An after-work idea

An after-work idea
 
Content

November 2014

 

To offer enhanced safety to customers in the power generating industry, a team at Bosch Rexroth developed a hydraulic component that continuously checks its functions. Project manager Dirk Bracht reports on the unusual development history of the patented STSS – a Self-Testing Switching System.

 

Four years ago our team equipped a power plant in the Netherlands with new hydraulic components for the high-pressure bypass valves associated with the steam supply. Once the work was finished we met for dinner, but we couldn’t get our minds off the job. While we were talking a colleague exclaimed: “Our customer actually needs a far better solution!” Bypass stations are always present in power plants and must respond quickly to sudden and large changes in pressure. One instance is an emergency shut-down, also known as a “trip”. The turbine-generator configuration is shut down in less than a second and all the steam has to be redirected around the turbine. The turbine inlet valves have to close quickly and the bypass system must open. That system comprises a pressure reduction valve and a water injection system. The bypass system limits the temperature of the intermediate superheater by redirecting the steam back to the boiler.

In the past: annual inspections

This means that the hydraulic elements opening the bypass system have to work properly when the need arises. Systems that are used infrequently, however, often fail in an emergency situation, simply because they are never exercised at other times. One of the first problems here is gumming. Since the systems are located close to the boiler, the hydraulic fluid is exposed to extreme heat. It quickly ages and becomes gummy. Another problem is silting. Spool valves always have a small amount of internal leakage. Contaminants in the fluid then collect in the plunger area and this narrows the clearance. Microscopic particles clog the remaining gap and act as wedges which, the first time the plunger is moved, block the valve completely. And that is exactly at the moment when valve closure is most urgent. That is why the European Union requires power plant operators to inspect the bypass valves once a year. This is an expensive and elaborate procedure because – in addition to the personnel, equipment and documentation costs – the losses in power plant output must also be considered. What’s more, the inspector can only establish that the system functions properly at a specific point in time – during the inspection. Forecasts are not possible.

The essential components in the STSS: hydraulic safety blocks and the controller.

The essential components in the STSS: hydraulic safety blocks and the controller.

 

A new idea: testing every 20 minutes

And this brought our little after-work group around to an idea. Why not build a hydraulic system that continuously checks itself? We sketched a rough circuit diagram and additional drawings on beermats at hand and a notepad provided by the restaurant owner. Four hours later we parted ways but with that “Eureka” feeling – something engineers always fervently seek but seldom experience. Our STSS – the Self-Testing Switching System – embodies this entirely new approach.

The combination of hydraulic equipment, sensors, electronics and software monitors itself regularly and logs the results, too. The operator can determine at any time whether the safety equipment is functioning perfectly. Since the STSS has only poppet valves instead of spool valves, there is no leakage and silting is eliminated. In order to prevent gumming entirely, two valves are connected in series in the same oil circuit. To suppress any motion at all, the system opens only one of the two valves at a specified testing interval. The logic circuit, which is active, taps a small quantity of fluid from the high-pressure side of the cylinder and the same quantity of fluid flows back to the low-pressure side. At a 20-minute inspection interval, the fluid will be replaced completely in four to eight weeks. This has no negative effect on the certainty that the steam valve will close. Prior to the test, the system is isolated completely from the supply pressure. The amount of fluid taken from the system’s high-pressure side is so small that it will have no effect on valve closing power. The fluid required for this purpose is replaced once again following the test. In addition, it can be adapted flexibly to any desired combination of cylinders and steam valves and the amount consumed can be set at one of four steps. In this way the STSS always operates with two-thirds more closing pressure than is necessary to ensure a tight seal. This means that there is more than enough leeway.

Trials of the STSS: Proximity sensors register the position of the control plunger in the cartridge valve, attached to a cylinder

Trials of the STSS: Proximity sensors register the position of the control plunger in the cartridge valve, attached to a cylinder

 

The system is engineered to be fault-tolerant. Should testing ever show that a valve is not working ideally (due a coil failing, for instance), the system will be notified of this fact. Maintenance can be carried out for the STSS even during ongoing operations, without the safety level of the whole system being affected. This failsafe feature is governed by a separate safety PLC to ensure that the system is powered down in case of a malfunction – and that the steam valve reliably opens. Since the STSS is redundant in its design and continuously tests itself, it can be used in safety circuits up to the SIL 3 category. No extra expense is incurred for a system like this and the inspection costs that are saved will amortize the system completely within the first inspection cycle.

Safety is always in demand

Shortly after our get-together I started researching whether anyone else had had the same idea. And then, in due course, came the moment of certainty: We are the first! After three years of development work, in partnership with other firms, as well, the STSS was officially unveiled at the 2014 Power-Gen fair in Cologne. Originally developed for bypass valves in power plants, the application potentials have been expanded enormously. Wherever safety and reliable functioning are of prime importance , the STSS can make a contribution to reaching this goal.

 
Dirk Bracht

Dirk Bracht

Sales Industry Sector Energy Technology
Bosch Rexroth AG
Lohr am Main, Germany