1105

6.3 Actuators: Digital, Electric,Hydraulic, Solenoid

C. S. BEARD (1970)

B. G. LIPTÁK (1985, 1995)

P. M. B. SILVA GIRÃO (2005)

Types: 1. Digital 2. Electromechanical (linear and rotary)2a. Stepping motors in smaller sizes2b. Reversible motor gears for larger sizes 3. Hydraulic and electrohydraulic (the pump can be driven by stepping or

servomotors) 4. Solenoid

Energy Sources: Electric, hydraulic or both

Speed Reduction Techniques: Worm gear, spur gear, or gearless

Torque Ranges: 0.5–30 ft · lbf (0.7–41 N · m) for type 2a, and 1–250,000 ft · lbf (1.4–338,954N · m) for type 2b actuators with gearboxes

Linear Thrust Ranges: The maximum of about 500 lbf (2224 N) output force can be obtained from type2a actuators; 100–10,000 lbf (445–44,500 N) can be obtained from type 2b ones.The thrust capability of type 3 actuators can exceed 100,000 lbf (445,000 N).

Speeds of Full Stroke: Small solenoids can close in 8–12 msec. Throttling solenoids can stroke in about1 sec. Electromechanical motor-driven valves stroke in 5–300 sec. Electrohy-draulic actuators generally move at 0.25 in./sec (6.35 mm/sec) but can be speededup by the use of hydraulic accumulators.

Costs: 0.25 in. (6.35 mm) solenoid pilot in stainless steel costs about $50; with a two-way design in explosionproof plastic construction it costs about $250, and whenbuilt for high-temperature service it costs about $300.

An electromechanical, type 2a actuator for 0.5–1 in. (12.7–25.4 mm) small ballvalve can be obtained in explosionproof construction with limit switches foron/off service for $750; with positioner and feedback potentiometer such anactuator costs about $1500.

S

R

PH

EH

M M

Spring-loadedpneumatic cylinder

actuator

Dual actingpneumatic cylinder

actuator

Electric solenoidactuator shown

with manual reset

Pneumatic-hydraulicand electro-hydraulic

actuators

Pneumatic and electricrotary motor

actuators

Flow sheet symbols

© 2006 by Béla Lipták

1106 Control Valve Selection and Sizing

A typical rotary actuator with 300 ft · lbf (407 N· m) torque rating costs about$2500.

The cost of larger type 2b or type 3 actuators can exceed $10,000.

Partial List of Suppliers: ABB Group (1, 2) (www.abb.com) Aeroflex Incorp. (2) (www.aeroflex.com) Allenair Corp. (2, 3, 4) (www.allenair.com) Auma Actuators Inc. (1, 2) (www. auma-usa.com) ASCO-Automatic Switch Co. (Division of Emerson) (4) (www. asco.com) Bafco Inc. (3) (www.bafcoinc.com) Barksdale, Inc. (3) (www.barksdale. com) Bettis-Emerson Process Management (2, 3) (www.emersonprocess.com/valveau-

tomation/bettis) Bodine Electric Co. (2) (www.bodine-electric.com) Bosch-Rexroth Corp. (2, 3, 4) (www.boschrexroth.com/BoschRexroth/corpo-

rate/en/index.jsp) Bray Controls-Bray International, Inc. (2, 4) (www.bray.com) Bürkert Fluid Control Systems (1, 4) (www.buerkert.com) Butler Automatic, Inc. (3) (www.butlerautomatic.com) Center Line-Crane Co. (2) (www.cranevalve.com/products.htm) Circle Seal Controls Inc. (4) (www.circle-seal.com) Clark-Cooper Corp. (4) (www.clark-cooper.com) Curtiss-Wright Flow Control Corp. (3, 4) (www.cwfc.com) Danaher Corp. (2, 3) (www.danaher.com) Danfoss (4) (http://us.ic.danfoss.com) Detroit Coil Co. (4) (www.detroitcoil.com/PAGES/mainfram.html) DeZurik/Copes-Vulcan (1, 2, 3) (www.dezurik.com) Eaton Corp. (2) (www.eaton.com) El-O-Matic-Emerson Process Management (2) (www.emersonprocess.com/home/

products/index.html) Engineering Measurements Comp. (Emco) (1) (www.emcoflow.com) Emerson Process Management (4) (www. emersonprocess.com) ETI Systems, Inc. (1, 2) (www.etisystems.com) Exlar Corporation (1, 2) (www.exlar.com) Flo-Tork Inc. (3) (www.flo-tork.com) GE Water Technologies (1, 2) (www.gewater.com) Hoke, Inc. (2) (www.hoke.com) Honeywell Automation and Control (1, 2) (www.honeywell.com/acs/index.jsp)Humphrey Products Co.(4) (www.humphrey-products.com/home.htm) Invensys-Eurotherm (2, 3) (www.eurotherm.com) Jordan Valve (2) (www.jordanvalve.com) Kammer Valves-Flowserve Corp. (1, 2) (www.flowserve.com/valves/control/index.

stm) Keane Controls Corp. (4) (www.fluidprocess.com/Keane/Keane.htm) Keystone Valve USA Inc. (2, 3) (www.keystonevalve.com) Leslie Controls, Inc. (1, 2) (www.lesliecontrols.com) Limitorque Corp.-Flowserve Corp. (1, 2) (www.limitorque.com) Metso Automation Inc. (1, 2, 3) (www.metsoautomation.com) McCanna Inc.-Flowserve Corp. (2) (www.mccannainc.com) Micro Mo Electronics Inc. (2) (www.micromo.com) Nihon Koso Co. Ltd; (1, 2, 3) (www.koso.co.jp/e/seihin.html) Norgren-Herion (4) (www.herionusa.com) OCV Control Valves (1, 2, 3, 4) (www.controlvalves.com) Oil City Valve Automation (2, 3, 4) (www.ocvauto.com) Oilgear Co. (3) (www.oilgear.com) Oriental Motor USA Corp. (2) (www.orientalmotor.com) Parker Hannifin Corp. (1, 2, 3, 4) (www.parker.com) Plast-O-Matic Valves Inc. (2, 4) (www.plastomatic.com) Regin Hvac Products Inc. (2) (www.regin.com) Rotork Controls Inc. (2, 3) (www.rotork.com)

© 2006 by Béla Lipták

6.3 Actuators: Digital, Electric, Hydraulic, Solenoid 1107

Saint-Gobain Performance Plastics-Furon Fluid Handling Div. (4)(www.furon.com)

Samson Controls, Inc. (1, 2, 3) (www.samson-usa.com) Servo Systems Co. (2) (www.servosystems.com) Servotronics Inc. (4) (www.servotronics.com) Shafer-Emerson Process Management (3) (www.emersonprocess.com/valveauto-

mation/shafer) Shore Western Mfg. Inc. (3) (www.shorewestern. com) Siemens AG (1, 2, 3) (www.siemens.com) SMAR International Corp. (1, 2) (www.smar.com) Snap-Tite Inc. (4) (www.snap-tite.com) Sonceboz Corp. (2) (www.sonceboz.com) Spirax Sarco (2) (www.spiraxsarco.com) Superior Electric Co. (2) (www.superiorelectric.com) Thunderco Inc. (3) (www.thunderco.com) Tyco Valves (2, 3) (www.tycovalves.com) Valcor Scientific (4) (www.valcor.com) Valtek-Flowserve Corp. (3) (www.flowserve.com/Valves/control/index.stm)Vetec Ventiltechnik GmbH (2, 3) (www.vetec.de) Worchester Controls-Flowserve Corp.(2, 3) (www.worcestercc.com)

Note: More information may be obtained for instance at www.thomasregisterdirectory.com, particularly at www.thomasregisterdirectory.com/actuators/actuators_categories.html, at www.medibix.com, or at www.globalspec.com.

