The Facts about Wind Power Pitch Control Accumulators, by the People who Know Figure 6: The Olaer Composite Accumulator Solution
our experience has shown that low temperature need not be a significant product selection criterion for either piston or bladder accumulators, and no ‘survival issues’ for the bladder at operating temperatures below -20 degrees C have been recorded.
This is because, for the vast majority of the product life, it is operating at moderate temperatures of 5–25 degrees C, with only minor spikes outside that range. Indeed, operating outside this range towards lower temperature is not recommended, due to the well-known change in hydraulic oil viscosity leading to the loss of hydraulic circuit functioning.
Fatigue characteristics at these normal working temperatures are much more important selection criteria. Furthermore, our laboratory tests have shown that low temperature working actually improves the lifetime characteristics of the bladder, slowing down any fatigue (see Figure 7).
Selecting piston seals that function at low temperatures is expensive and may not be the optimum solution from a lifetime perspective. There is also the potential for ‘gumming’ of the seals at low temperature. We are carrying out further laboratory tests to examine and validate some of these phenomena.
Figure 7: Typical Low-temperature Lifetime Results
6,000 5,000 4,000 3,000 2,000 1,000 0
-25 -20 -15 -10 -5 Temperature (°C)
Kinetic viscosity Source: OLAER accelerated test equipment. 0 5 10 15 20 25 • Lifetime
1,200 1,000 800 600 400 200 0
Non-significant Product Selection Criteria – Often Incorrectly Cited
Here are the often-quoted advantages of piston accumulators over bladder accumulators that are of no practical consequence in this application:
• high flow rates – flow rates in this application do not exceed 100 l/min, well within the capability of a bladder accumulator;
•
high compression ratios – compression ratios in this application do not exceed 4:1, well within the known acceptable performance limits of bladder accumulators; and
unlimited size – the main packaging constraint is the space within the turbine hub and weight. The geometry of the piston can present a packaging/mounting issue and weight is a drawback of the piston type.
product, as moving a heavy product around 85 m in the air is a dangerous operation. However, it is also increasingly a factor for offshore turbines, where weight reduction is a necessity to reduce the overall installation costs of the turbine.
Olaer is also offering the Olaer Composite Accumulator Solution to our wind power customers (see Figure 6). Based on our existing class-leading wind power bladder product, but using a liner wrapped in carbon fibre, we are able to reduce the weight of a 20 litre accumulator to around two-thirds of its previous weight. This is also a robust solution for offshore because of the excellent corrosion characteristics.
On the latest generation turbines, this innovation can reduce the combined nacelle, hub and rotor weight by approximately one tonne.
Low-temperature Performance
At low temperature, piston accumulators, in theory, can be presented as having better performance than bladder accumulators. However,
54 Conclusions
We are very proud of the position we have reached after nearly 20 years of meeting the ever-increasing needs of our industry-leading customer base for improving the quality, predictability and reliability of products.
This experience has led us to conclude that, when all of the key selection criteria are considered, the bladder accumulator product is the most appropriate for the wind power pitch application. This does not mean that those customers that are using the piston solution will have poor results; just that, due to all the work completed so far, we believe it has been demonstrated that the bladder accumulator product is a more optimised solution.
Ultimately, however, whatever the customer’s preference, at Olaer we have proven that we can engineer and supply both bladder and piston accumulators to a world-class standard, helping our customers meet the needs of this demanding market. n
MODERN ENERGY REVIEW – VOLUME 4 ISSUE 1
Lifetime (Kcycles)
Oil kinetic viscosity (mm2/s)
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