The wind power and petroleum industries may be seen as rivals, yet oil is essential throughout a wind farm to ensure smooth operation. As such, it is necessary to select the right accumulators for both hydraulic and lubrication applications.
Wind turbine manufacturers typically speak of having availability levels in the range of 97% to 98%, but a 2008 study by British wind consultants Garrad Hassan found actual availability levels to be lower. In North America, turbines were averaging an availability of 94% to 95%.
The reasons for downtime vary widely. Sometimes a fault condition is the result of voltage or current being too high or too low. Grid stability issues can cause downtime, as can rotor overspeed conditions, vibration protection alarms and problems with the pitch rate.
Some of these issues are normal in any new equipment design experiences, and more mature turbine designs usually have a higher mean time between fault (MTBF) than newer models. Downtime caused by overheating or premature failure of bearings, gearboxes, or other components due to inadequate or uneven oil pressure is unacceptable.
This issue is not limited to the wind industry. In 2001, for example, the Nuclear Regulatory Commission (NRC) found that “inadequate adherence to maintenance instructions” resulted in “a loss of lubrication and subsequent bearing failure” on an auxiliary feed water pump turbine at Calvert Cliffs Nuclear Power Plant in Lusby, Md. This failure resulted in one of only seven “yellow findings,” the second highest level in terms of potential safety hazard, that the NRC issued from 2001 to 2005. As NRC Regional Administrator Hubert J. Miller wrote, “The finding has substantial importance to safety due to the equipment’s intended function of removing decay heat if called upon, as well as the length of time this condition existed.”
With wind turbines, we are not concerned with radioactive safety, but even short-term loss of flow can lead to excessive wear and premature replacement. More severely, it can lead to failure of the braking pitch or yaw controls, which in high-wind conditions can result in the loss of the entire turbine. This can be more complicated from a maintenance standpoint, because while a single steam turbine generator can consistently produce hundreds of megawatts, it takes hundreds of wind turbines to produce the same output, vastly magnifying failures of a lubrication system. Furthermore, wind turbine controls must operate continually to compensate for ever-shifting wind speeds and direction.
Given the multitude of hydraulic and lubrication systems in each turbine, and the dozens or hundreds of turbines at each site, it is essential to select a high quality hydraulic system, whether as part of the original equipment or as an upgrade to boost reliability. To prevent unnecessary damage or catastrophic failure in that system, properly sized and maintained accumulators are needed to provide a temporary supply of lubricating oil when the oil pump fails. In hydraulic systems, accumulators provide rapid response and minimize pressure fluctuations.
Function of accumulators
Hydraulic accumulators are energy storage devices that smooth out the pulsation of oil pumps and provide short-term oil pressure during switchover between oil pumps. They help maintain a constant oil pressure during temporary changes in demand. In hydraulic systems, accumulators provide storage for fluid at a constant pressure allowing quick and precise movement of actuators. The number of accumulators in a wind turbine varies by manufacturer, but they typically have several with sizes ranging from about 10 in.3 up to 25 gallon.
Lube oil systems consist of three elements: a pump, a reservoir and an accumulator. Lube oil system accumulators (LOSA) prevent bearing damage and increase bearing life by supplying oil to the bearings when a power failure shuts down the pump, or when changing between the primary and backup oil pump.
Hydraulic and lubrication oil systems, with their accumulators, are placed throughout the hub, nacelle and ground equipment, serving a wide range of functions. These include the hydraulics used for pitch control and braking, and the lubrication of the gearbox, turbine, drive train and other components. When specifying an accumulator to mount within the hub, one has to be selected that would withstand continuous shaking and vibration.
Types of accumulators
The size, design and strength of an accumulator depend upon how it is to be used. There are several types of accumulators including:
Spring accumulators use a spring-loaded piston in a cylinder. As the oil line pressure increases, more oil flows into the cylinder and compresses the spring, with the spring pressure matching the hydraulic pressure. When pressure drops, the spring forces the oil back out of the cylinder into the system. Spring-loaded accumulators have three primary shortcomings. As the spring expands, the pressure gradually drops. Also, their moving parts wear and need replacement. Repeated compression and expansion of the spring fatigues the metal and reduces the amount of pressure the spring can provide, limiting their usefulness in high-cycle applications as the metal will quickly fatigue and lose elasticity.
Gravity-loaded accumulators use weights to drive the piston and provide desired pressure. Their advantage is that they supply a near constant pressure. They are, however, larger, heavier and more costly than other types of accumulators. They too have moving parts, which require maintenance. If the packing on the piston wears and develops a leak, the oil will gradually migrate to the top of the piston, adding to weight and reducing the effective amount of oil in the accumulator. Because of their weight and space constraints and maintenance requirements, it is not practical to install one at the top of a 100-m tower.
Gas-loaded accumulators, such as non-separator designs, are the simplest and can store the greatest amount of oil; however their drawbacks make them unsuitable for high-pressure applications. Because there is no barrier separating the gas from the oil, the gas may be absorbed by the fluid, particularly at higher pressures. When the pressure drops, the absorbed gas forms bubbles in the oil, causing sponginess in the system, which may damage the equipment through cavitation.