INTRODUCTION

This section covers a variety of valve actuators, except thepneumatic ones, which are discussed in Section 6.4. Thevarious accessory items such as positioners, handwheels,limit switches, potentiometers, and the like are also discussedin Section 6.2. The discussion of digital actuators in thissection is intended to complement Section 6.12 on intelligentvalves and Section 6.18 covering digital control valves.

The discussion in this section begins with some selectionand application guidelines, which is followed by the descrip-tion of the five actuator categories: 1) digital, 2) electrome-chanical, 3) electrohydraulic, 4) motors and pumps, and5) solenoids. The section ends with a brief discussion of thetrends in valve actuation, including the features of micropro-cessor-based “smart” and intelligent actuators.

SELECTION AND APPLICATION

The following are some of the characteristics to consider in theapplication and selection of all types of actuators. Table 6.3agives a summary of advantages, disadvantages, and applicationsfor some of the designs.

Actuator Types

Valve actuator types discussed in this section belong to oneof the following categories: (a) electric actuators, which usea motor to drive a combination of gears to generate thedesired torque or thrust level. This category includes (i) rodlinear actuators, whose output rod provides linear motion via

a motor-driven ball or ACME screw assembly. In this design,the actuator’s load is attached to the end of a screw, or rod,and is often unsupported; (ii) rodless linear actuators, whoseload is attached to a fully supported carriage. Rodless linearactuators provide linear motion via a motor-driven ball screw,ACME screw, or belt drive assembly; and (iii) electric rotaryactuators that use a motor to drive components rotationally.

The next category (b) are the hydraulic and electrohydrau-lic valve actuators, which convert fluid pressure into motion.They include (i) linear actuators or hydraulic cylinders, whichuse a cylinder and hydraulic fluid to produce linear motionand force, and (ii) hydraulic rotary actuators that use pressur-ized hydraulic oil to rotate mechanical components.

The third category (c) are the linear solenoids that convertelectrical energy into mechanical work via a plunger with anaxial stroke in either a push or pull action. They can be ratedfor continuous duty (100% duty cycle operation, continuousduty solenoids) or for off-on applications, less than 100%duty cycle (intermittent duty solenoids).

The last category (d) are digital actuators, which nowinclude all types of valve actuation solutions where the valveis digitally controlled.

Pneumatic actuators are still the technology favored byvalve actuators buyers. Nevertheless, the market share ofelectric, hydraulic, and electrohydraulic actuators is increas-ing, while the overall use of solenoid values has dropped.1

Actuator Features

Speed and Torque Ranges Speed requirements vary fromless than 1 rpm to about 160 rpm. The upper limit of availabletorque is about 250,000 ft · lbf (339,000 N · m) with gearboxes.

© 2006 by Béla Lipták

1108 Control Valve Selection and Sizing

Stem thrusts and rotational drive torques are limited only bythe size of the motor used and the ability of the gear, bearings,shafts, and so on to carry the load. Speed of operationdepends on the gear ratios, adequate prime move power, andmeans of overcoming the inertia of the moving system forrapid stopping. This is most important for proportional con-trol uses. Some actuators have a limited selection of drivespeeds, while others are furnished in gear ratios in discretesteps of 8–20% between speeds.

Manual Operation Manual operation is sometimes neces-sary for normal operational procedures, such as start-up, orunder emergency conditions. Only units that are rotating veryslowly or those with low output should have continuouslyconnected handwheels. Most units have the handwheel onthe actuator with a clutch for demobilizing the handwheelduring powered operation. Clutches are manual engage andmanual disengage, or manual engage and automatic disen-gage upon release of the handwheel. Others are manualengage when the handwheel is rotated, with the motor re-engaging when the handwheel is not being rotated; or powerre-engage, which takes the drive away from the operator uponenergization, leaving the handwheel freewheeling.

Electrical Equipment Most of these actuators include muchof the electric gear within the housings of the unit. Compo-nents such as limit, auxiliary, and torque switches and posi-tion or feedback potentiometers are run by gearing to stemrotation, so they must be housed on the unit. Installation isoptional concerning pushbuttons, reversing starters, lights,control circuit transformers, or line-disconnect devices in anintegral housing on the unit. Any or all of these componentsmay be located externally, such as in a transformer, switch,or control house. The enclosures must be designed to satisfyNEMA requirements for the area.

High Breakaway Force Resistance to opening requires amethod of allowing the motor and gear system to develop

speed to impart a “hammer blow,” which starts motion of thevalve gate or plug. Selection of motors with high startingtorque is not always sufficient. The dogs of a dog-clutch rotatebefore picking up the load, or a pin on the drive may movewithin a slot before picking up the load at the end of the slot.Systems are used that delay contact for a preselected time oruntil the tachometer indicates the desired speed of rotation.

Torque Control for Shutdown Torque control for shutdownat closure or due to an obstruction in the valve body isaccomplished in numerous ways, but each one uses a reactionspring to set the torque. When rotation of the drive sleeve isimpeded, the spring will collapse, moving sufficiently tooperate a shut-off switch.

Position Indication An indicator can be geared to the stemrotating gear, but it becomes a problem when actuator rota-tion varies from 90 to as much as 240 revolutions. Gearingof a cam shaft operating the position indicator or auxiliaryswitches must be calculated to obtain a fairly uniform angle.Upon correct gearing, an indicating arrow or transmittingpotentiometer can be rotated.

Maintenance of Last Position This is no problem when theactuator includes a worm gear or stem thread. Use of spurgears can cause instability when positioning a butterfly orball valve. Status quo is obtained by use of a motor brake orinsertion of a worm gear into the system.

Protection against Stem Expansion The status quo abilityof a thread or worm gear is detrimental when the valve itselfis subjected to temperatures high enough to expand the stem.This expansion, when restrained, can damage the seat or plug,bend the stem, or damage the actuator thrust bearings. Oneof the original patents for this type of actuator includedBelleville springs to allow the drive sleeve to move with thethermal expansion and relieve the linear force.

