Modeling PRV / PSV controlled by pressure at remote location in the model

 Product(s): WaterGEMS, WaterCAD Version(s): 08.11.XX.XX, 10.XX.XX.XX Area: Modeling

Problem

How do I model a pressure control valve i.e. PRV or PSV that is controlled by pressure elsewhere in the model, i.e. remotely real time-controlled valves (RTC valves)?

Background

Unlike a conventional pressure reducing valve - PRV which monitors pressure at its outlet, a remote PRV monitors pressure at a remote location, which may be a high point in the model or important locations where pressure needs to be maintained. It sends the signal back and has a PLC adjust the pressure setting on the valve or simply changes the valve position in the valve. It looks for a setting at the valve that gets the right pressure at the pressure sensor i.e. distant node / remote location.

This approach monitors pressure heads at remote point(s) in the system, usually the critical node(s) with the lowest pressure head and signals to the control valve the reading so that a programmable logic controller (PLC) can set a valve position at the source to maintain the minimum desired pressure at the remote point.

Solution

As of version 10.02.02.06, the PRV and PSV elements only monitor pressure at the valve location. Related Enhancement # 278426

This is challenging to model either with a PRV or a TCV in WaterGEMS/WaterCAD because if the pressure drops at the remote node, we would need to open the TCV more or increase the PRV setting. The problem is that there may not be a one-to-one correspondence between setting and target pressure depending on system demand and other things. As this setting can change every time-step as there is change in demand and hydraulics of the network. A real valve will hunt with a control loop until it gets the pressure right, but it cannot be interpreted/represented in model yet.

For example, suppose you want to maintain 50 psi at a remote location. During the night, you can set the valve at the source to 60 psi to get 50 at the target. However, as demand peaks, you may need to set it to 65 psi to get 50. If there is a fire, it may need to open wide. A conventional PRV cannot currently be set in this way in WaterGEMS or WaterCAD.

If this is the only source feeding the pressure zone, then you can develop a relationship between demand in the zone and TCV setting. But if there are multiple sources, this won’t work because we aren’t controlling them in model.

In conclusion, a pressure control may not be needed when the leakage rate is low as long as the pressure is not so high as to result in excessive pipe breakage. Remotely real time-controlled valves (RTC valves) are most beneficial when the leakage rate is high, especially in the case of peaked demand pattern and in large systems. In between those two extremes, conventional PRVs are best for pressure control.