6.3
Actuators: Digital, Electric, Hydraulic, Solenoid C. S. BEARD
(1970)
B. G. LIPTÁK
(1985, 1995)
P. M. B. SILVA GIRÃO
S R
Spring-loaded pneumatic cylinder actuator
Dual acting pneumatic cylinder actuator P
(2005)
H
Electric solenoid actuator shown with manual reset E
H
Pneumatic-hydraulic and electro-hydraulic actuators M
M
Pneumatic and electric rotary motor actuators
Flow sheet symbols
Types:
1. 2. 2a. 2b. 3.
Digital Electromechanical (linear and rotary) Stepping motors in smaller sizes Reversible motor gears for larger sizes 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 (1.4–338,954 N · m) for type 2b actuators with gearboxes
Linear Thrust Ranges:
The maximum of about 500 lbf (2224 N) output force can be obtained from type 2a 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 about 1 sec. Electromechanical motor-driven valves stroke in 5–300 sec. Electrohydraulic actuators generally move at 0.25 in./sec (6.35 mm/sec) but can be speeded up by the use of hydraulic accumulators.
Costs:
0.25 in. (6.35 mm) solenoid pilot in stainless steel costs about $50; with a twoway design in explosionproof plastic construction it costs about $250, and when built for high-temperature service it costs about $300. An electromechanical, type 2a actuator for 0.5–1 in. (12.7–25.4 mm) small ball valve can be obtained in explosionproof construction with limit switches for on/off service for $750; with positioner and feedback potentiometer such an actuator costs about $1500.
1105 © 2006 by Béla Lipták
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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:
© 2006 by Béla Lipták
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/valveautomation/bettis) Bodine Electric Co. (2) (www.bodine-electric.com) Bosch-Rexroth Corp. (2, 3, 4) (www.boschrexroth.com/BoschRexroth/corporate/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)
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/valveautomation/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 the pneumatic ones, which are discussed in Section 6.4. The various accessory items such as positioners, handwheels, limit switches, potentiometers, and the like are also discussed in Section 6.2. The discussion of digital actuators in this section is intended to complement Section 6.12 on intelligent valves and Section 6.18 covering digital control valves. The discussion in this section begins with some selection and application guidelines, which is followed by the description of the five actuator categories: 1) digital, 2) electromechanical, 3) electrohydraulic, 4) motors and pumps, and 5) solenoids. The section ends with a brief discussion of the trends in valve actuation, including the features of microprocessor-based “smart” and intelligent actuators.
SELECTION AND APPLICATION The following are some of the characteristics to consider in the application and selection of all types of actuators. Table 6.3a gives a summary of advantages, disadvantages, and applications for some of the designs. Actuator Types Valve actuator types discussed in this section belong to one of the following categories: (a) electric actuators, which use a motor to drive a combination of gears to generate the desired torque or thrust level. This category includes (i) rod linear actuators, whose output rod provides linear motion via
© 2006 by Béla Lipták
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, whose load is attached to a fully supported carriage. Rodless linear actuators provide linear motion via a motor-driven ball screw, ACME screw, or belt drive assembly; and (iii) electric rotary actuators that use a motor to drive components rotationally. The next category (b) are the hydraulic and electrohydraulic valve actuators, which convert fluid pressure into motion. They include (i) linear actuators or hydraulic cylinders, which use a cylinder and hydraulic fluid to produce linear motion and force, and (ii) hydraulic rotary actuators that use pressurized hydraulic oil to rotate mechanical components. The third category (c) are the linear solenoids that convert electrical energy into mechanical work via a plunger with an axial stroke in either a push or pull action. They can be rated for continuous duty (100% duty cycle operation, continuous duty solenoids) or for off-on applications, less than 100% duty cycle (intermittent duty solenoids). The last category (d) are digital actuators, which now include all types of valve actuation solutions where the valve is digitally controlled. Pneumatic actuators are still the technology favored by valve actuators buyers. Nevertheless, the market share of electric, hydraulic, and electrohydraulic actuators is increas1 ing, while the overall use of solenoid values has dropped. Actuator Features Speed and Torque Ranges Speed requirements vary from less than 1 rpm to about 160 rpm. The upper limit of available torque is about 250,000 ft · lbf (339,000 N · m) with gearboxes.
