Locating Pressure Relief Valves (PRVs)

Pressure relief valves (PRVs) are critical safety devices that protect pressure vessels and piping systems from catastrophic over-pressure events. Proper placement, orientation, and venting are essential to ensure reliable operation. This guide explains how PRVs work, where they should be located, how to design discharge piping, and what U.S. standards apply.

1. What Is a Pressure Relief Valve?

A PRV is a spring-loaded valve that opens automatically when internal pressure exceeds a preset limit. It vents excess pressure to prevent rupture or equipment damage. PRVs are required by U.S. codes such as ASME Section VIII and API 520/521, and must be installed without isolation valves between the PRV and the protected equipment.

Refer to this diagram as you read the placement and installation guidelines below.

Proper PRV location, orientation, and discharge design are essential for safe pressure relief. Following ASME and API guidelines ensures compliance and protects personnel and equipment.

2. Proper PRV Location

PRVs must be mounted directly on the vessel or pipeline they protect. Common connection types include flanged or threaded nozzles. To ensure uninterrupted protection:

• No isolation valves are allowed between the PRV and the process.
• Use a dedicated nozzle as close as possible to the pressure source.
• For systems requiring continuous protection during maintenance, use dual PRVs on a manifold with a selector valve that always leaves one PRV active.

3. Orientation and Cleanliness

PRVs should be mounted vertically to keep the inlet port clean and prevent blockage. If vertical mounting is not possible:

• Extend the nozzle and bring it vertical before connecting the PRV.
• Never mount a PRV horizontally in liquid service—it may block or fail to lift.

4. Discharge Piping Design

The outlet of a PRV must vent safely and remain unobstructed. Design considerations include:

• Use discharge piping at least the same diameter as the PRV outlet.
• If long discharge piping is needed, increase diameter by one nominal size to reduce backpressure.
• Grate the outlet to prevent birds nesting inside.
• Orient the outlet to prevent rainwater accumulation above the valve.
• Drill a 3 mm (1/8 in) drain hole just after the PRV outlet to remove water buildup.

5. PRV Testing and Set Pressure

PRVs are tested on calibrated rigs using water or compressed air. The valve is pressurized until the disk lifts, and the lift pressure is recorded. If the set pressure is incorrect:

• Adjust the spring tension.
• Re-test to confirm proper lift pressure.

PRVs must be sized to vent enough flow to prevent pressure from exceeding the vessel’s design limit. Use API 520/521 formulas or manufacturer sizing tools.

6. Expansion and Noise Considerations

When a PRV vents gas or steam into a lower-pressure environment, the volume expands dramatically. This causes:

• High-velocity flow through the discharge pipe.
• Loud noise and vibration.
• Large volumes of vapor and liquid discharge.

Ensure discharge piping is supported and routed to a safe location.

7. Materials and Corrosion Prevention

Select PRV materials compatible with the process fluid and environment. Avoid galvanic corrosion between dissimilar metals. For corrosive or contaminated services:

• Use stainless steel or corrosion-resistant alloys.
• Inspect regularly for buildup or corrosion.

Adapted from original work by Mike Sondalini, Maintenance Engineer.