WHEN SHUT-OFF DEVICES NEED A MAKEOVER

Autor: Roland Gruber , 05.02.2013

At the high-head power plants built in this region some 40 to 60 years ago, the shut-off devices are in many cases quite old. So most operators of such plants are now having their shut-off components completely restored.

After all, the shut-off devices are the most important security-relevant feature along the motive water stream of a hydropower station. Nowadays, however, putting the restoration plans into practice can get quite complicated, as by now some of the original manufacturers have gone out of business. In many cases the original machine blueprints have been lost as well. Specialising in solutions to these problems, Carinthian hydropower specialist EFG uses latest technologies to restore the complex shut-off mechanisms of yesteryear to showroom condition – and performing all the required installation work along the way.

Since engineers have been building high-head power plants in alpine regions, safety has been an important concern. Especially in storage and pumped storage power plants, which may have a gross head of several hundred feet, one can imagine the enormous pressure that the water column exerts on the shut-off components day in, day out. The ability to control this pressure load and withstand possible pressure surges in case of a machine standstill is a central requirement of the applicable security directives. These are designed to protect not just people’s property but, much more importantly, the people themselves. As a result, shut-off devices for controlling the motive water stream were developed as an indispensable security measure to cut off the water stream in case of an emergency. Hydropower engineers in the past have come up with a whole range of shut-off  mechanisms of varying designs. This includes everything from shut-off or pipe burst valves to various shut-off sluices, and the commonly used spherical valve, wedged gate valve, and annular valve systems. The respective designs and constructions reflect the different applications and environments in which the shut-off devices are used. Pipe burst valves are fitted at the intake point of a penstock, where the pressure is relatively low. Installed at the other end of the penstock are turbine shut-off devices, which function as a sort of ‘final link’ in the chain of safety mechanisms and are designed to withstand much higher pressures. The devices used at the lower shut-off point are usually ball valves or spheric al valves. The former are more common in smaller-sized hydropower stations, whereas the latter are typically installed in large-scale facilities.

LOST KNOW-HOW
Especially during the 1950s and up until the 1970s, a large number of regular and pumped storage power plants were built in alpine regions to satisfy the growing hunger for electricity. Today, these facilities have reached a technical age when many of the other components are already in desperate need of restoration. This applies particularly to the shut-off mechanisms, which in many cases were operated unchanged since their installation – up until recently, that is. “Over the past few years quite a few large-scale power plants have been putting a revision of their shut-off systems on their agenda. This is partly due to the age of the components, but it is also a requirement to ensure the necessary ‘re-granting of water rights’. From the operator’s point of vie w, the problem is that manufacturing the devices from scratch is usually very expensive. Also, finding specialists who have the necessary know-how for a professional reconditioning is becoming increasingly difficult. Sometimes the reason for this is the fact that the original manufacturers have gone out of business or no longer support the particular model, or the original blueprints are lost,” exp lains Werner Goldberger, Managing Direct or of EFG Turbinenbau, which has its head office in the Carinthian town of Feldkirchen. This market situation was what got Goldberger’s firm involved in the issue, eventually making it one of the leading providers of reconditioning services for shut-off mechanisms. Hydropower specialist EFG, which is primarily known in the industr y for its high-quality turbines, has also been making its mark with a growing series of reconditioning and revitalisation projects. This, as Werner Goldberger points out, was one of the basic preconditions for EFG’s successful entry into the market for shut-off systems.

WORKING FOR THE ‘BIG PLAYERS’
Barely ten years ago, EFG won the contract for reconditioning its first spherical valve. The contract was awarded by ÖBB (Austrian Federal Railways), one of Austria’s largest hydropower providers. The contract called for a full reconditioning of the shut-off system at the Spullersee railway power station. It was the successful completion of this project that sparked the decision to venture further into this new segment. The list of customers that have sought the services of EFG over the following years ready like a Who’s Who of Austrian hydropower: IKB, Verbund, Kelag, as well as numerous private operators. Increasingly, requests have also been coming in from across the border. For the hydropower specialists in Feldkirchen this development represents more than just the startup of a second business unit next to their turbine manufacturing operation, but also a significant growth in valuable know-how. Says Goldberger, “Of course we are learning new things with every new reconditioning project. It’s not just interesting professionally to know about the technical approaches that the hydropower engineers have chosen for their designs. We are also interested in acquiring the knowledge and know-how that is needed to professionally recondition any common type of shut-off device. This is a definite advantage in the market. Personally, I must confess that working on a complex piece of machinery like a spherical valve can sometimes be even more demanding and more interesting than most turbine constructions for new power plants.