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Control Valve Selection and Sizing
TABLE 6.3a 2 Applications, Advantages, and Disadvantages of Various Actuator Designs Actuator Types
Advantages
Disadvantages
Applications
Electromechanical
High thrust High stiffness coefficient Powered by electricity or pneumatics
Complex design No mechanical fail safe Large, heavy structure
Linear or rotary valves 2–36 in. body size
Electrohydraulic
High thrust High stiffness Fast speeds Powered by electricity; no pneumatic source required
Complex design Large, heavy structure Hydraulic temperature sensitive
Linear or rotary valves 2 in. to unlimited
Electric (servomotor or stepping motor)
Direct interface with computer system
Large structure Low thrust No mechanical fail safe Slow speed
Linear valves 1 /2 –2 in. body size
Stem thrusts and rotational drive torques are limited only by the size of the motor used and the ability of the gear, bearings, shafts, and so on to carry the load. Speed of operation depends on the gear ratios, adequate prime move power, and means of overcoming the inertia of the moving system for rapid stopping. This is most important for proportional control uses. Some actuators have a limited selection of drive speeds, while others are furnished in gear ratios in discrete steps of 8–20% between speeds. Manual Operation Manual operation is sometimes necessary for normal operational procedures, such as start-up, or under emergency conditions. Only units that are rotating very slowly or those with low output should have continuously connected handwheels. Most units have the handwheel on the actuator with a clutch for demobilizing the handwheel during powered operation. Clutches are manual engage and manual disengage, or manual engage and automatic disengage upon release of the handwheel. Others are manual engage when the handwheel is rotated, with the motor reengaging when the handwheel is not being rotated; or power re-engage, which takes the drive away from the operator upon energization, leaving the handwheel freewheeling. Electrical Equipment Most of these actuators include much of the electric gear within the housings of the unit. Components such as limit, auxiliary, and torque switches and position or feedback potentiometers are run by gearing to stem rotation, so they must be housed on the unit. Installation is optional concerning pushbuttons, reversing starters, lights, control circuit transformers, or line-disconnect devices in an integral housing on the unit. Any or all of these components may be located externally, such as in a transformer, switch, or control house. The enclosures must be designed to satisfy NEMA requirements for the area. High Breakaway Force Resistance to opening requires a method of allowing the motor and gear system to develop
© 2006 by Béla Lipták
speed to impart a “hammer blow,” which starts motion of the valve gate or plug. Selection of motors with high starting torque is not always sufficient. The dogs of a dog-clutch rotate before picking up the load, or a pin on the drive may move within a slot before picking up the load at the end of the slot. Systems are used that delay contact for a preselected time or until the tachometer indicates the desired speed of rotation. Torque Control for Shutdown Torque control for shutdown at closure or due to an obstruction in the valve body is accomplished in numerous ways, but each one uses a reaction spring to set the torque. When rotation of the drive sleeve is impeded, the spring will collapse, moving sufficiently to operate a shut-off switch. Position Indication An indicator can be geared to the stem rotating gear, but it becomes a problem when actuator rotation varies from 90 to as much as 240 revolutions. Gearing of a cam shaft operating the position indicator or auxiliary switches must be calculated to obtain a fairly uniform angle. Upon correct gearing, an indicating arrow or transmitting potentiometer can be rotated. Maintenance of Last Position This is no problem when the actuator includes a worm gear or stem thread. Use of spur gears can cause instability when positioning a butterfly or ball valve. Status quo is obtained by use of a motor brake or insertion of a worm gear into the system. Protection against Stem Expansion The status quo ability of a thread or worm gear is detrimental when the valve itself is 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. One of the original patents for this type of actuator included Belleville springs to allow the drive sleeve to move with the thermal expansion and relieve the linear force.
6.3 Actuators: Digital, Electric, Hydraulic, Solenoid
Mounting Methods Industry has dictated a set of dimensions for the mounting flanges and bolt holes for newly manufactured valves. Retrofit mounting requires adaptation to existing valves. Mounting requires a plate to match the existing valve, which is screwed into the yoke upon removal of 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 adaptability to 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 are used for both proportional and on-off service. The coils that open and close the contacts are energized by an open-center double-pole switch, which can be incorporated in the automatic control circuit. For manual control, the starter may be of the type that maintains contact, requiring the open or close button to be held in position. Some units are also wired for momentary depression so that the actuator runs until it reaches the limit switch, or until a stop button is depressed. Proportional Control Proportional control of these large units can be accomplished by including the coils of the reversing starter in a proportional control circuit. This requires a position feedback, which may be a potentiometer. For this type of control, a Wheatstone bridge circuit would be used. The reversing starter controls any voltage or phase required by the motor. A transducer to transform any of the accepted electronic controller outputs (e.g., 1–5 mA) to a resistance relative to controller output permits use of the actuator in these systems. A smaller unit, with a force output of 1460 lbf (6494 N) at stall 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 eliminates stem position feedback into the controller. A DC signal “x” (Figure 6.3b) from a process transmitter provides the loop with its measurement, such as temperature, as well as such high-responds systems as flow. A differential amplifier responds to the magnitude and the polarity of an internally modified error signal, which triggers silicon-controlled rectifiers (SCRs) to obtain bidirectional drive. The synchronous motor can be driven in either direction, depending upon the relation to the set point.
DIGITAL VALVE ACTUATORS Digital actuators can accept the output of digital computers directly without digital-to-analog converters. Only simple onoff elements are needed for their operation. The number of n output positions that can be achieved is equal to 2 , where n is the number of inputs. Accuracy of any position is a function of the manufacturing tolerances. Resolution is established by the number of inputs and by the operating code selected for
© 2006 by Béla Lipták
s Inputs
x
Analog s–x–kr computing amplifier
1109
Meter 0 Differential amplifier s–x–kr=0
Zero reference
Sampling program circuitry Isolation transformer
SCR A
SCR B Link
115 V 50/60 cycles
Phase-shift network Driving motor
Alternate loads Load A
Load B
s = instantaneous set point x = instantaneous transmitter signal k = adjustable transmitter compensation r = instantaneous rate of change of transmitter signal
FIG. 6.3b Proportional motor control circuit with position feedback.
a given requirement. The smallest move achievable is called a 1-bit move. The code may be binary, complementary binary, pulse, or special purpose. A three-input piston adder assembly produces eight discrete bit positions. The adders in Figure 6.3c are shown in the 6-bit extended position. The interlocking pistons and sleeves will move when vented or filled through their selector valves. This same adder can be used to position a four-way spool valve, with a mechanical bias to sense position. The spool valve controls the position of a largediameter piston actuator or force amplifier. Use of a DC motor featuring a disc-armature with low moment of inertia has created another valve actuator particularly adaptable to a digital input. Brushes contacting the flat armature conduct current to the armature segments. Incremental movement is caused by half-waves at line frequency for rotation in either direction. The rotation of the armature is converted to linear stem motion by use of a hollow shaft internally threaded to match the valve stem. Actuator output is 5,000 lbf (22,240 N) maximum for noncontinuous service at a rate of 0.4 in./sec (10 mm/s) through a valve stroke of 3 in. (76.4 mm). The actuator is de-energized at stroke limits or at power overloads by thermal overload relays.
