Product(s):	WaterGEMS, WaterCAD
Version(s):	CONNECT Edition, V8i
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 10.04.00.108, 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.

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.

See more in the following journal papers on this subject:

• Economic Analysis of Pressure Control for Leakage and Pipe Burst Reduction - Creaco, Walski, ASCE J. Water Resour. Plann. Manage., 2017,
• Operation and Cost-Effectiveness of Local and Remote RTC - Creaco, Walski, ASCE J. Water Resour. Plann. Manage., 2018

The conclusion in these papers is that a conventional PRV is good enough in most cases, while remote control costs more and is subject to instability.

Possible Workarounds:

1. If the pressure zone is fed by a single source, consider developing a relationship between demand in the zone and a valve setting in order to achieve comparable results. This approach could use a valve node such as a TCV where the headloss coefficient is adjusted to throttle the valve, or another valve like an actual PRV where the setpoint pressure is adjusted. You would set up a series of logical controls where flow in the source pipe is the condition and valve headloss/setting is the action. You would need to observe several points of data to determine if there is a correlation. You could also consider using a pattern for the valve simply based on the demand pattern (instead of controls which add more complication)
   
   
2. Similar to #1 above, you could determine if there is a correlation between the pressure at the PRV and the pressure at the control point (remote location). For example if the remote location pressure setpoint is 1.5 bar and the pressure at the PRV discharge side tends to always be about 0.2 bar higher than the pressure at the critical point, then you could set the PRV pressure setting to 1.7 bar.

See Also

How does the Pressure Reducing Valve (PRV) work in WaterGEMS and WaterCAD?

How can I make a PRV/PSV active after closing it with controls?

Technical paper on-Economic Analysis of Pressure Control for Leakage and Pipe Burst Reduction