U VALVE ACTUATORS AS P MAINTENANCE T FOR … VALVE ACTUATORS AS PREDICTIVE MAINTENANCE TOOLS FOR MOVs By: Chris Warnett Rotork Controls, Inc. 675 Mile Crossing Boulevard Rochester,

-1-

USING VALVE ACTUATORSAS PREDICTIVE MAINTENANCE

TOOLS FOR MOVs

A USER’S GUIDE

-2-

USING VALVE ACTUATORSAS PREDICTIVE MAINTENANCE

TOOLS FOR MOVs

By: Chris WarnettRotork Controls, Inc.

675 Mile Crossing BoulevardRochester, NY 14624

716-328-1550FAX: 716-328-5848

E-mail: [email protected] site: http://www.rotork.com

© 2000 by Rotork Controls, Inc. All rights reserved.

-3-

INTRODUCTIONAt the beginning of the

twentieth century, before valveactuators were invented,virtually all valves were oper-ated manually.

Experienced personnelcould tell the condition of avalve’s seat, stem, or packingsimply by the feel of the oper-ating handle as the valve wasopened or closed.

For example, if the handlewas harder to turn than theprevious operation, then theoperator could deduce that thestem was ready for regreasing.If the valve required more andmore effort to provide shutoff,then the operator could de-duce the condition of the seat.

Similarly, if the valve waseasy to operate, then perhapsthe seal packing neededadjustment.

Furthermore, because theoperator was standing next tothe valve in order to open orclose it, a visual inspection ofthe valve easily could bemade.

As a result, predictivemaintenance, although notnecessarily scientific, wasachievable by this simplehands-on method.

However, with the adventof automation, the direct

interface between the operat-ing personnel and the valvehas been eliminated.EARLY VALVE ACTUATION

The first successful valveactuators had an inherentdevice, which sensed theamount of torque being deliv-ered to the valve by the actua-tor. When it hit a preset limit, itwould cause the actuator to shutoff. This torque-sensing mecha-nism consisted of balancing thelinear force of an actuator’sworm shaft against a spring orsprings. (See Figure 1.)

It was a simple step to fit aninstrument such as a potentiom-eter or Linear Variable Differen-tial Transformer (LVDT) to theend of the worm shaft of anactuator and get a continuousmeasurement of the actuator’storque output.

In the early 1980s, this wasdone extensively in the nuclearpower industry where it wasimportant to monitor the condi-tion of certain critical nuclearvalves.

These devices were fitted tovalve actuators so that thetorque demand could berecorded and plotted against theposition of the valve. Althoughthese devices were expensiveto fit and operate, they werejustified by the extreme safety

-4-

requirements needed for operat-ing nuclear power plants.

When critical safety valveswere first installed, a “footprint”torque profile of the valve wastaken. Subsequent torqueposition readings on thesevalves were compared to theoriginal “footprint.” Problemswith the valve or actuator thencould be identified by comparingthe data.MAJOR BREAKTHROUGH

In the mid-1980s, Rotorkintroduced a robust digitalcommunication system to themarketplace – PakScan – whichenabled large numbers ofactuators to be linked togetherusing a single, shielded, twistedpair of wires.

The PakScan systemutilizes a master station as aloop management device whichtalks via the twisted shieldedpair to PakScan field units ineach actuator.

Because the communicationbetween the master station andthe actuator is digital, the numberof wires no longer restricts theamount of information that can begathered. The master stationmanages the loop by constantlyscanning each actuator anddetecting any changes in status.

It stores the information andrelays it back to a host DCS orPLC on demand. The masterstation communicates to thehost using standard ModbusRTU protocol.

Figure 1. Torque-sensing mechanism used in electric actuators.

-5-

The development of thePakscan system was an impor-tant step in the control of valveactuators, because a largeamount of data could be col-lected without the prohibitivecost of large numbers of copperwires to retrieve information.

When this equipment waslinked to a Rotork IQ electricactuator, then true valvediagnostic capability wasintroduced.

The breakthrough of the IQenabled continuous torque andposition information to begathered by the actuator itself.

When the data was trans-mitted via a Rotork PakScantwo-wire communication

system to PakVision software,then continuous monitoring oftorque and position allowed forcomparison between the foot-print torque characteristic of thevalve during installation andsubsequent torque profiles.

The original PakVisionsoftware (and the latest ver-sion, which is called InVision™software) is a Rotork Man/Machine Interface (MMI) forthe PakScan system.