1110
Control Valve Selection and Sizing
Solenoids
Normally closed solenoid
Normally open solenoid 6
5
4
3
2
1
Adders (hydraulic D/A converters) Four-way valve Mechanical feedback beam
Return Bias piston Pressure
Actuator (force amplifier)
FIG. 6.3c Digital valve actuator.
Operation of the actuator requires application of a thyristor (SCR) unit designed for this purpose. This unit accepts pulses from a computer or pulse generator. The thyristor unit consists of two SCRs with transformer, triggers with pulse shift circuit, facility for manual actuator operation, and the previously mentioned thermal overload relays. The output consists of half-waves to pulse the armature of the actuator. Modules are also available for process control with the necessary stem position feedback, slow pulsing for accurate manual positioning, and full-speed emergency operation. The digital-to-pneumatic transducer shown in Figure 6.3d is used to convert a controller output to a pressure signal for operating a pneumatic valve. The concept of digital valve actuator has been extended to other valve actuating systems such as stepping motors, which are discussed in the following paragraph dedicated to
Zero spring
Range spring
Motor
Stops
Pivot Pivot Zero adjust 25 PSIG (172.5 kpa) filtered supply pressure
Beam
Fixed orifice
FIG. 6.3d Digital-to-pneumatic transducer.
© 2006 by Béla Lipták
Non-rotating nut Nozzle 3–15 PSIG (0.2–1 bar) Output pressure
electromechanical actuators, and to the so-called smart or intelligent actuators briefly introduced below in Smart and Intelligent Actuators. The discussion of digital actuators in this section is intended to complement Section 6.12 on intelligent valves and Section 6.18 covering digital control valves. It is in this context that some of the suppliers in the partial list of suppliers at the beginning of this section identified as providing digital valve actuators indeed provide modern smart valve actuating solutions. ELECTROMECHANICAL ACTUATORS Electric actuators can utilize a reversible electric motor provided with an internal worm gear to prevent drive direction reversal (back-drives) by unbalanced loads. These units can operate both linear and rotary valves. Servomotor drives can position valves in response to feedback signals from linear or rotary encoders. Two-phase AC servomotors are available with up to 1 HP rating, while direct current servomotors can meet higher loads. Stepping motors, which rotate the shaft by a discrete step angle when energized electrically, can also be used in valve actuators. The electromechanical device that rotates the shaft can be a solenoid used to operate a star-wheel or ratchet device; it can be the stepping movement of a permanent magnet, the flux of which causes poles or teeth to align and thereby affect rotation; or it can be a variable reluctance unit, where the rotor-stator poles or teeth are aligned by electric fields. When used as valve actuators, the stepping motors are well suited for direct digital control, and pulse feedback can be provided for accurate closed-loop positioning.
6.3 Actuators: Digital, Electric, Hydraulic, Solenoid
Of the above designs the actuators using reversible motors are the most often used variety and therefore they will be discussed in more detail in the following paragraphs.
Output shaft Adjustable cams
Reversible Motor Gear Actuators Lever-operated valves and dampers are available with torques of 5–500 ft · lbf (6.8–678 N · m) and with full stroke speeds of 10–60 seconds. For very large valves or dampers, this type of actuator can deliver torques of 150–10,000 ft · lb f (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 return the valve to a safe condition upon electric failure. The motor gear actuator consists of an electric motor connected to a gear train; the gear train rotates a stationary drive nut, which in turn drives the threaded or keyed valve stem up or down. As the valve plug contacts the seat, the resistance is transmitted to a Belleville spring, which at a preset limit interrupts the motor power circuit. For throttling applications, the actuator can be provided with a positioner that compares the external control signal (analog or digital) with an internal position feedback signal and keeps turning the motor until the error between the two signals is eliminated. Rotary Output Actuators Worm Gear Reduction The actuator shown in Figure 6.3e is an example of the use of a double worm gear reduction to obtain output speeds of around 1 rpm with an input motor speed of 1800 rpm. The worm gear is self-locking, so it prevents the load from moving downward by back-driving the motor. However, the worm gear is less than 50% efficient, so more power is used compared to spur gears. Also shown is a handwheel for manual operation during power loss. Spur Gear Reduction The spur gear actuator in Figure 6.3f has very low power loss through the relatively efficient spur Geared limit switches
Prime mover
Hammer blow Gear drive Drive sleeve
Torque control
FIG. 6.3e Electric actuator with worm gear reduction.
© 2006 by Béla Lipták
De-clutching handwheel
1111
Travel limit switches
(Optional) potentiometer Auxiliary switches
FIG. 6.3f Spur gear reduction.
