Water PRV Sequence of Operation
A High-Performance Water Pressure Reducing (PRV) Station requires valves that are properly sized for the flow rate and pressure drop of the zone they serve. Also, the High Flow and Low Flow valves must have the proper pressure setpoints to balance the flow rates across them. Cougar USA can help in the design, installation, and start-up of PRV’s to ensure constant pressure to the fixtures downstream of the PRV Station.
Water PRV Sequence of Operation Tech Talk Transcript:
Hi, I’m Tim Zacharias with Cougar USA on this Tech Talk, we’re going to be covering the sequence of operation for a Water Pressure Reducing Valve station or PRV station.
For this example, we are looking at a PRV station here that’s designed for a high-rise building. We’re generating a lot of pressure with our booster system, down the lower floor to be able to have usable pressure up at the top of the building, and in the lower floors, we’re having to knock that pressure back down to get it below 80 PSI.
So, for this example, our low flow PRV is going to be set at 70 PSI, our high flow will be set at 65 PSI, and our relief valve and pressure switch here are going to be set to 85 PSI, so 15 PSI above our set point. So, 70 PSI, 65 PSI, and 85 PSI here. The relief valve can’t be set to higher than 100 PSI by code, so usually, 15 to 20 PSI above your low-flow valve is where that set point is going to be. We’re going to start the sequence, assuming there’s no flow in the building, so both valves pressures are satisfied, we have no flow going through the valve system, and they are shut. Once you start to get a little bit of flow through the station here, let’s say it’s serving a few floors, a few restrooms kitchen, things like that, you start to see some activity. We’re going to see the pressure drop off and our low-flow valve here is going to start controlling it at that 70 PSI, so it’s going to modulate open close to maintain that 70 PSI going through the valve.
Now, as the flow demand increases, that flow rate increases, the low-flow valve is not going to be able to keep up and the pressure is going to start to drop off and once that pressure drop gets down to about 65 PSI your high flow valve is going to open up and start to control at the 65 PSI. So our low flow valve is all the way open, and we are controlling with the high flow valve here at the 65 PSI. Now, this one is going to continue to control until it’s satisfied. So, once the demand starts to slow down and the pressure is going to rise up again, and once this valve is satisfied at 65 PSI, it will close, and this valve will take over controlling at the higher pressure. Once that pressure comes back up, it will start to control again at the 70 PSI until there’s no flow and then it will shut off as well, and that cycle is just going to repeat as you have changes in flow rate across the system. Low-flow valve handle on the overflows; high flow in the high flows there.
Now, if you have an upset condition where one of these valves fails, it will fail open, which will allow the high pressure to come downstream. You’re going to see the relief valve open up at that 85 PSI and start to relieve the pressure and dump water to drain. At the same time, we’re going to sense that pressure here with the pressure switch and that is going to send the signal to our control panel here that I can simulate with the button and that is going to after that time delay it is going to close the block valve.
Now we have the time delay in there so that, you know, a short over-pressurization that is not caused by a failure valve won’t kick off this sequence, it has to be sustained in order to trip the alarm. So once the alarm is triggered here, it’s going to send a signal to the building automation using an output there and it’s also going to close the motor-operated butterfly valve or ball valve there that is our block valve. It is going to hold that valve closed until it is manually reset. So if we are to have this pressurization event and we allowed the valve to automatically reset. As soon as the valve closed, the pressure would drop downstream, the pressure switch would sense that, and send a signal to the control panel. It would allow the block valve to open. The block valve would open, we would have high pressure coming back thought. It would go through the field PRV, hit the relief valve again, over pressurize, hit the pressure switch. It would send a signal to the control panel, which would close the block valve again and you just be stuck in this circle of opening-closing the block valve, basically trying to maintain pressure with an MOV and that’s not going to be a good application or good control method for doing that.
So instead once we sense that high-pressure, we close the block valve and hold it close until it is manually, reset and opened. So before manually, resetting it, we want to close the downstream valves there, and then, press to reset. That’s going to open our block valve, allow the high-pressure to come up to these isolation valves here, and then at that point, you could slowly open these up, refill Downstream with water and see if the pressure goes past its set point. If it does, you can leave that valve closed and you can open up the other valve and allow it to control. Now, you can isolate this valve completely, do your service on it, and while you still have water flow through the other valve.
So, that is the breakdown of the sequence of operation for a Cougar Systems Water PRV station. If you have any questions, please feel free to reach out or check out the other videos on our website.
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