TABLE 6.3a Applications, Advantages, and Disadvantages of Various Actuator Designs2

Actuator Types Advantages Disadvantages Applications

Electromechanical High thrust High stiffness coefficientPowered by electricity or pneumatics

Complex designNo mechanical fail safeLarge, heavy structure

Linear or rotary valves2–36 in. body size

Electrohydraulic High thrustHigh stiffnessFast speedsPowered by electricity; no pneumaticsource required

Complex designLarge, heavy structureHydraulic temperature sensitive

Linear or rotary valves2 in. to unlimited

Electric (servomotor orstepping motor)

Direct interface with computer system Large structureLow thrustNo mechanical fail safeSlow speed

Linear valves 1/2–2 in. body size

© 2006 by Béla Lipták

6.3 Actuators: Digital, Electric, Hydraulic, Solenoid 1109

Mounting Methods Industry has dictated a set of dimen-sions for the mounting flanges and bolt holes for newlymanufactured valves. Retrofit mounting requires adaptationto existing valves. Mounting requires a plate to match theexisting valve, which is screwed into the yoke upon removalof the manual drive sleeve, welded or brazed to the yoke, or,for a split yoke, bolted to the yoke.

Adaptability to Control This feature includes adaptabilityto many voltages and to single- or polyphase supplies.Polyphase motors of 240–480 V and 60 Hz predominate.Single-phase motors up to about 2 horsepower (hp) (1,492 W)are used. Reversing starters with mechanical interlocks areused for both proportional and on-off service.

The coils that open and close the contacts are energizedby an open-center double-pole switch, which can be incorpo-rated in the automatic control circuit. For manual control, thestarter may be of the type that maintains contact, requiring theopen or close button to be held in position. Some units are alsowired for momentary depression so that the actuator runs untilit reaches the limit switch, or until a stop button is depressed.

Proportional Control Proportional control of these largeunits can be accomplished by including the coils of thereversing starter in a proportional control circuit. Thisrequires a position feedback, which may be a potentiometer.For this type of control, a Wheatstone bridge circuit wouldbe used.

The reversing starter controls any voltage or phaserequired by the motor. A transducer to transform any of theaccepted electronic controller outputs (e.g., 1–5 mA) to aresistance relative to controller output permits use of theactuator in these systems.

A smaller unit, with a force output of 1460 lbf (6494 N) atstall and 500 lbf (2224 N) at a rate 0.23 in./sec (5.84 mm/s),uses solid-state control of the motor. At the same time it elim-inates stem position feedback into the controller.

A DC signal “x” (Figure 6.3b) from a process transmitterprovides the loop with its measurement, such as temperature,as well as such high-responds systems as flow. A differentialamplifier responds to the magnitude and the polarity of an inter-nally modified error signal, which triggers silicon-controlledrectifiers (SCRs) to obtain bidirectional drive. The synchronousmotor can be driven in either direction, depending upon therelation to the set point.

DIGITAL VALVE ACTUATORS

Digital actuators can accept the output of digital computersdirectly without digital-to-analog converters. Only simple on-off elements are needed for their operation. The number ofoutput positions that can be achieved is equal to 2n, where nis the number of inputs. Accuracy of any position is a functionof the manufacturing tolerances. Resolution is established bythe number of inputs and by the operating code selected for

a given requirement. The smallest move achievable is calleda 1-bit move.

The code may be binary, complementary binary, pulse,or special purpose. A three-input piston adder assembly pro-duces eight discrete bit positions. The adders in Figure 6.3care shown in the 6-bit extended position. The interlockingpistons and sleeves will move when vented or filled throughtheir selector valves. This same adder can be used to positiona four-way spool valve, with a mechanical bias to senseposition. The spool valve controls the position of a large-diameter piston actuator or force amplifier.

Use of a DC motor featuring a disc-armature with lowmoment of inertia has created another valve actuator partic-ularly adaptable to a digital input. Brushes contacting the flatarmature conduct current to the armature segments. Incre-mental movement is caused by half-waves at line frequencyfor rotation in either direction. The rotation of the armatureis converted to linear stem motion by use of a hollow shaftinternally threaded to match the valve stem.

Actuator output is 5,000 lbf (22,240 N) maximum fornoncontinuous service at a rate of 0.4 in./sec (10 mm/s)through a valve stroke of 3 in. (76.4 mm). The actuator isde-energized at stroke limits or at power overloads by thermaloverload relays.

FIG. 6.3b Proportional motor control circuit with position feedback.

InputsAnalog

computingamplifier

ss–x–kr

xMeter

Differentialamplifiers–x–kr=0

0 Zeroreference

Samplingprogramcircuitry

Isolationtransformer

Link

Phase-shift network

Driving motor

Alternate loads

SCRA

SCRB

115 V50/60cycles

Load A Load B

s = instantaneous set pointx = instantaneous transmitter signalk = adjustable transmitter compensationr = instantaneous rate of change of transmitter signal

© 2006 by Béla Lipták

1110 Control Valve Selection and Sizing

Operation of the actuator requires application of a thy-ristor (SCR) unit designed for this purpose. This unit acceptspulses from a computer or pulse generator. The thyristor unitconsists of two SCRs with transformer, triggers with pulseshift circuit, facility for manual actuator operation, and thepreviously mentioned thermal overload relays. The outputconsists of half-waves to pulse the armature of the actuator.

Modules are also available for process control with thenecessary stem position feedback, slow pulsing for accuratemanual positioning, and full-speed emergency operation.

The digital-to-pneumatic transducer shown in Figure 6.3dis used to convert a controller output to a pressure signal foroperating a pneumatic valve.

The concept of digital valve actuator has been extendedto other valve actuating systems such as stepping motors,which are discussed in the following paragraph dedicated to

electromechanical actuators, and to the so-called smart orintelligent actuators briefly introduced below in Smart andIntelligent Actuators. The discussion of digital actuators inthis section is intended to complement Section 6.12 on intel-ligent valves and Section 6.18 covering digital control valves.

It is in this context that some of the suppliers in the partiallist of suppliers at the beginning of this section identified asproviding digital valve actuators indeed provide modernsmart valve actuating solutions.

ELECTROMECHANICAL ACTUATORS

Electric actuators can utilize a reversible electric motor pro-vided with an internal worm gear to prevent drive directionreversal (back-drives) by unbalanced loads. These units canoperate both linear and rotary valves. Servomotor drives canposition valves in response to feedback signals from linearor rotary encoders. Two-phase AC servomotors are availablewith up to 1 HP rating, while direct current servomotors canmeet higher loads.

Stepping motors, which rotate the shaft by a discrete stepangle when energized electrically, can also be used in valveactuators. The electromechanical device that rotates the shaftcan be a solenoid used to operate a star-wheel or ratchetdevice; it can be the stepping movement of a permanent mag-net, the flux of which causes poles or teeth to align and therebyaffect rotation; or it can be a variable reluctance unit, wherethe rotor-stator poles or teeth are aligned by electric fields.

When used as valve actuators, the stepping motors arewell suited for direct digital control, and pulse feedback canbe provided for accurate closed-loop positioning.

FIG. 6.3c Digital valve actuator.