PakVision and InVisionsoftware allow operators notonly to control field devices,but also monitor their status onconfigurable screens on astandard IBM-type PC.

PakVision and InVision

Figure 2. Rotork PakVision and InVision software take data from the IQactuator via the PakScan system and display it as easy-to-read bargraphs.

-6-

software can also keep com-prehensive logs of statuschanges, alarms, and operatoractions. This software is oftenused as a backup for the mainhost MMI software. It’s alsovaluable to maintenancepersonnel as an off-linerecordkeeping and devicemonitoring tool.

With these tools, motoroperated valve end users areable to monitor the torquedemands of their valves andcan view sudden changes intorque demand or trends indemand over a period of time.(See Figure 2.)

Another developmentassociated with the IQ actuator

is that it enabled data to bestored in the actuator itselfutilizing a data logger card.

The data logger cardstores the condition of 32discreet signals associatedwith actuator components,including monitoring positionswitches, local/remote selec-tors, “Stop” button, and otherdata as well as torque andposition information.

By downloading this informa-tion into a hand-held computer, itcan be removed offsite for detailedanalysis. (See photo below.)LATEST ADVANCE

The innovations of theearly 1990s have now beenenhanced and modified to

You can use the Rotork data logger to download vital information foroffsite analysis.

-7-

provide highly accurate infor-mation and analysis of motoroperated valves.

The latest Rotork IQ featuresa built-in data logger, whichrecords information from anadvanced and highly accuratetorque-sensing mechanism,which constantly monitors theactuator’s torque output byaccurately measuring theworm shaft-force reactionelectronically.

This information, togetherwith position data and theother discreet component data,are stored in the actuator forfuture analysis.

When a motor operated

valve is first commissioned in aplant, pipeline, or other facility,the initial torque demand isrecorded in the data-loggingchip as the “footprint.”

This serves as a referencepoint for future torque readingsso comparisons can be madeand the condition of the valvediagnosed. (See Figure 3.)

The data from the datalogger can be collected locallyutilizing a PC with an infraredlink connected to the actuatorindicator window. The PC canrun its diagnostic analysisutilizing IQ-Insight software.

Also, actuators fitted withPakScan can transmit data

Figure 3. Rotork IQ actuators can gather continuous torque and positioninginformation. Above is an IQ torque profile using IQ-Insight™ software.

-8-

directly via a PakScan masterstation to either the remotehost controller, PLC, DCS, orto a PC running diagnosticsoftware, such as InVision.HOW TO USE THE DATA

The question is: What doesone do with the data once it iscollected?

Usually, the most usefuldata relates to torque, soRotork has concentrated onthis. The IQ-Insight data alsoallows analysis of frequency ofoperation and other param-eters, so contactor wear orstem wear can be explained bystudying the operation recordof IQ-Insight or Pakvision

software files.Let’s examine the different

valve torque elements. Thesecan be broken down into sevenmain components:• Valve Seal or Packing

Friction• Valve Shaft Bearing

Friction• Closure Element on

Seat Friction• Closure Element In-

Travel Friction• Hydrodynamic Force

on Closure Element In-Travel

• Stem Piston Effect• Valve Stem Thread

Friction.

The above image is a view of a Rotork IQ-Insight screen showingfrequency of operation data.

-9-

Most of these elements arepresent in all types of valves,but in varying degrees ofmagnitude.

For example, the closureelement travel friction in abutterfly valve is negligible,whereas in a lubricated plugvalve, it is more significant.

Valve actuators are de-signed to limit their torque to apreset level (torque switchsetting) in the closing direction.An increase in torque abovethis level will stop the actuator.

In the opening direction,the torque switch is bypassedfor the initial unseating opera-tion. The resulting torqueprofile is therefore useful inanalyzing the valve condition.

PROFILES OF DIFFERENTVALVE TYPES

Let’s look at the different typesof valves and how these differentelements build up the torqueprofile of the valve.Wedge Gate Valve

The significant torquerequirements for opening orclosing a wedge gate valveoccur during the final travelgoing closed and the initial travelgoing open.

During the remaining por-tion, the torque demand is madeup mainly of packing friction andthread friction of the Acme-threaded shaft.

As the valve seats, thehydrostatic force on the closureelement (the disc), increases the

Valve Shaft Bearing Friction – negligible.

Closure Element on Seat Friction – wedge effect.