STEP-BY-STEP PROCEDURE
Generally, the reconditioning of these machine parts proceeds in several stages. Matthias Viertler, a seasoned machine engineering expert at EFG, describes them as follows: “First, we take out the shut-off device and transport it to back our workshop. Of course, we keep a seamless documentation of the entire process from the start. That’s not just for our own benefit, it also benefits our clients. Back at the workshop we completely dismantle the device and sandblast the parts. Next, it’s off to non-destructive materials testing and a whole series of other tests. At that point we consult with the client to determine the next steps and the remaining process. This gives us the chance to assess the actual condition of the individual parts in detail. Which parts are still in working condition? Which ones are defective? Which ones need reconditioning or re-manufacturing? These are the main questions at that stage. This usually leads to a combination of welding work and manufacturing of new parts. After that, new gaskets are put in and the corrosion protective coating is applied. The finished device then undergoes various function tests and an intense test run to verify its pressure bearing capabilities. After the factory acceptance test with the client, it’s over to the installation team again, who reinstall the
device.”

LATEST METHODS FOR BEST RESULTS
Quite obviously, the devil is in the detail at every single step in the process. The list of potential problems that the EFG specialists may be confronted with is virtually endless. “It’s not uncommon that we come across invalid labels on some parts that were just left unchanged. Certifiers today would never approve parts like that. All parts are expected to be state-of-the-art technically to get an approval. It’s up to the operator to provide all the missing technical documentation. Together with appropriate certified bodies we are happy to assist our clients with this. By now it’s not much of a problem any more if the original blueprints and documentation are no longer available – which is not at all uncommon. In that case we just measure the parts at a 1:1 scale using our modern scanning systems. The data is then transferred electronically to the CAD software and transformed into a 3-D model. This allows us not only to retrieve and process 3-D models fairly quickly and easily, it also offers clients a complete technical documentation of their shut-off device,” says Daniel Zaminer.

DAMAGE CAUSED BY SAND AND PEBBLES
The types of damage shut-off devices may sustain are as varied as the types of shut-off devices themselves. “When you take a look inside a spherical valve after half a century of use, you’ll notice that it’s primarily sediment, pebbles, sand and pieces of wood that cause mechanical damage. The small grooves that are cut into the steel surfaces under high pressure keep eating away at the metal slowly but steadily and cause the system to leak over time,” explains Mathias Viertler. Of course, general wear and tear contributes to this effect as well. The more repositioning movements a shut-off device has to perform, the more intense the steady load on the sealing gaskets becomes. The bearing bushes may also be chewed away under the pressure, unless they are lubricated properly and regularly. This applies particularly to spherical valves, due to the one-sided surface pressure that is typically exerted on this type of device. “We found that the rack-and-pinion assembly in the gearing of the spherical valve were also in urgent need of repair. Here is a quick rundown of how these parts work: The sprockets of the pinion shift the gear rack, which turns the ball head into its closing position. The pressurised slider plate then ensures proper sealing of the device so no water can get through. In case the sprockets are damaged beyond repair, the flange clearance can be restored with a remanufactured pinion and refinished gear rack; this is achieved by shifting the profile of the sprokkets. Even if it does not sound very exciting,” says Viertler, “rebuilding the sprockets from scratch does take a lot of effort.” Where damage is concerned, the pressure level is a crucial factor, especially in turbine shut-off devices. “High pressures above 70 or 80 bars – which are quite common – may warp even large steel parts. That is why the bolted connections on the partition of a spherical valve are positioned tightly next to each other and are hydraulically pre-loaded due to the intense preload forces. It takes a lot of attention and precision to open and tighten such block flange connections to avoid possible warping, which could lead to the journal pin getting stuck,” says the expert.

