Roller Screw Electric Actuator Retrofits for Steam Turbine Governor Valves

 

In the history of steam  turbine control improvements few upgrades have afforded as much advantage as the installation of electric actuators for governor valve control.  Plants which have opted for electric actuator turbine control have enjoyed far less maintenance, much improved performance, and reduced operating costs over the previous hydraulic operators.  In this treatise the reason for these improvements will be discussed and in addition the cautions which must be observed in applying electric actuators to turbines are also outlined to provide a full picture.

 

1.0    The Inverted Roller Screw Electric Actuator

An inverse roller screw electric actuator is a conventional electric motor (three-phase R-S-T wound) which rotates an internal-thread cylinder.  Inside the cylinder a planetary set of roller screws are coupled to the output shaft.  Since the threads have little or no backlash when loaded, any rotation of the motor shaft results in a corresponding planetary gear set motion and output shaft stroke.

 

Inverted Roller Screw Internals (Cut-Away Sample)

 

 

 

(Images courtesy of the Exlar Corporation, http://www.exlar.com)

    An additional key electric actuator component is the rotational feedback device, termed resolver or encoder.  Feedback precision for an electric actuator is typically 12-bit (4096 steps) per shaft revolution.  Typical standard model electric actuator performance specifications may be summarized as:

        SPEED:        0.5  to 10.0 inches/second

        LOAD:          100 lbs. to 10,000 lbs

        STROKE:      1 inch to 12 inch

        PITCH:          0.1 to 0.5 inch/revolution

Custom models may be found with more exotic specifications.

Lubrication is usually by grease.  In high temperature areas a low pressure oil lube system may be used which also extends load capability.

 

2.0    The Vector Servo Drive (VSD)

Two VSD with line filters mounted in an RFI/EMI shielded, air conditioned enclosure.

    By itself, the electric actuator would have very few practical applications, limited primarily to "open and close" valve control.  In a parallel development a new genre of electric motor controller, the Vector Servo Drive (VSD), has been made available which transforms electric actuators into superior motor modulating control devices.  VSDs became feasible for more than exotic spacecraft applications when Digital Signal Processor (DSP) and Insulated Gate Bipolar Transistors (IGBT) became available in the past decade.  Unlike their relatively slow microcomputer predecessors, DSP are fast enough to build and control three-phase output to an electric motor for position control based upon resolver feedback.  IGBTs allow the fast switching determined by the DSP to result in motor operation level currents.  The combination in VSD provide a perfect reliable interface between a digital turbine governor and the electric actuator.

    Although certainly manageable by careful application engineering, VSD also have USE CAUTIONS:

    2.1    VSD are potentially high level EMI/RFI emitters.  Cabinets with special cable entry shielding and secure ground paths are required.  Ignoring emissions could impact the operation of other systems near the VSD.

    2.2    Some VSD manufacturers do not program their products under software quality assurance programs.  Be warned! Avoid these, or suffer long term software failures with poor recovery support.

    2.3    The larger the VSD's current output capability the more heat will be generated requiring external cooling.  Keep them small.

    2.4    VSD actuator position loop closure may "fight" a high speed governor speed loop unless signals are integrated between controllers.  LCC designs stability data and status exchange between the VSD and LCC Governors.  Older governors do not have this capability.

    As in most retrofit scenarios, a "catalog cut piece together" approach is seldom sufficient to design a good system, while a thoughtfully integrated system is unmatched in performance and reliability.

 

3.0     Electric Actuator Mounting

     LCC uses three mounting strategies depending upon the linkage mass, backwind characteristics needed, and pivot geometry of the application.  One method does not suffice for all governor valves.  

3.1    DIRECT ACTING

    The Electric Actuator may directly stroke the governor valve stem as shown above.  Consideration must be given to insure correct alignment between the actuator shaft and valve stem, for the resolver stroke to be compensated for thermal expansion, and the prevention of steam packing leaks reaching the actuator.  Direct acting applications should be horizontal, not vertical, to avoid excess heat environments.  Vertical applications are possible, but require extensive heat load reviews.

