Pressure Relief Valves vs. Safety Relief Valves: Understanding the Difference

In pressurized systems, overpressure protection is critical to ensuring safety, preventing equipment damage, and maintaining compliance with industry standards. Two of the most common types of overpressure protection devices are pressure relief valves (PRVs) and safety relief valves (SRVs).
Though the terms are often used interchangeably, they serve distinct purposes and operate under different conditions. Understanding these differences is essential when selecting the correct valve for your application.

1. The Purpose of Overpressure Protection

Every pressurized system, whether it carries steam, gas, air, water, or chemicals, must have a reliable method of relieving excess pressure. Overpressure can result from equipment failure, blockages, thermal expansion, or process upsets.
Relief valves are designed to open automatically when the system pressure exceeds a predetermined setpoint, discharging fluid or gas until safe operating conditions are restored.

2. What Is a Pressure Relief Valve (PRV)?

A pressure relief valve is typically used in liquid service applications, such as water, oil, or hydraulic systems. It opens gradually, in proportion to the increase in system pressure above its setpoint.

Key Characteristics of a PRV:

• Operation: Opens gradually as pressure rises above the set pressure.

• Function: Maintains system pressure at a safe level in liquid systems.

• Application: Common in hydraulic systems, pump discharge lines, and liquid process systems.

• Reseating: Closes once pressure drops slightly below the setpoint, preventing system shock or fluid hammer.

Example: In a water distribution system, a PRV protects against overpressure caused by pump surges or thermal expansion.

3. What Is a Safety Relief Valve (SRV)?

A safety relief valve is designed primarily for gas or vapor service, such as air, steam, or compressed gases. It opens rapidly and fully, allowing immediate discharge to prevent catastrophic overpressure.

Key Characteristics of an SRV:

• Operation: Opens fully, instantly releasing pressure when the setpoint is reached.

• Function: Protects systems from sudden overpressure in gases or vapors.

• Application: Common in boilers, pressure vessels, air compressors, and steam systems.

• Reseating: Closes once the system returns to safe operating pressure.

Example: In a steam boiler, an SRV prevents an explosion by quickly releasing steam when internal pressure exceeds safe limits.

4. The Combined “Safety Relief Valve”

In many applications, particularly those that can handle both liquids and gases, manufacturers offer combination safety relief valves. These hybrid designs are suitable for mixed-phase flow, allowing engineers to protect systems that may alternate between liquid and vapor states.

They meet codes such as ASME Section VIII for pressure vessels, ensuring compliance and safety under variable conditions.

5. Design and Code Differences

6. Selection Considerations

Choosing the correct valve type depends on the medium, system pressure dynamics, and regulatory requirements.
Consider the following when specifying a relief valve:

• Fluid Type: Gas or liquid determines valve type.

• Response Speed: Rapid vs. gradual opening based on system needs.

• Discharge Capacity: Must match maximum overpressure conditions.

• Installation Environment: Materials and seal compatibility.

• Applicable Codes: Compliance with ASME, API, or local standards.

7. Summary

Both pressure relief valves and safety relief valves are vital components in pressurized systems, but they serve distinct roles:

• PRVs are ideal for liquid systems, providing smooth, controlled relief.

• SRVs are essential for gaseous or vapor systems, delivering fast, full-capacity pressure release.

Selecting the correct valve ensures system integrity, protects personnel and equipment, and maintains compliance with safety regulations.