MAINTENANCE of Measurement and Control Equipment

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Now that the instruments and control systems are installed and operating, the responsibility reverts to maintenance for keeping the control equipment in good working condition and for ensuring that the operation of these systems meets the design intent. In addition, keep in mind that not only is the maintenance to be done correctly but also that any alterations must comply with all established codes, such as the electrical code in effect at the site.

It’s imperative to underline that the content of this unit is only a memory jogger and should not be taken "as is" because statutory, technical, and corporate needs vary from one site to the other. Post-installation and maintenance requirements vary with plant needs and specifics.

Maintenance activities in general, and in particular for instruments and control systems, consist of a large portion of human interrelations and teamwork. The two main activities of the maintenance team are:

1. plant improvements and modifications (generally a pre-planned activity) and,

2. plant maintenance (corrective and preventive types).

Maintenance is typically done by plant personnel; however, the help of outside contractors is sometimes needed. In such cases, contract maintenance programs with outside contractors are generally implemented.

Maintenance must at all times be kept in the designer's mind when instruments and control systems are being specified and designed. Items that are inaccessible, badly designed, or difficult to calibrate will be poorly maintained, eventually deteriorate, adversely affect process performance, and may end up in the garbage-a waste of time and money and a poor design effort.

Management has a few basic responsibilities to maintenance personnel and to the public at large. They include ensuring and maintaining a safe work environment, as well as providing maintenance personnel with the proper training, tools, and procedures to work safely and efficiently.

Because no product is absolutely perfect, everything eventually fails. The function of maintenance is to ensure the continued, reliable operation of the equipment on demand. It should be mentioned at this point that ISO 9000 states: "Sufficient control should be maintained over all measurement systems used in the development, manufacture, installation, and servicing of a product to provide confidence in decisions or actions based on measurement data." The above definition of measurement systems includes related computer software.

A maintenance shop should be clean and have sufficient tools in good condition to perform the required work. Generally, maintenance personnel will assess their needs based on the scope of their work and responsibilities. Maintenance personnel should always remember that modifications to approved equipment may void the approval of such equipment.

Maintenance activities can be broken down into different steps. This breakdown may be required for estimates, scope of work, and job descriptions. The breakdown is an example that should be adjusted to fit particular applications.

___1 Example of a typical maintenance activity.

The following topics cover common maintenance items. Obviously, not all maintenance items can be covered in this unit. However, the selected items in this unit give a good under standing of the requirements to be covered.


Maintenance is successfully accomplished through a combination of technical know-how and experience. Maintenance and post-installation activities will widely vary from corporation to corporation.

Even within the same corporation, variations from site to site will occur. However, a good maintenance program generally includes:

• an understanding of the maintenance activities,

• a clear definition of the maintenance organization,

• a set of procedures, in conformance with the vendors' recommendations, to maintain all equipment, in particular those performing critical and safety functions or located in hazardous areas,

• a system for the maintenance of records, where engineering data is always kept up to date with the site modifications,

• training (to be provided where needed),

• the availability of required spare parts,

1) Receive a request for maintenance.

2) Select the required procedures, tools, and manpower to do the job.

3) Get a work permit from the operator.

4) Isolate the process.

5) Remove the instrument from the process.

6) Decontaminate the instrument.

7) Perform the maintenance activity, part of which is the diagnosis of the problem.

8) Recalibrate the instrument, which includes:

• collecting the required technical information,

• ensuring it’s the correct information for the instrument in question,

• selecting the calibration equipment,

• connecting the instrument to be calibrated,

• calibrating, and,

• disconnecting the calibrated instrument.

9) Prepare for instrument reinstallation.

10) Reinstall the instrument.

11) Check its correct operation.

12) Advise the operator.

13) Complete the required paperwork.

• cost monitoring,

• accessibility to maintenance manuals for all items of control, and,

• an analysis of equipment histories, accessible through records of calibration and maintenance results, that examines repeated failures.

Types of Maintenance:

The two main types of maintenance are corrective maintenance and preventive maintenance.

