"High Performance" Safety Relief Valves


Auto Seat Technology® is patented soft seat relief valve design that increases seat life,
reduces repairs, and gives the valve a consistent set pressure.


Direct Spring Operated
Safety Relief Valve

  • NON-RISING STEM: When adjusting the set pressure the adjustment screw does not move up or down, instead it moves the adjustment bushing up and down increasing and decreasing spring tension. The adjustment bushing never turns so it does not transmit a torque into the disk. This helps extend the seat life of the valve.
  • MECHANICAL STOPS: Because the adjustment screw in a non-rising stem valve is always in the same place it can be used as a mechanical stop for the disk. This allows the disk to open the same amount each time the valve opens. This gives the valve the same capacity every pop. The mechanical stop is positioned so that the disk is always held up by the flow of the valve until the pressure drops low enough to reclose the valve. The mechanical stop also does not allow the spring to be over compressed, reducing stress on the spring.
  • FULLY GUIDED SPRING: Guided from top to bottom, a fully guided spring is only compressed vertically so the spring keeps consistent tension from one pop to the next.
  • FULLY GUIDED DISC: The disk is aligned on the top through the adjustment screw and at the bottom by the radius on the disk. When a relief valve begins to vent, the flow of the valve pulls everything toward the outlet. Guiding the disk allows the disk to only move up and down and not to the sides resulting in consistent opening and reseating. The radius on the bottom of the disk also helps insure that the valve reseats after each pop. If the disk is not centered on the nozzle at reclose, the radius on the bottom of the disk helps to realign the disk.
  • LOW SPRING RATES: A spring rate is the amount of force the spring exerts for how much it is compressed. Mercer Valve uses low rated springs to help keep stresses out of the spring. The more force that the spring is exerting, the higher the stresses in the spring. When a spring is over stressed the properties of the spring are changed, affecting the set pressure of the valve.
  • LIP SEAT SEAL: On reclose the seat flexes, allowing the brute impact of the disk reclosing to be taken by the hard nozzle, while still providing the soft seat seal. Also before the valve reaches set pressure, the pressure is applied underneath the lip seal, pushing the seat against the disk. This gives a tight seal up to the set pressure and extends the seat life.

Pilot Operated
Safety Relief Valve

Pilot operated valves continue using
Auto Seat Technology®
in both the pilot and main valves, increasing seat life, reducing repairs, and giving the valve a consistent set pressure.

Pop / Snap Acting or Modulating - Pilot Operated Safety Relief Valve

Snap Acting: At set pressure the valve snaps to full lift. The system pressure will relieve until the pre-set blowdown is achieved. Blowdown is the difference between set pressure and reseat pressure. Nominal blowdown is 7 to 8% so the valve will reseat 7 to 8% below set pressure.

At set pressure the valve gradually begins to open as needed in response to the system overpressure. When the system pressure has been relieved the modulating valve will reseat very close to set pressure. This is very different from the snap action valve which abruptly "snaps" open, then closed. Modulating valves produce far less shock on the system and there is considerably less wear and tear on the main valve components. Another key reason for using a modulating valve is the economic savings due to minimized product loss because the valve reseats near set pressure, rather than 7 to 8% below set pressure.

Mercer Valve Co., Inc.
Safety Relief Valve Catalogs