Closure Element In-Travel Friction – negligible.

Hydrodynamic Force on Closure Element In-Travel –negligible.

Valve Stem Thread Friction – function of thread form andlubrication.

Valve Seal or Packing Friction – function of stem diameter.

Stem Piston Effect – function of pressure; negligible below1,000 psi.

TYPICAL CHARACTERISTICS OFA WEDGE GATE VALVE

-10-

seating friction, and finally thewedging effect of the disc in theseat causes a rapid increase intorque demand until seating isaffected.Similarly, when unseating thevalve, the disc has to beunwedged and the hydrostaticforce of the differential pressureacross the valve has to be over-come as the valve is opened.

Once the valve is crackedopen and the differential pressurehas dissipated, then the torque

The photo above showsautomated wedge gate valves.

Packing Too Tight

Stem NeedsLubrication

FootprintTorqueProfile

TorqueDemand

Opening Torque Characteristics of a Typical Wedge Gate Valve

Closed Valve Position Open

(FIGURE 4)

GalledSeat

-11-

demand drops off significantly. InFigure 4, you can see the totaltorque profile of the valve. Should,for example, the valve stempacking be overtightened, then animmediate increase in torqueprofile would be recorded. Shouldthe lubrication on the threaddeteriorate over time, then youwould see an incremental in-crease in overall torque per thediagram.

Alternatively, should thevalve seat become galled ordeteriorate, then you would seean increase in the unseatingtorque required.Ball Valve

The main contributors oftorque to a ball valve are:1. The shaft bearing friction.This could include the friction

of any ‘o’ ring seals which arefrequently used to seal ballvalve shafts.2. The ball (closure element)on seat friction during closingand opening.

As can be seen from thediagram (See Figure 5), shaftbearing friction is usually aconstant as is the ball on seat

Example of a ball valve.

Valve Seal or Packing Friction – constant.

Valve Shaft Bearing Friction – function of differential pressure.

Closure Element on Seat Friction – function of differentialpressure.

Closure Element In-Travel Friction – constant.

Hydrodynamic Force on Closure Element In-Travel –negligible.

Stem Piston Effect – none.

Valve Stem Thread Friction – none.

TYPICAL CHARACTERISTICS OFA BALL VALVE

-12-

GalledSeat

FootprintTorqueProfileTorque

Demand

Opening Torque Characteristics of a Typical Ball Valve


(FIGURE 5)

-13-

friction during travel, but at theclosed position, just prior toclosing and during opening,the ball on seat friction isincreased significantly. This isdue to the differential pressureacting on the ball in the floatingball design, or on the seat in thetrunion design, forcing the balland seat together to providesealing.

Should the ball or the seatbecome scored or pitted, thenthe torque demand at theclosed position would increase.Butterfly Valve

The main elements oftorque in a butterfly valve are:1. Bearing friction, togetherwith valve shaft seal friction,which is typically an ‘o’ ring andtherefore constant.

2. Closure element on seatfriction; in other words, the discon seat friction.3. Hydrodynamic forces on theclosure element in travel.

Because the disc of abutterfly valve is directly in themiddle of the fluid stream, thendynamic forces could act onthe disc to exert a backdrivetorque on the actuator.

Problems with the bearings on

Pictured above are Rotork IQactuators on butterfly valves.


Valve Shaft Bearing Friction – constant.

Closure Element on Seat Friction – high at closure.

Closure Element In-Travel Friction – none.

Hydrodynamic Force on Closure Element In-Travel –mid-travel.

Stem Piston Effect – none (not applicable).

Valve Stem Thread Friction – none (not applicable).

TYPICAL CHARACTERISTICS OFA BUTTERFLY VALVE

-14-

Problems with Bearingsor Stem Seal

DamagedSeats

Change of Flow Rate,Viscosity, Etc.


TorqueDemand

Opening Torque Characteristics of a Typical Butterfly Valve


(FIGURE 6)

-15-

a butterfly valve show up as anincrease in torque across theentire stroke of the valve.

Damaged seats or damageddiscs affect the closing andopening torque over the first fewdegrees of valve travel.

Changes in flow rates, fluidviscosity, temperature, etc. affectthe dynamic torque demand in themiddle of the stroke; however, thiswould not necessarily be anindicator of a problem with thevalve itself.Lubricated Plug Valve

The torque elements in alubricated plug valve are moreconstant than other valves dueto the relatively high frictionbetween the closure element –the plug, and the valve.

