Over the last ten years, turbine operators have been converting their mechanical trip systems to redundant electronic controls in order to improve the reliability of their trip systems and to reduce their exposures to turbine overspeed accidents during testing. This was a big improvement, but it has exposed another weak link in the trip chain: the hydraulic trip solenoid.
One solenoid valve can’t be tested without tripping the turbine. Two solenoids in series can be tested, but both solenoids must open to affect a trip. Some manufacturers offer a two-out-of-three voting arrangement that can be tested on-line, but not repaired on-line. And these triplicated arrangements depend on precision orifices and a narrow fluid viscosity range to be effective.
The Quadvoter provides a solution to all these problems. With the Quadvoter, no single component failure can cause a trip and no single component failure will prevent a trip. And all the active components can be repaired online without bypassing the trip functions.
Invensys has developed test logic to fully automate the test sequence. Each step of the test procedure is monitored and can be recorded for test documentation. Testing can be initiated by the operator or it can be scheduled to automatically proceed.
Each step of the automated test sequence has a “pass” criteria. If any step in the sequence fails, the test is aborted and the operator is notified. The failure details are presented to the maintenance personnel to allow quick troubleshooting.
A collateral benefit of installing a Quadvoter trip system is the elimination of expensive OEM trip system components. The Quadvoter uses industry-standard NFPA hydraulic components so high replacement parts prices and long leadtimes are eliminated.
The Invensys Quadvoter is the most reliable, most available and most testable turbine trip system available.
The Quadvoter is placed in series with the hydraulic trip header to keep the steam trip valves open during operation. The Quadvoter quickly dumps the trip fluid to the reservoir during a trip action. It can be provided for dump-style systems or for systems requiring a pressure source.
In any safety system, the key component in reducing the system’s probability of failure on demand is testing. The Quadvoter provides both the means to test all the active components and instrumentation to judge the results. Pressure transmitters monitor the state of each solenoid valve. During testing, they provide feedback to the control system to allow confirmation of the success of each step in the test. Optional proximity switches provide an additional level of redundancy and allow performance monitoring of each solenoid valve.
Isolation valves allow each pair of solenoid valves to be taken out of service and replaced without compromising the system’s ability to trip, if required.