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Users did not realize the flow characteristic was sometimes quick-opening and in many cases, too flat for large openings. Even more insidious was that the backlash and stiction was more than an order of magnitude larger than valves originally designed for throttling service. Adding insult to injury, poor precision pistons (e.g., link arm, rack and pinion, and scotch yoke) and less expensive positioners (e.g., spool instead of relay type) were used that deteriorated the resolution by a factor of five or more. The result was a dramatic increase in nonlinearity, dead time, response time, dead band and stick-slip.
Users were clueless as to the source of the limit cycles and increased process variability. Pneumatic positioners offered no position readback. Even more deceptive was the fact that many installations of smart positioners had actuator shaft rather than internal closure member position feedback. The actuator shaft would move even though the closure member did not. Diagnostics and response test metrics from the positioner said things were not too bad. The smart positioner was basically lying.
Nearly all control loops in the chemical industry depend upon the manipulation of flow by the use of a final element such as a control valve. It's generally taken for granted that, when a controller changes its output, there's an actual change in the position of the closure member of the valve (plug, ball or disk). However, the specification of control valves doesn't adequately emphasize the very basic requirement that the positioner respond in a timely manner or even at all, and this has resulted in shortcomings that introduce variability into the process.
Before the advent of smart HART and fieldbus positioners, feedback measurements of position were rare because a separate position transmitter had to be installed and wired. The user generally wasn't aware that differences in valve, actuator and pneumatic positioner design were the source of cycling in the process.
Typically, besides traditional factors such as size and materials of construction, control valve specifications have focused on minimizing leakage through the valve at shutoff and emissions to the environment from packing. Too often, to reduce project costs, plants pick on/off valves to address requirements. This can create performance problems that can't be fixed simply by adding a smart positioner. While installing a smart positioner is always beneficial, an incorrect feedback mechanism in the valve design can give a false indication of performance.
To avoid problems, always consider five basic valve requirements—linearity, dead time, response time, resolution and dead band. They can give crucial guidance and justification for a final element that leads to better control. Rangeability and sensitivity also are important, but, as we'll see, properly meeting the other requirements will address them.
To get on a common basis, we need to define process gain for a self-regulating process as the final percent change in the controlled variable divided by the percent change in valve position. Note that the calibration span of the transmitter for the controlled variable is a factor. Because the changes seen in data historians for process variables are in engineering units, they must be converted to percent of scale. The maximum allowable controller gain is inversely proportional to the process gain. The process gain for flow is the slope on a plot of percent flow versus percent valve position (travel).
The plot should reflect the installed flow characteristic, not the inherent trim characteristic. This accounts for the reduced pressure drop available to the control valve at higher flows, because of the increase in pressure drop in the rest of the system from frictional losses and a decrease in pump discharge pressure. The changing valve drop makes an equal-percentage trim more like a linear characteristic, and a linear trim more like a quick-opening characteristic. The effect increases as the valve pressure drop as a percent of the total system pressure drop is decreased.
In Figure 1, we see the process gain gets too low for travel above 80% of a sliding stem valve. The control loop must make large changes in position to change the flow. For similar conditions, a ball or butterfly with a 60° maximum rotation would see a corresponding excessive loss of sensitivity at about 60% travel, a typical problem for high-capacity valves.
If the pressure drop across the control valve is large compared to the pressure drop in the rest of the system, as in pressure letdown, reagent, surge and vent valves, the installed characteristic is the inherent characteristic. For an equal-percentage trim, the nonlinearity is extreme (process gain can change by a factor of 50) because the slope of the characteristic is proportional to flow. If a pH loop directly throttles a reagent valve on a static mixer, this change in slope on the valve characteristic compensates for a change in process gain for pH that is inversely proportional to flow.
A quick-opening trim characteristic provides initially a very high process gain followed by a very low process gain. This nonlinearity is accentuated in the installed characteristic and is generally undesirable because it magnifies resolution problems near the seat, and causes an excessive loss of sensitivity even at mid-range throttle positions. Pinch valves and isolation valves designed for on/off service tend to have this characteristic.