Pictured above is a lubricated plugvalve. The illustration wasprovided courtesy of NordstromValves, Inc.



Closure Element on Seat Friction – rises somewhat withdifferential pressure.

Closure Element In-Travel Friction – constant.

Hydrodynamic Force on Closure Element In-Travel –none (not applicable).



TYPICAL CHARACTERISTICS OFA LUBRICATED PLUG VALVE

-16-

Problem with Seats or Bearings


TorqueDemand

Opening Torque Characteristics of a Typical Lubricated Plug Valve


(FIGURE 7)

-17-

Bearing friction and closureelement on seat friction arerelatively constant throughoutthe stroke. Hydrostatic forcesduring closing and the first fewdegrees of opening wouldincrease the torque demand.A typical lubricated plug valvetorque demand is representedin Figure 7.

Problems with the bearingsor the seats would reflectuniform torque increase acrossthe stroke.Resilient Seated Plug Valve

The torque characteristic of aresilient seated plug valve reflectsa relatively high seating andunseating torque to constantbearing and seal friction through-out the stroke.

As the closure element does

not touch the body of the valveor the seat during travel, there isno closure element travelfriction, similar to a butterflyvalve. A typical torque curve isrepresented in Figure 8.

Problems with the valveseating would be reflected in thetorque demand during closing

Pictured above is a DeZurikRubber Lined Eccentric Plug Valve.



Closure Element on Seat Friction – high at closure.

Closure Element In-Travel Friction – none.

Hydrodynamic Force on Closure Element In-Travel –low.



TYPICAL CHARACTERISTICS OFA RESILIENT SEATED PLUG VALVE

-18-

Problem with Bushingson Shaft or Stem Seal


TorqueDemand

Opening Torque Characteristics of a Typical Resilient Seated Plug Valve


(FIGURE 8)

Valve SeatingProblem

-19-

and opening. A problematicbushing on the shaft wouldmanifest itself as a constanttorque increase throughout thestroke as indicated in Figure 8.Sluice Gate

In the closed position, thefull head of fluid is acting againstthe gate.

As the gate opens, the headdecreases relatively graduallyand in proportion to the positionof the gate.

Because sluice gates areexposed to the elements, stemlubrication should be a regularmaintenance item.

This can be easily monitoredas a gradual increase in torqueacross the entire open stroke.

Above is a Rotork IQ actuatoroperating a sluice gate. To assureoptimum operation, sluice gate stemsneed to be lubricated regularly.



Closure Element on Seat Friction – wedge effect.

Closure Element In-Travel Friction – guide friction againstclosure element (gate).

Hydrodynamic Force on Closure Element In-Travel –none, but hydrostatic force on gate throughout stroke.

Stem Piston Effect – none.

Valve Stem Thread Friction – function of thread form andlubrication.

TYPICAL CHARACTERISTICS OFA SLUICE GATE

-20-

Thread NeedsLubrication


TorqueDemand

Opening Torque Characteristics of a Typical Sluice Gate


(FIGURE 9)

Bent Stem

-21-

EXAMPLES OF PREDICTIVEMAINTENANCE BENEFITS

Below are a few examples ofthe benefits of predictive mainte-nance in various applications.Wastewater Industry

Most wastewater plantsutilize sluice gates to controlflow through their facilities.

The sluice gates often aremounted outdoors with thestems exposed to the elements.This means that any lubricationthat may be utilized on thestems frequently is subjected tothe drying effects of the sun, theerosive effects of dirt and water,as well as contamination fromairborne particulate.

As the sluice gates areoperated and the stems passthrough the drive nut of theactuator, the drive nut experi-ences a certain amount ofwear. This is directly propor-tional to the torque demand ofthe sluice gate.

When the sluice gate isinitially installed and the stemis well lubricated, the mini-mum level of torque is neededfor operation. After a period ofsix months, assuming therehas been no additional attentionpaid to the stem of the sluicegate, the torque demand willincrease as the lubricationdeteriorates.

Rotork IQ actuators are used extensively throughout wastewatertreatment facilities.

-22-

A further six months wouldshow yet a further increase intorque demand. If no preven-tive maintenance is performedon the stem, then eventuallythe torque demand will reach apoint where the efficiency ofthe drive is so low that thedrive nut in the actuator couldexperience rapid and cata-strophic wear.

