 There are approx. 13,000 wind turbines in Germany, with another 1,000 being added each year. To keep the operating costs and insurance premiums from rising uncontrollably, these should all be thoroughly checked over.  Prolonging the filter lifetime via combination glass-fibre filter elements greatly reduces the operating costs of wind generators.  An additional bypass filtration system has a positive effect on the lifetime of hydraulic and gear oils.  Not to be underestimated: a ventilating filter with a 3 µm insert plus granules stops dirt, sand and condensation from getting into the hydraulic system in the first place. |
The reasons for the frequency of faults or failures are the source of much speculation. What is clear is that future systems will have to have much greater redundancy built in for longer running times. The same goes for hydraulic systems, which tend to dominate in larger-scale plants. From pod adjustment through rotor blade pitching and hydraulic braking to gear lubrication, oils are used to ensure smooth operation. In other words: there is plenty of scope for action. In the filtration of hydraulic fluids in particular, confirms Dipl.-Ing. Peter Botka, head of the Engineering & Service department of filter manufacturer RT-Filtertechnik, there is a considerable ratio potential. The reason for this is clear: most HLP oils used in hydraulic units have to be filtered to a specification of 5 µm due to the servo valves used. This calls for the installation of pressure filters, which are accordingly cost-intensive. In addition, the wind energy sector relies on return flow filters in order to remove the majority of the dirt from the hydraulic fluid before it enters the pump and causes havoc. RT-Filtertechnik on the other hand has spent years developing a filter system which performs the suction and return flow functions to perfection. Contained in a single housing are both a return flow and a suction filter which protects the pump against particles. In combination with an effective bypass filtration system, this allows an alternative filtration concept which is highly attractive in terms of costs. In ideal conditions, the theoretical lifetime of a wind generator can amount to up to 150 000 operating hours. For the hydraulic and gear lubrication systems, this means regular 24-hour shifts. We now know that – aside from the design aspect – contamination by particles in combination with condensation and the constant oil temperature are critical factors in the life expectancy of a gear or hydraulic system. Indeed, they are the source of gear damage in over 80 percent of cases. Although wind generators are fixed installations, their filtration systems have to be devised according to the conditions encountered in the utility vehicle sector – with dramatic temperature variations, high air humidity and extreme contamination with solid impurities such as sand. The usual service interval for hydraulic filters nowadays is six months. According to Peter Botka, the solution is simple: ”Our glass-fibre filter elements are divided into fine and medium gauges. That way you get a much longer lifetime for the required oil purity.“ Depending on loading and despite the use of hydraulic accumulators, the pump in the hydraulic unit may have to be activated up to 20 times an hour. If the ambient temperature is low, this means that there is always a risk of cavitation. At the same time, the pump bearings operate in a mixed-friction environment, thus generating additional abrasion in the system. Another phenomenon occurs in the hydraulic pitch adjustment. For a 90 degree rotation, the hydraulic cylinders execute the full stroke. This means that the dirt and rust particles and traces of water left in them flow through the entire hydraulic system in one go. Given this situation, the filters cannot act immediately and a large proportion of dirt passes through the system, only being filtered out after several circuits. In addition, the oil volumes in the hydraulic units contained in the pods of the wind generators are usually calculated according to the recommended values for mobile hydraulics – i.e. the pump output times 0.5 to 1. This means that dirt in the system is recirculated frequently and has little chance to settle at the bottom of the tank. Air and condensation contained in the oil are also flushed through the system. Strong fluctuations in the hydraulic tank mean that much more dirt can get in. All the above factors have an adverse effect on the hydraulic and gear fluid, and this can lead to faults. With a comprehensive ’purity package’ consisting of suction/return flow filter, bypass filtration and air breather for the oil tanks, plant operators can ensure maximum protection against such risks. The use of suction/return flow filters brings distinct advantages in itself. The oil overpressure of 0.5 bar supplied by the precharge valve integrated in the filter improves the suction in the feed pump circuit. That way there are no problems if the pump has to undergo frequent cold starts. What’s more, the two-in-one design greatly reduces expenditure in terms of costs and maintenance. To improve the oil purity even further, RT-Filtertechnik will also be using an additionally installed bypass filtration system. Although this type of filtration has a chequered history in wind generators, the benefit of additional bypass filtration of the gear oil is now beyond question. “Before long manufacturers and operators are bound to realise that hydraulic systems also need to be equipped with this in order to avoid damage and faults” remarks Peter Botka. Such additional filtration costs around EUR 2,000, i.e. 0.5 thousandth of the acquisition costs of a wind generator. Changing the oil alone is quite likely to cost EUR 3,000. If the oil lifetime is doubled by the new filtration concept of suction/return flow filter, bypass filter and ventilating filter, the investment will have paid for itself in a very short time. An important argument in favour of bypass filtration is that it allows condensation to be filtered out. Moreover, the system has its own gear pump in the bypass to the hydraulic main pipe which runs round the clock at low pressure. Not only does this ensure extra-fine filtration with a beta value 3 µm of 3.100, but the temperature balance of the oil also has an extremely positive influence on the entire plant operation, keeping the oil at a low viscosity even when the ambient temperature is low. As a finishing touch, a tank air breather with integrated 3 µm glass fibre filter is fitted to prevent a continuous flow of dirt particles into the system. |
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