Normally open solenoid Normally closed solenoid

Four-way valve

Return

Pressure

Bias piston

Mechanical feedback beam

Adders (hydraulic D/A converters)

Actuator (force amplifier)

Solenoids

6 5 4 3 2 1

FIG. 6.3d Digital-to-pneumatic transducer.

Zeroadjust

Zerospring

Rangespring

Beam

Fixed orifice

Pivot

Outputpressure

NozzleNon-rotating nut

3–15 PSIG(0.2–1 bar)

Pivot

Motor Stops

25 PSIG(172.5 kpa)

filteredsupply

pressure

© 2006 by Béla Lipták

6.3 Actuators: Digital, Electric, Hydraulic, Solenoid 1111

Of the above designs the actuators using reversiblemotors are the most often used variety and therefore theywill be discussed in more detail in the following paragraphs.

Reversible Motor Gear Actuators

Lever-operated valves and dampers are available with torquesof 5–500 ft · lbf (6.8–678 N · m) and with full stroke speedsof 10–60 seconds. For very large valves or dampers, this typeof actuator can deliver torques of 150–10,000 ft · lbf

(203–13,558 N · m) and a full stroke speed of 30–300 seconds.The power consumption of these larger units ranges from

0.5–5 kVA operating on 460 V three-phase power supply.Some of these designs can be obtained with springs to returnthe valve to a safe condition upon electric failure.

The motor gear actuator consists of an electric motor con-nected to a gear train; the gear train rotates a stationary drivenut, which in turn drives the threaded or keyed valve stem upor down. As the valve plug contacts the seat, the resistance istransmitted to a Belleville spring, which at a preset limit inter-rupts the motor power circuit.

For throttling applications, the actuator can be providedwith a positioner that compares the external control signal(analog or digital) with an internal position feedback signaland keeps turning the motor until the error between the twosignals is eliminated.

Rotary Output Actuators

Worm Gear Reduction The actuator shown in Figure 6.3eis an example of the use of a double worm gear reduction toobtain output speeds of around 1 rpm with an input motorspeed of 1800 rpm. The worm gear is self-locking, so itprevents the load from moving downward by back-drivingthe motor. However, the worm gear is less than 50% efficient,so more power is used compared to spur gears. Also shownis a handwheel for manual operation during power loss.

Spur Gear Reduction The spur gear actuator in Figure 6.3fhas very low power loss through the relatively efficient spur

gears. Ordinarily the load could back-drive this system, buta friction member at the motor end of the gear train minimizesthis undesirable action. Of course, if the load has a frictionalcharacteristic, it will not impose a back-driving torque on theactuator.

Actuators for larger outputs are externally mountedmotors. Two motors operating a single gear train have beenused to obtain 3,750 ft · lbf (5084 N · m) of torque through90° rotation in 75 sec. An actuator has been designed foraccurate rotary positioning that develops 5000 ft ⋅ lbf

(6800 N · m) at stall and will rotate 90° in 10 sec. SCRs ener-gize the motor as commanded by a servotrigger assemblyhoused separately.

Electrical gear may include adjustable cams to operatelimit and auxiliary switches. A potentiometric feedback cal-ibrated to the rotation of the actuator is required for use witha control circuit. The unit shown in Figure 6.3f was developedto slide over and be keyed to the shaft of a boiler damper.Actuators of this type must be adaptable to mounting on andoperating a variety of quarter-turn valves. Because of thedifficulty in setting limit switches to accurately stop the valvein the shut position, it is advisable to incorporate a torque-limiting device to sense closure against a stop.

Opening can be controlled by a limit switch. One com-pact unit contains the features noted with an output of 750ft · lbf (1017 N · m) of torque. The unit is powered by a motorwith a high-torque capacitor and includes a mechanical brake,feedback potentiometer, limit switches, and a de-clutchablehand wheel.

Flex-Spline Reduction Figure 6.3g is an example of aunique single-stage, high-reduction system. Instant break-away and efficient transfer of prime mover power is obtained

FIG. 6.3e Electric actuator with worm gear reduction.

Hammerblow

Drivesleeve

Torquecontrol

Geardrive

De-clutchinghandwheel

Gearedlimit

switchesPrime mover

FIG. 6.3f Spur gear reduction.

Travellimit

switches

Outputshaft

Adjustablecams (Optional)

potentiometerAuxiliaryswitches

© 2006 by Béla Lipták

1112 Control Valve Selection and Sizing

with a modified concentric planetary system consisting of asemiflexible gear within a rigid gear. A three-lobed bearingassembly transmits power to the gears by creating a deflectionwave transmission that causes a three-point mesh of the gear-ing teeth on 30% or more of the external gearing surface.

The semiflexible geared spline has fewer teeth than thenonrotating internal gear it meshes with. The spline slowlyrotates as it is pressed into the larger gear by the bearings onthe motor shaft.

Linear Output Actuators

Motors and Rack Figure 6.3h uses a worm and a rack andpinion to translate horizontal shaft motor output to verticallinear motion. Maximum force output is approximately 1500lbf (6672 N) at about 0.1 in./min (2.5 mm/min). A continu-ously connected handwheel, which must rotate the rotor ofthe motor, can be used when there is short stem travel andrelatively low force output.

The actuator is designed with a conventional globe valvebonnet for ease of mounting. Units operate on 110 V and havebeen adapted to proportional use with a 135 ohm Wheatstonebridge or any of the standard electronic controller outputs.

Motor and Travelling Nut Linear unit consists of the motor,gears, and a lead screw that moves the drive shaft (Figure 6.3i).A secondary gear system rotates cams to operate limit andauxiliary switches.

The unit may have a brake motor for accurate positioningand a manual handwheel. The bracket on the rear end allowsthe actuator to rotate on the pin of a saddle mount, so that thedrive shaft can be pinned directly to the lever arm of a valve

or a lever arm fixture that fits the shaft configuration of a plugcock or ball valve. Maximum output is 1600 lbf (7117 N) at5 in./min (127 mm/min).

Rotating Armature An internally threaded drive sleeve in thearmature of the motor is used to obtain a linear thrust up to6,600 lbf (29,360 N) at a rate of 10 in./min (254 mm/min).Bearings in the end cap support the drive assembly (Figure 6.3j).The drive stem is threaded to match the drive sleeve and is keptfrom rotating by a guide key.

FIG. 6.3g Flex-spline gear reduction.

FIG. 6.3h Rack and pinion assembly converts the rotation of a horizontal,motor-driven shaft into vertical, linear motion.

FIG. 6.3i Electric quarter-turn actuator with linear output and with limitswitches.

