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How does the Pressure Reducing Valve (PRV) work?
Pressure reducing valves (PRVs) throttle (ie. restrict) flow to prevent the downstream pressure or hydraulic grade from exceeding a user-defined value.
Note: For a complete list of the PRV attributes explained in detail, see the help article "Pressure Reducing Valve (PRV) Attributes". A couple of notes for PRV attributes are included below.
Valve "Status (Initial)":
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.
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:
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:
Next, check how the system behaves when the PRV is completely closed by setting "Status (Initial)" to Closed and then computing the model:
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.
Pressure Setting (Initial)
Status (Calculated)
0psi
Closed (1)
60psi
Active (2)
100psi
Inactive (3)
(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
Hydraulic Grade Setting (Initial)
125ft
Closed (4)
190ft
Active (5)
265ft
Inactive (6)
(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.
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.
When using PRV downstream of tank or reservoir, it shuts off PRV and flow through it is zero.
In this case you should check for PRV setting (initial), it might be less than HGL of that of tank / reservoir, which means it will shut off the PRV i.e. not allowing it to open, hence status is closed. To make the PRV work, you should have Hydraulic Grade Setting (Initial) > or close to tank / reservoir HGL.
There are 3 possibilities in this case as below.
It also depend on how much water is being consumed in the lower pressure zone. As demand increases, possibility of case 3 increases, so you should also check the downstream demands.
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
How does the Pressure Sustaining Valve (PSV) work?