Aerospace technology has been applied to demanding industrial applications in order to gain performance improvement, increased efficiency, and longer life. Parker's technology innovation and development apply to check valves, manifolds, and switches for accurate control of flaps, rudders, and flight control surfaces in aviation. Applying the latest advancement to motion and control for gas turbine design within Power Generation plants, such as combined-cycle units, is helping to keep equipment running longer and more reliably. One example is electrohydraulic servovalves (EHSV). The advancement of EHSVs took off in the 1950s, largely due to the adoption of permanent magnet torque motors as the first stage (as opposed to solenoids). This resulted in greatly improved response times and a reduction in power used to control the valves. Parker Hannifin purchased Denison International and their Abex product line in 2003 to support our EHSV product solutions.
Power plant gas and steam turbine applications require precise control of their “fuel”, whether this is natural gas, fuel oil, or steam. These engines may also have a need for the actuation of inlet guide vanes (IGV) and fuel blending stop/ratio valves. Electrohydraulic servovalves enable a control signal to be converted to the precise movement of an actuator, which in turn will control the fuel valves or IGV.
One of the initial design objectives for servovalves was electrohydraulic control of flight control surfaces on aircraft. These demanding, critical applications resulted in designs that were close to fail-safe, with redundant coils
For the turbine system, the fuel gas control valve on an actuator is the primary interface between a complex control system and the mechanical part of the plant. Maintaining that link is a cornerstone for producing power.
The servovalve is comprised of two major parts: the valve, which is a precision, close tolerance, matched spool and sleeve; and the electrical force motor called a torque motor. Combining an electrical device (torque motor) with a mechanical device (spool and sleeve) with a mechanical feedback spring results in a servovalve that provides an output flow precisely proportional to the input current.
To achieve high precision in performance, exacting levels of manufacturing are required to assure the proper size and fit of the valve components. In service, the valve components must maintain their relative positions and condition to assure continued operation within requirements. Electrohydraulic servovalves as seen in the schematic below are two-stage, with a servo control portion on top and a hydraulic portion below. The control portion on top is the electrical actuation that moves the jet-pipe within the servo. The bottom portion or second stage is the hydraulic control which manages the downstream actuator position and opening/closing of the gas or steam valves, inlet guide vane position, or stop/ratio valve.
A contemporary four-way servovalve is illustrated in Figure 1 This unit is shown in the neutral or null position. Supply pressure is applied to the pressure port and to the jet-pipe (usually one common supply connection). Jet-pipe flow is directed into a flow divider or receiver. In the null position, flows and pressures are equal in the passageways leading to the ends of the spool, thus there is no net force pushing the spool in either direction.
Upon application of an electrical signal to the torque motor, the armature deflects (as shown in Figure 2), causing the jet-pipe to displace and direct the jet flow into only one of the two receiver ports. The flow into one receiver passageway acts upon one end of the spool, causing the spool to move. The spool movement results in one cylinder port being opened to the supply port and the other cylinder port being opened to the return port.
As the spool moves it acts upon the feedback spring, which in turn pulls the jet-pipe back over the receiver null position (illustrated in Figure 2). This balance between input current, spool position, and feedback spring force results in a particular flow to be passed for each particular input signal to the servovalve. When the polarity of the input signal changes, Flow from the other cylinder port results.
Servovalves are used to accomplish many tasks. Most commonly they are mounted on linear or rotary actuators so that they will transform the electrical command signal into linear or rotary motion output of the actuator. Quite often this concept is used for position control of a machine platform.
Current trends in power plant utilization often demand numerous starts and stops of the turbines, and maintenance intervals are being extended as long as possible. These operating parameters have resulted in electrohydraulic control system oil contamination (particulate and varnish formation). Dirty EHC system oil may then cause critical use servos to become sluggish and even fail, tripping the plant off-line.
Parker’s JET-PIPE™ servovalve design offers performance advantages over traditional “flapper style” servos. With only one larger 0.008” diameter control orifice when compared with a flapper servos five 0.002” orifices, the Parker servovalve orifice is more difficult to plug with contaminated oil. The JET-PIPE™ servo, if plugged, will not fail in a manner that results in the downstream actuator fully extending or retracting. This type of uncontrolled movement of the turbines control valves would result in a trip or even damage to the engine.
Like most hydraulic system components, all servovalves like to be used with a fluid free of excessive particle contamination as well as a reasonable chemical composition to avoid chemical erosion. It is difficult to generalize in describing how clean a system should be due to the great variance between requirements with different applications.
One guide that can be generally used is document AS4059, published by SAE International. This document, titled Aerospace Fluid Power - Cleanliness Classification for Hydraulic Fluids, classifies varying levels of contamination. Servovalves have been found to operate quite satisfactorily in systems with a contamination level equal to, or below AS4059, Class 7, which corresponds to the following:
In terms of filtration, a well-maintained system with filtration of 10-micron nominal and 25-micron absolute has been found to be satisfactory in most applications. Fluid chemical composition should be monitored as well as the fluid and system manufacturer recommendations followed to maintain the proper chemical composition.