Continouslyconnectedhandwheel

Reversiblelow-inertiaAC motor

Motor drivepinion gears

Load connectionlinear motion

Terminalstrip

MotorElectric brake

Eye

Shaft

Screw drive

Auxiliary switches-limit switches located

on opposite sideTraveling

nutSwivelflange

© 2006 by Béla Lipták

6.3 Actuators: Digital, Electric, Hydraulic, Solenoid 1113

Thrust-limit switch assemblies are mounted in each endof the housing to locate the hollow shaft in mid-position.When the linear movement of the drive stem is restricted ineither direction, the limit switch involved will operate to shutdown the unit. Thermal cut-outs in the motor windings offeradditional overload protection. Strokes are available from2–48 in. (51–1219 mm).

The unit has been adapted for proportional control byuse of an external sensing position for feedback. For use asa valve actuator, it must be mounted so that the drive stemcan be attached to the valve stem, or a suitably threaded valvestem must be supplied.

Rotary to Linear Motion An electric proportional actuator(Figure 6.3k) is designed for continuous rotation of a drive-sleeve on a ball-screw thread. 3,000 lbf of thrust (13,335 N)is obtained at a stem speed of 1 in./min (25 mm/min).

One or two DC signals are used separately or numericallyadded or subtracted. Triacs operate on position error to con-trol a DC permanent magnet motor that positions a stemwithin an adjustable dead band. Degree of error and rate ofreturn are sensed by a lead network to determine the directionand time that the motor must run.

Stem reversals are almost instantaneous. The back emf ofthe motor is used as a velocity sensor and is fed into a circuitthat allows adjustment of the speed of drive sleeve rotation.Gain can be adjusted to control oscillation of the stem.

Stem position feedback is by a linear variable differentialtransformer (LVDT). Use of the DC motor allows for torquecontrol through sensing of motor current. A manual hand-wheel is furnished that can only be used when the unit is de-energized by a manual/automatic switch.

Application of the linear requirement of a valve necessitatesa linkage for translation from rotary to linear motion. Use of alinkage (Figure 6.3l) provides a thrust for operating the valve.

FIG. 6.3j Electric actuator with rotating armature has been adapted for proportional control.

FIG. 6.3k Proportional electrical drive converts rotary to linear motion.

FIG. 6.3l Electric actuator with linkage to convert rotary motion to linear.

Lift scaleMotor shaft

Shaft adjustmentscrew

Washer ARelief springmechanism

Motor

Linkage arm

Linkage arm clip

Lift adjustmentlocknut

Locknut

Washer B

Bolt

Screw

Cylinder

Stem buttonclamp

Stem buttonStem

Bracket

Allen set screw

© 2006 by Béla Lipták

1114 Control Valve Selection and Sizing

ELECTROHYDRAULIC ACTUATORS

The main features of electrohydraulic actuators have beensummarized in Table 6.3a. They are often used where instru-ment air is unavailable or where the required actuator stiff-ness or thrust cannot be obtained from pneumatic actuators.They are heavier than pneumatic actuators; therefore, in orderto avoid straining the bonnet, vertical upright installationsare recommended.

Electrohydraulic actuators are superior to electromechan-ical ones in the areas of speed, positioning without overshoot,actuator stiffness, and complexity. Electrohydraulic actuatorscan be stepping motor or servovalve driven. In the servovalvedesigns the pumps are running continuously, while in thestepping-motor configuration they run only when the valveneeds repositioning.

Electrohydraulic actuators can be mounted directly ontothe stems of valves that are 6 in. (152 mm) or smaller andcan be provided with springs to guarantee a fail-safe positionin case the hydraulic fluid is lost. The actuator usually con-sists of a hydraulic cylinder, a pump with motor, some feed-back linkage on valve position, and a balancing arrangement.The balancing or positioning mechanism compares the exter-nal control signal with the valve position and actuates thehydraulic system if repositioning is required.

In spring-loaded designs the hydraulic fluid might drivethe valve in one direction (opening it, for example), whilethe spring drives it in the other direction (closing it, forexample). Open, closed, or last-position failure positions areavailable. When higher thrusts, longer strokes, or rotaryvalves are involved, the fail-safe position can be provided bystoring energy in a hydraulic accumulator instead of using aspring.

Hydraulic actuators are sensitive to viscosity changescaused by ambient temperature variations and, therefore, insubfreezing temperatures must be provided with heaters,which can require substantial heat-up periods before thevalve can be operated. Because the hydraulic fluid is incom-pressible, the actuator is “stiff” and provides stable andaccurate positioning with hysteresis and dead band within5% of span.

Their speed of operation is similar to pneumatic actuators—0.125–0.25 in. (3.2–6.4 mm) per second of stemmovement—but can be speeded up substantially by the useof high-pressure accumulators. Intelligent, programmableelectrohydraulic actuators can be provided with bidirectionalhydraulic gear pumps, which are driven by microprocessor-controlled stepping motors.

External Hydraulic Source

The term “electrohydraulic” has been applied to actuatorsystems in which the hydraulic pressure to one or moreactuators is supplied by a hydraulic mule. The hydraulicpower is supplied to the actuator by electrical control means.

In the broad sense, the use of two three-way solenoidsor one four-way solenoid externally mounted to the actuatorconstitutes an electrohydraulic system.

More extensively, “electrohydraulic” applies to a propor-tionally positioned cylinder actuator. This requires a servo-system, which is a closed loop within itself. A servo-systemrequires one of the standard command signals, which is usu-ally electrical but can be pneumatic. This small signal, whichoften requires amplification, controls a torque motor or voicecoil to position a flapper or other form of variable nozzle.This positions a spool valve or comparable device to controlthe hydraulic positioning of a high-pressure second-stagevalve.

The second-stage valve directs operating pressure to thecylinder for very accurate positioning. Closing the looprequires mechanical (Figure 6.3m) or electrical feedback tocompare the piston position with the controller output signal.

The electrical feedback can be a servoamplifier, illus-trated in connection with a linear hydraulic actuator shownin Figure 6.3n.

Hermetically Sealed Power Pack

A much more compact electrohydraulic actuator combinesthe electrohydraulic power pack with the cylinder in onepackage. Many of these actuators are designed as a trulyintegral unit. An electric motor pump supplies high-pressureoil through internal ports to move the piston connected to thestem (Figure 6.3o).

The small magnetic relief valve is held closed during thepower stroke until de-energized by an external control or

FIG. 6.3mTwo-stage servovalve with mechanical feedback.

Commandsignal

Hydraulicsupply Spool

valve

Feedbacksprings

Torquemotor

T

Hydraulicsupply

Servoloop

Actuatorposition

Feedbackcam &

follower

© 2006 by Béla Lipták

6.3 Actuators: Digital, Electric, Hydraulic, Solenoid 1115

emergency circuit to allow the spring to cause a “down” stroke.The same unit can be used to cause the spring to return to theup position using a Bourdon switch to produce force limit.

Motor and Pump Combinations

Reversible Motor and Pump A reversible motor can beused to drive a gear pump in a system to remove oil from oneside of the piston and deliver it to the other side (Figure 6.3p).

The check valves allow the pump to withdraw oil from thereservoir and position the directional control valve in order topressurize the cylinder. Reversing the motor (and pump)reverses the direction.

