Model 600-PO - Steam Safety Valve

Product Brand: Target Rock

A solid history of reliability in hundreds of thousands of hours of reactor operation.

Target Rock has been manufacturing ASME Section III pilot-operated safety valves for commercial nuclear power plants since 1967. The heart of our technology is a special bellows machined out of solid bar stock that has nhundreds of thousands of hours of reliable reactor operation. This unique piece of machining is used as a pressure sensor to actuate the valve at the set pressure.

The advantage of piloted valves is they are pressure seated; the higher the pressure the greater the seating force and these valves are known for their tight seating performance.

General Specifications:
Size Main steam safety relief valve:
  • 6 in. (DN150) to 10 in (DN250)
Pressurizer safety valve
  • 6 in. (DN150) to 6 in (DN150)
Applications Pressurizer safety valve
Main steam safety/safety-relief valve
Quality Class ASME III Class 1, ASME QME-1


Pilot Operated Valve: Purpose

A pilot-operated valve is a type of relief valve that is a significant emission control device. It is a self-contained system that does not require external power or pressure to perform. They are mainly used on liquid storage tanks or other process vessels to prevent structural damage during excess internal pressure or vacuum.


  • Offers a smaller package on the larger pipe sizes.
  • Variety of options for control.
  • Little or no maintenance.
  • Requires no external power source.
  • Tighter seal as the system pressure approaches.
  • It can be installed in a variety of piping configurations.
  • The Control pilot can be mounted remotely.
  • Some designs allow for changes in orifice size within the main valve.
  • It can be used in engines.

How does a pilot valve work?

The pilot valve using system media and pressure to automatically control the actuator pressure to open or close the main valve. It does so depending on the pressure setting of the pilot valve vs. the actual system pressure. When the pilot-operated valve reaches set pressure, it opens and releases the pressure. It is then free to open, and the main valve exhausts the pressure at the bottom of the dome. It opens either to the main exhaust pipe or into the atmosphere. 

There are typically two functioning modes: snap-acting and modulating.

Snap Acting

Snap acting pilot-operated valves work to the set pressure operation. When it reaches set pressure, the valve snaps to open and can release excess pressure. When the pressure drops below the set pressure, the piston can then reseat. 


The pilot-operated valve is designed to open gradually so that less of the system fluid is lost during pressure relief—the piston inside lifts to the proportion of the overpressure event. 

Pilot Operated Valve Applications

At Curtiss Wright, we provide pilot-operated valves that are used in a variety of applications.

  • Liquid storage tanks
  • Process vessels
  • Crude oil pipelines
  • Emergency and SIS controls

What is the difference between a pilot-operated valve and a direct-operated valve?

While both have their advantages and disadvantages and are more suited to different applications, direct-acting valves directly connect with the opening and closing armature. Still, pilot-operated valves rely on the use of process fluid to pilot the valve’s operation. 

Boiling water reactor solution

This technology is currently installed in boiling water reactors worldwide in the main steam safety relief valve and in pressurized water reactors in the pressurizer safety valve application. For applications where both safety and relief functionality are required, Target Rock offers an optional actuator using our solenoid valve technology.

Features and Benefits
Features Benefit
Differential Pressure Mechanism Unaffected by change to state of process fluids
Chatter Free No chatter when valve set on steam passes water
Stability Unaffected by two-phase flow
Historic Solution Developed in the 1960s
Dependable Has never failed in thousands of reactor hours of service
Simple Simplified valve design

Recommended applications

  • Pressurizer safety valve
  • Main steam safety valve
  • Main steam safety relief valve
Model 600-PO - Steam Safety Valve
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