gears. Ordinarily the load could back-drive this system, but a friction member at the motor end of the gear train minimizes this undesirable action. Of course, if the load has a frictional characteristic, it will not impose a back-driving torque on the actuator. Actuators for larger outputs are externally mounted motors. Two motors operating a single gear train have been used to obtain 3,750 ft · lbf (5084 N · m) of torque through 90° rotation in 75 sec. An actuator has been designed for accurate rotary positioning that develops 5000 ft ⋅ lb f (6800 N · m) at stall and will rotate 90° in 10 sec. SCRs energize the motor as commanded by a servotrigger assembly housed separately. Electrical gear may include adjustable cams to operate limit and auxiliary switches. A potentiometric feedback calibrated to the rotation of the actuator is required for use with a control circuit. The unit shown in Figure 6.3f was developed to slide over and be keyed to the shaft of a boiler damper. Actuators of this type must be adaptable to mounting on and operating a variety of quarter-turn valves. Because of the difficulty in setting limit switches to accurately stop the valve in the shut position, it is advisable to incorporate a torquelimiting device to sense closure against a stop. Opening can be controlled by a limit switch. One compact unit contains the features noted with an output of 750 ft · lbf (1017 N · m) of torque. The unit is powered by a motor with a high-torque capacitor and includes a mechanical brake, feedback potentiometer, limit switches, and a de-clutchable hand wheel. Flex-Spline Reduction Figure 6.3g is an example of a unique single-stage, high-reduction system. Instant breakaway and efficient transfer of prime mover power is obtained
1112
Control Valve Selection and Sizing
Reversible low-inertia AC motor Continously connected handwheel
Motor drive pinion gears Load connection linear motion
FIG. 6.3g Flex-spline gear reduction.
with a modified concentric planetary system consisting of a semiflexible gear within a rigid gear. A three-lobed bearing assembly transmits power to the gears by creating a deflection wave transmission that causes a three-point mesh of the gearing teeth on 30% or more of the external gearing surface. The semiflexible geared spline has fewer teeth than the nonrotating internal gear it meshes with. The spline slowly rotates as it is pressed into the larger gear by the bearings on the motor shaft. Linear Output Actuators Motors and Rack Figure 6.3h uses a worm and a rack and pinion to translate horizontal shaft motor output to vertical linear motion. Maximum force output is approximately 1500 lbf (6672 N) at about 0.1 in./min (2.5 mm/min). A continuously connected handwheel, which must rotate the rotor of the motor, can be used when there is short stem travel and relatively low force output. The actuator is designed with a conventional globe valve bonnet for ease of mounting. Units operate on 110 V and have been adapted to proportional use with a 135 ohm Wheatstone bridge 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 and auxiliary switches. The unit may have a brake motor for accurate positioning and a manual handwheel. The bracket on the rear end allows the actuator to rotate on the pin of a saddle mount, so that the drive shaft can be pinned directly to the lever arm of a valve
© 2006 by Béla Lipták
FIG. 6.3h Rack and pinion assembly converts the rotation of a horizontal, motor-driven shaft into vertical, linear motion.
or a lever arm fixture that fits the shaft configuration of a plug cock or ball valve. Maximum output is 1600 lbf (7117 N) at 5 in./min (127 mm/min). Rotating Armature An internally threaded drive sleeve in the armature of the motor is used to obtain a linear thrust up to 6,600 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 kept from rotating by a guide key. Motor Terminal strip
Electric brake
Eye Shaft Screw drive
Swivel flange
Traveling nut
Auxiliary switcheslimit switches located on opposite side
FIG. 6.3i Electric quarter-turn actuator with linear output and with limit switches.
6.3 Actuators: Digital, Electric, Hydraulic, Solenoid
1113
FIG. 6.3j Electric actuator with rotating armature has been adapted for proportional control.
Thrust-limit switch assemblies are mounted in each end of the housing to locate the hollow shaft in mid-position. When the linear movement of the drive stem is restricted in either direction, the limit switch involved will operate to shut down the unit. Thermal cut-outs in the motor windings offer additional overload protection. Strokes are available from 2–48 in. (51–1219 mm). The unit has been adapted for proportional control by use of an external sensing position for feedback. For use as a valve actuator, it must be mounted so that the drive stem can be attached to the valve stem, or a suitably threaded valve stem must be supplied. Rotary to Linear Motion An electric proportional actuator (Figure 6.3k) is designed for continuous rotation of a drivesleeve 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 numerically added or subtracted. Triacs operate on position error to control a DC permanent magnet motor that positions a stem within an adjustable dead band. Degree of error and rate of return are sensed by a lead network to determine the direction and time that the motor must run. Stem reversals are almost instantaneous. The back emf of the motor is used as a velocity sensor and is fed into a circuit that 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 differential transformer (LVDT). Use of the DC motor allows for torque control through sensing of motor current. A manual handwheel is furnished that can only be used when the unit is deenergized by a manual/automatic switch. Application of the linear requirement of a valve necessitates a linkage for translation from rotary to linear motion. Use of a linkage (Figure 6.3l) provides a thrust for operating the valve. Lift scale Motor shaft Motor Linkage arm Linkage arm clip Lift adjustment locknut Locknut
Screw
Shaft adjustment screw Washer A Relief spring mechanism Washer B Bolt
Cylinder Stem button clamp Stem button Stem Bracket
Allen set screw
FIG. 6.3k Proportional electrical drive converts rotary to linear motion.
© 2006 by Béla Lipták
FIG. 6.3l Electric actuator with linkage to convert rotary motion to linear.