When the motor is de-energized, the system is “lockedup.” For proportional control, feedback is necessary fromstem position to obtain a balance with the control signal.

Jet Pipe System A very old control system for a cylinder,the jet pipe, is employed in an electrohydraulic actuator. Anelectromechanical moving coil in the field of a permanentmagnet is used to position a jet that can direct oil to one endor the other of the cylinder actuator (Figure 6.3q). Force-balance feedback from stem position creates the balance withthe controller signal.

Hydraulic Control of Pinch Valves Controlled hydraulicpositioning of a sleeve valve is obtained with a moving coiland magnet to position the pilot (Figure 6.3r), which controlspressure to the annular space of the valve. Feedback is in theform of a Bourdon tube, which senses the pressure suppliedto the valve and moves the pilot valve to lock in that pressure.

Multiple Pump The multiple pump system consists of threepumps running on the shaft of one prime mover (Figure 6.3s).There is one pump for each side of the piston and one for

FIG. 6.3n Servovalve-operated electrohydraulic linear valve actuator. (Courtesy of Valtek, Flowserve Corp.)

Pressurefilter

Servovalve(A)

Servovalve(A)

Servovalve command

Position feedback

Positiontransducer

(B)

Pistonwith lowfriction

sealsHighly

polishedcylinderbore (C)

Completeplumbing

Sectionedview

Actuator(C)

4–20 mAcommand

signal

Integralmanifoldend cap

Protective cap

Permanent magnet

Industry standardmounting flange

Split stemclampfor easydisassemblyValtekstandardyoke-to-bodymounting

Exteriorview

Sonic wand

Analogabsolute LDT(position sensor)(B)

Servo-amplifier

Pressurecompensated

variablevolumepump

FIG. 6.3o Hermetically sealed electrohydraulic power pack.

M MPistonPiston

Travel limit“pull type”

Force limit“push type”

© 2006 by Béla Lipták

1116 Control Valve Selection and Sizing

the control circuit. The force motor tilts a flapper to exposeor cover one of two control nozzles. The flow through arestricting nozzle allows pressure to be transmitted to oneside of the piston or the other.

Force-balance feedback is created by a ramp attachedto the piston shaft, which positions a cam attached to thefeedback spring. Upon loss of electric power, the cylindershut-off valves close to lock up the pressure in the cylinderand assume a status quo. A bypass valve between the cyl-inder chambers allows pressure equalization to make use ofthe manual handwheel.

Vibrating Pump A vibratory pump is used in a power packmounted on a cylinder actuator (Figure 6.3t). A 60 Hz alter-nating source causes a plunger to move toward the core, and,upon de-energizing, a spring returns it. The pump operateson this cycle and continues until the piston reaches the endof its stroke, when a pressure switch shuts it off.

The solenoid that retains the pressure is de-energized byan external control circuit. Maximum stem force is 2,500 lbf

(11,120 N) at a rate of about 1 in./min (25 mm/min), or 5,000lbf (22,240 N) at 0.3 in./min (7.6 mm/min).

Two-Cylinder Pump A two-cylinder pump, driven by a uni-directional motor, injects pressure into one end of a cylinderor the other, depending upon the positions of two solenoidrelief valves (Figure 6.3u). The solenoid on the left is closedto move the piston to the right, with hydraulic pressurerelieved through the other relief valve. Motion continues untilthe valve it is operating is seated.

The build-up of cylinder pressure operates the pressureswitch at a predetermined setting to de-energize the motorand both solenoid relief valves. This locks the hydraulic

FIG. 6.3p Pump with reversible motor combination, where the actuator is“locked” when the pump is de-energized.

FIG. 6.3q Electrohydraulic actuator with jet pipe control.

Piston

Piston

Directionalcontrol valve

Centering springs

Internalreliefvalve

Checkvalve

#4

Checkvalve

#3

Checkvalve

#3

Checkvalve

#4Checkvalve

#2Checkvalve

#1

Checkvalve

#2Checkvalve

#1

Tank line

Motor running forward

Cylinder

Pistonhydraulically

locked

Internalreliefvalve(as

required)

Internalreliefvalve

Motor shut off

Reservoir (Oil)

Input signal Feedbackspring

Motor

Pump

Suction

Feedbacklinkage

to jet pipe

Forcemotor

Jet pipe

Shut-offvalve

FIG. 6.3r Electrohydraulic control of a jacketed pinch valve.

Bourdonelement

Feedbackspring

Hydraulicpressureto valve

Rubbersleevetype

slurryvalve

FilterReliefvalve

Gear pump

Start-up filter

Oil reservoir

Electricmotor

AC

Input

Capacitor

DC+–Input

© 2006 by Béla Lipták

6.3 Actuators: Digital, Electric, Hydraulic, Solenoid 1117

pressure in the cylinder. Switching of the three-way switchwill start the motor and reverse the sequence of the reliefvalves to move the piston to the left. At full travel (which isthe up position of a valve stem), a limit switch shuts downthe unit. Open-centering the three-way switch at any pistonposition will lock the piston (and valve stem) at that point.

Remote control is accomplished by manipulating theopen-center switch. Automatic control is acknowledged byincluding this open-center function, which may be solid state,in the control circuit. A potentiometer or LVDT that sensesthe stem position is required for feedback.

Stem output is 6,000 lbf (26,690 N) at a rate of 3 sec/in.(0.12 sec/mm). Stem travels up to 7 in. (178 mm) are available,

FIG. 6.3s Hydroelectric valve actuator with three pumps running on the shaft of one mover.

Inputsignal

Force motor Air bleed

Feed back cam

“R”-restriction

3 section pumpwith biult-inrelief valves

By-passvalve

Pressure to hydraulic amplifierPressure to upper cylinderPressure to lower cylinderSuction to pump

Drain offconnection

Cylindershut-offvalves

Feed backspringZero

spring

R1 R2

FIG. 6.3t Electrohydraulic actuator with vibratory pump.

Actuate “up”

Actuatorspring

Actuatorpiston

Valvestem

Checkvalve

Neutral

ReliefvalveIndicator stem

Signal

SolenoidSolenoid

valve

Pistonspring

Plunger

Coil

Oil sumpCore

Presssureswitch

FIG. 6.3u Two-cylinder pump-type electrohydraulic actuator.

Motor (or solenoidair valve)

Solenoidreliefvalve

Pressureswitch

Solenoidreliefvalve

Limitswitch

Limitswitch

R2 R1

Intake

Relay

Toreservoir

Toreservoir

Powersupply

Toreservoir

(ree-wayswitch

Intake Pump

Actuator cylinder

© 2006 by Béla Lipták

1118 Control Valve Selection and Sizing

although longer travels are feasible. The entire system isdesigned in a very compact explosionproof package that canbe mounted on a variety of valve bonnets. The speed ofresponse to energizing and de-energizing the control circuitmakes it feasible to adapt the unit to digital impulses.

SOLENOID VALVES

Solenoids move in a straight line and therefore require a camor other mechanical converter to operate rotary valves. Theseactuators are best suited for small, short-stroke on-off valves,requiring high speeds of response. Solenoid-actuated valvescan open or close in 8–12 ms.

