On-board accumulator

31 July 2014

A request for a fast-acting failsafe stop valve actuator for a nuclear plant firefighting system required an innovative design. Rotork-Hiller worked with its customer for well over a year before an order was placed.

A nuclear plant specified a very compact pneumatically-operated actuator for a plant firefighting system. With no springs for the failsafe function, it had to be provided with a stored energy accumulator.

The application was specified as:

  • A safety-related actuator application in a nuclear power plant
  • Environmental and seismic qualifications to IEEE 382, IEEE 344, and IEEE 323, as well as plant-specific requirements
  • Y-pattern type globe valve
  • 23.6 kN (5,300 lb) thrust requirements
  • 35mm (1.375 in) required stroke
  • 45 Barg (652 psi) available supply pressure
  • Mechanical spring prohibited
  • Fast operation (close stroke time of less than three seconds)
  • Compliance with European design codes (CE & PED).

Adding a conventional accumulator would have exceeded the dimensional constraints of the specification, so Rotork-Hiller engineers conceived a radical new design that incorporated the accumulator within the pneumatic actuator.

The design not only eliminated the size problem but also freed space to mount the actuator controls. These include filters, regulators, solenoid valves and pressure switches, contained within overall actuator dimensions of 470 x 584 x 680 mm (181/2 x 23 x 263/4 inches) and weight of less than 150 kg (340 lbs). Redundancy is built into the controls, both for protection of control components and functionality. Manual operation is available through locally- operated valves on control panel. Stroke time is less than three seconds.

The relatively short stroke and availability of high-pressure nitrogen were key factors that allowed Rotork-Hiller to come up with a compact solution. The controls reduce the customer's supply pressure of 45 Barg (652 psig) to 31 Barg (450 psig), which is pumped into the accumulator to provide approximately 3.8 litres (1 US gallon) of storage capacity. The actuator delivers a maximum thrust of 27.6 kN (6,209 lbs).

An intermediate head was put in place after the initial cylinder, and then a secondary tube was installed prior to the cap. These components were held together using common tie rods that extend from the front head, through the intermediate head and the cap, to where the tie rod bolts can be installed to pull the assembly together. The accumulator has a pneumatic port in the cap, while the cylinder contains ports in both the front and intermediate heads (allowing the rod to extend and retract). The accumulator is pneumatically-connected to the cylinder using tubing and other control components.

"Adding a conventional accumulator would have exceeded the dimensional constraints of the specification, so Rotork-Hiller engineers conceived a radical new design that incorporated the accumulator within the pneumatic actuator."

Normally, the actuator holds the valve in the closed position with solenoids de- energised. The valve can be opened by energising both solenoids, which will cause the actuator rod to retract. Both solenoids must be energised to prevent inadvertent operation of the actuator.

If electrical power is lost, the solenoids will default to their de-energised state and the valve will move to the closed position (or remain there if it is already in that position). The accumulator is sized to provide one emergency stroke if the pneumatic supply is lost from the inlet. The actuator can be set to manual operation by transitioning the locally- mounted manual valves to their alternative states in a set sequence.

All electrical components were electrically terminated in one junction box ensuring the plant only has to have one point of connection on the actuator.

About the vendor

Rotork-Hiller is an actuator company that offers bespoke designs. Hiller, a manufacturer and distributor of safety- related actuators and associated controls, has an installed base of 82 of the 104 operating nuclear power plants in the USA, and 135 commercial nuclear plants worldwide. Hiller is a member of the Rotork Group of flow control companies.

Front side of failsafe stop valve actuator
Integrating the accumulator
Rear side of failsafe stop valve actuator
Pneumatic schematics

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