Is Your Control Valve an Imposter?

Beware the Disguised Positioner

By Greg McMillan

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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.

Turning on a Dime or at Least a Quarter

Dead time and response time quantify dynamic response. The dead time, Td, is the time to a first change in closure member position after a change in signal. The response time, T86, is the time required for the position to reach 86.5% of its final value and includes the dead time. These parameters are defined in the ISA standard and report for the test and measurement of the response of the complete control valve assembly.

If you're looking just at the actuator, you can estimate the pre-stroke dead time and stroking time from an individual fill or exhaust parameter for the actuator type, and volume divided by the fill or exhaust flow coefficient for the positioner, I/P or booster. The response time for large changes is estimated as 86% of the desired change in valve position (%) divided by the slewing rate (%/sec.). For changes between 1% and 10%, the actuator response time becomes relatively fixed, except for large valves and dampers.

It always was rather obvious that large valves were slow because it takes time to fill or exhaust enough air in a large actuator volume to make a change in actuator pressure large enough to overcome the torque or friction load to move the closure element. However, until recently, it wasn't known that the response time of even small valves was dramatically slower, to the point of almost no response, because of the design of pneumatic positioners.

Figure 2 shows how the response time increases to 10 to 100 seconds from 1 to 2 seconds as the size of the step change in signal decreases below 1%. Note that in this plot we are primarily seeing the effect of the positioner because the step changes aren't big enough to see stroking-time limitations. Spool-type positioners have an extremely slow response, to the point of no response for changes less than 0.2%. The response time for even good positioners can rise by an order magnitude for small steps.

HART and fieldbus digital positioners generally have eliminated this positioner resolution problem. When you also consider that pneumatic positioners tend to lose their calibration and have no position feedback or diagnostics, there's considerable justification in terms of performance and maintenance for replacing such positioners.

In the days of analog control, a guideline advised using boosters instead of positioners on fast loops. With digital process loops and smart positioners, this no longer is an issue. It's essential that every control valve have a smart digital positioner. A booster, if needed on a large actuator to reduce pre-stroke dead time and stroking time, should be installed on the outlet of the positioner with the booster bypass adjusted to prevent cycling by allowing the positioner to see a small portion of the actuator volume.

Achieving Fine Results

Resolution and dead band play a crucial role in valve response and highly depend upon the total valve package. Resolution is the smallest change in signal in the same direction that will result in a change in position. Pneumatic positioners can adversely affect this, but an even bigger potential problem originates from friction in the packing and seating through a behavior known as stick-slip, where the valve closure member doesn't move (sticks) and then breaks free and jumps to a new position (slips). Older designs of high-temperature and environmental packing, as well as manual tightening of the packing beyond specifications can cause the resolution to deteriorate to 10% or worse.

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