What is the difference between Pressure Relief Valves (PRV) vs Pressure Control Valves (PCV)?

A pressure relief valve (PRV) protects equipment by opening to discharge fluid when pressure reaches a specified setpoint. A pressure control valve (PCV) regulates flow or pressure during normal operation to maintain desired process conditions. PRVs provide overpressure protection, while PCVs provide operational control.

Both may be needed in the same system.

Pressure "Relief" vs. Pressure "Control" Valves

Pressure relief valves (PRVs) and pressure control valves (PCVs) both influence system pressure, but they serve fundamentally different purposes.

Pressure relief valves protect equipment against overpressure. They open at a specified pressure to discharge fluid and limit further pressure buildup.

Pressure control valves regulate pressure during normal operation. They adjust flow or pressure to maintain desired process conditions as system demands change.

In many industrial systems, both valve types are used together: a PCV manages normal operating conditions, while a PRV provides protection against specified overpressure scenarios.

How Does a Pressure Relief Valve Work?

A pressure relief valve is designed to discharge fluid when system pressure reaches a specified setpoint.

In a conventional spring-loaded design, a calibrated spring holds the valve closed. As system pressure increases, the force acting on the valve disc increases. When the specified set pressure is reached, the valve begins opening according to its design, allowing fluid to escape through a designated discharge pathway.

Depending on the valve type, opening may be gradual or rapid. The valve typically closes again after pressure falls sufficiently below the setpoint.

Pressure relief valves are used to protect equipment against conditions such as:

• Blocked outlets or other process upsets
• Excessive pressure from pumps or compressors
• Thermal expansion of trapped liquids
• External fire exposure, where applicable to the system's relief design

A PRV must be properly selected and sized for the applicable overpressure scenarios. Its set pressure, relieving capacity, discharge arrangement, and installation must meet the relevant system requirements and codes.

How Does a Pressure Control Valve Work?

A pressure control valve regulates system pressure by adjusting the flow of fluid.

For example, a pressure-reducing valve can maintain a desired downstream pressure even when upstream pressure or flow demand changes, provided the valve operates within its designed range.

Pressure control valves can operate through different mechanisms:

• Self-operated regulators use process pressure acting on a diaphragm, piston, or similar mechanism to adjust the valve without an external electronic controller.
• Actuated control valves receive signals from a controller, which uses pressure measurements to adjust valve position.

Depending on the application, PCVs may regulate upstream pressure, downstream pressure, or pressure differential.

Unlike pressure relief valves, they are generally intended to manage pressure as part of normal system operation rather than provide independent emergency overpressure protection.

Why Are Both Valves Sometimes Needed?

Consider a process system supplied through a pressure-reducing control valve. During normal operation, the PCV regulates flow to maintain the required downstream pressure. However, if the control valve fails open, excessive upstream pressure could reach downstream equipment.

That failure scenario could then lead to pressures exceeding the downstream system's allowable pressure. In this case, adding a pressure relief valve is one possible protective measure.

The PCV manages normal pressure conditions, while the PRV protects against specified overpressure events.

A standard pressure control valve should not be assumed to provide the same protection as a properly specified pressure relief device.

Selection and Maintenance Considerations

The correct valve depends first on whether the application requires pressure regulation or overpressure protection.

For pressure relief valves, important considerations include:

• Set pressure and required relieving capacity
• Applicable overpressure scenarios
• Fluid properties and operating temperature
• Inlet pressure losses, discharge backpressure, and installation requirements
• Applicable codes, inspection, and testing requirements

For pressure control valves, important considerations include:

• Required pressure range and control accuracy
• Flow capacity and operating conditions
• Valve sizing and response characteristics
• Actuator or regulator configuration
• Compatibility with the process fluid

Both valve types require appropriate inspection and maintenance. Relief valves must remain capable of performing their protective function, while control valves must maintain reliable pressure regulation.

A Note About Terminology

The abbreviation PRV can cause confusion because it is used for both pressure relief valve and pressure reducing valve in different industries.

These devices perform different functions. Equipment specifications should clearly identify the intended valve type, function, and applicable standard rather than relying on the abbreviation alone.

Additionally, safety valves, relief valves, and safety-relief valves may have distinct definitions and operating characteristics under applicable codes.

Final Thoughts

Pressure relief valves and pressure control valves are designed to solve different problems.

A PRV protects equipment against specified overpressure conditions. A PCV regulates pressure during normal system operation.

Understanding this distinction is an essential part of the job when it comes to understanding and communicating valve system components.