Pressure Relief Valve Basics: Spring-Loaded vs Pilot-Operated Designs

Pressure relief valves (PRVs) are the last line of defense protecting vessels, piping, and equipment from overpressure conditions that could cause catastrophic failure. Unlike control valves that modulate process conditions continuously, pressure relief valves remain closed during normal operation and open only when the pressure exceeds the set pressure, discharging fluid to reduce the pressure to a safe level. Understanding the two main types of pressure relief valves, spring-loaded and pilot-operated, and their respective applications helps engineers select the appropriate relief device for each overpressure protection scenario.

Spring-Loaded Pressure Relief Valves

Spring-loaded pressure relief valves are the most widely used type of relief device. A disc or poppet is held closed against the inlet nozzle by a calibrated spring. When the inlet pressure rises to the set pressure, the pressure force on the disc overcomes the spring force and the disc lifts, allowing fluid to escape. As the valve opens, the disc lift increases with pressure above the set point (overpressure), and the valve reaches rated capacity at typically 10 percent overpressure (the accumulation). When the pressure drops below the set pressure by the blowdown amount (typically 7 to 10 percent), the spring closes the valve. Spring-loaded PRVs are direct-acting, self-contained, and require no external power or control signals.

• Set pressure: inlet pressure at which the valve begins to open

• Overpressure: pressure above set pressure at which rated capacity is achieved (typically 10%)

• Blowdown: pressure reduction below set pressure at which valve reseats (typically 7 to 10%)

• Conventional PRV: open bonnet design, spring force affected by back pressure on outlet

• Balanced bellows PRV: bellows isolates spring from back pressure, suitable for variable back pressure

Pilot-Operated Pressure Relief Valves

Pilot-operated pressure relief valves (POPRVs) use a small pilot valve to sense inlet pressure and control a larger main valve through pneumatic or hydraulic signals. The main valve is held closed by process pressure applied to a piston or dome area that is larger than the inlet seat area, creating a net downward closing force proportional to system pressure. When the pilot senses that inlet pressure has reached the set point, it vents the dome pressure, allowing the net pressure force to lift the main valve disc. The main valve opens fully and instantly at set pressure, providing very accurate set pressure control and eliminating the simmer and chatter problems common in spring-loaded valves near set pressure.

Selection Criteria

Spring-loaded PRVs are the standard choice for most overpressure protection applications due to their simplicity, reliability, and low maintenance requirements. Pilot-operated designs are preferred when tight tolerances on set pressure are required (the pilot can be set within plus or minus 1 percent versus plus or minus 3 percent for spring-loaded designs), when the normal operating pressure is close to the relief set pressure (pilot-operated valves can operate at up to 98 percent of set pressure without simmer), when the relief valve outlet has significant back pressure that varies with flow (pilot-operated designs are immune to back pressure effects), or when the required capacity is very large (pilot-operated designs achieve full lift and full capacity immediately at set pressure without requiring overpressure buildup).