This example describes working of a pressure relief valve (PRV). The model is composed of components from Pneumatics.Elements.

The relief valve modeled here is assumed to be a 2-way single 2-land spool and sleeve arrangement with no leakage and referenced from [1].

The example shown is parameterized with typical values for the sizing and flow characteristics of the valve and orifices [2].

This example demonstrates the pressure relief cracking open to maintain the maximum pressure in the circuit. The input flow to the system starts from 2 seconds and ramps to around 0.025 kg/s at 8 seconds and remains constant after that. The role of the PRV is to crack open and bypass flow to the tank through port_A to port_B flow when the pressure exceeds certain level. The load pressure to the system starts ramping up from 1 bar at 2 seconds and reaches a maximum of 3 bar at 4 seconds and remains there till the end of simulation.

The pressure in the system at control_port rises after 2 seconds as the flow starts to ramp up. As the pressure builds up and crosses 4 bar, the PRV starts to crack open and bypasses the flow. This can be seen by looking at the m_dot at the port_B of the PRV (negative sign of flow means that air flowing out of port_B). In the end (12-15 secs), it can be seen that the mass flow through the metering orifice is around 0.015 kg/s whereas bypass flow is around 0.01 kg/s. This shows that PRV is spilling close to 40% of the mass flow from the input flow into the bypass thereby maintaining system pressure in the vicinity of 5-6 bars.


[1] Merritt, H.E., "Hydraulic Control Systems," John Wiley Sons, Inc., 1967.

[2] Chandrasekar S., and Tummescheit H., "Physical Design of Hydraulic Valves in Modelica," Proceedings of the 10th International Modelica Conference, Lund 2014.