The Epic noncontact position transmitter represents a significant departure from previous attempts to reliably measure stem position of linear control valves. As opposed to conventional devices, no linkages, levers, rotary or linear seals are utilised. A static slide-by actuation assembly attached to the valve stem, is the only 'moving' part in the entire transmitter. Sensing is performed totally by noncontacting means, based upon characterisation of flux strength as a function of axial position.
A solid-state sensor is employed, the output of which is a function of both flux density and excitation voltage. That property allows the use of an internal bridge circuit for highly accurate zero adjustment where one arm of the bridge is comprised of the sensor and the other comprised of a resistive divider for zero reference.
Transmitter output is 4-20 mA, 2-wire, with compliance voltage ranging from 14,5 to 35 V. Output updating takes place 20 times/s, allowing use of the transmitter for applications heretofore not possible with transmitters of other types. The inherent reliability of the transmitter permits use of advanced control strategies where knowledge of valve position can be used in predictive and other algorithms.
Predictive diagnostics
Epic provides operating personnel the capability of bringing a higher degree of intelligence to the final control element. On-board electronics allow remote monitoring and diagnostics that continuously monitor control valve positioning performance, allowing comparison with the ideal. Reporting of deviations affords plant personnel the ability to detect problems before they become serious enough to trigger an alarm or shutdown.
Need for diagnostic capability
In terms of diagnostics, one of the most common problems in control valves is a form of 'stickiness' developing in the packing or guide bushings.
In addition, most pneumatic actuators do not provide infinite positioning 'stiffness'. That is, a sudden decrease of force required to stroke the valve will cause the actuator to stroke further than intended with the actuator air signal held constant.
In practice, the mass of actuator and valve moving parts is low enough that a rapid jump in position will occur due to the fall in friction at the onset of stroking. The jump is rapid enough that the positioner cannot correct quickly enough to prevent the resulting overshoot.
As age, wear and other factors take their toll, the jump effect often becomes more pronounced. The resulting overshoot may become severe enough that the positioner continually tries to correct the situation, resulting in a constant 'hunting' condition.
Aiding maintenance with early detection and diagnostics
With proper signal processing the Epic transmitter is ideal for detecting the onset of such effects. Early diagnosis allows maintenance or other corrective action to be scheduled, therefore avoiding a severe problem. The key in detection lies in determination of maximum rate of position change or alternatively, in a comparison between rate of change of position signal and the actual position change of the valve.
Rate of valve position change may be determined by taking Epic transmitter readings at finite intervals, perhaps once per second. The change in position during each interval may be compared with a maximum allowable value. In essence, the technique described here amounts to taking a first time derivative of position by numerical means.
Valve & Automation
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