Pressure relief valves are used to prevent the overpressurizing of process equipment and piping.  They operate automatically at a preset pressure to vent fluid and relieve the overpressure.

The following terminology applies to pressure relief valves:

Relief valve – primarily for liquid service.
Safety valve – for steam, gas, and vapor service.
Safety relief valve – for liquid or vapor service.

Using a back pressure valve (BPV) control and the Prevent Backflow features, the operation of these valves can be modeled in PIPE-FLO.

First, a quick review on how the BPV control and Prevent Backflow features work.

Back Pressure Valves – A BPV maintains the set pressure on the high pressure or upstream side of the valve and only allows enough flow through the valve to maintain the upstream BPV set pressure.

Prevent Backflow - When backflow is prevented in a pipeline, the pipeline closes if the flow through the pipeline reverses during a calculation.  Therefore, if the pressure at the inlet of the pipeline is less than the pressure at the outlet, the pipeline will be closed to prevent reverse flow.

Below is a system in which a relief valve will be installed in the system to prevent the pressure at node A from going above 250 psig.  The system is modeled from just upstream of the relief valve to the system outlets (outlet pressure = 25 psig).  The pressure at the start of the model can vary from 50 to 300 psig.



To model the operation of the relief valve in PIPE-FLO, a BPV set at 250 psig is installed and Prevent Backflow is specified for Pipe1.  This configuration is shown in the figure below.

If the pressure at node A is less than 250 psig, Pipe1 will close since the pressure at the inlet of the pipeline is less than the pressure at the outlet of the pipeline (Poutlet = set value of BPV = 250 psig).

If the pressure at node A is greater than 250 psig, Pipe1 will open since the pressure at the inlet of the pipeline is greater than the pressure at the outlet of the pipeline (Poutlet = set value of BPV = 250 psig).

To see how this works in PIPE-FLO, download the attached file for PIPE-FLO 2009.

Open the project file and note that the pressure upstream of the relief valve is set to 50 psig and the BPV set pressure is 250 psig.  Click the calculate button.  The pressure at node A is 49.91 psig, which is less than the set value of the BPV.  Therefore, Pipe1 remains closed and no flow leaves the system through the relief valve piping attached to node A.

From the Lineup drop-down box on the toolbar, select the "Upstream P=300 psig" lineup.  (If you cleared the calculated results, click the Calculate button again after you select the lineup.)  Note that the Pipe1 has opened (since the pressure at node A is greater than the BPV set value of 250 psig), and flow is now leaving the system through the relief valve piping.