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Supplement to Fisher� FIELDVUE� DVC6000Series Digital Valve Controllers InstructionManual

Implementation of Lock-in-Last Strategy

Many applications require a valve assembly toremain in the position it was prior to a specificcontrol system failure (lock-in-last position).Functionality and safety can be designed into valveassemblies with Fisher� FIELDVUE� digital valvecontrollers by utilizing an array of accessories. Thefollowing paragraphs describe standard solutions fora number of generic lock-in-last applications withDVC6000 Series digital valve controllers.

Note

This instruction manual supplement isfor use with HART� communicatingFIELDVUE DVC6000 digital valvecontrollers only.

Lock-in-Last on Loss of SupplyPressure

Once the supply pressure falls below a minimumvalue, the digital valve controller can no longerposition a valve assembly; the valve will start to goto the actuator “at rest” position. The locking devicesenses this change and activates, preventing the

FISHER 164A THREE-WAYSWITCHING VALVE

A

C

D

B

FIELDVUEDVC6000SERIES

FISHER 67CFRFILTER/REGULATOR

AIRSUPPLY

SUPPLYOUTPUT

Figure 1. Lock-in-Last on Loss of Supply Pressure for anAssembly with a Single-Acting Actuator

remaining air from venting. The valve is “locked-in”this new position.

Note

The locked-in valve position dependson the exhausting speed of theactuator.

Assemblies with Single-ActingActuatorsA Fisher 164A three-way switching valve should beused for locking a single-acting actuator in place onloss of supply pressure. Figure 1 is a schematicrepresenting proper assembly layout. In normal

Instruction Manual SupplementD103261X012February 2009 FIELDVUE� Digital Valve Controllers

FIELDVUE� Digital Valve ControllersInstruction Manual Supplement

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FISHER 377LTRIP VALVE

E

D

F C

B

A

FIELDVUEDVC6000SERIES

FISHER 64REGULATOR

AIRSUPPLY

OUTPUT

SUPPLY

INPUT

OUTPUT

Figure 2. Lock-in-Last on Loss of Supply Pressure for anAssembly with a Double-Acting Actuator

operation, the digital valve controller output passesthrough the switching valve from port A to port B andon to the actuator.

In a fail condition, the pressure at port D is below theswitching valve’s set point, causing the switchingvalve to trip. This closes port B, which locks thepressure in the actuator. Port C is plugged so thedigital valve controller output will not exhaust to theatmosphere.

Assemblies with Double-ActingActuators

Similar to the locking strategy used withsingle-acting actuators, the locking strategy withdouble-acting actuators also uses a valve for lockingsupply pressure in the actuator. For assemblies withdouble-acting actuators, a Fisher 377L trip valve isused as the locking device. The 377L trip valve hastwo output ports for locking pressure on both sidesof a double-acting actuator. Figure 2 is a schematicrepresenting proper assembly layout.

Connection of the digital valve controller output to adouble-acting actuator determines the action of theactuator. Figure 2 shows the digital valve controllerproviding lower cylinder pressure through ports Dand E, and upper cylinder pressure through ports Aand B. When the supply pressure falls below the set

point of the trip valve, the trip valve closes ports Dand A and connects port B to C and port E to F.Because C and F are plugged, the control valve islocked in place by locking pressure on both sides ofthe actuator piston. The assembly returns to normaloperation once supply pressure is restored at the tripvalve INPUT port.

Note

Double-acting actuators with a 377Ltrip valve require a Fisher 64 or 95Hregulator. Regulators with smaller flowcapacities may cause the trip valve tocycle (lock and unlock repeatedly) dueto air flow demand as the assemblyattempts to reset. Use a Fisher 252 or262C pilot filter to filter supply air.

Both the 164A switching valve and the377L trip valve have a deadband thatmust be overcome. The switchingvalve must be calibrated to reset uponrestoration of adequate supplypressure to the regulator. The 377L tripvalve has only one spring selection.However, for minimal deadband, thelightest appropriate spring should beselected for the 164A switching valve.

Lock-in-Last Using SolenoidValvesSolenoid valves are used with valve assemblies inmany ways. Their electrical control can be utilized incombination with switches and controller logic toperform a number of functions.

Note

Solenoid valves placed between theoutput of a DVC6000 Series digitalvalve controller and the input to anactuator require a minimum Cv of 0.49.Greater restrictions can affect theresponse of the assembly. Anappropriate three-way solenoid valvefor use with the digital valve controlleris the ASCO� 8320 Series solenoidvalve from ASCO Valve, Inc.

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32

1

24 VDCPOWER SUPPLY

FISHER 67CFRFILTER/REGULATOR

AIRSUPPLY

FIELDVUEDVC6000 SERIES

ASCO 8320 SERIESSOLENOID VALVE

(ENERGIZED)

OUTPUT

SUPPLY

Figure 3. Lock-in-Last Strategy for an Assembly with aSingle-Acting Actuator Using a Solenoid Valve

Assemblies with Single-ActingActuatorsA three-way universal solenoid valve can be placedbetween the digital valve controller output and theactuator input. Switching the valve assembly from anunlocked state to a locked state is controlled byswitching power on and off to the solenoid valve.Figure 3 depicts proper assembly layout.