Corrective maintenance is performed when breakdowns occur (or are about to occur). It’s an unscheduled activity. Another type of maintenance, known as predictive maintenance, relies on the monitoring of sensors (e.g., vibration, temperature, pressure) to warn that a breakdown will soon occur. In many cases, predictive maintenance is considered part of corrective maintenance.

Preventive maintenance is predictable because it’s scheduled ahead of time and performed generally at preset time intervals. Preventive maintenance reduces downtime by avoiding or reducing the unexpected problems. This type of maintenance requires a set frequency generally linked to scheduled shutdowns.

The debate over the advantages and disadvantages of corrective maintenance vs. preventive maintenance is interesting and ongoing in many plants. On one hand, failures that could occur at the wrong time (as they generally do) can result in the loss of production or, even more importantly, affect human safety. On the other hand, if preventive maintenance is scheduled over too long a period, a breakdown will occur before the preset maintenance interval. If it’s scheduled over too short a period, it will waste money and can even increase the chances of a breakdown due to the increased potential of introducing human errors or defective components. To have the preventive maintenance set at the correct schedule is quite difficult; there fore, the dilemma still exists, and the debate goes on. In any case, preventive maintenance should not be established just for the sake of implementing this method. It should be based on facts and figures.

Preventive maintenance is based on frequency that requires review on a regular basis (annually seems to be a norm). In most cases, the frequency of maintenance is based on the equipment manufacturer's recommendations and past experience through the analysis of maintenance records. If, after a number of consecutive inspections, the equipment is consistently in good condition, then the frequency of maintenance can be eased. A good maintenance program should indicate how preventive maintenance is scheduled and planned (including identifying the need for spare parts).


Some corporations find it essential to identify the maintenance organization. This is done by establishing who does what, how they interface, and what the official lines of authority and responsibility are. This need depends on the corporate philosophy and on the way a company does business. When identifying a maintenance organization, detailed job requirements and necessary skills are defined and can include, For example, loop-tuning skills (see appendix G for job descriptions).

All maintenance activities, including inspection and testing, must be performed by competent personnel. Competency is achieved through proper training that includes instructions on the various types of protection and installation practices and on the general principles of electrical area classification. In addition, suitable refresher training must be provided as needed.

Maintenance personnel have a variety of duties. These include troubleshooting control loops and the removal, repair, calibration, and reinstallation of many components of instrumentation and control hardware.


Through proper training, the maintenance of control equipment can be correctly accomplished.

Maintenance should be performed according to the vendors' maintenance manuals. Training includes both classroom and supervised hands-on experience at the plant with qualified personnel. Specialized equipment, such as analyzers and programmable electronic systems, require specific training. In addition, specialized skills, such as loop-tuning capabilities, are obtained through training and hands-on experience acquired over the years.

Training of maintenance personnel includes many components. Some are obvious, such as technical know-how, while others have more to do with safety. Safety training can include:

• understanding and implementing work permit procedures,

• identifying work hazards and eliminating them where possible,

• mastering the use of tools and protective clothing,

• understanding the function of safety guards, interlocks, safety signs, tags, and barriers,

• avoiding loose clothing, jewelry, and long hair that can entangle in equipment, and,

• reporting accidents and emergency conditions, as well as identifying any limitations that can reduce the safety of a job to be done (including any unsafe activities and any hazards).


Maintenance records are required to support all maintenance activities, regardless of whether they are corrective or preventive. They are used as historical data for reasons that vary from setting the frequency of preventive maintenance to providing legal and insurance documentation. Sometimes maintenance records are coded to facilitate entry into computer systems, where database managers or spreadsheets handle data collection and retrieval.

ISO 9000 states: "Procedures should be established to monitor and maintain the measurement process itself under statistical control, including equipment, procedures, and operator skills." Maintenance documentation requirements vary from plant to plant, but in general, the mini mum includes a copy of maintenance records, all process and instrumentation diagrams ( PIDs), all instrumentation and control documentation, and the area classification documents, as well as a set of vendor manuals.