3.2    LARGE FULCRUM RACK DRIVE

    Fulcrum rack motion must account for the one-dimensional pivot radius change over the stroke distance.  For large racks with high moment weights this is best accomplished with a trunion bearing supporting the actuator at mid-span.  The trunion bearing handles the greater loads and the base is easily aligned while mounted to a cover plate installed over the former hydraulic operator footprint.

 

3.3    SMALL PIVOT DRIVE

Lower force balances of small pivot drives permit clevis mounted actuators on dowelled alignment plates.


3.4    ENGINEERED FABRICATED BRACKETS

    When adapting brackets and mounting plates need to be fabricated they are fully engineered by LCC for the application.  Because these pieces mount on turbines they must have high safety factors for structural integrity, solid documentation in case a replacement is ever needed, and well finished so as not to corrode.  For these reasons all LCC retrofit mounting hardware is designed and assembled to engineering drawings and all steel pieces are powder-coat painted for high quality long lasting finish in the harsh turbine environments.  In contrast, some retrofits by others have left all bracket construction to "field form and fit" at the point of installation with poor documentation and a quick coat of paint.

 

4.0    Application Design Points

    To be successful the electric actuator specification must meet all the demands of the turbine valve application without introducing any marginal operating issues.  It is in this respect that LCC's experience in electric actuator retrofits provides a definite project advantage.  Design points that must be factored into specification are:

4.1    Force and Stroke Requirements

    Sizing electric actuators is not a simple matter.  Several recent installations by others have evidenced  inexperience in electric actuator selection and design.  In one case the installation vender specified a governor valve actuator with five times the force capability that the valve control required in order to overpower a massive closing spring.  The closing speed of the actuator was misinterpreted as being far faster than needed which resulted in selecting the large spring, now serving as a constant operating load.  This is a poor design for several reasons.  Any closure spring addition becomes additional steady state actuator load.  This added load demands more steady state motor current to maintain any position.  This added current generates excess heat and greater parasitic power loads.  The excess mechanical loads increase roller screw stress and shorten maintenance free run periods.  Arbitrarily retaining the original hydraulic operator's closing spring force is also not advised.  The hydraulic operator closing spring was designed to vacate the cylinder area of oil, back-pumping it to drain.  To perform this task quickly took a much larger force than to backwind a replacement electric actuator.

    The best method for sizing electric actuator force is to perform a thorough calculation of all steam, leverage, balance piston, friction, and seating forces.  A safety factor is then applied for specification.  As a final caution, electric actuator force ratings are at stall, not at rated velocities.

    Stroke specification also has details to consider.  On fulcrum pivot valve systems the sequential lift poppet valves  seat themselves with gravity and neither require nor should have any actuator closing force.  In many direct-acting applications a closing force is needed for valve seating but a closing position is troublesome because the valve stem thermal growth is faster than the valve body resulting in either overload closure or no closure at all.  In LCC applications  the proper valve seating method is designed into the VSD setup.

4.2    Emergency Closure, Actuator Power Off Response

 In most North American generation turbines the stop or throttle valves provide emergency rapid closure (less than 0.5 seconds) and steam shut off, while the governor valve closure criteria is slower (typically 3 to 5 seconds).  In Europe both throttle and governor valves generally have rapid closure criteria.  In each case a closure speed necessary can be determined and met with an electric actuator installation.  An additional consideration is the "Fail Safe" criteria in which the actuator must be closed within a specified time in an emergency even if the feed power is lost. This criteria is analogous to the reaction of a hydraulic system upon loss of oil feed pressure.  LCC engineers electric actuator retrofits meet power off closure in two levels.  First, if the closure times are modest, the criteria may be met with simple selection of the proper back-wind force actuator providing the required closing times when driven by the steam force with or without a closure spring.  Second, if additional closure response is needed LCC provides a SafeCloze arrangement using a small self-contained hydraulic reset and trip operator in series with the electric actuator.  The advantage of the SafeCloze solution to rapid closure needs is that the electric actuator does not have to constantly overcome the closing spring force (the power cylinder accomplishes this) while the power cylinder does not have to provide control modulation.  See SafeCloze for additional information.