1114
Control Valve Selection and Sizing
ELECTROHYDRAULIC ACTUATORS The main features of electrohydraulic actuators have been summarized in Table 6.3a. They are often used where instrument air is unavailable or where the required actuator stiffness or thrust cannot be obtained from pneumatic actuators. They are heavier than pneumatic actuators; therefore, in order to avoid straining the bonnet, vertical upright installations are recommended. Electrohydraulic actuators are superior to electromechanical ones in the areas of speed, positioning without overshoot, actuator stiffness, and complexity. Electrohydraulic actuators can be stepping motor or servovalve driven. In the servovalve designs the pumps are running continuously, while in the stepping-motor configuration they run only when the valve needs repositioning. Electrohydraulic actuators can be mounted directly onto the stems of valves that are 6 in. (152 mm) or smaller and can be provided with springs to guarantee a fail-safe position in case the hydraulic fluid is lost. The actuator usually consists of a hydraulic cylinder, a pump with motor, some feedback linkage on valve position, and a balancing arrangement. The balancing or positioning mechanism compares the external control signal with the valve position and actuates the hydraulic system if repositioning is required. In spring-loaded designs the hydraulic fluid might drive the valve in one direction (opening it, for example), while the spring drives it in the other direction (closing it, for example). Open, closed, or last-position failure positions are available. When higher thrusts, longer strokes, or rotary valves are involved, the fail-safe position can be provided by storing energy in a hydraulic accumulator instead of using a spring. Hydraulic actuators are sensitive to viscosity changes caused by ambient temperature variations and, therefore, in subfreezing temperatures must be provided with heaters, which can require substantial heat-up periods before the valve can be operated. Because the hydraulic fluid is incompressible, the actuator is “stiff” and provides stable and accurate positioning with hysteresis and dead band within 5% of span. Their speed of operation is similar to pneumatic actuators — 0.125–0.25 in. (3.2–6.4 mm) per second of stem movement — but can be speeded up substantially by the use of high-pressure accumulators. Intelligent, programmable electrohydraulic actuators can be provided with bidirectional hydraulic gear pumps, which are driven by microprocessorcontrolled stepping motors.
In the broad sense, the use of two three-way solenoids or one four-way solenoid externally mounted to the actuator constitutes an electrohydraulic system. More extensively, “electrohydraulic” applies to a proportionally positioned cylinder actuator. This requires a servosystem, which is a closed loop within itself. A servo-system requires one of the standard command signals, which is usually electrical but can be pneumatic. This small signal, which often requires amplification, controls a torque motor or voice coil to position a flapper or other form of variable nozzle. This positions a spool valve or comparable device to control the hydraulic positioning of a high-pressure second-stage valve. The second-stage valve directs operating pressure to the cylinder for very accurate positioning. Closing the loop requires mechanical (Figure 6.3m) or electrical feedback to compare the piston position with the controller output signal. The electrical feedback can be a servoamplifier, illustrated in connection with a linear hydraulic actuator shown in Figure 6.3n. Hermetically Sealed Power Pack A much more compact electrohydraulic actuator combines the electrohydraulic power pack with the cylinder in one package. Many of these actuators are designed as a truly integral unit. An electric motor pump supplies high-pressure oil through internal ports to move the piston connected to the stem (Figure 6.3o). The small magnetic relief valve is held closed during the power stroke until de-energized by an external control or Hydraulic supply Command signal
Feedback springs
Spool valve
T
Torque motor Hydraulic supply Feedback cam & follower
Servoloop
External Hydraulic Source The term “electrohydraulic” has been applied to actuator systems in which the hydraulic pressure to one or more actuators is supplied by a hydraulic mule. The hydraulic power is supplied to the actuator by electrical control means.
© 2006 by Béla Lipták
Actuator position
FIG. 6.3m Two-stage servovalve with mechanical feedback.
6.3 Actuators: Digital, Electric, Hydraulic, Solenoid
Integral manifold end cap
Servovalve command Position feedback
Pressure compensated variable volume pump
Position transducer (B)
Pressure filter
Actuator (C) Servovalve (A)
Servoamplifier
Servovalve (A) Piston with low friction seals Highly polished cylinder bore (C) Complete plumbing
4–20 mA command signal
1115
Protective cap Analog absolute LDT (position sensor) (B) Permanent magnet Sonic wand
Industry standard mounting flange
Split stem clamp for easy disassembly Valtek standard yoke-to-body mounting Sectioned view
Exterior view
FIG. 6.3n Servovalve-operated electrohydraulic linear valve actuator. (Courtesy of Valtek, Flowserve Corp.)
emergency circuit to allow the spring to cause a “down” stroke. The same unit can be used to cause the spring to return to the up position using a Bourdon switch to produce force limit.
Motor and Pump Combinations Reversible Motor and Pump A reversible motor can be used to drive a gear pump in a system to remove oil from one side of the piston and deliver it to the other side (Figure 6.3p).
Piston M
Piston
M
The check valves allow the pump to withdraw oil from the reservoir and position the directional control valve in order to pressurize the cylinder. Reversing the motor (and pump) reverses the direction. When the motor is de-energized, the system is “locked up.” For proportional control, feedback is necessary from stem 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. An electromechanical moving coil in the field of a permanent magnet is used to position a jet that can direct oil to one end or the other of the cylinder actuator (Figure 6.3q). Forcebalance feedback from stem position creates the balance with the controller signal. Hydraulic Control of Pinch Valves Controlled hydraulic positioning of a sleeve valve is obtained with a moving coil and magnet to position the pilot (Figure 6.3r), which controls pressure to the annular space of the valve. Feedback is in the form of a Bourdon tube, which senses the pressure supplied to the valve and moves the pilot valve to lock in that pressure.
Travel limit “pull type”
Force limit “push type”
FIG. 6.3o Hermetically sealed electrohydraulic power pack.
© 2006 by Béla Lipták
Multiple Pump The multiple pump system consists of three pumps running on the shaft of one prime mover (Figure 6.3s). There is one pump for each side of the piston and one for
1116
Control Valve Selection and Sizing
Directional control valve Centering springs
Piston
Hydraulic pressure to valve
Bourdon element Feedback spring
Rubber sleeve type slurry valve
DC + – Input
Internal relief valve
Check valve #4
Check valve #2
Check valve #3
Check valve #1
Filter Internal relief valve
Capacitor AC
Relief valve Gear pump
Input Electric motor
Tank line
Start-up filter
Motor running forward Cylinder Piston
Oil reservoir
Piston hydraulically locked
Check valve #4
Check valve #2
Check valve #3
Check valve #1
Reservoir
FIG. 6.3r Electrohydraulic control of a jacketed pinch valve.
