How does the Pressure Sustaining Valve (PSV) work in WaterGEMS and WaterCAD?

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

How does the Pressure Sustaining Valve (PSV) work?

Pressure Sustaining Valves (PSVs) throttle (ie. limit or restrict) flow to maintain a user-defined upstream pressure or hydraulic grade. 

When a PSV is throttling flow:

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

Situations to use

PSVs are useful in situations where unregulated flow would result in inadequate pressures (too low) in the upstream portion of a system. They are also often used to model pressure relief valves. 

The valve can be in one of three states.

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

Reverse flow through the PSV

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

When the PSV'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:

Example

When the PSV's "Status (Initial)" is set to inactive (ie. fully open):

  • Upstream Pressure = 60psi
  • Upstream Hydraulic Grade = 170ft

For: "Setting Type"  = Pressure

Pressure Setting (Initial)

Status (Calculated)

55psi

Inactive (1)

65psi

Active (2)

80psi

Closed (3)

(1) If the PSV's "Pressure Setting (Initial)" is below the upstream 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 PSV fully open, the upstream pressure is already higher than required and any throttling would just increase the pressure more.

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

(3) If the PSV's upstream pressure can't be raised to the value defined for "Pressure Setting (Initial)", then the "Status (Calculated)" is set to Closed to provide the maximum upstream pressure.

Similarly, for: "Setting Type"  = Hydraulic grade

Hydraulic Grade Setting (Initial)

Status (Calculated)

160ft

Inactive (4)

175ft

Active (5)

200ft

Closed (6) 

(4) If the PSV's "Hydraulic Grade Setting (Initial)" is below the upstream 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 PSV fully open, the upstream hydraulic grade is already higher than required and any throttling would just increase the hydraulic grade more.

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

(6) If the PSV's upstream hydraulic grade can't be raised to the value defined for "Hydraulic grade Setting (Initial)", then the "Status (Calculated)" is set to Closed to provide the maximum upstream hydraulic grade.

See Also

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

Why do I see such a large headloss through my FCV, PSV or PRV? 

Anonymous
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