Applies To
	Product(s):	WaterGEMS, WaterCAD
	Version(s):	All
	Area:	Modeling

Problem

How does the Pressure Reducing Valve (PRV) work?

Solution - How it works

Pressure reducing valves (PRVs) throttle (partially close) to prevent the downstream pressure or hydraulic grade from exceeding a user-defined maximum value.

PRV Effects on the System

• The upstream hydraulic grade and pressure increase.
• The downstream hydraulic grade and pressure decrease.

Practical Applications

• To separate pressure zone boundaries in water distribution networks.
• To avoid downstream pressures that could have damaging effects on the system (eg. burst pipes, cause relief valves to open).

Steps to Create

1. Select PRV from the layout toolbar.
2. Place it in the network.
3. Define the PRV's attributes.

Valve "Status (Initial)":

• Active: Valve is partially opened to limit the user-defined downstream pressure or hydraulic grade.
• Inactive: Valve is fully open.
• Closed

Minor Loss Coefficient:

A PRV's minor loss coefficient is only a concern when the valve is not throttling flow (ie. wide open). This is because the PRV introduces loss to meet a required downstream hydraulic grade or pressure.

Note: For a complete list of the PRV attributes explained in detail, see the help article "Pressure Reducing Valve (PRV) Attributes".

 

Modeling Considerations

Reverse flow through the PRV

Reverse flow through a PRV is allowed when the "Status (initial)" is set to Inactive.

 

When the PRV's "Status (initial)" is set to Active, reverse flow is not allowed and "Status (Calculated)" will be set to Closed. If you need to model reverse flow and keep the valve status as Active, consider a bypass pipe with a check valve:

 

Determination of the PRV headloss

The headloss across the PRV is calculated based on the difference between the upstream and downstream HGL's. The upstream and downstream HGL's  are determined from the boundary conditions of the model, which include tank, reservoir, or valve hydraulic grades.

Troubleshooting

PRV not working as expected

You may see for example, a PRV calculated status of "Closed" when the downstream pressure is above the setting. This can happen if the valve is unable to reduce pressure simply by opening (even when closed all the way, pressure downstream is higher). Meaning, the closure of the valve cannot possibly reduce the downstream pressure because of another, downstream water source providing the higher pressure. 

Example

First, check how the system behaves without the PRV by setting the PRV's attribute "Status (Initial)" to Inactive (ie. fully open) and then computing the model:

• Downstream Pressure = 90psi
• Downstream Hydraulic Grade = 260ft

 

Next, check how the system behaves when the PRV is completely closed by setting "Status (Initial)" to Closed and then computing the model:

• Downstream Pressure = 35psi
• Downstream Hydraulic Grade = 130ft

Therefore, the PRV can control the downstream pressure to be between 35-90psi, or control the hydraulic grade to be between 130-260ft.

 

Now, switch "Status (Initial)" to Active to let the PRV's initial settings determine the valve status. The scenarios below give an idea of when the PRV's calculated status would be inactive, active and closed.

 

 

(1) If the PRV's downstream pressure can't be lowered to the value defined for the "Pressure Setting (Initial)", then the "Status (Calculated)" is set to Closed to provide the minimum downstream pressure.

(2) If the PRV's "Pressure Setting (Initial)" is below the downstream pressure when "Status (Initial)" is set to Inactive, and throttling the PRV will allow the "Pressure Setting (Initial)" to be maintained, then "Status (Calculated)" is set to Active (ie. partially open).

(3) If the PRV's "Pressure Setting (Initial)" is above the downstream pressure when "Status (initial)" is set to inactive, then the "Status (Calculated)" is set to Inactive (ie. fully open) as there is no throttling necessary. This is because even with the PRV fully open, the downstream pressure is already lower than required and any throttling would just decrease the pressure more.

 

Similarly, for: "Setting Type"  = Hydraulic grade

 

(4) If the PRV's downstream hydraulic grade can't be lowered to the value defined for "Hydraulic Grade Setting (Initial)", then the "Status (Calculated)" is set to Closed to provide the minimum downstream hydraulic grade.

(5) If the PRV's "hydraulic Grade Setting (Initial)" is below the downstream hydraulic grade when "Status (Initial)" is set to Inactive, and throttling the PRV will allow the "Hydraulic Grade Setting (Initial)" to be maintained, then "Status (Calculated)" is set to Active (ie. partially open).

(6) If the PRV's "Hydraulic Grade Setting (Initial)" is above the downstream hydraulic grade when "Status (Initial)" is set to inactive, then the "Status (Calculated)" is set to Inactive (ie. fully open) as there is no throttling necessary. This is because even with the PRV fully open, the downstream hydraulic grade is already lower than required and any throttling would just decrease the hydraulic grade more.

PRV upstream of tank or reservoir

When using a PRV upstream of a tank or reservoir (known HGL), the PRV may automatically close (zero flow). In this case, check the PRV's Initial setting (HGL or pressure). If it is less than the HGL of the tank / reservoir, it will fully close because the downstream HGL will still be greater than the setting. Meaning, the high HGL provided by the tank ends up closing the PRV. 

In order for a PRV to work on the upstream side of a tank or reservoir, the setting will need to be an HGL that is greater than the downstream tank HGL.

If you are doing this to model a top-fill tank, use a PSV instead, or the built-in feature in the tank element instead. See: Modeling top fill tanks and throttling inlet valves

How does the PRV operate during a transient simulation in HAMMER?

During a transient simulation in HAMMER, by default, PRVs are assumed to operate at a fixed position based on the initial conditions, unless the modulation option is used. See: Using Modulating PRVs

Modeling a "Two-way" PRV

To model a two way PRV where pressure is reduced in either direction, consider modeling two PRVs in parallel with the opposite orientation, like the following screenshot. The "Downstream pipe" properties of the PRV can be used to change the orientation. Depending on the flow direction, only one PRV will throttle at a time, because the other one will automatically close to prevent reverse flow.

See Also

How does the Pressure Sustaining Valve (PSV) work?