Improve control loop performance

Specification of control valves doesn’t adequately emphasize the very basic requirement that valve position respond in a timely manner or even at all — leading to process variability.

By Gregory K. McMillan, Emerson Process Management

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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 [3, 4].

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 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. Various positioners on small actuators all exhibit much longer times for smaller step changes. Source: Ref. 3.

Figure 2. Various positioners on small actuators all exhibit much longer times for smaller step changes. Source: Ref. 3.

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 [5].

Achieving fine results

Resolution and deadband 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.

A more insidious source of resolution problems is the high seat or seal friction particularly associated with valves designed for isolation. On/off (block) valves aren’t control valves and vice versa. If an application must prevent leakage, install an on/off valve whose action is coordinated with the opening and closing of a control valve. If a valve must operate near the seat due to rangeability requirements, it’s essential to ask the vendor for resolution measurements near the seat.

Don’t rely on stated resolution because this normally is for the valve at mid-position where the seating or sealing friction is lower. High friction and the discontinuity in process gains near the seat cause loops to oscillate around the split range point where there is a transition from one valve to another [6]. The realistic rangeability of a control valve (the largest flow divided by the smallest controllable flow) is set by the installed characteristic and resolution near the seat. Furthermore, repeatability isn’t as important as simply the ability of the valve to respond, as the process loop will correct for changes in the magnitude of the response.

Deadband is the smallest change in signal in the opposite direction that will result in a change in position. The deadband also is known as backlash and lost motion because it primarily originates from shaft connections and linkages. It’s particularly noticeable in rotary valves when there’s a translation of linear motion of an actuator to rotary motion of the ball or disk. Don’t count on rotary actuators as a solution; these actuators typically have been designed for on/off service and consequently don’t generally provide adequate resolution and deadband specifications. The effect is aggravated by high sealing friction and may result in shaft windup where the actuator shaft twists but the closure member is stuck and then considerably overshoots the desired position when it breaks free.

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