solved your problem. 
 
To model the waste line, Just put a PSV in the waste line and have it only open when the pressure exceeds some predefined value (and hope it never does).

Tom Walski · Answer

There are actually several solutions to your problem that are more energy efficient than what you propose. 
 
1. The obvious solution is to simply turn off some pumps when the pressure gets too high. The remaining pumps will run further out on their pump curves and the pressure will drop. Use the Combination pump curve feature to study this. 
 
2. Install some different size pumps. This will give you a huge range of flows that you can deliver. If you can't change out existing pumps, you can trim the impellers on a few but you need to be careful to analyze the resulting combinations. 
 
3. Install Variable Frequency Drives (VFD) on a few of the pumps that will enable you to turn them down. You need to study the effect of doing this as you can end up with poor operating points in terms of efficiency. Plus VFD's are pretty expensive. 
 
4. Install a valve as you suggest (model it as a PSV) but instead of discharging to atmosphere, recycle the water to the suction side of the pumps. In this way you may waste some energy but you won't waste water. 
 
Before doing any of this, analyze the demands downstream so that you know the overall distribution of demands. I'm assuming there are no storage tanks downstream because if there are, you can just run enough pumps to fill the tank(s) and turn off a few until the tank levels drop and then turn some of the pumps back on. 
 
The energy tools in WaterGEMS will enable you do perform the analyses you'll need to quantify the energy use and other benefits of each alternative. 
 
Best wishes