Internal relief valve (as required)
(Oil)
Motor shut off
FIG. 6.3p Pump with reversible motor combination, where the actuator is “locked” when the pump is de-energized.
Feedback linkage to jet pipe Input signal
Motor
Feedback spring Pump
Force motor
Jet pipe
Shut-off valve
FIG. 6.3q Electrohydraulic actuator with jet pipe control.
© 2006 by Béla Lipták
Suction
the control circuit. The force motor tilts a flapper to expose or cover one of two control nozzles. The flow through a restricting nozzle allows pressure to be transmitted to one side of the piston or the other. Force-balance feedback is created by a ramp attached to the piston shaft, which positions a cam attached to the feedback spring. Upon loss of electric power, the cylinder shut-off valves close to lock up the pressure in the cylinder and assume a status quo. A bypass valve between the cylinder chambers allows pressure equalization to make use of the manual handwheel. Vibrating Pump A vibratory pump is used in a power pack mounted on a cylinder actuator (Figure 6.3t). A 60 Hz alternating source causes a plunger to move toward the core, and, upon de-energizing, a spring returns it. The pump operates on this cycle and continues until the piston reaches the end of its stroke, when a pressure switch shuts it off. The solenoid that retains the pressure is de-energized by an 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 5,000 lbf (22,240 N) at 0.3 in./min (7.6 mm/min). Two-Cylinder Pump A two-cylinder pump, driven by a unidirectional motor, injects pressure into one end of a cylinder or the other, depending upon the positions of two solenoid relief valves (Figure 6.3u). The solenoid on the left is closed to move the piston to the right, with hydraulic pressure relieved through the other relief valve. Motion continues until the valve it is operating is seated. The build-up of cylinder pressure operates the pressure switch at a predetermined setting to de-energize the motor and both solenoid relief valves. This locks the hydraulic
6.3 Actuators: Digital, Electric, Hydraulic, Solenoid
Input signal
Force motor Cylinder shut-off valves
1117
Air bleed Feed back cam
Feed back spring
Zero spring
By-pass valve
R1 R2
“R”-restriction Drain off connection 3 section pump with biult-in relief valves
Pressure to hydraulic amplifier Pressure to upper cylinder Pressure to lower cylinder Suction to pump
FIG. 6.3s Hydroelectric valve actuator with three pumps running on the shaft of one mover.
pressure in the cylinder. Switching of the three-way switch will start the motor and reverse the sequence of the relief valves to move the piston to the left. At full travel (which is the up position of a valve stem), a limit switch shuts down the unit. Open-centering the three-way switch at any piston position will lock the piston (and valve stem) at that point. Remote control is accomplished by manipulating the open-center switch. Automatic control is acknowledged by including this open-center function, which may be solid state, in the control circuit. A potentiometer or LVDT that senses the 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, Actuate “up”
Intake Solenoid relief valve
Presssure switch
Relief valve
Solenoid Solenoid valve
Actuator spring
Pump
Intake
Solenoid relief valve
Relay R2
R1
To reservoir
Signal
Neutral
Indicator stem
Motor (or solenoid air valve)
Pressure switch
Limit switch
Actuator cylinder
Limit switch
Piston spring
Actuator piston
Check valve
Plunger
Valve stem
Coil Oil sump Core
FIG. 6.3t Electrohydraulic actuator with vibratory pump.
© 2006 by Béla Lipták
To reservoir Power supply
To reservoir Three-way switch
FIG. 6.3u Two-cylinder pump-type electrohydraulic actuator.
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Control Valve Selection and Sizing
although longer travels are feasible. The entire system is designed in a very compact explosionproof package that can be mounted on a variety of valve bonnets. The speed of response to energizing and de-energizing the control circuit makes it feasible to adapt the unit to digital impulses. SOLENOID VALVES Solenoids move in a straight line and therefore require a cam or other mechanical converter to operate rotary valves. These actuators are best suited for small, short-stroke on-off valves, requiring high speeds of response. Solenoid-actuated valves can open or close in 8–12 ms. Their fast closure is not always an advantage: In water systems, it can cause water hammer. They are limited to pressure drops below 300 PSIG (20.7 bars), although when provided with pilots, levers, or double seats, they can handle higher pressure drops. They are available in two- or three-way designs, with power requirements ranging from 10–30 W with 6–440 VAC or 6–115 VDC power supplies. Solenoids are reliable devices, and they can provide multimillion cycles on liquid service. Solenoids (consisting of a soft iron core that can move within the field set up by a surrounding coil) are used extensively for moving valve stems. Although the force output of solenoids may not have many electrical or mechanical limitations, 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 magnetic core attached to the stem and disc, and a solenoid coil (Figure 6.3v). The magnetic core moves in a tube that is closed at the top and is sealed at the bottom; this design eliminates the need for packing. A small spring assists the release and initial closing of the valve. The valve is electrically energized to open. Housing
FIG. 6.3w When valve packing is required, the increased friction can be overcome by a solenoid valve design with two or more strong return springs.