    Should the actuator hold position, slowly close, slowly open, or rapidly close upon a loss of drive power?  This decision is sometimes preordained by the Emergency Closure criteria.  In the remaining application the proper selection of power off response can save a future plant shut down when properly engineered.  By adjusting specifications the electric actuator can behave in any manner desired on loss of supply power, including the ability to hold the last position.  For small process turbines this ability can be very useful to survive short term power outages without major turbine-dependent process upsets.

4.3    Environment

    Electric actuators are tough but not tolerant of extreme temperatures.  Usually they may be mounted in the same locations as the previous hydraulic operators since they share similar ambient temperature operation ranges.  In some installations, however, the mounting location for the electric actuator will be different to adjust for changed fulcrum mechanical advantage or to permit individual valve actuation in a previous sequential rack arrangement.  In these installations additional thermal protection such as forced oil lubrication and insulated and ventilated enclosures may be needed.

4.4    AC Feed Voltage

    Selection of AC feed voltage should consider actuator current loads, local availability, and feed reliability.  Smaller applications do well using low voltages while larger (2000 lbs+) applications are almost always benefited using 400+ VAC.

4.5    Local Positioning Capability

LCC Local Control Station

 Many VSD manufacturers provide computer connection ports for temporary connection of laptops operating calibration and drive setup software.  This is fine for it's intended purpose, but not the best for local positioning purposes.  It is recommended to have the ability to locally position actuators for maintenance and emergency backup control purposes.  A small local operator station which can also display alarms and operating parameters is a good investment.  LCC supplies the Model 231 Operator Control Station for this purpose.

4.6    Diagnostic Condition Monitoring

    Unlike hydraulic operators which have few condition monitoring parameters available, electric actuators coupled to VSD have a wealth of diagnostic capabilities, if these resources are tapped. Typically the following parameters are provided for conditioning monitoring:

        VSD Heat Sink Temperature

        Immediate Drive Current

        Average Drive Current

        Resolver Phase and Turns

        Bus Voltage

    Monitoring these parameters, alarms are generated for:

    ○    High Immediate Current

    ○    High Average Current

    ○    High Heat Sink Temperature

    ○    High/Low Bus Voltage

    ○    Short Circuit

    By establishing operating current profiles (see sample below), long term baseline operation can detect changes indicative of valve linkage friction increase or binding.  Because the immediate force provided by the electric actuator is proportional to the drive current the valve system can be under continuous condition monitoring which can detect abnormal conditions well before the problem stage.

 

5.0    Advantages Over Hydraulic Operators

    This section is presented to provide system engineers and technicians with solid, defendable arguments for choosing electric actuators for turbine governor valve control which may be presented to management

 

5.1    Dynamic Response is Much Improved

    When any mechanical (pilot operated) or displacement feedback (LVDT) hydraulic operator is positioning governor valves the immediate small step response characteristic is never known.  Since small corrections are by far the majority in turbine control this introduces a very vague outcome and poor repeatability.  The property that causes this lack of small change repeatability is termed "hysterisis", and is defined as the input signal correction to change the direction (i.e., open to close or close to open) of the operator stroke.  Depending upon the previous history of input signals the operator may at any time be anywhere within the hysterisis bandwidth, thus the unknown immediate response. This is illustrated in the following figure.  Notice that due to the hysterisis, a variable amount of input (demand) signal change is required to obtain the first finite output motion.

