Accelerated testing of the two accumulator designs shows one best for wind turbine hydraulics.
Wind turbines typically use hydraulics to control yaw, pitch, and braking systems. Yaw controls keep the turbine pointed into the wind. Pitch controls adjust the angles of the blades to ensure high and constant power output, while protecting the equipment from over speed. Where pitch control is accomplished hydraulically, accumulators are used with a power unit and pitch cylinder to feather the blade pitch. Their use let the turbine continually extract maximum power during fluctuating wind speeds. Additionally, during extreme wind conditions or power failure, the accumulator serves as an auxiliary power storage device to quickly and safely rotate the blade to a null position.
Accumulators are also used as a fail-safe source of auxiliary power to activate the calipers of both the high-speed shaft and yaw-brake systems. The brake force holding time requirements for unplanned shutdowns are significantly increasing as wind turbines grow in size. For instance a hold time of up to 500 hours is not uncommon.
Both piston and bladder accumulators are used in, wind turbines. While design decisions may appear fairly straightforward, each type of accumulator has advantages and limitations that must be considered. These include response time, system speed, mounting orientation, fluid type, pressure, temperature, diagnostic characteristics and maintenance and service intervals as well as the cost for servicing and downtime.
Recently, accelerated tests of piston-versus-bladder accumulators for wind turbines point to precharge loss as a major service and safety issue with bladder accumulators. But before detailing the test results, it is helpful to look at the differences and tradeoffs in design, function, and performance.
These consist of a cylindrical body, sealed by a gas cap, with a charging valve at the gas end, and a hydraulic cap at the hydraulic end. A lightweight piston separates the gas side of the accumulator from the hydraulic side. This design offers the greatest efficiency and flexibility in most wind-turbine applications due to a wide range of sizes and low gas pre-charge loss in wind-turbine testing. Advantages include:
· High flow rates
· High and low-temperature tolerance
· High compression ratios
· Ability to withstand external centrifugal forces
· Unlimited sizes and mounting
These use a non-pleated, flexible rubber bladder housed in a steel shell. The bladder has a pre-charging valve at the gas
end of the shell and stores nitrogen within. A poppet valve, normally held open by spring pressure, allows fluid flow through the hydraulic port and protects the fully extended bladder from extruding out of the fluid port. Advantages of the design include:
· Generally universal application (wind turbines are the exception)
· Contamination tolerant
· Medium flow rates
· Compliant to high-frequency pulsations
· Quick response
These differ for piston and bladder accum-ulators. In a bladder accumulator, the compression ratio can be no greater than 4:1. In a piston accumulator it can be 10:1 or greater as long as the design pressure is not exceeded. The difference is that beyond a 4:1 ratio, the bladder can be crushed by hydraulic fluid pressure to the point of damage. For a piston, it is not as critical because it simply travels down the bore and stops against the end cap. Hence, a major reason to choose a piston over a bladder accumulator is that the compression ratio will exceed 4:1.
Differences and tradeoffs
In high flow rates: Most bladder accum-ulators have a capacity from 10 in.3 to 15 gallons, with a maximum flow rate of 220 gallons per minute (gpm). Any more than that has potential to damage the bladder. Piston accumulators can have flow rates of up to 818 gpm. With standard seals, the limit on piston accumulators is 120-in./s (ips) of piston speed. All piston accumulator flow rates are based on piston speeds of 120 ips. For systems with flow rate requirements in excess of 220 gpm, the only option is a piston unit.
Extreme low or high temperatures: Bladders are made of rubber and look much like a balloon. The rubber compounds meet different operational temperature requirements from -40 to 250°F (-40 to 121°C). The tradeoff to using low temp-erature bladder compounds is higher gas permeation rates through the bladder at working or elevated temperature. But regardless of temperature, there is an inherent loss of precharge that occurs over time due to gas permeation. If not checked on a regular basis, the loss of precharge leads to poor performance and premature bladder failure. Gas permeation cannot be avoided with bladder units.
Piston units, depending on the type of seal used, have an application temperature range of -45 to 320°F (-43 to 160°C), If the wind turbine location involves temperatures below -40°F (-40°C), or higher than 250°F (121°C), piston units are the only choice.
Gradual piston failure: Bladder failures are sudden and allow their stored nitrogen into the hydraulic system. Piston accumulators, because of their small seal, fail gradually. Thus, even when the unit begins to fail, the migration of N2 from the gas side to the fluid side is slow, leaving sufficient time to correct in a scheduled service.
Unlimited size: Piston units can range from 5 in.3 to more than 100 gallons (379 liters). These accumulators can be custom produced in any size up to rated working pressures in excess of 689 Bar (10,000 psi), and in bore sizes from 1.5 to 25-in. (38 to 636-mm).
Bladder sizes were established by industry many years ago so that size options in the U.S. are limited to 10 in.3, 1pt, 1qt, 150 in.3, and 1, 2.5, 5, 10, 11, 15 gallons). So, if a system only requires a 3-gal. unit (11 liters), by industry standards designers must use a 5-gal. (19 liters) unit.
Sensors can be added to piston accumulators on bore sizes from 4-in. ID and up. One sensor can determine the location of the piston. Knowing the piston location in the operational cycle lets a small programmable controller (PLC) determine if the pre-charge is correct. Should the unit loose gas pressure, an immediate signal would indicate that the piston is not positioned where it should be.
Bladder accumulators can only accommodate pressure sensors. The only way to check precharge in a bladder unit is to take a sensor reading while there is no hydraulic system pressure, which would require shutting down the wind turbine.
Piston versus bladder accelerated test findings
Recently, Parker completed a Piston versus Bladder Pre-Charge Loss Test for a major wind turbine OEM. The accelerated test was modeled after an actual frequency trace off an active wind turbine. Both accumulator designs were subjected to the accelerated frequency test. The test emulated three years of operation in nine months. After the equivalent of 1.2 years of operation, the bladder accumulator lost 9 Bar (130 psi), while the piston accumulator lost less than 2 Bar (22 psi). This trend continued throughout the test.
Based on this test, the bladder accumulator lost precharge at a rate 4.5 times greater than the piston unit. Recommended practice is a variation of no more than 10% of the specified pre-charge value. In this test instance the specification is 80 Bar (1,160 psi). With a 9 Bar (130 psi) loss, the unit exceeded the 10% recommendation. Based on these findings, the bladder unit would have required service in 12 months.
Restoring precharge to a bladder accumulator requires taking a wind turbine offline for two to three hours. A failed bladder can push repair time to more than eight hours. In both instances, a low-pressure alarm will trigger a shutdown.
The service charge, according to one OEM, is $7,500 per day for downtime under warranty. Should there be a shutdown due to a pre-charge alarm, the time it takes to get a repair crew onsite (with the right parts) can be added to the downtime service fees.
Loosing precharge also changes the available hydraulic fluid volume. A pre-charge too low can rapidly produce a severe safety issue – especially when the accumulator is a fail-safe source of auxiliary power.
Bladder accumulators are an excellent choice in applications where contamination, response time and frequency compliance are critical. Pre-charge loss, however, is inherent in bladder accumulators and occurs over time due to gas permeation through the bladder. How long will a precharge last in a given wind turbine application? The “life factors” are the bladder compound, size (surface area) of the bladder, temperature, cycle rate and compression ratio. The use of piston accumulators in wind turbine applications is, in fact, a cost savings insurance policy. WPE
-Charles Taylor, Engineering Project Manager, Alternative Energy, Parker Hannifin Corp., Global Accumulator Dev.
Filed Under: Components