Their fast closure is not always an advantage: In watersystems, it can cause water hammer. They are limited to pressuredrops below 300 PSIG (20.7 bars), although when provided withpilots, levers, or double seats, they can handle higher pressuredrops. They are available in two- or three-way designs, withpower requirements ranging from 10–30 W with 6–440 VAC or6–115 VDC power supplies. Solenoids are reliable devices, andthey can provide multimillion cycles on liquid service.

Solenoids (consisting of a soft iron core that can movewithin the field set up by a surrounding coil) are used exten-sively for moving valve stems. Although the force output ofsolenoids may not have many electrical or mechanical limi-tations, their use as valve actuators has economic and core(or stem) travel limitations, and they are expensive.

A solenoid valve consists of the valve body, a magneticcore attached to the stem and disc, and a solenoid coil(Figure 6.3v). The magnetic core moves in a tube that is closedat the top and is sealed at the bottom; this design eliminatesthe need for packing. A small spring assists the release andinitial closing of the valve. The valve is electrically energizedto open.

Stronger springs are used to overcome the friction ofpacking when it is required (Figure 6.3w). Reversing thevalve plug results in reverse action (open when de-energized).

Even stronger stroking force can be obtained by usingthe force amplification effect of a mechanical lever, in com-bination with a strong solenoid (Figure 6.3x).

Using a solenoid to open a small pilot valve (Figure 6.3y)increases the port size and allowable pressure drop of sole-noid-operated valves.

FIG. 6.3w When valve packing is required, the increased friction can be over-come by a solenoid valve design with two or more strong returnsprings.

FIG. 6.3v The operation of a direct-acting solenoid valve involves the lifting of the plunger when energized.

Solenoid coilHousing Shading coil

Coil connections

Magnetic flux paths

Spring compressedStemSpring retainer

Body

Outlet OutletIntlet

Disc Orifice

EnergizedDe-energized

Direction of flow through valve

Stationary core (plug nut)

Core tube

Movable core (plunger)

BonnetSpring

Inlet

© 2006 by Béla Lipták

6.3 Actuators: Digital, Electric, Hydraulic, Solenoid 1119

Small solenoid pilot valves are widely employed to supplypressure to diaphragms or pistons for a wide range of outputforces. Pilot operation applies pressure to a diaphragm orpiston or may release pressure, allowing the higher upstreampressure to open the valve. A good example is the in-line valve(Figure 6.3z).

Most solenoid valves are designed to be continually ener-gized, particularly for emergency shutdown service. Thus the

power output is limited to the current whose I2R-developedheat can be readily dissipated.

Using a high source voltage and a latch-in plunge over-comes the need for continuous current. The single-pulsevalve-closing solenoid is disconnected from the voltagesource by a single-pulse delatch solenoid and hence does notheat up after it is closed. A pulse to the delatch solenoidpermits the valve to be opened by a spring.

Three-way solenoid valves with three pipe connectionsand two ports are used to load or unload cylinders or dia-phragm actuators (Figure 6.3aa). Four-way solenoid pilot

FIG. 6.3x The force generated by the solenoid is amplified in the lever-typeactuator.

FIG. 6.3y The solenoid valve can close against higher pressure drop, if thesolenoid operates a small pilot valve.

Open Closed

Cushionedclosing

Manualopening

FIG. 6.3z Pilot-operated in-line solenoid valve, where the pilot serves to applypressure to the main poppet piston.

FIG. 6.3aa The operation of a normally closed three-way solenoid valve.

Armaturefor pushsolenoid

Pilot poppet

Pilot flow

Pilotmetering

orificePilot flowentrance

MainpoppetPilot

exhaust

Returnspring Outlet

Inlet

Vent

De-energized position

Energized position

© 2006 by Béla Lipták

1120 Control Valve Selection and Sizing

valves are used principally for controlling double-actingcylinders.

Modulating Solenoid Valves

Modulating magnetic valves (Figure 6.3bb) utilize spring-loaded low-power solenoids to provide throttling action. Theonly moving part in this design is the valve stem, which hasthe valve plug attached to one end and an iron core attachedto the other.

The valve opening is thereby a function of the voltageapplied across the solenoid. (Figure 6.3cc illustrates thechange in plunger force as a function of applied voltage andplunger air gap.)

Such throttling solenoids are frequently used in theHVAC industry and in other applications where the valveactuators do not need to be very powerful. The actuator thrustrequirements are lowered by balancing the inner valvethrough the use of pressure equalization bellows or floatingpistons. The positive failure position of these valves is pro-vided by spring action.

Throttling solenoids are typically available in 1/2–8 in.(127–203 mm) sizes and are limited by the pressure differenceagainst which they can close. They are available in two- orthree-way designs and can handle water flows up to 5000 gpm(19 m3/m).

The use of throttling solenoids can completely eliminatethe need for instrument air in the control loop. These valves

FIG. 6.3bb Throttling solenoid valve design with LVDT used to measure the stem position for feedback.

Modulating valvesLVDT

Indicatortube

Bonnettube

Ventport

ON/OFF valves positionreed switches

Magnet

Core rod

Solenoid assembly

Plunger

Fixed core

Return spring

Main discPilot disc

Flow

Flow

Inlet orifice

Pilotdisc

Maindisc

Pilot valvedischarge

orifice

Main disc

Pilot disc

Return spring

Valve inlet

© 2006 by Béla Lipták

6.3 Actuators: Digital, Electric, Hydraulic, Solenoid 1121

offer a higher speed and better range than their pneumaticcounterparts. Some manufacturers claim a range of 500:1 anda stroking time of 1 sec. The design illustrated in Figure 6.3bbcan be driven directly from microprocessor-based buildingautomation systems.

The design shown in Figure 6.3bb also includes a sepa-rate positioner, which accepts a 4–20 mA DC input from thecontroller and delivers a DC output signal to the throttlingsolenoid. Valve position feedback is obtained through the useof a linear variable differential transformer mounted directlyon the valve.

SMART ACTUATORS

In addition to specifying the thrust, travel, failure position,control signal, power supply, speed, electrical area classifi-cation, and ambient conditions that an actuator must meet,one can also consider the use of microprocessor-based smartsystems and of some other types of intelligent systems.

One of the trends in process automation is the use ofdistributed control and, in some cases, to locate the controlleras close as possible to the final control elements (controlvalves). Often the control signal is digital (digital valve con-trol, or DVC3). It is in this respect that an increasing numberof features have been added to valves, valve actuators, andpositioners through the integration of microprocessors withthem.

Applications

Microprocessors have been included in valve actuators andvalve positioners for some time (Figure 6.3dd), but the degreeof exploitation of the possibilities of the smart and intelligentactuators for the purposes of process control is highly depen-dent on data access and thus on digital networking capabilities.