 

 In contrast the immediate dynamic response of the electric actuator is always known and is perfectly predictable.  Since a direct coupling occurs through the roller screw between the motor rotational position and the driven linkage or valve stem, no uncertainty bandwidth is present which is a function of previous directional operation.

    The removal of hysterisis and the resulting control precision permits accurate position loop gains to be implemented which solves the old hydraulics valve "hunting" problems of overshoot in corrective positions to maintain position control.  Such excessive oscillations wear out turbine components and can create overall plant output oscillations when severe.

5.2    Outage Schedules Benefit

    Lube oil based hydraulic operator positioned valve systems require the full turbine oil system to be assembled with all bearing covers in place and lines secure before the governor valves can be tested.  In outage planning, this activity is forced to the end of the scheduled maintenance since the thrust bearing pedestal is usually the last turbine re-assembly step.  All the time to stroke the valves and test hydraulic control components is essentially critical path, and often becomes a major schedule delay issue.  With electric actuators the control system check outs can be scheduled at any time during the outage well out of critical path determination since the turbine need not be reassembled to test the controls.

    High pressure synthetic hydraulic operator positioned valve systems have a similar critical path window due to the need of all valves to be in operating position and hydraulic feeds hard-piped and ready.  Disassembly of turbine components often requires piping removals which pushes this maintenance to the end of work also.

5.3    No Dependence on Oil Systems

    Lube oil systems have major contaminant entries both before and after filters.  Rust particles from steel reservoirs and pipes, paint, gasket sealant, sand, and numerous other foreign matter easily find entry into lube oil systems.  Even minor accumulation of this debris in a pilot relay land edge upsets the valve positioning loop and induces control oscillations.  Synthetic segregated oil systems are also prone to contamination and extremely fine filtration standards must be constantly be observed to avoid contamination of the even more sensitive E/H converters.  LCC electric actuator retrofits do not use any oil system so all of these control related oil problems disappear.  The actuator roller screws are simply re-greased at normal outage periods which provides lubrication for several years.

    Oil systems do not contribute anything to turbine control, but are simply a means of converting the AC-powered motor-operated oil pump energy to forces capable of positioning the steam admission valves.  The LCC electric actuator retrofit uses the same AC-powered motor, but through the inverse roller screw mechanism and VSD control, directly positions the steam admission valves and skips all the extra hardware and equipment with associated failure modes and maintenance.

5.4    I&C Technician Friendly Calibration and Maintenance

    Back in the 1950's and 1960's almost all major turbine and many major process controls were mechanical and hydraulic in design.  Technical schools and industrial training focused on hydraulic fits, oil maintenance, and other details that technicians needed to know to work on these systems.  Times have changed.  Today very little is available in "old school" hydraulics, but extensive training is available in electronics and AC drives.  By upgrading to electric actuators a plant keeps its equipment in tune with the training familiarity of its staff.  Further to this point, the numbers of qualified Turbine OEM service people with sufficient familiarity to be of assistance are dwindling through retirements. Even if people are still available, the scarcity means that each plant rolls the dice over an outage-extending hydraulic servomotor episode for each start up.

5.5    Spare Parts for the Future

    Almost all of the refurbishment parts of hydraulic operators and servomotors are mechanical fabrications.  Long ago OEMs made these parts themselves, but in the slow down in new plant constructions in the 1970's almost all small parts fabrications began to be jobbed out to local machine shops.  This continues today, with very few parts stocked for immediate delivery.  The normal processing on a hydraulic operator part order is for the OEM to bid the fabrications upon receipt of RFQ.  Unfortunately costs are very high.  Many parts need special surface hardening which often calls for an additional shop visit.  As profits and commissions are tacked on at successive stops the part assumes a very high price.  A recent pilot relay and bushing set for a small turbine servomotor was quoted at $20,000 and a six month delivery.  It does not take long to find trouble operating equipment under this type of spare parts availability.