Stronger springs are used to overcome the friction of packing when it is required (Figure 6.3w). Reversing the valve plug results in reverse action (open when de-energized). Even stronger stroking force can be obtained by using the force amplification effect of a mechanical lever, in combination 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 solenoid-operated valves. Magnetic flux paths
Shading coil
Solenoid coil
Stationary core (plug nut)
Coil connections
Core tube Movable core (plunger)
Stem Spring compressed
Spring retainer
Bonnet
Body
Spring
Outlet
Inlet
Disc
Outlet
Intlet
Orifice
Direction of flow through valve De-energized
Energized
FIG. 6.3v The operation of a direct-acting solenoid valve involves the lifting of the plunger when energized.
© 2006 by Béla Lipták
6.3 Actuators: Digital, Electric, Hydraulic, Solenoid
1119
Armature for push solenoid
Pilot poppet Pilot flow
Pilot exhaust
FIG. 6.3x The force generated by the solenoid is amplified in the lever-type actuator.
Main poppet
Pilot metering orifice
Pilot flow entrance
FIG. 6.3z Pilot-operated in-line solenoid valve, where the pilot serves to apply pressure to the main poppet piston.
2
Small solenoid pilot valves are widely employed to supply pressure to diaphragms or pistons for a wide range of output forces. Pilot operation applies pressure to a diaphragm or piston or may release pressure, allowing the higher upstream pressure to open the valve. A good example is the in-line valve (Figure 6.3z). Most solenoid valves are designed to be continually energized, particularly for emergency shutdown service. Thus the
power output is limited to the current whose I R-developed heat can be readily dissipated. Using a high source voltage and a latch-in plunge overcomes the need for continuous current. The single-pulse valve-closing solenoid is disconnected from the voltage source by a single-pulse delatch solenoid and hence does not heat up after it is closed. A pulse to the delatch solenoid permits the valve to be opened by a spring. Three-way solenoid valves with three pipe connections and two ports are used to load or unload cylinders or diaphragm actuators (Figure 6.3aa). Four-way solenoid pilot
Return spring
Outlet
Manual opening
Cushioned closing
Inlet
Vent De-energized position
Open
Closed Energized position
FIG. 6.3y The solenoid valve can close against higher pressure drop, if the solenoid operates a small pilot valve.
© 2006 by Béla Lipták
FIG. 6.3aa The operation of a normally closed three-way solenoid valve.
1120
Control Valve Selection and Sizing
valves are used principally for controlling double-acting cylinders. Modulating Solenoid Valves Modulating magnetic valves (Figure 6.3bb) utilize springloaded low-power solenoids to provide throttling action. The only moving part in this design is the valve stem, which has the valve plug attached to one end and an iron core attached to the other. The valve opening is thereby a function of the voltage applied across the solenoid. (Figure 6.3cc illustrates the change in plunger force as a function of applied voltage and plunger air gap.)
ON/OFF valves position reed switches
Magnet
Such throttling solenoids are frequently used in the HVAC industry and in other applications where the valve actuators do not need to be very powerful. The actuator thrust requirements are lowered by balancing the inner valve through the use of pressure equalization bellows or floating pistons. The positive failure position of these valves is provided by spring action. Throttling solenoids are typically available in 1/2 –8 in. (127–203 mm) sizes and are limited by the pressure difference against which they can close. They are available in two- or three-way designs and can handle water flows up to 5000 gpm 3 (19 m /m). The use of throttling solenoids can completely eliminate the need for instrument air in the control loop. These valves
Modulating valves LVDT
Indicator tube
Return spring
Core rod
Pilot disc Valve inlet Solenoid assembly
Main disc
Bonnet tube
Plunger
Inlet orifice
Pilot valve discharge orifice
Fixed core Return spring
Pilot disc Main disc
Flow Main disc Pilot disc
Vent port
Flow
FIG. 6.3bb Throttling solenoid valve design with LVDT used to measure the stem position for feedback.
© 2006 by Béla Lipták
Plunger force pounds (Kg)
6.3 Actuators: Digital, Electric, Hydraulic, Solenoid
120 (54) 110 (50) 100 (45) 90 (41) 80 (36) 70 (32)
Anodized aluminum housing
Modulating spring valve characteristics
B
1121
Microprocessor control board
VA valve actuator
Motor 100 V
60 (27) 50 (23) 40 (18) 30 (14) 20 (9) 10 (4.5) 0
90 V 80 V 70 V A
60 V
ON/OFF valve spring characteristics
0
O-ring seals cover & shaft Anodized aluminum bracket Stainless steel shaft Brass nickel plating
.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0 (2.5) (5) (7.5) (10) (13) (15) (18) (20) (23) (25)
Position indicator Heavy duty stainless steel geartrain New universal VK coupler valve kit Nickel plated swivel adapter
Plunger air gap in inches (mm) Valve
FIG. 6.3cc The force available to the pilot plunger of a throttling solenoid depends on the voltage and the air gap.
offer a higher speed and better range than their pneumatic counterparts. Some manufacturers claim a range of 500:1 and a stroking time of 1 sec. The design illustrated in Figure 6.3bb can be driven directly from microprocessor-based building automation systems. The design shown in Figure 6.3bb also includes a separate positioner, which accepts a 4–20 mA DC input from the controller and delivers a DC output signal to the throttling solenoid. Valve position feedback is obtained through the use of a linear variable differential transformer mounted directly on the valve.
SMART ACTUATORS In addition to specifying the thrust, travel, failure position, control signal, power supply, speed, electrical area classification, and ambient conditions that an actuator must meet, one can also consider the use of microprocessor-based smart systems and of some other types of intelligent systems. One of the trends in process automation is the use of distributed control and, in some cases, to locate the controller as close as possible to the final control elements (control valves). Often the control signal is digital (digital valve con3 trol, or DVC ). It is in this respect that an increasing number of features have been added to valves, valve actuators, and positioners through the integration of microprocessors with them.