Bladder-type accumulators have proven best for most wind turbine applications. Their vessels are typically made of carbon steel and certain designs can withstand pressures up to 3000 psi. (See sidebar for details on sizing of accumulators.) Inside the pressure vessel is a bladder made of Nitrile compound (Buna-N) or other material as appropriate. Because of its high flexibility and low weight, the bladder has a very rapid response time. If the working pressure is expected to be below 500 psi, a screen can be welded inside the flange to keep the bladder from extruding through the fluid port. At higher pressures, the bladder may extrude through the screen, so a plug and poppet assembly is used. As the pressure drops, the bladder pushes against the poppet and closes the fuel port, keeping the bladder inside the vessel.
Bladder-type accumulators are installed vertically with a gas valve molded into the top of the bladder, and a fluid port at bottom of the vessel. The bladder is precharged to 70 to 80% of the minimum working pressure of the system; this pressure must be periodically verified. Typically nitrogen is used because it is very stable and non-reactive even under pressure. Air is not recommended because of its corrosive properties and risk of explosion under high pressure.
The wind industry is rapidly changing and is still working its way up the learning curve. Sometimes the lessons learned make the news, such as when Suzlon had to replace all 1200 of its earlier-generation blades due to cracking or when the Vestas turbine near Hornslet, Denmark suffered a brake failure and a lose blade sheared the tower, generating several million views on YouTube.
Most problems, however, are far less spectacular, idling potentially productive units, lowering reliability statistics and adding to maintenance costs. But here is one area where wind has a major advantage over fossil plants. When you detect an opportunity for improvement you don’t have to conduct extensive engineering studies and wait for a regulator-approved spring shutdown to see whether the change boosts output and reliability. (And then wait another year to reverse the change if it didn’t work out.) Instead, when reliability problems are encountered, new components can be tested on a single unit and, if the change proves successful, expand it to the rest of the units one-by-one.
Joe Cheema is engineering manager for Fluid Energy Controls Inc., Los Angeles. For information, call (323) 721-0588 or visit www.fecintl.com.
Sizing a bladder accumulator
By selecting a properly sized accumulator and maintaining it according to the manufacturer’s instructions, wind farm operators can eliminate the costly and catastrophic damage caused by overheating, wear and premature component failure. Selecting the right size of accumulator and the correct precharge pressure requires an understanding of underlying principles. Bladder accumulators operate based on Boyle’s Law, which states that the product of the pressure, P, and volume, V, of a fixed quantity of gas is a constant, C, assuming temperature remains constant (PV= C).
In simple terms, if you double the pressure, you halve the volume. Because the expansion or contraction of the bladder takes place in less than a minute, there is no transfer of heat into or out of the gas as the pressure changes. Given that the polytropic constant for nitrogen, N, is 1.4, the formula for a nitrogen charged bladder becomes:
P1 × V11.4=P2 × V21.4
Applying this data to the sizing and operation of an accumulator, one gets the following:
V1= Size of the accumulator required, in.3 This is the maximum volume of gas in the accumulator bladder at the pre-charge pressure, P1.
Vx= The volume of lube oil to be discharged from the accumulator, in.3 This is the volume of lube oil demanded by the system. Vxis a function of the lube oil system for a particular type of turbomachinery and can be obtained from manufacturer’s specifications.
P1= Precharge gas pressure of the accumulator, psia. This pressure is always less than the minimum system pressure, P3.
P2= Maximum system operating pressure, psia.
V2= The compressed volume of gas at maximum system pressure P2, psia.
P3= The minimum system pressure, psia, at which the additional volume of oil (V3) is required.
V3= The expanded volume of gas at minimum pressure P3, in.3
So let’s see how this would apply to sizing an accumulator that requires a flow rate of 15 gpm at 100 psig system pressure and a maximum operating pressure of 115 psig. If the main oil pump shuts down, system pressure must be maintained within 10% of the system pressure for 15 seconds while the stand-by pump accelerates from an idle condition to operating speed.
In this case, the volume of fluid needed by the accumulator is:
Vx = 15 gpm ×0.25 min ×231 in.3/gallon = 866.25 in.3
Minimum system pressure (within 10% of the system pressure) is:
P3= (100 x 0.90) + 14.7 = 90 + 14.7 = 104.7 psia.
Maximum operating pressure is
P2= 115 + 14.7 = 129.7 psia.
Precharge pressure of the accumulator is 70% of minimum system pressure, so:
P1= 0.70 ×104.70 = 73.29 psia.
Inserting the above values into the formula below yields the volume of the accumulator required:
V1= Vx×(P31/N÷P11/N) ÷(1– (P31/N÷P21/N))
This formula yields a volume of 7878.28 in.3 (34.11 gallon).
Of course, there won’t be any accumulators made in that exact size, so the next larger size should be selected, not the next smaller one. There is no harm in being able to provide additional oil when needed, but there is a risk of damage if the undersized accumulator runs out of oil too soon.