Under normal operating conditions the solenoid isenergized and supply air flows from the digital valvecontroller output to the actuator input. In the failstate, power is removed from the solenoid causingthe solenoid valve to close, locking air pressure inthe actuator. Port 1 of the solenoid valve is plugged,preventing actuator air pressure from exhausting tothe atmosphere.

Assemblies with Double-ActingActuatorsAssemblies with double-acting actuators require a377L trip valve to lock pressures on both sides of theactuator piston. Figure 4 is a schematic of adouble-acting actuator with lock-in-last capabilityshown in its normal operating mode. The solenoidvalve is placed in series with the signal port of thetrip valve. When tripped, the solenoid valve exhauststhe signal pressure to the trip valve to atmosphere.This simulates a loss in supply pressure, causing thetrip valve to lock pressure in the actuator.

2

3

1

E

D

F C

B

A

24 VDCPOWER SUPPLY

FISHER 64REGULATOR

AIRSUPPLY

FISHER 377LTRIP VALVE

ASCO 8320 SERIESSOLENOID VALVE

(ENERGIZED)

FIELDVUEDVC6000 SERIES

FISHER 252AIR/GAS FILTER

OUTPUT

SUPPLY

OUTPUT

Figure 4. Lock-in-Last for an Assembly with a Double-ActingActuator Using a Solenoid Valve

The solenoid valve in this solution does not require aCv greater than 0.49. Flow capacity of this valve canbe much smaller. A proper solenoid valve for thisassembly is the 8320 Series three-way solenoidvalve. As with all solenoid valves, ensure that thesolenoid valve operating pressure differential ratingis adequate for the supply pressure.

Lock-in-Last on Loss of SupplyPressure and/or Loss of LoopCurrentMost applications require a valve assembly to be ina lock-in-last fail state not only on loss of adequatesupply pressure but also on loss of loop power.Lock-in-last on loss of supply pressure is quitestraight forward. Lock-in-last on loss of loop powercan be more complex.

Most control loops operate on a 4-20 mA controlsignal. A solenoid valve, adequately sized for theapplication, requires more than 4 mA to energize.Therefore, the control loop cannot be used toenergize the solenoid. Instead the solenoid must bepowered by a separate 24 volt power source. Thusan additional device is required to monitor thecurrent to the digital valve controller and control thepower to the solenoid.

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PHOENIXSWITCHMCR-2SP/UI

A

C

2

3

D

1

B

+

−

+ −

+

+−

−21

12

8713

FISHER 67CFRFILTER/REGULATOR

AIRSUPPLY

ASCO 8320 SERIESSOLENOID VALVE

(ENERGIZED)

24 VDC POWER SUPPLY

FISHER 164ATHREE-WAY

SWITCHING VALVE

4-20mA

FIELDVUEDVC6000SERIES

OUTPUT

SUPPLY

Figure 5. Lock-in-Last on Loss of Loop Current and/or SupplyPressure for an Assembly with a Single-Acting Actuator

A current threshold switch can be used to monitorthe 4-20 mA signal to the digital valve controller.Upon loss of this signal, or when the signal fallsbelow the threshold, the threshold switch will openan internal relay. This relay, if placed between asolenoid valve and its power supply, will essentiallyopen and close the solenoid valve. The solenoidvalve can be placed in line with the tubing to theactuator to provide the lock-in-last function.

To ensure adequate response time of a lockingsystem to a loss of loop current, a threshold switchwith a maximum deadtime of 0.025 seconds shouldbe used. The locking system will be faster than theresponse of the positioning system to the failure.

One switch that meets the maximum deadtimerequirements is the Phoenix Contact Dual SetpointModule, Model MCR-2SP/UI.

Note

The MCR-2SP/UI switch is notprovided by Emerson ProcessManagement. Order this switch fromPhoenix Contact.

Assemblies with Single-ActingActuatorsAssemblies with a single-acting actuator use a 164Athree-way switching valve in conjunction with asolenoid valve. Figure 5 is a schematic of asingle-acting actuator assembly with lock-in-lastcapability on loss of supply pressure or loop current.Under normal operating conditions power is suppliedto the solenoid valve and adequate supply pressureis available to the switching valve. Upon loss ofsupply pressure, the set point of the switching valveis exceeded and the switching valve trips.

Upon loss of loop current, the relay of the currentthreshold switch opens. This cuts power to thesolenoid valve, causing the solenoid valve to trip.When the solenoid valve trips, the supply pressurethat was holding the switching valve open isexhausted to atmosphere. This simulates a loss ofsupply pressure, causing the switching valve to trip.

