K Controls Expands Range of Position Transmitters
The information from the transmitter can be used to diagnose valve performance in order to prepare the requirements 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.
With modulating control valves, closed loop controllers 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 valves response to a changing input signal. 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 the process quality and overall system performance will suffer. 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. The performance of the control loop can be affected by gradual wear of the plug and seat within the valve, or a build-up of deposits in the valve or its surrounding pipework.
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”. In turn, this 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.
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.
Valves are sometimes partially stroked to an agreed schedule and in accordance with planned maintenance procedures. The PLC monitors the movement via the position transmitter and issues an alarm if the time taken exceeds a pre-defined limit. Valve movement can be graphically displayed and any irregular patterns can be investigated. This process ensures the valves are in good working order and ready to react if a programmed shut down of the system is required in the event of an emergency.
Most K Controls position transmitters contain a potentiometer driven from the actuator via a gear set. A clutch drive system 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 resistance of the potentiometer is measured by a resistanceto– current transducer circuit in the enclosure. Noncontact sensing devices are optionally available.
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. The latest versions allow this to be done by simply pushing and holding a button at each end of travel.
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.
Wireless position transmitters are also available where installing hard wired technology would be costly and disruptive. K Controls has a wide range of solutions for use in hazardous or hostile environments. Versions are available with coated aluminium or stainless steel enclosures, Exi or Exd protection for use in Zones 1, 2, 21 and 22 for use with hazardous gasses or combustible dusts and for use in the temperature range -60°C to +120°C. Versions with the following approvals are available: ATEX, IECEX and CSA for Canada and the USA. There are special versions for sub-sea use to a depth of 2000 metres.
If you require accurate valve movement data to both improve the performance of the control loop and or give early warning of deteriorating control valve performance, the chances are K Controls have a solution.
K Controls Ltd
Tel: 01895 449601
Published in Valve User Magazine Issue 20
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