Some think their existing conventional valve positioners provide the accuracy needed. However, Jo Kirkbride, Actuator & Positioning Product Manager for ABB, argues that there are too many other advantages at stake to ignore the drive to go digital.
The spread of communications technologies such as fieldbus means that smart valve positioners have really come into their own in recent years. Virtually all new-build control schemes will now incorporate smart positioners, while most existing installations tend to upgrade to smarter systems whenever their old analogue controls need to be replaced. Whether it’s a new-build project or an upgrade, the advantages of going digital are clear.
The benefits begin with installation and commissioning. Unlike conventional positioners, which must be mechanically configured in a process that can take up to three or four hours per valve, smart positioners use an autostroke routine to set themselves to the valve in as little as two to three minutes. Since many large sites will have hundreds of valves, this can have a significant impact on scheduling for a new-build or downtime for an upgrade project.
In addition, many valves may be sited in inaccessible locations or an inhospitable environment, which means that the ability to communicate with them remotely from set-up onwards will yield significant savings. Smart positioners are typically equipped with at least one communications protocol and sometimes more. For example, ABB positioners can communicate using 4-20mA, HART, Profibus or FOUNDATION Fieldbus.
Because they are controlled electronically and offer a much higher data resolution, smart positioners are less vulnerable to drift and tend to stick closely to their programmed tasks over a longer time than traditional positioners. This is also due to the fact that the components are generally smaller, which makes them less vulnerable to disturbances such as plant vibrations.
This feature, coupled with auto-stroking, means that many users of smart positioners find that they never have to reset their units after commissioning.
Even if they do need attention, however, smart positioners can be recalibrated at the touch of a button using the same routines as commissioning. This goes even further towards minimising any downtime.
Reduced air costs
Cutting energy costs is paramount in today’s climate of escalating prices. Any chance to reduce the energy used to generate compressed air is especially valuable, since around 90% of the energy consumed by a compressor ends up as waste heat, not compressed air. This effectively makes compressed air ten times more expensive than electricity.
So it’s important when choosing a smart positioner to check its air consumption. The following example provides an idea of the type of savings that can be achieved using a smart positioner.
Let’s assume that we have standardised on one of the top five positioners in the UK market and that these positioners spend 50% of their operational time in a steady state position (steady set-point). Assuming a compressed air cost of 1p per scfm (standard cubic feet per minute), the cost of this wasted energy, in the worst case, can be as much as £2,106.78 per year.
Now let’s assume that we replace these positioners with an ABB TZIDC smart positioner. Based on the above, the intelligent control of compressed air consumption provided by the TZIDC will save between £500 & £1,200 per year per positioner.
In many instances, particularly those with fairly constant or predictable loads, the amount of wear on control valves is likely to be very limited. For such applications it is usually sufficient to use positioners that are typically expected to handle 100,000 strokes. However, for processes where changes are likely to be more frequent or less predictable or where accurate control is critical, it is essential to use equipment that can deliver a consistently fast response for as long as possible.
Compared with manual control systems, today’s smart positioners provide scope for significant maintenance cost savings. For a start, remote communications mean that engineers no longer have to physically check every unit to pinpoint those that need attention.
Moreover, some of today’s smart positioners now also offer data gathering and diagnostic capabilities that can help to reduce unplanned maintenance to an absolute minimum. A simple example might be counting valve open/close cycles in order to predict the amount of wear on the valve trim, stem or seals.
When they first arrived on the market, smart positioners found a home predominantly in industries with the most to gain, such as large chemical plants.
However, their entry was restricted when it came to certain applications, such as those in hazardous areas where all-pneumatic systems were still the norm. That is no longer the case.
For example, ABB’s range, including its new EDP300 positioner, features an intrinsically safe, stainless steel Exia-rated version and SIL2 variant for safety-critical applications.
The continuing developments in smart positioner technology, coupled with the benefits of enhanced cost efficiency, greater accuracy and reduced maintenance, make them increasingly attractive for both new and retro-fit installations in all industries.
Furthermore, their ability to offer improved energy performance over traditional devices makes them ideal for businesses looking for new ways to beat the continuing escalation in fuel prices currently affecting UK industries.