Note

Use a single power source for both theanalog output (AO) card providingloop current to the digital valvecontroller and the solenoid valve. Thisensures power will be maintained tothe solenoid valve so long as power ismaintained to the AO card.

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2

3

1

+−

−+

ED

F C

B

A

+

+

−

−

1 2 7

13

8 12

FIELDVUEDVC6000SERIES

4-20 mA

FISHER 377LTRIP VALVE

24VDCPOWER SUPPLY

PHOENIX SWITCHMCR-2SP/UI

FISHER 64REGULATOR

FISHER 252AIR/GAS FILTER

AIRSUPPLY

ASCO 8320 SERIESSOLENOID VALVE

(ENERGIZED)

OUTPUTSUPPLY

OUTPUT

Figure 6. Lock-in-Last on Loss of Loop Current and/or Supplyressure for an Assembly with a Double-Acting Actuator

Assemblies with Double-ActingActuators

The same principle used for assemblies withsingle-acting actuators is used for assemblies withdouble-acting actuators. Figure 6 is a schematic ofthe double-acting assembly with lock-in-lastcapability shown under normal operating conditions.

Note

DVC6000 Series digital valvecontrollers require approximately 0.5seconds to initialize upon power up.During this time, the digital valvecontroller cannot provide control. Inthe case where control of a previouslylocked valve is restored to a digital

valve controller prior to the completionof initialization, the valve will traveltoward its fail-safe position. A relaywith an engagement deadtime, such asthe MCR-2SP/UI from Phoenix Contact,can be used to delay the release of thelocking system until the digital valvecontroller has completely initialized.An engagement delay of 1.0 secondswill adequately minimize the dip intravel.

Note

While the valve is in a locked state, theset point from the digital valvecontroller may change in response tochanges in process conditions. Thischange in set point may result in aprocess bump when control isrestored to the digital valve controller.

Related Documents� Bulletin 62.1:DVC6000 − Fisher� FIELDVUE�

DVC6000 Series Digital Valve Controllers(D102758X012)

� Fisher� FIELDVUE� DVC6000 Series DigitalValve Controllers Instruction Manual (D102794X012)

� Fisher� FIELDVUE� DVC6000 Series DigitalValve Controllers Quick Start Guide (D102762X012)

These documents are available from your EmersonProcess Management sales office. Also visit ourwebsite at www.FIELDVUE.com.

Note

Neither Emerson, Emerson ProcessManagement, nor any of their affiliatedentities assumes responsibility for theselection, use, or maintenance of anyproduct. Responsibility for theselection, use, and maintenance of anyproduct remains with the purchaserand end-user.

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Phoenix Threshold Switch Specifications(1)

Switch Model Number

MCR-2SP/UI−DC

Size

44.5 X 76.2 X 108 mm (1-3/4 X 3 x 4-1/4 inches)

Mounting

Mounts in control room (DIN rail mount)

Wiring

Connect switch in series between control loopand field device

Control Loop + to switch terminal 2Control Loop – to switch terminal 1

24 VDC Power Supply to switch connectionsPower Supply + to switch terminal 8Power Supply − to switch terminal 7

Connect switch in series between the solenoidvalve and 24 VDC power supply

Power Supply + to switch terminal 12Power Supply − to switch terminal 13

Configuration

Set point 1: 15% (3 mA)Engagement Deadtime: 1 second

Switch SettingsSwitch Setting Description

1234

OnOnOffOff

Hys. 2Hys. 1Out 2Out 1

5678

OnOnOnOn

InInSP 2SP 1

1. The Phoenix threshold switch is not supplied by Emerson Process Management. Order the switch from the manufacturer, Phoenix Contact (www.phoenixcontact.com). For completeinstallation details, see the instruction manual provided by the switch manufacturer.

Emerson Process Management Marshalltown, Iowa 50158 USASorocaba, 18087 BrazilChatham, Kent ME4 4QZ UKDubai, United Arab EmiratesSingapore 128461 Singaporewww.Fisher.com

The contents of this publication are presented for informational purposes only, and while every effort has been made to ensure their accuracy, theyare not to be construed as warranties or guarantees, express or implied, regarding the products or services described herein or their use orapplicability. All sales are governed by our terms and conditions, which are available upon request. We reserve the right to modify or improve thedesigns or specifications of such products at any time without notice. Neither Emerson, Emerson Process Management, nor any of their affiliatedentities assumes responsibility for the selection, use or maintenance of any product. Responsibility for proper selection, use, and maintenance ofany product remains solely with the purchaser and end-user.

�Fisher Controls International LLC 2006, 2009; All Rights Reserved

Fisher and FIELDVUE are marks owned by one of the companies in the Emerson Process Management business division of Emerson Electric Co.Emerson Process Management, Emerson, and the Emerson logo are trademarks and service marks of Emerson Electric Co. HART is a markowned by the HART Communication Foundation. All other marks are the property of their respective owners..