Digitize and optimize preventive maintenance for process instrumentation
Defining scope, optimizing tasks, and using digitalization
By Fawaz AlSahan, CAP, SCE
Maintenance can be made more efficient, helping make operations more efficient and profitable, by leveraging new digital technologies. Conventional time-based preventive maintenance (PM) is widely used throughout industry but has shortcomings that can be overcome with smart sensors and analytics (digital transformation). This enables process facilities to move into performance-based and shutdown-based preventive maintenance.
The common categories for preventive maintenance are:
- Time-based preventive maintenance: Conducted as a defined procedure during a preset interval.
- Performance-based preventive maintenance: Conducted when the instrument performance goes below a certain limit. Performance monitoring is done either online (via an online instrument) or off-line (via a portable instrument).
- Shutdown-based preventive maintenance: Conducted during plant shutdown.
The diagram shows the ISO categorization for maintenance in general and preventive maintenance in particular.
It is worth highlighting that conventional time-based preventive maintenance is widely used and has the following shortcomings:
- Forty to 60 percent of PM tasks add no value to operations.
- Some PM tasks are similar.
- Many tasks require removing the instruments or valve to the shop, while condition monitoring is enough to determine reliability.
- New technologies and digital instrument features are not used to optimize tasks.
- Some PM tasks have a high probability of causing unnecessary shutdowns and operation interruptions.
Using smart sensors and analytics (digital transformation) resolves these shortcomings and enables process facilities to move into performance-based and shutdown-based preventive maintenance.
Defining PM scope, tools, and frequency
To have the right preventive maintenance for instrumentation, first clearly define the type and scope of PM. Proper tools need to be selected, and the right frequency defined. The scope of PM can include a certain task to be performed. This can be a time-based visual check, time-based functionality check, removal and overhauling, or a condition-based task.
Second, address who will perform the PM task, such as a field operator or maintenance technician. Determining this point requires agreement between the parties, with clear tasks and a system to collect and communicate the findings.
The tools used for PM tasks are either instrument asset management systems (IAMSs), hand-held tools, an instrument built-in keypad, hand tools, or shop activities. The frequency of a PM task is based on prior knowledge from similar equipment, manufacturer or supplier data, reliability data, and performance prediction, and defines the task as online or shutdown activity.
Strategies to optimize PM
Optimizing the preventive maintenance for instrumentation is needed to eliminate unnecessary activities and unnecessary cost. Excessive preventive maintenance may also cause nuisance trips or operation upset. This is another motive to optimize the PM tasks.
Implement optimization with the following strategies: improve the design and apply technologies, capitalize on instrumentation self-diagnostics and online condition monitoring, and use the redundancy approach, once justifiable.
Design and technology
Nowadays, available technologies, like smart sensors, bring a new dimension of reliability and minimize maintenance requirements. Capitalizing on the following technologies and design approach will highly reduce the PM scope and time:
- single-rod, guided-wave radar (GWR) for level measurement
- pressure transmitters instead of process-actuated switches
- smart pressure and vibration fork switches (with display and/or diagnostics) instead of conventional blind switches
- diaphragm/remote seal pressure transmitters instead of tubing-based transmitters
- digital vibration transmitter/switch instead of mechanical switches
- smart valve
- single-rod GWR and two-wire noncontact radar for inventory tanks application
- electrochemical gas detector for H2S gas detection
- infrared gas detector for combustible gas detection
- configuring a discrepancy alarm between adjacent control and safety transmitters
- avoiding soft-seated control valves and instead installing a metal-seated control valve next to a tight shutoff rotary isolation valve
Diagnostics and online condition monitoring
Available instruments are now smart and have internal diagnostics (analytics) and digital communication. These features are effectively used to improve the preventive maintenance program and eliminate or highly reduce traditional practices. Diagnostic data is obtained and collected using:
- smart sensors (like smart transmitters, smart positioners, and smart pressure switches)
- an analytics data platform (IAMS, which receives the data from the smart instruments via a wired or wireless connection and generates a status message with a recommendation)
- digital connectivity, like Foundation fieldbus (FF) with physical layer diagnostics
Figure 1. Maintenance categorization
Source: ISO 14224:2016
Redundancy is having a permanent or temporary reference to compare the installed instrument performance and reading to. Below are some examples:
- having dual circuits (or more) for axial and radial vibration, bearing temperature, and fired equipment flame monitoring
- applying two-out-of-two voting, if safe and practical, to avoid nuisance trips and reduce excessive maintenance
- installing control and shutdown transmitters with the same calibration range and configuring discrepancy alarms for them
- having permanently or temporarily installed pressure gauges to compare to the nearby pressure transmitters' readings
- local level gauges (sight glass, magnetic level indicator) or infrared cameras to cross check the level instruments' (displacer, differential pressure, radar, etc.) readings
- checking the online temperature sensor reading with a temperature gauge, a test temperature element, a portable temperature detector, or with an infrared detector/camera.
