Product category:
Valve positioners and actuators
News Release from: K Controls | Subject: Valve Position Transmitters
Edited by the Processingtalk Editorial
Team on 04 February 2008
Review of uses for "Valve Position
Transmitters"
Position transmitters are used to monitor the position of modulating control valves or the opening and closing characteristics of critical isolating valves
The information from the transmitter can then be used to diagnose valve performance in order to warn of the requirement for early preventative maintenance or to automatically adjust various control parameters in the positioner or controller The ultimate objective is to improve process quality and increase plant throughput
This article was originally published on Processingtalk on 16 Aug 2005 at 8.00am (UK)
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Deteriorating valve performance can also lead to environmentally damaging fugitive emissions.
Closed control loops will try to compensate for deteriorating valve performance but eventually it will be impossible to maintain a given set point.
Backlash in mechanical linkages and stiction in valve glands and seats can be detected by tracing the valve response to a changing input signal.
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The trace of the valves actual position relative to a steadily increasing input signal can be recorded when the valve is new and then compared to other movement traces recorded throughout its life.
It is also possible to check that a control valve has been sized properly when first installed.
For example if a valve is only controlling by spending most of its time close to the fully open or fully closed positions it is likely that it is too large or too small and control will be compromised.
Speed of response, hunting or overshoot can also be monitored with a position transmitter.
If a control valve responds to a signal change too slowly, hunts, or overshoots the desired position significantly before settling into control, then process quality and overall system performance will suffer.
Signal increments need to be greater than those used when testing for backlash or stiction.
Speed, hunting and overshoot problems could imply air leakage, incorrect actuator sizing or inappropriate positioner gain settings.
It is also possible to use the position transmitter to trace valve position against flow rate.
A control valve can continue to function well without backlash, stiction, actuator or positioner problems but gradual wear of the plug and seat in the valve or a build up of deposits in the valve or surrounding pipework can affect performance of the control loop.
Similar movement traces can be produced for critical isolating valves.
Limit switches can confirm that the desired position has been reached but a position transmitter can determine if the valve is moving smoothly or hesitating somewhere in mid travel.
Limit switches can also be fitted in a position transmitter.
A valve will no longer control when it is fully open or closed although the control loop will keep trying.
This can cause the control algorithm to "wind-up".
This in turn can cause the valve to oscillate for a while even when the process variable returns to the controllable range.
Some controllers can adjust the algorithm to eliminate control "wind-up" when end of travel is detected via the limit switches.
There are a number of different types of position transmitter.
The simplest is a potentiometer driven from the actuator via a gear set.
A voltage is applied across the varying resistance of the potentiometer and the resultant current is measured to determine valve position.
The potentiometer can be gear driven through a clutch drive system which prevents damage to the potentiometer and gears through over travel of the input shaft.
Limit stops are provided within the drive to ensure the wiper in the potentiometer remains within the limits of electrical rotation.
The range of resistive output will be a ratio of the mechanical rotation to the electrical rotation.
Temperature and the length of interconnecting cable can affect the resistance of the device and for this reason it is recommended the potentiometer be used as a voltage divider to eliminate the effects of these fluctuations.
To avoid the effects of ambient electrical disturbance, screened cables are recommended when connecting the potentiometer to the associated equipment.
When adapting to linear applications lever arm linkage systems are often used and this conversion from linear to rotary motion will have some effect on the linearity of the device.
This rarely has any practical impact on the performance of a control loop.
Backlash in the mechanism can be limited by including a spring within the lever arm that reduces the tolerances in the linkage.
Alternatively a rack and pinion can be used to convert linear to rotary motion.
LVDTs and RVDTs.
Linear and Rotary Variable Differential Transformers can be used in place of potentiometers.
They are accurate and as they do not have contacting surfaces they are not affected by vibration.
However they tend to be expensive and difficult to adapt from one application to another.
An "optical encoder" driven transmitter senses position optically via an LED shining through a slotted disk.
Output is rarely affected by vibration or temperature.
They are expensive and consume a lot of power.
They do not give instantaneous output values.
Condensation on the disk at low temperatures can be a problem.
"Hall effect" transmitters use a magnetic coupling and accuracy is variable.
They do not have contacting surfaces so there is little if any wear due to vibration.
The addition of a resistance-to-current transducer circuit in the enclosure with the potentiometer can overcome many of the problems associated with varying lengths of interconnecting cable.
The transducer is powered entirely from a two wire current loop and will generate a 4-20mA signal that will vary with changing valve position.
A standard twisted pair of wires is all that is required and the signal is relatively immune to noise.
The 4-20mA signal can be matched to the open and closed positions of the valve by adjusting the "zero" and "range" settings on the transducer.
Some versions allow this to be done by simply pushing and holding a button at each end of travel.
As was mentioned earlier, conversion from linear to rotary motion will have some effect on the linearity of the device.
Some 4-20mA transducers can be programmed for linearisation.
Resistance can be plotted against valve travel during calibration.
Once programmed for a given linkage geometry the same polynomial can be used for different transmitters on valves with the same physical characteristic.
A resistance-to-current transducer circuit with a built in HART facility permits data acquisition or transmission to and from the position transmitter.
This is very useful for remote monitoring or calibration.
Digital data is transmitted by superimposing it on top of the 4-20mA analogue signal.
HART stands for "Highway addressable remote transducer".
A hand-held HART calibrator can be connected to the cable at any point between the position transmitter and the control room.
The hand-held device can then be used to remotely calibrate the ends of travel and also input the date of calibration and valve tag number.
The same device can be used to read valve position, calibration date and tag number from the position transmitter.
This can be particularly useful if the position transmitter in located in a hazardous or physically inaccessible location.
Ex certified versions of the hand-held calibrator are available for use in hazardous areas.
Transducers that operate entirely digitally are now available that use the Profibus or Foundation Fieldbus protocols.
These offer similar benefits to HART plus the possibility of multi-dropping a number of position transmitters on a single fieldbus spur.
Some electro-pneumatic positioners are available with integrated position transmitters although sometimes there is a preference for a separate unit driven by a separate linkage on the other side of the linear actuator yoke.
This would enable the position transmitter to detect a failure in the positioner linkage or a failure of the positioner integrated position feedback system.
Some "Smart" positioners have built in software that monitors various actuator and positioner parameters and selects an appropriate algorithm to optimise performance.
Foundation Fieldbus "Smart" positioners are capable of local control (via internal function blocks) and this releases the remote controller to perform other functions.
If Foundation Fieldbus "Smart" positioners are fitted to a control valve from the same manufacturer they are often programmed with detailed information about the characteristics of the valve itself and this can give better local control performance than a positioner sourced from a third party.
Even control valves fitted with "Smart" positioners can benefit from a separate position transmitter.
There can be concerns about using a device which is purely software based to provide position feedback.
The use of an independent position transmitter provides a more robust indication of valve position and can also detect a positioner or linkage failure.
Position transmitters can be engineered for hazardous and or hostile environments.
Carbon steel, coated aluminium or stainless steel enclosures for safe area use or with ATEX/IECEx intrinsically safe Exi or flameproof Exd approvals are available for surface or sub-sea use.
In summary: electro pneumatic positioners, pneumatic positioners or control valves operated directly via I to P converters can all benefit from the data provided to a remote controller by a position transmitter to both improve the performance of the control loop over time and give early warning of deteriorating control valve performance.
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