Fluid Handling: Pick The Proper Positioner

Oct. 11, 2021
The necessary level of communication is an important factor

Positioner selection deserves due diligence — and consideration of several factors before making any firm decision. There’s certainly not a typical one-size-fits-all approach for positioners; an uninformed choice can lead to undesirable consequences. On the one hand, selecting a positioner with features that aren’t helpful for a particular process likely will lead to spending more than necessary. On the other hand, a purchasing decision made solely on price may result in a positioner that either does a substandard job or needs replacing to perform a required function.

Many vendors categorize the positioners in their line as good, better and best. While that’s helpful, it’s not enough to pin down the optimum choice. Having a clear understanding of the requirements of a given process and the features of the positioners available will guide the best decision.

When going through the positioner selection process, you typically should focus on two key elements: control and communication. In other words, what precision do you need in controlling the valve, and how much feedback is necessary during the process?

As far as control, the main objective is to facilitate an environment where the valve performs at its very best for the given application. Several options exist depending on the nature of the process. A basic local loop may be well served by a pneumatic positioner, perhaps interfacing to the controller with a current/pressure (IP) transducer, or by an analog electro-pneumatic positioner. When you desire greater capabilities, consider one of the newer microprocessor-based positioners with advanced internal algorithms that can perform more-comprehensive tasks. For example, they can be tuned to maximize performance or adapt the tuning parameters to the loop as time goes on to improve the responsiveness of the valve.

However, although important, control isn’t something that many operators need dwell upon. As the communication levels improve, there isn’t much fear about inadequate control. In other words, if the communication is robust, the control level will be as well.

Communication Considerations

Determining appropriate communication is much more nuanced. Originally, typical positioners relied on one-way communication; they would act on an analog (3–15-psi pneumatic or 4–20-mA electrical) signal from the controller. Smart positioners with digital communication capabilities started to appear in the 1980s and 1990s. Today, many positioners feature two-way communication: the device gets a signal from the control system about required valve position and also sends back digital information, e.g., actual valve position. Such a feedback signal can allow operators to determine whether the positioner is, in fact, following the system commands. Additionally, diagnostics became available with the smart positioner revolution. Now, plant staff can monitor the internal components of the positioner and even the external behavior of the valve to spot when the valve is behaving in an unexpected way. Some manufacturers offer multiple levels of diagnostics from which to choose.

You should strive for the optimum amount of communication needed for the process. With an under-specified communication level, you might not get notifications of potential problems or the valve might not operate as efficiently as possible. On the other hand, over-specifying the amount of desired communication could be a costly mistake. It doesn’t make a great deal of sense to pay for technology that is either unnecessary or unutilized.

Basic analog pneumatic and electro-pneumatic positioners still can play a valuable role. The technology is reliable and mature — and performs the job it was designed to do very well. For example, many refineries rely on a large number of basic pneumatic positioners that simply take the 3–15-psi control signal from the controller and modulate the air pressure to the actuator. With no electrical components, the devices fit with the old refinery maxim “no wire, no fire.” These and basic electro-pneumatic positioners may adequately handle what’s needed under some circumstances; so, upgrading to more-advanced technology wouldn’t offer the return on investment required to make such an upgrade feasible. Such positioners lack any kind of digital communication or microprocessor electronics and usually provide a basic level of accuracy and repeatability (0.5–1%).

A bit more capable are advanced electro-pneumatic positioners. These use an analog 4–20-mA input, similar to the basic device, but boast microprocessor-based electronics that allow easy configuration and tuning. However, these analog electro-pneumatic positioners still don’t have any kind of diagnostics or digital communications. They do provide a good level of accuracy and repeatability (0.25–0.5%).

The next step up are HART positioners. Such devices come in two basic styles: with zero or just basic diagnostic capabilities; or advanced diagnostics. These positioners usually have a high level of accuracy and repeatability (0.1–0.25%).

HART devices have a digital communications capability thanks to a frequency-shift-key digital signal on top of the 4–20-mA signal going to the positioner. Thus, they are compatible with a 4–20-mA controller. HART positioners are designed to continuously communicate with a control system that has digital communication capability. In addition, someone in the field with a suitable handheld device can communicate with such a positioner to get certain diagnostic information or do an auto-calibration, even if the positioner isn’t communicating with the control system digitally.

Smart HART positioners have the widest range of optional capabilities, including international electrical certifications, “fail freeze” functions that lock the valve in its last position on loss of communications or air pressure, explosion-proof or intrinsically safe construction, and others. One of the most common options available is a 4–20-mA position signal. A device with this option has two additional terminals that provide a 4–20-mA position signal connected to an analog input in the control system to give feedback to confirm the valve is moving to the commanded position.

With a digitally integrated control system, you don’t need the extra analog input because that information is available in the digital data stream being communicated back and forth between the control system and the positioner.

As mentioned earlier, suppliers offer differing levels of diagnostic capabilities in their positioners. Depending on the sensors and internal parts built into the positioner, you may be able to receive information such as the air pressure value, shaft position, housing temperature, etc. Having that digital feedback can foster achieving the best reliability in a control system because operators can determine when certain things of concern are happening. For example, has the air pressure gone away for a particular valve? Is the valve responding correctly? A valve that stops before it gets to the desired position could generate a software alarm that alerts the operator the valve isn’t performing the intended way.

Taking this one step further, you can build diagnostic algorithms inside the positioner or control system that look at all the digital data and trigger an alarm if the valve did not go all the way open. Maybe the valve or actuator has jammed. Maybe air pressure isn’t adequate. Maybe some physical damage has occurred. Positioner diagnostics can warn the plant about any of these occurrences.

Keep in mind that optimal communication and diagnostic ability often can significantly impact the bottom line.

Infrastructure Issues

Unfortunately, limitations due to infrastructure can impact options. For example, a plant may have a variety of different types and generations of input/output (IO) panels that connect to field devices. Sometimes in such cases, an IO panel is an earlier revision that can’t talk to these field devices, precluding advanced communications. A variety of other issues also can affect a plant’s ability to reach that desired communication level. For example, if the plant has a bunch of programmable logic controllers, as opposed to a distributed control system (DCS), they may lack full communication capabilities — and, thus, may not be able to support an advanced diagnostic system that allows for predictive maintenance.

Fortunately, positioners are available with diagnostic and data historian technology built right in. Thus, even if the control system can’t support this capability, those positioners store the diagnostic data internally — which can be retrieved using a handheld device or laptop computer. So, a plant can do predictive maintenance without the expense of a seven-figure DCS system by using a predictive maintenance software application. Or it can upload these data to the control system once full communication capabilities are established, preserving potentially years of diagnostic data.)

The optimum positioner, of course, depends upon the given application. For example, a plant looking into doing more predictive maintenance should select positioners with the level of diagnostics to accurately predict when failures will happen in the equipment. At the same time, perhaps only five loops in the plant may really require the top-tier level of diagnostics. The rest don’t need that level of diagnostics and never will use it, so paying for those capabilities makes no sense.

When determining the level of technology necessary, it’s critical to understand the needs of the system and options available. Every process is a little different, which is why working with a professional who understands all the options available — after developing a deep understanding of a process’ specific needs — is the ideal approach.

PETER JESSEE, PE, is a process control application engineer for Valin Corp., San Jose, Calif. DAVE FAHLGREN is customer service director for Valin. Email them at [email protected] and [email protected].

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