This can be prevented bymonitoring the increase intorque demand of the sluicegate over time such that therate of increase in torquedemand can be quantified, andthe optimum point for stemlubrication predicted.

This predictive mainte-nance can not only increasethe life of the sluice gate, butalso eliminate the potential forcatastrophic failures, whichcan occur when conditionsbecome so bad that the drivestem of the actuator failsallowing the sluice gate to dropinto its seat.Power Industry

In power plants, there aremany high temperature linescontaining steam or water.

Often, these lines arecontrolled by gate or globevalves to isolate flow duringstartup and shutdown of aplant.

Plants that are not on abaseload may find that theyare cycling twice a day. Underthese circumstances, when thevalve stem is withdrawn fromthe valve, it cools to the ambi-ent temperature outside of thepipeline insulation, while thevalve remains at the tempera-ture of the fluid media.

However, on closing, thevalve stem is now inside thevalve and absorbs residualheat, increasing in temperatureuntil it reaches equilibrium withthe valve body.

This means there is thepotential for the valve stem toexpand inside of the valvebody, generating a thrust overand above that delivered bythe actuator to provide tightshutoff.

In these circumstances,valve stems can take a perma-nent offset or valve seats canbecome galled.

If the valve stem has beenslightly bent, the situation canbe detected by the torqueprofile of the actuator from alocalized high spot in thetorque chart somewhere in themid-portion of the stroke. Asthe plant continues to cycle,the offset of the stem wouldincrease as the greater theoffset, the lower the resistance

-23-

The power industry has found Rotork IQ actuators to be reliable.

to bending. This would result ingreater and greater torquedemand in the mid-stroke areaas the valve stems takes agreater and greater offset.

This increasing torquetrend in mid-stroke can bedetected utilizing torque profil-ing methods.

Given sufficient time tocatch the failure mechanism,the valve stem can be removedand straightened, so the costof replacing the valve stem canbe avoided. In these circum-stances, where differential

thermal expansion is the rootcause of the problem, a moreresilient mounting can beprovided to avoid repetition ofthe failure mechanism.

Similarly, if the seats aregalled in the valve, then theunseating torque will increase.As the valve continues tocycle, the galling will getprogressively worse, displayingan increased torque demand inthe unseating torque curve.Refining Industry

One of the most arduousduties for a valve is coker

-24-

Rotork IQ actuators in a refinery industry application.

service in an oil refinery.The coking process is a

batch process whereby firstone coker is in process whilean adjacent coker is beingemptied. Constant cycling isrequired of the coker valvesevery few hours.

The coke media itself isextremely tough on the valveand tends to stick to the seatsand other parts of the valve.

Frequently, heavy-duty ballor plug valves are used in thisservice. Coker operatorsmonitor these valves carefullyto anticipate maintenanceproblems.

Because they are a high-maintenance item, downtime is

inevitable; however, if problemscan be anticipated, then theimpact can be minimized.

Not only can torque profil-ing on coker valves indicatecoke buildup on the valve, butalso the rate of buildup and thedeterioration of the valve’seffectiveness. By monitoringthe increase in torque at theends of stroke and over thestroke itself (depending on thevalve), quarter-turn valves canbe maintained at exactly theright interval.

This can reduce unplanneddowntime and allows replace-ment equipment to bepreordered so last-minuterushes can be avoided.

-25-

Comprehensive Web SiteTo find out more about

Rotork valve actuators andapplications, we invite you tovisit our Web site atwww.rotork.com.

The site offers a compre-hensive overview of actuatorapplications, comments byusers of Rotork actuators, anoverview of Rotork productsand specifications, worldwidesales addresses and contacts,and much more.

Also, from the Web site,you can order free CDs thatprovide in-depth sizing andspecification information aboutRotork electric and fluid poweractuators.For More Information

For more information abouttorque profiling and predictivemaintenance, contact:Mr. Chris WarnettRotork Controls, Inc.675 Mile Crossing BoulevardRochester, NY 14624 USATelephone: 716-328-1550Fax: 716-328-5848Email: [email protected]

Rotork is 9001 certified andhas registered approval byFoundation Fieldbus.

Documents

U VALVE ACTUATORS AS P MAINTENANCE T FOR … VALVE ACTUATORS AS PREDICTIVE MAINTENANCE TOOLS FOR MOVs By: Chris Warnett Rotork Controls, Inc. 675 Mile Crossing Boulevard Rochester,