Does Flow Loop Make Valve Positioner Unnecessary?

by | Feb 10, 2015 | Valves, Actuators & Regulators | 0 comments

In the Process Control LinkedIn group, a question was asked:

Positioners seem very popular now, but most lines with valves have flow meters attached. Won’t a flow loop make the positioner redundant?

When asked for clarification, the original poster explained:

As far as I understand them, a positioner simply moves the stem to a particular position governed by the control signal (unlike a ‘non-positioner’ valve that might not always produce the same stem position for a particular signal, and probably won’t get there as quickly). However, the purpose of the valve is to deliver a particular flow, not set a particular stem position, so a flow controller (I think) is a better way to achieve this. Cascading a flow controller onto a positioner is pointless as the secondary (positioner) loop is where the dynamics are.

This question spurred quite a bit of discussion, some heated, with 85 comments as I currently write this post. I wanted to highlight two posts that have been submitted but not yet posted. The first is from Greg McMillan:

Gregory K. McMillan

Leading Process Control author, Greg McMillan

A feedback loop in the process industry should have a valve positioner and a flow measurement. The use of external reset feedback and the positive feedback implementation of the integral mode will prevent an upper PID output from changing faster than a lower PID (e.g., secondary flow) can respond and will also prevent this lower PID output from changing faster than the control valve can respond (given fast readback of actual valve position). Thus, the violation of the cascade rule does not cause excessive oscillation.

I have never seen a case where a positioner should not be used. The high gain smart digital valve positioner reduces the amplitude of deadband from backlash, eliminates confusing offset during manual operation, deals with the integrating response of actuator pressure, prevents positive feedback from a booster and diaphragm actuator (potentially dangerous situation), provides ability to tune for different types of actuators, provides readback of actual valve position for visibility and external reset feedback.

The addition of a volume booster on the output of the positioner can reduce valve dead time to less than 0.1 second and increase slewing rate to 100% per sec. If this is not fast enough, then a pulse width modulated variable frequency drive (VFD) is used with speed to torque cascade control in the drive. There are many design considerations with a VFD not commonly recognized. Most overlooked is the severe loss in turndown when the static pressure approaches the total head.

I am an advocate of flow loops as well. The flow loop compensates for pressure disturbances, provides better regulation of process stoichiometry leading to better composition control by the use of lower flow loops and coordinated flow ratio control, and more accurate feedforward control to preemptively correct for feed and utility disturbances (e.g. flow and temperature changes). For startup, the cascade control system can be operated with just the lower loop in service (e.g. flow ratio control) until operating conditions are reached.

However, a secondary flow loop may cause the PID output response to be slower than desired gas and liquid pressure control. Here the PID output should go directly to a fast valve (booster on the output of a positioner) or a VFD. I would still have a flow measurement for better process knowledge.

Flow measurements enable closure of material and energy balances, tracking down disturbances, better inputs for data analytics and verification of process simulations, online process metrics, and an accurate relative gain analysis (RGA) for interactions.

See my February 9, 2015 Control Talk Blog post Flow Control and Valve Positioner Tips on the Control Global website for more details and case histories including the unsafe case of a booster without a positioner.

Emerson’s Mark Coughran added to the discussion highlighting specific control scenarios:

Emerson's Mark Coughran

Mark Coughran
Process Control Consultant

The idea that Bruce raises may be theoretically possible given only the dynamic considerations cited. And in the 1970’s literature it was arguable for the positioners of the time. However, in practice the static requirements trump the dynamics. Final control elements require small dead band, resolution, and repeatability for a variety of reasons. For a pneumatic control valve, this can only be accomplished with a valve positioner. Let us assume that the loop performance requirements include holding the process value (PV) close to the setpoint.

I think everyone on this thread has agreed that valve positioners are useful when the process dynamics are relatively slow. This will include temperature, level, and large-volume gas pressure processes, and any case where the process transmitter has a time constant of two or more seconds.

Second, also concerning the dynamic response, there is a very large fraction of loops that operate in a DCS with an execution interval of one second or slower. In this case it would not be wise to try for a loop time constant (Lambda) faster than three seconds. So here again, the valve positioner achieving its loop response in less than one second is helpful and appropriate in the cascade.

Composition control is often key to the profitability and safety of the plant. Control of pH, density, excess O2, and many others requires good static performance (dead band) hence valve positioners.

As a practical matter of hardware, the following valve/actuator applications require a positioner: springless actuators—piston and some rotary vane actuators; rotary valves with nonlinear relationship between actuator pressure and rotation; actuator pressure higher than an I/P can deliver (very common today); bench set as discussed by others; nonlinear characterization of the signal-to-travel; plants that use diagnostics for preventive or predictive maintenance.

For more subtle reasons, the following valve/actuator applications require a positioner: high friction in the valve packing, valve guiding, or actuator; split-range loops; valves that have a position transmitter for display in the control room; large piston actuators with volume boosters; stroking time (100% travel) specified as five seconds or less.

This leaves only the case of a flow or liquid pressure loop containing a fast process transmitter, small globe valve, PTFE packing, low-pressure spring & diaphragm actuator, and a PID running faster than 1/sec (probably in a single-loop or PLC controller). 20 years ago I tried this experiment in a laboratory flow loop. I encourage Bruce to try the experiment and compare an I/P to a modern positioner.

I do not see a positioner as merely a PID controller applied to a valve position loop. A modern positioner is a motion controller with feedback loops on position, velocity, and acceleration. It uses two stages of amplification (signal relay and power relay) and other tricks to cope with friction, impedance matching to the actuator, and other needs already discussed. An I/P only provides the signal amplifier piece. The positioner enables the valve to move quickly so that the process PID controller can handle the easy dynamics—self-regulating or integrating, one- or two-dominant lags. Friction is not an easy dynamic for a simple PID controller.

Process/Industrial Instruments and Controls Handbook, 5th EditionMark wrote a chapter, Control Valve Response, for Greg’s book Process/Industrial Instruments and Controls Handbook, 5th Edition written in the late 1990s. It is an excellent resource to gain a deeper understanding of the dynamics of the control valve in relation to the dynamics of the process being controlled.

Add your thoughts to this LinkedIn discussion or join other valve and process control experts in the Improve & Modernize and Valve Controllers & Positioners groups in the Emerson Exchange 365 community.

Update: I received some great thoughts from Emerson’s Norman Render that I wanted to share with you:

Emerson's Norman Render

Norman Render
Sales Development & Support Director

Mark’s final comments about friction not being an easy dynamic for a simple PID controller is so true. What must also be considered in the real world is that friction will change over time, posing even more challenges for the simple PID controller. Stem wear or more likely tightening of the packing (to stop emissions) will all alter the loop dynamics.

Modern digital valve controllers can measure the friction during operation and alert the operator if it goes outside set limits. Equally they have adjustable tuning parameters to enable the control valve response to be tuned to the control loop needs. Good control valve performance can dramatically improve loop and therefore plant performance, so replacing older style positioners or I/P’s can dramatic ROI.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.

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