Introduction

These valves are designed to bypass high pressure feedwater around a group of heaters in the event of high water level in the shell caused by a defective tube, weld or drain system. The time available to give protection to a turbine in modern feed systems is short but these valves can operate in approximately 5 seconds in a 300mm bore pipe system with the minimum of hydraulic shock. Protection is achieved in the system by installing fast closing tee and changeover valves in the pipework around a heater or group of heaters. These valves are equally applicable in new or established stations. The motive power employed is the feedwater pressure and the system is designed to ‘fail safe’ in that the heaters will be by-passed if either electric or pneumatic supplies fail.

As an alternative to the medium operated valves, a spring loaded bypass valve can be supplied. This is the simplest form of bypass which relieves at a set pressure and is fail safe.

The Dewrance design has the advantages of a pressure seal bonnet and external spring adjustment whilst the valve is still under pressure.

Dewrance Feed Water Heater Isolation systems are designed to be “FAIL SAFE”. Failure of the controlling electric or pneumatic supplies will cause the main changeover and outlet valves to close and so isolate the feed side of the feed heater or heater bank. Float switches on feed water heaters are intended to have contacts normally made and to break with rising water level. Relays and solenoid are normally energised and are to be continuously rated.