SIS preventive maintenance
Diagnostics provided by the logic solver, input devices (like transmitters), and output devices (like emergency isolation valves) should support the overall preventive maintenance program for the safety instrumented system (SIS) by reducing the required physical preventive maintenance. The safety requirement specification (SRS) document should be developed after the safety integrity level (SIL) study/verification is conducted. The SRS specifies the required testing intervals for the SIS equipment (logic solver, input and output devices).
To define and further optimize the required preventive maintenance for SIS devices, do the following for each safety instrumented function (SIF):
- Review the SIL study and SIL verification report or conduct a study.
- Check the test interval (TI) for input and output devices recommended in the study report.
- Increase the TI to the maximum number (such as matching the plant total shutdown interval) and confirm if the SIL requirement is still achieved. The preventive maintenance interval is based on the maximum possible TI.
- For emergency isolation valves, if the TI for these valves (i.e., full stroke test) cannot meet the plant shutdown window, introduce partial stroke testing (PST) with a weight of 60 percent of the total stoke test. TI for the total and partial valve stroke shall then be clearly defined.
Figure 2. Probability of failure on demand calculation and safety integrity level.
Operator tasks versus technician tasks
There should be two different PM tasks. One is conducted by the maintenance crew, and one is conducted by the operation crew. Operation tasks are visual checks, visual inspections, or simple test and observation tasks. The maintenance crew PM tasks include detailed test procedures, calibration, and physical testing that requires hand tools and communication devices.
The objective of segregating the operation and maintenance tasks is to optimize the testing intervals and duration. Some tasks are simple, requiring a long time to prepare the work permit and the system that needs PM. Also, due to the internal mandatory requirements, the operations team must be involved in these PM tasks. Simple examples are stroke testing for control valves and emergency isolation valves and testing gas detectors.
Analysis of PM findings
There is great value in reviewing and analyzing PM findings in terms of preventing failures and maintaining equipment reliability and availability. This requirement is clearly addressed in industry references (figure 3). Doing analysis requires the PM executer to write accurate PM findings. The analysis of the compiled findings helps identify the PM tasks that add value and eliminate the "value-wasting" PM tasks. Also, it helps to identify repeated failures and to recommend revisiting some system designs. Moreover, analyzing PM findings helps improve PM tasks by better managing spare parts, highlighting optimization opportunities and life-cycle costs, and capturing and applying lessons learned across the plant or company.
Figure 3. Maintenance work order
Source: EN 13460
Impact on instrumentation preventive maintenance
Digital transformation will change the world of preventive maintenance for instrumentation and equipment to more "predictive analytics," where data is used to predict performance and failures. The "big data" collected from the instruments can be used for making decisions about when maintenance is actually needed. Hence, 70 percent or more of conventional PM tasks for both process instrumentation and equipment could be deleted. Digital transformation will address the major maintenance challenges of aging equipment and the aging workforce, and can highly cut cost, time, and manpower.
Solutions like wireless instruments, automatic work permits, risk-based maintenance, remote maintenance, robotics, and self-calibrating instruments will be used more and more. Printed PM sheets for equipment and instruments will be replaced by additional smart sensors, like acoustic detectors, video, and infrared cameras. Digital transformation can also extend monitoring into the instrumentation utilities (i.e., the quality of instrument air and power supply).
Digital transformation will also provide more efficient and safer PM by remote condition monitoring and built-in analytics and can provide faster resolutions to problems by having more data and more efficient spare parts management.
Control valves are another beneficiary of digitalization, where the cloud can be used to obtain data from the field, predict the life and performance, and also estimate the time for overhaul. Video and augmented reality, used to monitor and train the maintenance crew on valve maintenance, will be part of this digital transformation.
PM procedures for process instrumentation
This section highlights recommended PM tasks for common instruments. It covers SIS input and output devices, control valves, motor operated valves (MOV), transmitters, gas and flame detectors, thermowells, process flowmeters, flare flowmeters, process actuated switches, level gauges, vibration circuits, and skids instruments.
An analytical approach
This article gave guidelines for digitizing and optimizing the instrumentation PM program, an important task that every company needs to perform to avoid an unnecessary maintenance work load, eliminate operational losses, cut unnecessary costs, and digitally transform conventional PM into analytical maintenance. The scope of this exercise included revisiting the instrument type, PM scope, PM frequency, the PM task performer, and the task sheet.
Using smart sensors and analytics (digital transformation) lets process facilities move into performance-based and shutdown-based preventive maintenance. They eliminate or greatly reduce the conventional preventive maintenance for instrumentation, and replace it with an analytical approach, which uses data to monitor the instrument performance and predict failures.
Recommended preventive maintenance for process instrumentation