How To Tell When Process Instruments Need A Healthcheck

By Dr Jonathan Farrington, Technical Manager – Service, ABB Measurement & Analytics

It is easy to take the efficiency of your installed instrumentation for granted. After all, if it’s measuring, it must obviously be working. Yet the reality is that this assumption could actually be losing you money. Not only could you be losing potential revenue through impaired or lost production, but the cost of rectifying an instrument problem could often end up costing you more than if you had simply serviced the device throughout its lifetime. Moreover, an impaired measurement could also affect your ability to comply with environmental and safety legislation, exposing your organisation and its representatives to legal action and potentially ruinous financial penalties.

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Constraints on costs, limited in-house technical resources, a lack of technical expertise or installations being located in hard to reach areas are just some of the common factors that can lead to instruments not being maintained in accordance with a manufacturer’s guidelines. Where sites have had multiple owners or frequent changes in maintenance teams, for example, it is also not unknown for documents for specific instruments to be lost, or for users to lose track of where devices are installed.

The following are some suggested pointers to help you assess whether an instrument may be in need of a health check, which may highlight the need for servicing, upgrading or replacement with a better alternative.


Calibration – are you doing it properly?

Correct instrument calibration is vital for accurate and repeatable measurement performance. Instruments such as pressure and temperature sensors and transmitters and flowmeters will all have been calibrated when they were manufactured to check their performance under a known set of operating conditions.

Although this calibration will be valid when the instrument is first installed, it cannot be assumed that it will remain so indefinitely. Arduous processes especially will cause instruments to drift, such that a failure to routinely take a device out of service and calibrate it could lead to a measurement error. Drift is also more common on older instruments compared to the new generation of instruments, which feature improved electronics, with self-checking routines built-in, and a more robust mechanical design. Even these devices will still need to be checked, as their electrical components can be affected by small chemical and / or physical changes with time, resulting in unavoidable long term drift.

The calibration of an instrument can also often be compromised as soon as it is installed. In most cases, installers will calibrate a device to the installation using their own devices, effectively over- riding the original factory calibration. The resulting new calibration will only be as good as the devices they are calibrating against, which may themselves not be properly calibrated.

It is important to be aware that any of these factors could affect a manufacturer’s guidelines when it comes to calibration frequency. Even where a manufacturer recommends a longer period between calibration checks, the installation environment can impact on the performance of the transmitter and/or primary sensing element. In such applications, more frequent calibrations, or at least inspections, may be necessary.

Standards such as section 7.6 of ISO 9001:2015 oblige companies to maintain and calibrate their measurement instruments on a regular basis, the frequency of which should be dictated by the specific requirements and demands of the application. Additional requirements include the need for instruments to be clearly labelled with information including calibration status, the date when the next calibration is needed, and the need for protection against accidental damage and deliberate interference.

Instruments must also have been calibrated by a qualified testing and calibration laboratory. ISO/IEC 17025 stipulates key management and technical requirements for ensuring that laboratories are operating the correct quality management systems and that any tests and calibrations are performed to the correct accuracy and reliability.

Where multiple instruments are used, for example for redundancy in safety critical processes, it is also advisable to check the calibration of each instrument and recalibrate if necessary, with the frequency of checking being determined by the characteristics and operating requirements of the application. This will avoid measurement discrepancies between each instrument and ensure that each instrument is operating accurately and safely.

Depending on the type of instrument and the nature of the production process in which it is being used, it can be desirable to be able to check and adjust its calibration without having to remove it from the line. This not only prevents disruption caused by removing and replacing the instrument, but also helps to avoid the introduction of any external factors that could affect its calibration.

While the calibration of many types of instruments can be verified in-situ, it should be emphasised that verification is not the same as calibration, and should never be considered as a replacement. If the verification reveals that an instrument’s calibration has wandered significantly, then it must either be returned to the manufacturer for recalibration or replaced.

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Are your instruments giving you the data you need?

Another area to look at is whether your installed instruments are delivering the right information. A study by the ARC Advisory Group estimates that global process industry losses due to unscheduled downtime and poor quality are in the order of $20 billion, or five percent of annual production. Of these losses, almost 80 percent are avoidable and 40 percent are attributable to operator error.