© 2006 by Béla Lipták
FIG. 6.3dd Microprocessor-based electric valve actuator. (Courtesy of ETI Systems, Inc.)
Applications Microprocessors have been included in valve actuators and valve positioners for some time (Figure 6.3dd), but the degree of exploitation of the possibilities of the smart and intelligent actuators for the purposes of process control is highly dependent on data access and thus on digital networking capabilities. The tendency is to provide new actuators with some sort 4 of network connection. As discussed in more detail in Sections 4.16 and 6.11, Foundation fieldbus, HART, and Profibus are some of the leading suppliers. Open architectures, such as Foundation fieldbus, have made positive contributions, because by broadening the market they allowed more users to take advantage of better performing actuating systems at lower prices. Moreover, the compatibility problems between different fieldbus systems are in the process of being resolved, and it is hoped that through international standard3 ization “plug and play” capabilities will soon be available for such devices. The intelligence provided by microprocessors can and is used for valve tuning, semiautomatic calibration, and data collection for maintenance and diagnostic purposes. It is also used for standalone control when that might improve the positioning, the protection, or the communication of the valve. Table 6.3ee lists some of the more common applications of smart and intelligent actuator systems.
1122
Control Valve Selection and Sizing
TABLE 6.3ee 5 Applications for Integrated Intelligent Systems • 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.
Microprocessor-based systems are available in watertight or explosionproof (NEMA 6 to 7) construction and can tolerate ambient temperatures from – 40 to 185°F (– 40 to 85°C), as well as the presence of moisture, fungus, or dust. They can incorporate sensors, an electronic positioner, or an electronic proportional, integral, derivative (PID) controller, which can operate off a digital or analog external set point. Locating the controller card at the valve and dedicating it to full-time control is an advantage on very fast critical loops, such as compressor surge protection, where otherwise surge could evolve while the central DCS control system is scanning/updating other loops. Smart systems can also take advantage of the lower cost of digital communications over a single loop of two-conductor or fiber-optic cable (Figure 6.3ff). This method of communication substantially reduces the wiring cost of installations that can control up to 250 actuators, pumps, or solenoids. The smart valves can be remotely calibrated and reconfigured and can be used to detect such performance changes as pressure changes resulting from a fouled pump or pipeline. They can also limit the valve travel to stay within the range where the required characteristics (gain) are available. It is perhaps at the maintenance level that the most benefits can be obtained through the use of smart and intelligent 6 systems. Smart actuator circuitry can protect electric motor operators from burning out when the valve is jammed or from reverse phasing. More importantly, by close monitoring of
© 2006 by Béla Lipták
Master station
FIG. 6.3ff Multiple intelligent actuators can be monitored and controlled by a two-wire multiplexer loop. (Courtesy of Rotork Controls Ltd.)
the valve, the required information can be obtained to evaluate the valve and actuator performance and thereby replace preventive and corrective maintenance by predictive maintenance with the benefits of improved process performance and products quality at a lower cost.
References 1.
2. 3.
4.
5. 6.
Harrold, D., “A Changing Landscape,” Control Engineering, December 2003, www.manufacturing.net/ctl/article/CA339684? text = valve + actuators. Scott, A. B., “Control Valve Actuators: Types and Application, InTech, January 1988. Harrold, D., “Making Valve Controllers/Positioners Smarter is Smart Business,” Control Engineering, January 2003, www.manufacturing. net/ctl/index.asp?layout=articlePrint&articleID=CA269795. Hoske, M. T., “Implementing Industrial Networks,” Control Engineering, July 1998, www.manufacturing.net/ctl/article/CA197636?text= valve+actuators. Price, V. E., “Smart Valve Intelligence Takes Many Forms,” InTech, August 1992. Merritt, R., “A Real (Valve) Turn-On” Control Design, August 2002, www.controldesign.com/Web_First/CD.nsf/ArticleID/JFEY-5BMRJD.
6.3 Actuators: Digital, Electric, Hydraulic, Solenoid
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© 2006 by Béla Lipták
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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. Kervin, D., “Zero-Crossing Triac Drivers Simplify Circuit Design,” Control Engineering, Vol. 29, No. 3, p. 76, 1982. Koechner, Q. V., “Characterized Valve Actuators,” Instrumentation Technology, March 1977. Liantonio, V., “High Pressure Modulating Solenoid Valve for Steam/Gas Service,” InTech, January 1988. Lipták, B. G., “Control Valves in Optimized Systems,” Chemical Engineering, September 5, 1983. Lockert, C. A., “Low Load Technique Controls Motor Energy Losses,” Control Engineering, Vol. 29, No. 2, 1982, p. 142. MSS SP-86-2002, “Guidelines for Metric Data in Standards for Valves, Flanges, Fittings and Actuators,” 2002. Price, V. E., “Smart Valve Intelligence Takes Many Forms,” InTech, August 1992. Pyotsia, J., “A Mathematical Model of a Control Valve,” 1992 ISA Conference, Houston, TX, October 1992. Valve & Actuator User’s Manual, British Valve & Actuator Association, 2002. Scott, A. B., “Control Valve Actuators: Types and Application,” InTech, January 1988. Shinskey, F. G., “Dynamic Response of Valve Motors,” Instruments and Control Systems, July 1974. Usry, J. D., “Stepping Motors for Valve Actuators,” Instrumentation Technology, March 1977. VDI/VDE 3845, “Industrial Process Control Valves. Interfaces between Valves, Actuators, and Auxiliary Equipment, 1998.”