Two other areas should receive particular attention.
On new system start-up, flush the system thoroughly prior to the installation of servovalves. Defective servovalves with very low operating time are sometimes returned after having been installed in a new system. These units are often found with jammed spools due to trapped chips, weld slag, plastic tape, etc. This system contamination was built into the system between the filtration and servovalve and probably could have been removed by prior flushing.
When an element of the system has a failure that is suspected to have caused the generation of contamination, flush the system and service the filtration system.
Premature failures of electrohydraulic servovalves (EHSV) on fuel control valves were causing headaches and consuming maintenance budgets for the team at Marcus Hook Energy Center. Originally owned by NextEra and operated by Florida Power & Light, the fleet manager of this 790 MW combined cycle operation reached out to Parker for help in finding a solution.
Each of the three GE 7FA.03 turbines was running 6,200 hours per year on average with 210 starts. The expected service life of the OEM-supplied fuel control servos was 32,000 hours but they were failing every six months (3,100 hours) and cleaned, repaired, or replaced at every other outage. The flapper style EHSV's were supplied as OEM equipment from GE.
Parker’s JET-PIPE™ electrohydraulic servovalve with decades of success in flight control systems for commercial passenger planes and military fighter aircraft was selected for a side-by-side test and evaluation. A total of twelve Parker JET-PIPE™ electrohydraulic servovalves were installed.
Improves turbine availability and reliability while extending the service life of servos on critical engine control applications
Contamination resistant, erosion-tolerant, designed to last
GE Specification 312A6077
Dirty hydraulic fluid and varnish are two primary enemies of EHSV’s. GE's Lube Oil Recommendation Document GEK32568K discusses lube oil varnish formation and the negative impact on turbine availability and reliability.
Particulate (dirt) contamination in an oil system is the result of the oil physically breaking down, wear of components that are exposed to the oil stream, or external contaminants that wind up in the oil. Particulate formation in a hydraulic system that supports servovalves is a concern, as servos have very small internal orifices, as well as extremely tight tolerances between the hydraulic spool and sleeve (sometimes as tight at 0.00004”).
Varnish formation, as a result of moisture, acid formation, thermal and chemical degradation, can also greatly affect the operation of servovalves. Varnish can clog supply pressure filter, build up in low flow areas of the servo, and slow or stop the second stage spool from moving when commanded.
The condition of system fluid at Marcus Hook is especially demanding. A fluid analysis was not allowed but visual inspection of oil from an open port in the image above shows signs of contamination.
Parker JET-PIPE™ technology is far less prone to contamination, a key advantage in power generation “dirty” environments. Parker EHSVs offer a first to second stage gap that is four times larger than that of the nearest competitor. The unique jet construction enables most designs to receive and pass particles as large as 500 microns without malfunction. By allowing larger particulates to pass through the system, Parker EHSVs can then use a coarser filter that helps prevent clogging of the filter assembly in a dirty fluid environment. Plus Parker EHSVs offer 75 percent pressure recovery and neutral fail-safe capability.
Parker JET-PIPE™ servos are also designed with strong resistance to varnish and pollutants and have the unique ability of "failure return to zero" and "fault safe." Meaning that it is trip resistant as the valve will move to a null position rather than a hard-over failure.
After 19,000 fired hours, 520 starts, and nearly three years of operation without incident, a single Parker JET-PIPE™ servovalve was removed for testing and analysis. Laboratory results document that the Parker JET-PIPE™ was within new performance requirements.
NOTE: The OEM valve had to be serviced six times.
As of December 2019, the Parker JET-PIPE™ EHSV’s have over 60,000 hours of trouble-free operation and no signs of weakening.
Marcus Hook Energy Center has saved thousands in repair costs and hundreds of man-hours in avoided maintenance.
The Parker JET-PIPE™ is fully approved by GE for heavy-duty gas turbines and has been added to GE specification 312A6077 under a long term agreement with Parker.
Made in the USA at the Parker Aerospace Control Systems Division in Dublin, Georgia, JET-PIPE™ is a “drop-in” replacement for existing OEM EHSV on fuel control valves making changeouts quick and simple.
Article contributed by
Jim Hoke, market development manager, Parker Hannifin, Power Generation, North American Power Generation new construction business development. Works with plant/project owners, as well as associated Engineering / Procurement / Construction companies on technical and commercial topics.
Tom Ulery, business development manager, Energy Team Parker Hannifin, North America Wind industry. He has many years of experience in hydraulic valves, as the applications manager for Hydraulic Valve Division.
Tim Bryarly. project engineer, EHSV project engineer, working in EHSV design, new product development and product support, Parker Aerospace, MFCD.
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