ABB’s own studies highlight that as many as 35 percent of trips into the field are for routine checks, 28 percent are for non-existent problems, 20 percent are for calibration shifts, six percent are fo ‘zero off’ issues and six percent for plugged lines.

Only four percent are actually for failed instruments. From these figures, it is easy to get an idea of the potential savings that could be achieved by equipping engineers with the knowledge and equipment they need in advance, rather than having to visit one or more devices to diagnose a problem.

One way that this can be achieved is through the use of smart instrumentation. Developments in communications technologies present new possibilities for gaining quick and easy access to an expanded range of instrument data.

In particular, the latest generation of smart instruments offer a range of capabilities that were not previously possible in conventional 4-20 mA analogue devices.

Foremost amongst these is the ability to obtain a clearer picture of what is happening in a process. The ability to convey a greater range of data from a device at faster speeds over a digital network enables real or near real-time data to be obtained which can be used to identify areas for possible improvement. When connected to a process visualisation system, smart instrumentation can provide an immediate overview of conditions, making it easier to make informed decisions about areas for greater efficiency.

Another benefit of smart instruments is their self-diagnostic capabilities. By assessing every aspect of the instrument’s performance and pinpointing potential issues before they develop, this technology can help enable improved deployment of engineering resources.

It is true that the added time and effort needed to configure and connect smart instruments to fieldbus systems means that they can cost comparatively more to install than their analogue counterparts. It is therefore necessary to understand in advance the potential scale of savings that could be achieved by using smart instruments, which can then be weighed against the cost of installing them.

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Could your measurement devices be causing you problems elsewhere?

Frequent or unexplained plant breakdowns can often be symptomatic of a measurement problem. Although an instrument may appear to be in good working order, it may actually be generating spurious information that can impact on equipment or processes further down the line.

This can be avoided through performing regular inspections, particularly where the measurements are critical to a production process. These inspections should include both a physical examination of the instruments themselves, and also any recorded data that can be used to check for deteriorating measurement performance.

In installations using older instruments, it may also be advisable to either upgrade or replace them with the latest technology.

Equally, problems with instrumentation may also be due to the characteristics of the installation environment. High vibration, extreme ambient temperatures and continuous exposure to the weather, can all impact on instrument performance.

Are your devices as accurate as they could be?

In the same way as calibration, the long-term accuracy of a measurement instrument cannot be taken for granted. One way is to check the current performance against the original specified accuracy. Referring to the original specification and/or historical data collected from the device can help to pinpoint any divergences in measurement performance, which can then be addressed if necessary. This may include recalibrating it, or, where the problem is due to wearing or failure of components, repairing or replacing it.

Another way is to verify a device by comparing it against a device with a known accuracy. This may be a device specifically for verification, or an identical ‘mirror’ device that can be set up to temporarily measure the same process, with the readings being compared to the device under test.

Are you complying with the latest legislation and standards?

Ensuring compliance with the latest legislation is essential in order to avoid the risk of financial penalties, plant shutdown and, in extreme cases, criminal prosecution.

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Typical areas to look at when assessing the capability of instrumentation to meet legislation include:

Is the equipment fit for purpose?

Is the equipment considered as a best available technique?

 Is the equipment suitably certified for use in the application?

Are there any special requirements relating to operation and/or maintenance of the equipment?

 Is the equipment covered by a documented maintenance strategy, stipulating the frequency of

   maintenance, the persons responsible and the scope and limits of any maintenance work?  

 Is there a maintenance history for the installed instruments, detailing the frequency and nature of    

   any work carried out?      

Does the measurement data need to be collected for reporting purposes?

 Is there a requirement for redundant measurement?

By considering each of these areas in turn, a health check strategy can be formulated to help ensure compliance.


Summary

With cost pressures and a shortage of sufficient skilled staff to carry out regular maintenance and inspection, it can be difficult to give every installed device the attention it needs. Turning to the instrument supplier or manufacturer for help can often provide the answer. Their in-depth understanding of their equipment and the conditions under which it can be used, coupled with their expertise and knowledge of the latest standards and legislation, means they are well-placed to help you find ways to optimise the efficiency of your measurement devices.

In conjunction with  other organizations,  ABB offers a broad range of health check and life cycle services to help you get the most from your installed instruments and analysers

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