Product(s):	WaterGEMS, WaterCAD
Version(s):	CONNECT Edition, V8i
Area:	Modeling

Problem

How to model a Combination Pressure Reducing Valve (PRV) and Pressure Sustaining Valve (PSV)?

How do a model a PSV and PRV in series which controls both the upstream and downstream pressure?

Solution

Application and Benefits

To maintain a minimum upstream and maximum downstream pressure or hydraulic grade.
This valve may be helpful when there is an upstream water source and a pump at a high elevation that distributes to a network at a lower elevation.
When an upstream pump shuts off, this valve would prevent the upstream network from emptying.
PRVs and PSVs can be modeled to use either hydraulic grade or pressure settings.

Example of this Valve

Cla-Val Model 692-01 Combination Pressure Reducing and Pressure Sustaining Valve

How to model

This can be modeled with a PSV and PRV in series with the correct orientation and HGL settings. Since this can be very difficult to solve hydraulically, you may need to select the "2.00.12" version of the solver in the "Engine Compatibility" calculation option, and introduce a "leak pipe" to help with the hydraulics as described in this article: Ill-conditioning User Notification for valves in a hydraulic model

Note that the PSV and PRV elements do not attempt to "set" the pressure to the exact values that you enter but rather the PSV maintains the upstream pressure to be equal to or greater than the setting and the PRV maintains the downstream pressure to be less than or equal to the setting. It may be possible in some cases for multiple solutions to exist for a combination PSV/PRV. Meaning, one valve or the other could be partially closed (throttling to induce the necessary headloss) to meet the same constraints. It is also possible in some cases for both settings to be physically impossible to maintain at the same time, so care must be taken to configure the arrangement properly.

Note that in some cases it may not be physically

Example of how to model

The following example model can be opened in the CONNECT Edition or V8i SELECTseries 6 of either WaterGEMS, WaterCAD, or HAMMER.

communities.bentley.com/.../combination-PRV-PSV-example.zip

Video

The above video is simulating a pump shut down.

Stage 1: As the upstream hydraulic grade decreases to 20ft (PSV's minimum hydraulic grade setting), the PRV gradually opens to maintain a downstream hydraulic grade of 10 ft (PRV's maximum hydraulic grade setting).

Stage 2: As the downstream hydraulic grade gradually decreases, the PSV gradually closes to maintain an upstream hydraulic grade of 20ft (PSV's minimum hydraulic grade setting).

NOTE: If you are using HAMMER, a transient analysis will not permit three pipes connected to the PSV or PRV. In this case, you will need to insert (split pipe with) an additional node on both sides of the valves (very close to the valve) and connect the bypass pipe to that node instead of connecting directly to the valve.

Stability problems

If you encounter stability problems where the network is unbalanced or you see unexpected results in the area of the combination PRV/PSV, you may need to adjust the advanced calculation options available with the 2.0.012 engine version, such as the Damping Limit. See more here

For the "Ill conditioning" message, try the "leak pipe" method described here: Ill-conditioning User Notification for valves in a hydraulic model

How to model a Combination Pressure Reducing Valve (PRV) and Pressure Sustaining Valve (PSV)