THE NEED FOR HIGH PRECISION IN MANUFACTURING
As a result, the challenges the reconditioners are faced with are varied and demanding. “Especially the details can get very tricky indeed. Take a task like grinding the ball for a perfect fit in the valve body, for example: It takes a lot of craftsmanship and an incredibly high level of manufacturing precision to get a perfectly tight system with clearances between the metal surfaces of one hundredth of a millimetre. But even a perfectly shaped ball head is not enough to make the valve 100 % watertight if there is even the slightest deviation in axial distance between the head and body,” says EFG Refurbishment Specialist Ernst Eberhard, quoting an essential guiding principle for this type of work: “Errors start with a compromise – and the point is to avoid error wherever possible.” Supplementary components such as the bypass system and filling nozzles play a rather important role as well. Working on them requires quite a bit of technical know-how and the necessary experience to cope with the many potential problems and challenges. And then, of course, there are the individual needs and wants of the operator that need to be considered as well. Such individual demands, says Matthias Viertler, may even concern things as specific as the abrasiveness level in sandblasting or the type and application of the corrosion protective coating. As he points out, “It’s essential that we cooperate with our client to meet the highly demanding requirements.”

DEADLINES: THE TOUCHSTONE OFPROFESSIONALISM
Looking back on the past ten years that EFG has been active in shut-off device reconditioning, its Managing Director can list quite a few interesting reference projects. The complete reconditioning of the spherical valves for IKB at their Untere Sill power station, the ÖBB power stations at Uttendorf and Schneiderau, and similar valve units for hydropower group Fragant/Kelag, for example. A particularly memorable project, he recalls, was the contract EFG was awarded for the reconditioning of four pipe burst valves, including the air valves, at the VERBUND’s hydropower plant near Hieflau in the Austrian province of Styria. What made this project so special was the extraordinary size of the components, which measured an impressive 3 metres in diameter. The other thing that made this project stand out was the extremely tight time schedule, which ‘squeezed’ the whole project into a time frame of only three months. “Especially with largescale power plants, the replacing or reconditioning of components requires long-term planning and meticulous timing of the shutdown. This is quite understandable, considering that downtimes mean a considerable loss of income for the operator. Of course, this puts quite a bit of pressure on us as the contractor. That particular contract really pushed us to the limit. But in the end it was a success, as we finished it all in time,” says the managing director.

EXPERIENCE WITH COMPLEX SYSTEMS
The more complex and more difficult the reconditioning of a shut-off device turned out to be, the more experience and know-how the Carinthian hydropower specialists were able to accumulate. By now, there is hardly any commonly used shut-off device that they have not dealt with so far. Even the extremely high-quality Von Roll systems, which can be found primarily in high-head facilities in Switzerland and Western Austria, are on EFG’s list of references. These devices are considered to be expensive and of very high quality, but also a great technical challenge. “In these constructions, the position of the rotary head is controlled by two rotationally symmetric, curvilinear hydraulic cylinders. In my opinion, this is the cleanest and best thought-out solution for any shut-off device in terms of engineering, and experience shows that it has very good maintenance qualities indeed. However, these systems are not exactly easy to repair, as their inner workings are a sort of ‘different world’ altogether. That’s why nowadays they are sometimes replaced by straight cylinders and a rack-andpinion construction – simply because the necessary knowledge is missing in the market. For us, reconditioning a Von Roll system is an exciting, very rewarding challenge, which we love to take on if we get a chance,” says Armin Pretis from Research and Development at EFG. Although a passionate turbine engineer, Werner Goldberger has developed a great deal of professional enthusiasm for the many different types of shut-off devices that are commonly installed in alpine high-head power stations. Thanks to EFG’s commitment and extensive know-how, this segment has grown into a sizeable contributor to the economic success of the mid-sized firm from Feldkirchen. The issues at the core of the reconditioning business are quite sensitive, as they are all tied to the essential aspects of power plant safety. The fact that some of the largest and most renowned hydropower operators have decided to put these safety issues into the able hand of the EFG engineers may well be seen as a high-profile seal of approval.

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