Maintenance records vary from one organization to the other, but required maintenance data, regardless of the format, includes:

• the tag number or description of the device,

• corrective/maintenance actions on the device,

• spare parts used,

• the name of the person performing the maintenance, and,

• the date of the maintenance.

In the case of corrective maintenance, two additional items are needed: a description of the complaint (or failure) and a description of the diagnostics.

___2 shows a typical example of a completed form used for corrective maintenance.

___2 A typical corrective maintenance form.


Maintenance personnel encounter numerous hazards in their day-to-day activities. Some of these hazards are described in the following pages and are summarized.

___3 Some of the hazards encountered in a plant environment.

Training, constant awareness, and compliance with the code requirements greatly minimize accidents. Before work begins, hazards should be identified, and if they cannot be eliminated, safety barriers should be implemented. The following represents a number of potential hazards facing plant personnel.

General Hazards:

The most common injuries occur due to improper lifting, falls, slips, and falling objects. Maintenance personnel should assess the weight to be moved and be trained on how it should be carried. Where the potential of falling exists, fall-arrest or travel-restraint systems should be used. Falling objects should be prevented by securing items located overhead (such as on scaffolding and roofs), and where this is not possible, the area should be barricaded to prevent access. Maintenance personnel should avoid shortcuts-an accident could be a lifetime of suffering and frustration.

The presence of buried cables and utility lines must be verified before any drilling, excavation, or driving ground rods is started. The use of tools must always be in accordance with the tool manufacturer, including the use of eye-, face-, and hand-protective equipment.

In noisy environments (where the noise level is above 85dBA), hearing protection must be used. In adverse weather conditions (e.g., high winds, snow storms, and electrical storms) out side work should immediately stop.

When spray painting, personnel should ensure good ventilation. Where air breathing is required, only equipment approved for that purpose can be used, and breathing air systems must not be mixed with any other system.

Maintenance personnel should keep fixed and portable fire equipment, as well as all emergency access routes, free from obstructions, such as ladders, scaffolds, and tools. They should not work alone in a battery room (where hydrogen is produced when wet cells are charged), and they should check that the ventilation is working properly before entering.

Improper lifting; Falls; Slips; Falling objects; Defective tools; Noise; Adverse weather conditions; Spray painting; Fire; Poor ventilation; Hazardous locations; Confined space; Live and exposed equipment.

Compressed gas cylinders should be stored and secured in approved racks designed for this purpose. Cylinders should be capped when not in use. Leaky or damaged cylinders should not be used and must be immediately returned to the supplier.

When temporary electrical cables are used, they should be protected from physical damage and, if exposed to traffic, should clearly be identified and protected.

Hazardous Locations:

The requirements and precautions for maintenance in hazardous locations must be kept in mind when setting the guidelines for maintenance. A work permit approved by the operator must always be obtained.

Maintenance work in hazardous areas is generally limited to disconnection, removal, or replacement of control equipment and cabling. Whenever possible, the repair, calibration, and testing of equipment should be performed in a safe area.

Because equipment used in hazardous environments possesses special features, the features must be maintained (e.g., explosion-proof boxes should not be altered or repaired on-site, and intrinsic barriers should be used as recommended by the vendor). The seemingly correct operation of such equipment does not mean that its integrity is protected (e.g., explosion-proof boxes may have dirty or corroded joints, and intrinsic barriers may not be grounded). It’s advisable that the repair of devices that provide safety in hazardous environments be performed only by the original manufacturer of such devices.

Where repair has been done on safety-related equipment, it’s a good plant policy to have a second person (such as a supervisor) inspect the work. If this is not done, the safety and the quality of work cannot be ensured. After all, anyone can make a mistake, but no one can afford deadly ones.

Uncertified electrically powered test equipment (including uncertified batteries) should not be used in a hazardous area unless (and if permitted by code):

• the route is covered by a hazard-free work permit,

• the equipment is adequately protected (e.g., equipment switched off with leads disconnected), or

• the equipment is so enclosed that the risk of it being surrounded by a hazardous atmosphere is insignificant (e.g., by enclosing it in a sealed bag). It should be noted that it may be permissible to use an uncertified voltage indicator to prove the effectiveness of an isolation, providing that the voltage indicator does not contain a voltage source.