6.0    Factory Testing

    Many turbine controls retrofitters, including most OEMs, supply a new system by shipping in the various parts and component pieces to the installation site where they first become acquainted.  Not LCC.  We think your plant is important enough that your new system should be fully tested before factory shipment, not debugged at your plant..  We don't think the customer's turbine should be a test mule either.  Full factory testing is not an easy nor inexpensive task.  LCC performs the test through the use of two very sophisticated testing systems developed from years of turbine controls experience, the Automatic Loading System (ALS) for actuator testing and the Turbine Speed Feedback Simulator (TSFS) for governor speed loop closure.

Computer Controlled Pneumatic Automatic Load System

    Using the combined ALS/TSFS the new governor and electric actuator system can be placed in full operating conditions.  The ALS provides control of the actuator working load by adjusting the pressure in the coupled pneumatic cylinders.  The pressure is calculated ten times per second to maintain actuator loads based upon the steam load, mechanical advantage, and all other forces which the actuator must work against.  The TSFS has inputs of actuator position and retains a computer model of the turbines speed transfer function which means it knows how the turbine reacts in speed to valve position under various steam pressure and load conditions.  The output of the TSFS is a pulse train speed signal which mimics the speed probe signals of the installed system.

    Factory testing provides assurance of component compatibility, a platform for pre-tuning control parameters, and a means of testing transient response without shock to the actual turbine.

 

7.0    Frequently Asked Questions

 

7.1    Can we install electric actuators in place of mechanical feedback linkages to position hydraulic servomotor pilots?

    Yes.  This will  improve the servomotor control.  This method also has a few drawbacks.  The stroke range of the pilot is very short, on the order of .050-inch.  Even using a softer pilot follow up spring the electric actuator stroke will be less than 1/2-inch, or on the margin of good position control with even the lowest of pitch actuator.  This method also retains the servomotor pilot and debris contamination and wear susceptibility weaknesses.  LCC generally advises that except for temporary runs, the whole servomotor be replaced.

7.2    If we have a functioning electronic governor, but it is not an LCC model, will LCC engineer an electric actuator retrofit using our existing governor?

    Yes.  We have successfully retrofit electric actuators on turbines running older digital designs like Woodward 505 and Triconex. These governors are adequate.  When still older analog systems are being used the upgrade to LCC Series 2 or LCC 200 Series provides spare parts availability and the exact matching control algorithms to position electric actuators (see 2.4 above) which competitor governors do not have.

7.3    What happens if the power feed is lost to the Vector Servo Drive?  Isn't this a New Vulnerability?  

    Part 1:    The actuator can react however you want on power loss.  It can hold position.  It can slowly close.  The reaction is optimized for the application.

    Part 2:    There is actually less vulnerability in that we can manage the power-off response.  It is also possible to bring redundant feeds and a transfer switch or install a UPS with battery backup to carry over potential power losses.  AC feed probably powers the existing Main Oil Pumps, so a loss with the hydraulic system guarantees an immediate shutdown.

7.4    LCC Seems to Favor Exlar Electric Actuators, Why?

    Yes, we do favor Exlar.  Exlar's inverted screw roller design is more robust and requires less maintenance than other candidates.  Exlar Electric Actuators that LCC have installed on steam turbines have amassed 50 unit years of trouble-free operation to date.  Exlar also was very helpful in providing engineering assistance for electric actuator application to the Naval Governor project.

7.5    If Electric Actuator Valve Control Is So Advantageous, Why Haven't the OEMs Offered It?

    The benign answer is that OEMs have so few people left with turbine controls insight that they can't staff an adequate re-design project.  A more cynical response is that the OEMs make far more money selling the job-shopped parts at huge mark up and billing long service trips than they would by selling an electric actuator upgrade.

7.6    What is the Lead Time for an Electric Actuator Installation?

    Typically sixteen weeks from order.  More exotic requirements may take a month or two longer.