Tim Probert, Deputy Editor

Technical failures in the power industry are not only difficult to avert but also extremely costly. In order to ensure the availability and reliability of steam turbines for power generation, Voith Turbo has developed an innovative monitoring and testing system based on hydraulic triple-redundancy, which protects against undesirable and consequently expensive turbine switch-offs.

Regular online testing and online maintenance to turbine equipment are extremely important to profitable operation, identifying when breakdowns are about to occur and predicting potential system failures without causing unnecessary shutdowns – this is the key to operational efficiency. Monitoring and testing of control oil dump arrangements during normal turbine operation to ensure that the system will trip in the event of an emergency or process disruption has always been a major requirement for operators.

The trip shut-off valve in the turbine is closed in less than 300 ms
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For absolutely safe and economical operation of the plant, Voith has developed a hydraulic trip shut-off module (TSM). The module, the German company claims, is the perfect solution for online monitoring and online testing of operating steam turbines. The TSM 2-o-o-3 is a revolutionary electro-hydraulic trip block, which enables regular testing to be carried during full turbine operation.

The new TSM safety device had its first outing into the market at the Turbomachinery Exposition in Houston, Texas, last year. The product is a reliable supplement for electronic speed monitoring. The TSM name affix ‘2-o-o-3’ – two-out-of-three – refers to the triple redundancy of the module. It avoids damage to turbines operating at overspeed when the load to be driven suddenly disappears. As such failures also present a danger to man and environment, steam turbines are subject to strict safety regulations. These include the reliable shut-down of the turbine in an emergency.

At an overspeed of approximately ten per cent, an electronic overspeed protection device is activated sending a trip command. The TSM has three identical on/off solenoids with slide valves and immediately disconnects the pressure oil of a hydraulic cylinder by means of a return spring.

Thanks to the spring action, the trip shut off valve of the turbine is closed in less than 0.3 seconds and the process is stopped. In the event of an involuntary release of the process, for example owing to a broken cable, unnecessary and, most of all, expensive downtimes are the result. This is precisely the situation when the Voith TSM comes in to protect the system.

A major advantage of this new development is the monitoring of the module during operation with the help of redundant position sensors. The switch position of the three systems is instantly reported to the control technology. Statistically, the probability of a complete failure is extremely low – one failure in 100 000 years.

The Crailsheim, Germany-based company claims that its new product offers improved reliability, increased system availability from automated testing and repair without shutdown, which means that there is no need for unreliable valve isolation. Voith says that this in turn eliminates operator error and the system allows for complete standardization across the plant. There is also a considerable reduction in trip times.

Two out of three selection

The TSM 2-o-o-3 unit contains three solenoids, each linked to sensors. As long as any two out of the three solenoids are operational the turbine will continue to run normally with no interruption of power output. In regular operation mode all solenoids are energized and the valve-closed position is picked up by the position sensors. The gauge shows that the turbine operation sequence continues uninterrupted.

Online testing of TSM 2-o-o-3 is simplicity itself. Any of the three solenoids can be tested during the operational cycle without shutting down the system and causing interruption to power generation.

The TSM module is particularly suitable for use in large steam turbine applications.
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The control system continually monitors all sensors and solenoids electronically whilst the turbine is running, significantly reducing the possibility of unexpected and costly shutdowns.

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If any two of the three channels are de-energized the turbine is immediately shut down. As soon as the second solenoid is deactivated the trip and throttle valve is closed and operation is stopped immediately. When solenoid 1 and 2 are de-energized shutdown occurs within 300 milliseconds (ms) and potential damage is avoided. When channels 1 and 3 are de-energized turbine operation is again stopped within 300 ms and de-energizing 2 and 3 achieves exactly the same result.

Voith says the module is simple to install and commission. The TSM provides a cost-effective and highly efficient turbine safety system. Additionally, replacing components can be achieved easily during the running sequence.

With no interruption to operation, components can also be regularly checked for wear and malfunction. The effect is no unplanned shutdowns, and no unexpected and costly downtimes.

The TSM is an electro-hydraulic switching assembly to drain pressurized oil of a quick-acting cylinder for steam turbine tripping. Three 4/2 slide valves activated by switching magnets are connected in series. The module is activated via a 2-out-of-3 selection, meaning the pressurized oil is only drained after de-energizing two solenoids.

Electric and hydraulic auxiliary energy are required for Trip Shutoff Module operation.

All arrangements necessary for Trip Shutoff Module operation are integrated in the three Voith actuating magnets (VSM).

Opening of steam valve

After connection of the voltage the Voith actuating magnets VSM 1 picks up and presses the respective control piston, which is guided in the control housing in a bore, against a resetting spring. The three 4/2 slide valves close, thus closing the connection from the output to the tank.

The oil streaming in via an orifice from the pump flows into the cylinder, moves the piston rod and activates the connected trip shut off valve towards opening direction.

Schematic setup of a quick-closing hydraulic cylinder arrangement
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Since the oil pressure supply is not stopped when tripping the system, it is necessary to also drain the oil flowing in towards tank. In order to limit this flow rate an orifice is placed downstream of pump connection in the TSM 2-o-o-3.

This orifice causes some slow motion of the cylinder piston rod towards opening direction.

Closing of steam valve

If two actuating magnets VSM of the TSM 2-o-o-3 are switched off in arbitrary combination and order, the respective 4/2 slide valve opens. After opening of any second 4/2 slide valve the connection between output and tank is reestablished and allows the oil flowing off the cylinder via the two valves. The quick-acting valve connected with the cylinder closes immediately.

Function of proximity switch

On the TSM 2-o-o-3 bottom side, three proximity switches are attached per valve parallel to the control piston moving direction. Two of these are connected as standard, with the third one provided as an option.

When switching off a valve, the resetting spring moves the piston towards the actuating magnet VSM, thus increasing the distance to the proximity switch. If the distance exceeds a certain value, the actuating magnet VSM outputs a signal. This signal is used to identify the switching condition (opening/closing) of the actuating magnet and thus to identify the respective 4/2 actuating valve. The switching condition is visualized by the lighting or not lighting up of a light-emitting diode on the proximity switch. If an 4/2 actuating valve is switched off, the diode lights up. Hence it is possible to identify an error or deviation from normal operation directly on the TSM 2-o-o-3.

The TSM is especially suited for large steam turbines with outputs of up to 1000 MW, and it can be retrofitted to existing installations. The TSM can be tested during operation without causing any interruption and allows the exchange of electronic components during operation, too.

The module has already been ATEX-certified for the European market. FM/CSA approval for the North American market has been applied for. Additionally, efforts are underway to acquire international SIL (Safety Integrated Level) standard.