Batteries (including lithium batteries commonly used for memory retention) should be handled according to the vendor's recommendations and disposed off in compliance with all the environmental regulations.

Confined Space:

A confined space is a location where the build-up of dangerous gases, vapors, fumes, and dusts or the formation of an oxygen-deficient condition can occur. Maintenance personnel must follow established confined-space entry procedures.

The environment of a confined space must be considered hazardous until proven otherwise by a competent person. Such a person must be trained and capable of identifying and assessing the hazards in a confined space and allow entry after the space is deemed safe or under special controlled conditions. In addition, such a person must have a rescue plan ready in case a dangerous situation suddenly develops.

A trained observer must be assigned whenever maintenance personnel are in a confined space.

The observer must advise the personnel of the potential hazards, remain outside the confined space, and stay in constant communication with the personnel in the confined space, ready to tell them to evacuate at the first sign of unusual symptoms.

Electrical Isolation:

Except for intrinsically safe circuits, instruments and control systems that contain electrically energized components and are located in a hazardous environment should not be opened. To be opened, the electrical energy should first be isolated through either fuse removal or by opening and locking the breaker in the open position.

Maintenance personnel should not work alone near live and exposed electrical equipment.

When protective electrical equipment (e.g., insulator covers and rubber gloves) is used, it should be evaluated by a certified testing laboratory, show no defects, and be used only within the approved voltage rating.

Circuit identification is essential. The necessary up-to-date documentation must ensure that the circuit identification is shown and is correct. This information provides the ability to safely isolate the equipment whenever maintenance is done. Maintenance personnel should always check that the actual tag numbers on the equipment and cables conform to the available documentation.

When equipment is withdrawn from service, exposed wires should not be left hanging loose.

They should be terminated in an appropriate enclosure and insulated. In addition, lock-out and tag-out procedures must be carefully followed.

Lock-out means that the motor starter and/or other sources of power are actually locked with a padlock and the energy source is isolated. Maintenance personnel working on locked-out equipment have their own locks and keys. Tag-out means that a tag is attached to the lock identifying the duration of the lock-out with signatures and dates.

Maintenance personnel should ensure that all activities (including alterations and repairs) com ply with the local electrical code. It must be kept in mind that modifications to approved equipment can void original equipment or system approval and should always be reviewed with the manufacturer.

Programmable Electronic Systems:

The maintenance of programmable electronic systems, including the replacement of components and modules, should be done in accordance with the vendor's recommendations, using competent personnel trained to do this type of work. Maintenance of some equipment can be performed with the equipment online and the power supply connected. This reduces shut downs and saves maintenance time.

Common maintenance issues related to programmable electronic systems (see unit 9) should be part of routine maintenance. They include making sure that ventilation passages are clean and clear of obstructions, monitoring the condition of enclosures, and checking the condition of all grounding.

Alarm and Trip Systems:

In the maintenance of alarm and trip systems, in particular the critical ones, adequate training is essential to ensure their correct operation (see unit 10). Maintenance personnel should have access to procedures and maintenance manuals. The manuals should indicate how the system operates, what its set points are, and what risks are associated with such a trip and its testing and maintenance.

It’s recommended to have another person, such as a supervisor, check all completed work.

This minimizes the possibility of errors, a vital requirement where safety is involved.

Sometimes, a safety trip must be bypassed--for example, when testing it or calibrating its components. In such cases, safety must be maintained. The bypass function should be clearly and constantly indicated to the operator. And all such activities must conform with set procedures.

Safety procedures related to the maintenance of critical alarms and trips should be reviewed on a regular basis to ensure their adequacy. These procedures should have sufficient details and not leave the interpretation of unclear information to maintenance personnel.

The maintenance of critical alarm and trips should include:

• a fault reporting system,

• the as-found and as-left condition of the system,

• the means to verify the calibration of the test equipment, and

• a record of all maintenance activities.

Next: Measurement / Control Equipment--CALIBRATION

Related: What The HELL is Measurement and Control?

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