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Jul
18

Commissioning Control Systems – Valve Positioners

When commissioning a control system, it is important to be aware of whether a regulating valve is equipped with a positioner. If a valve does not have a positioner, then as part of loop commissioning it may be necessary to install a positioner on the valve to achieve the desired control performance. For various reasons, a valve may be installed without a positioner. In this case, the valve will usually be fitted with an electro-pneumatic transducer, an “I-to-P,” that takes the 4–20 mA current signal from the control system and, based on the signal value, provides a 3–15 psi signal to the valve actuator. An example of a valve with an electro-pneumatic transducer is shown below.

The cost of an I-to-P transducer is much less than the cost of a valve positioner. However, in such an installation, there is no means to automatically adjust the air pressure to the actuator to ensure the required position is maintained by the valve. Consequently, it is common to have large offsets between the target valve position requested by the control system and the actual valve position. This offset is often caused by the force needed to overcome the friction of the valve packing.

As explained in chapter 5 of Control Loop Foundation – Batch and Continuous Processes, valve manufacturers’ guidelines on when to use a positioner versus an I-to-P transducer have changed dramatically over the last 40 years. In the early 1970s, many manufacturers of valves recommended valve positioners be used with only a limited number of applications. Plants constructed in that timeframe may still have valves that do not have a positioner. However, based on field experience, the recommendation today is a positioner should always be used with a valve. An example of a valve with a positioner is shown below.

If the valve is sticking, then it may not move as requested in response to a change in the control system input unless the valve has a positioner. Therefore, a positioner should be installed if the valve is to be used in a control application. There are very few installations that would not benefit from all valves being installed with a positioner.