The tendency is to provide new actuators with some sortof network connection.4 As discussed in more detail inSections 4.16 and 6.11, Foundation fieldbus, HART, andProfibus are some of the leading suppliers. Open architectures,such as Foundation fieldbus, have made positive contribu-tions, because by broadening the market they allowed moreusers to take advantage of better performing actuating systemsat lower prices. Moreover, the compatibility problemsbetween different fieldbus systems are in the process of beingresolved, and it is hoped that through international standard-ization “plug and play” capabilities3 will soon be availablefor such devices.

The intelligence provided by microprocessors can and isused for valve tuning, semiautomatic calibration, and datacollection for maintenance and diagnostic purposes. It is alsoused for standalone control when that might improve thepositioning, the protection, or the communication of thevalve. Table 6.3ee lists some of the more common applica-tions of smart and intelligent actuator systems.

FIG. 6.3ccThe force available to the pilot plunger of a throttling solenoiddepends on the voltage and the air gap.

120(54)110(50)100(45)90

(41)80

(36)70

(32)60

(27)50

(23)40

(18)30

(14)20(9)10

(4.5)0

0 .1(2.5)

.2(5)

.3(7.5)

.4(10)

.5(13)

.6(15)

.7(18)

.8(20)

.9(23)

1.0(25)

Plunger air gap in inches (mm)

Plun

ger f

orce

pou

nds (

Kg)

100 V

90 V

80 V

70 V

60 VA

B

ON/OFF valvespring characteristics

Modulatingspring valve

characteristics

FIG. 6.3ddMicroprocessor-based electric valve actuator. (Courtesy of ETI Sys-tems, Inc.)

Microprocessorcontrol board

Motor

O-ring sealscover & shaft

AnodizedaluminumbracketStainlesssteel shaft

Brass nickelplating

Anodizedaluminum housing

Position indicator

Heavy dutystainless steel

geartrain

Newuniversal

coupler

Nickel platedswivel adapter

Valve

VAvalve

actuator

VKvalve

kit

© 2006 by Béla Lipták

1122 Control Valve Selection and Sizing

Microprocessor-based systems are available in water-tight or explosionproof (NEMA 6 to 7) construction andcan tolerate ambient temperatures from –40 to 185°F (–40to 85°C), as well as the presence of moisture, fungus, ordust. They can incorporate sensors, an electronic positioner,or an electronic proportional, integral, derivative (PID) con-troller, which can operate off a digital or analog externalset point.

Locating the controller card at the valve and dedicatingit to full-time control is an advantage on very fast criticalloops, such as compressor surge protection, where otherwisesurge could evolve while the central DCS control system isscanning/updating other loops.

Smart systems can also take advantage of the lower cost ofdigital communications over a single loop of two-conductor orfiber-optic cable (Figure 6.3ff). This method of communica-tion substantially reduces the wiring cost of installations thatcan control up to 250 actuators, pumps, or solenoids. Thesmart valves can be remotely calibrated and reconfigured andcan be used to detect such performance changes as pressurechanges resulting from a fouled pump or pipeline. They canalso limit the valve travel to stay within the range where therequired characteristics (gain) are available.

It is perhaps at the maintenance level that the most ben-efits can be obtained through the use of smart and intelligentsystems.6 Smart actuator circuitry can protect electric motoroperators from burning out when the valve is jammed or fromreverse phasing. More importantly, by close monitoring of

the valve, the required information can be obtained to eval-uate the valve and actuator performance and thereby replacepreventive and corrective maintenance by predictive mainte-nance with the benefits of improved process performance andproducts quality at a lower cost.

References

1. Harrold, D., “A Changing Landscape,” Control Engineering, December2003, www.manufacturing.net/ctl/article/CA339684? text = valve + actu-ators.

2. Scott, A. B., “Control Valve Actuators: Types and Application, InTech,January 1988.

3. Harrold, D., “Making Valve Controllers/Positioners Smarter is SmartBusiness,” Control Engineering, January 2003, www.manufacturing.net/ctl/index.asp?layout=articlePrint&articleID=CA269795.

4. Hoske, M. T., “Implementing Industrial Networks,” Control Engineer-ing, July 1998, www.manufacturing.net/ctl/article/CA197636?text=valve+actuators.

5. Price, V. E., “Smart Valve Intelligence Takes Many Forms,” InTech,August 1992.

6. Merritt, R., “A Real (Valve) Turn-On” Control Design, August 2002,www.controldesign.com/Web_First/CD.nsf/ArticleID/JFEY-5BMRJD.

TABLE 6.3eeApplications for Integrated Intelligent Systems5

• Under-instrumented loops where additional information is required for optimization or improved control.

• Loops requiring very tight control with a fast update for the PID algorithm.

• Applications requiring a large turndown in flow measurement and control capabilities.

• Processes that need local supervisory and control capabilities to provide continuous operation or controlled shutdown upon loss of control signal.

• New or retrofit installations where there is insufficient space for conventional instrumentation systems.

• Critical systems requiring continuous monitoring and predictive diagnostics on the valve or process.

• Applications where changing conditions require the loop to be reconfigured to control diverse variables for best control.

• Standalone remote applications requiring programmable operation and remote interface for monitoring.

• New applications where an intelligent system can be used as a fully integrated system, reducing engineering, cost, and maintenance requirements.

• Specialized control functions such as gap control for very large turndown, or self-contained minimum pump recirculation systems for pump or compressor protection.

FIG. 6.3ff Multiple intelligent actuators can be monitored and controlled bya two-wire multiplexer loop. (Courtesy of Rotork Controls Ltd.)

Masterstation

© 2006 by Béla Lipták

6.3 Actuators: Digital, Electric, Hydraulic, Solenoid 1123

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Baumann, H. D., “Trends in Control Valves and Actuators,” Instrument andControl Systems, November 1975.

Berris, R., Hazony, D., and Resch, R., “Discrete Pulses Put Induction Motorsinto the Stepping Mode,” Control Engineering, Vol. 29, No. 1, 1982,p. 85.

Colaneri, M. R., “Solenoid Valve Basics,” Instruments and Control Systems,August 1979.

Cottell, N., “Electrohydraulic Actuation-Still in Control?”, IEE Colloquiumon Actuator Technology: Current Practice and New Developments,May 1996, London, U.K.

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Hammitt, D., “How to Select Valve Actuators,” Instruments and ControlSystems, February 1997.

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IEC 60534-6 Ed. 1.0 b:1985, “Industrial Process Control Valves. Part 6:Mounting Details for Attachment of Positioners to Control Valve Actu-ators,” 1985.

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IEC 60534-6-2 Ed. 1.0 b:2000, “Industrial Process Control Valves. Part 6-2:Mounting Details for Attachment of Positioners to Control Valves.Positioner Mounting on Rotary Actuators,” 2000.

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Liantonio, V., “High Pressure Modulating Solenoid Valve for Steam/GasService,” InTech, January 1988.

Lipták, B. G., “Control Valves in Optimized Systems,” Chemical Engineer-ing, September 5, 1983.

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© 2006 by Béla Lipták