Air and Oil Do Mix

Selecting the right accumulators for wind farm applications


By Viren Patel, Vice President, Fluid Energy Controls Inc.

The wind power and petroleum industries may be seen as rivals, yet oil is essential throughout a wind farm to ensure smooth operation. This requires selecting the right accumulators for both hydraulic and lubrication applications.

Wind turbine manufacturers typically speak of availability levels in the 97 percent to 98 percent range, but a 2008 study by British wind consultancy Garrad Hassan found that actual availability levels to be lower. In North America, turbines were averaging 94 percent to 95 percent availability.

The reasons for downtime vary widely. Sometimes a fault condition is the result of voltage or current being too high or too low. There can be grid stability issues. There are rotor overspeed conditions, vibration protection alarms and problems with the pitch rate.

Some of these issues are just part of the normal teething issues that any new equipment design experiences, and experience shows that more mature turbine designs have a higher mean time between fault (MTBF) than newer models. One area that is inexcusable, however, is downtime caused by overheating or premature failure of bearings, gearboxes or other components due to inadequate or uneven oil pressure.

This is not an issue that is 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 feedwater pump turbine at the 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, the NRC issued during the 2001 to 2005 time frame. 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 the concern is not 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. Complicating this from a maintenance standpoint, while a single steam turbine generator can consistently produce hundreds of megawatts, it takes hundreds of wind turbines to produce the same output, magnifying the opportunities for failure of a lubrication system. Furthermore, the 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. As part of that system, to prevent unnecessary damage or catastrophic failure, properly sized and maintained accumulators are needed to provide a temporary supply of lubricating oil when the oil pump fails. In the hydraulic systems, accumulators provide rapid response and minimize pressure fluctuations.

Function of Accumulators

Utilities use capacitor banks as energy storage devices to smooth power flows along a transmission grid. Similarly, hydraulic accumulators are energy storage devices that smooth the pulsation of oil pumps and provide short-term oil pressure during the switchover between oil pumps. They also help maintain a constant oil pressure during temporary changes of demand. In hydraulic systems, the accumulators provide a storage place for the fluid at a constant pressure allowing quick and precise movement of the actuators. The number of accumulators in a wind turbine varies by manufacturer. Typically, however, they will have several with sizes ranging from about 10 cubic inches up to 25 gallons.

Lube oil systems for 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, drivetrain and other components. When specifying an accumulator to mount within the hub, ones have to be selected that will withstand being subjected to continuous force and vibration.

Types of Accumulators

The size, design and strength of an accumulator depends on how it will be used. (See accompanying sidebar.) There are several types of accumulators including:

Spring Accumulators – This method uses a spring-loaded position 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. Then, when the 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, rather than providing a constant pressure. Since these types of accumulators have moving parts, those parts wear and need replacement. In addition, repeated compression and expansion of the spring fatigues the metal and reduces the amount of pressure the spring can provide. This limits their usefulness in high-cycle applications as the metal will quickly fatigue and lose its elasticity.

Gravity-Loaded Accumulators – These are similar to spring accumulators, but instead of using a spring, they use weights to drive the piston and provide the desired pressure. The advantage of this design is that it supplies a near-constant pressure. It is, however, larger, heavier and more costly than other types of accumulators. In addition, it has 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 its weight and reducing the effective amount of oil in the accumulator. These accumulators can be used for ground applications. Due to weight and space constraints, as well as the maintenance requirements, you might not want to install one at the top of a 100-meter tower.

Gas-Loaded Accumulators –Several types of accumulators use compressed gas to provide the pressure. These divide into two main categories: separator and non-separator accumulators. Non-separator accumulators do not have any barrier between the gas and the liquid. This is the simplest design and can store the greatest amount of oil. However, its drawbacks make it unsuitable for high-pressure applications. Since there is no barrier separating the gas from the oil, the gas may become absorbed by the fluid, particularly at higher pressures. Then, as the pressure drops, the absorbed gas forms bubbles in the oils, causing sponginess in the system that may damage the equipment through cavitation.

Bladder-Type Accumulators –Bladder accumulator vessels are typically made of carbon steel and certain designs can withstand pressures up to 3,000 psi. (See sidebar for details on sizing of accumulators.) The inside 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 rapid response time. If the working pressure will 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 percent of the minimum working pressure of the system. This pressure must be periodically verified. Typically nitrogen is used because it is stable and non-reactive even under pressure. Air is not a good choice because of its corrosive properties and risk of explosion under high pressure.

Boosting Uptime

The wind industry is rapidly changing and is still working its way up the learning curve. Most problems are less than spectacular, although they idle 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. Although there are far more components to contend with, when you detect an opportunity for improvement you don’t have to conduct extensive engineering studies and wait for a regulator-approved outage 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 you are having reliability problems, you can test new components on a single unit and, if the change proves successful, expand it to the rest of the units one by one.

Author: Viren Patel is a vice president for accumulator manufacturer Fluid Energy Controls Inc. in Los Angeles.

Sizing a Bladder Accumulator

By selecting the 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 the 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 the temperature remains constant (PV=C). In simple terms, if you double the pressure, you halve the volume. Since the expansion and contraction of the bladder take place in under a minute, however, no transfer of heat occurs into or out of the gas as the pressure changes. As a result, the formula for a nitrogen charged bladder becomes P1V11.4=P2V21.4 .

Applying this data to the sizing and operation of an accumulator, one gets the following:

V1 = Size of the accumulator required in cu. in. 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 cu. in. This the volume of lube oil demanded by the system. The VX value is a function of the lube oil system for a particular type of turbomachinery and can be obtained from the manufacturer’s specifications.

P1 = Precharge gas pressure of the accumulator in psia. This pressure is always less than the minimum system pressure P3.

P2 = Maximum system design operating pressure in psia.

V2 = The compressed volume of gas at maximum system pressure P2, psia.

P3 = The minimum system pressure, psia, at which the additional volume V3 of of lube oil is required.

V3 = The expanded volume of gas at minimum pressure P3 in cu. in.

So, let’s take a look at 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 percent 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 / 60 seconds) x 15 seconds x 231 (cu. in. per gallon) = 866.25 cu. in.

Minimum system pressure (within 10 percent of the system pressure):

P3 = (100 x 0.90) + 14.7 = 90 + 14.7 = 104.7 psia.

Maximum operating pressure:

P2 = 115 + 14.7 = 129.7 psia.

Polytropic constant for Nitrogen: n = 1.4.

Precharge pressure of the accumulator:

P1 = 70% of P3 = 0.70 x 104.70 = 73.29 psia.

By inserting the above values into the formula below, you can obtain the size of the accumulator required (V1):

This formula yields a volume of 7878.28 cu. in., or 34.11 gallons.

Of course there won’t be any accumulators made in that exact size, so the next larger size should be selected, not the next smallest one. There is no harm in being able to provide additional oil when needed, but there is a risk of damage if an undersized accumulator runs out of oil too soon.—VP

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