'Smart' Positioning Justifies a Valve Changeout at a Refinery
By J. Thurman Payne, P.E., PMP
Dresser Valve & Controls Div.-Masoneilan Operations
Plant operators are always interested in achieving optimum process yield. Yield improvement is a function of process knowledge and effective implementation of process control. Ultimately, successful process control depends on good measurement and positioning of valves. Both of these functions (measurement and positioning) must be fully integrated to obtain maximum operating efficiencies. A major factor for achieving good positioning is the amount of deadband and hysteresis within the control valve system; i.e., valve, actuation, and instrumentation. Reduced deadband and hysteresis permits more accurate positioning, thus improving total control loop performance.
Deadband is defined as the range through which an input signal may be varied, upon reversal of direction, without initiating an observable change in output signal. There are separate and distinct input/output relationships for increasing and decreasing signals. Deadband produces a measurable phase lag between input and output and is usually expressed in percent of total span.
Hysteresis is defined as that property of an element evidenced by the dependence of the value of the output, for a given excursion of the input, upon the history of prior excursions and the direction for the current traverse. It is usually determined by subtracting the value of the deadband from the maximum separation between upscale and downscale measurements. The distinguishable differences between hysteresis and deadband are the possible reversals of output for any small reversal of input when measuring hysteresis.
A valve with no positioner exhibits a hysteresis-plus-deadband range of 5-10% (of total span). A control valve with fingertight Teflon packing should produce a hysteresis-plus-deadband range of approximately 5%. A valve with Grafoil packing, fully tightened to meet strict environmental emissions, will exhibit a hysteresis-plus-deadband range of 20% to 40%. In general, a ball valve with Teflon packing would exhibit more friction than other types of valves and produce a hysteresis-plus-deadband range of about 30%. Valve hysteresis-plus-deadband range is also dependent upon size and type of actuator.
A valve with a conventional positioner exhibits a hysteresis plus deadband of 1-2%. Conventional positioners typically have a nonadjustable control "gain." This gain is selected so that the positioner will function well in most situations. The target hysteresis-plus-deadband range is 1% of full span, but occasionally a positioner will permit up to 2%. This fixed gain is a weak link in the control system, since it limits the gain of the entire loop. Normally, the same conventional positioner is used on ball valves, eccentric plug valves, butterfly valves, globe valves, etc. Since each valve exhibits different dynamic characteristics, the use of the same conventional positioner will result in various levels of performance.
Autotuning improves accuracy
Masoneilan's Smart Valve Interface (SVI; see Figure), a digital positioner and controller, has an autotuning feature that provides an automated solution for maximizing control valve system performance. Additionally, the operator can manually adjust any parameter for special process-loop conditions. A valve with an SVI exhibits a hysteresis plus deadband of ±0.040%.
Masoneilan's SVI includes an innovative positioner-tuning parameter (Deadzone) that is designed to deal with the stick-slip behavior (hysteresis plus deadband) that is normally associated with a high friction valve. This is particularly important for valves with Grafoil packing. The steady-state control in the SVI is more flexible and minimizes the system deadband.
With this positioner feature, each system (regardless of valve type, valve size, actuator size, etc.) can be individually tuned for optimized performance. This is achievable at the operator level where the positioner can be tuned or device tuning changed if the process loop dictates a need.
The current version of Masoneilan's SVI incorporates these improvements in positioner tuning parameters and controller tuning factors. Recent testing conducted with a competitive valve design revealed high hysteresis plus deadband in the range of 10% with no positioner. This same valve was tested using the original prototype version of the SVI positioner. The system cycled 1% and showed practically no cycling (0.08-0.12%) with the current version of the SVI. For high friction valves, the expected hysteresis-plus-deadband range with the SVI is normally from 0.05 to 0.3% based on our current findings.
SVI Pays Off for a Large Oil Refinery
Masoneilan's SVI provided improved control for a large oil refinery as evidenced by a 12% reduction in the setpoint standard deviation. This provided enough justification for the refinery to replace 265 conventional control valves with new valves equipped with SVIs. The refinery estimates a 10-month payback from their investment, which will permanently improve their process yield.
For more information: Dresser Valve and Controls Div. 85 Bodweil Street Avon, Massachusetts U.S.A. 2322-190. Tel: 508-586-4600, fax: 508-427-8971