Scheduled Maintenance Automation in Desalination Mega-plants

Scheduled maintenance (SM) accounts for 90% of all maintenance activities related to the desalination mega-plant.

At the core of SM is a set of tasks routinely performed on the equipment to maintain its warranted performance and reliability. If the task’s result deviates from the expected range, it triggers a corrective action. Hence, tasks may be interlinked, and corrective maintenance should also be included in SM.

The SM scope should answer the questions of what, how, who, and when. (The lack of definitive answers to these questions is one of the reasons driving the proliferation of PPP contracts in the water industry.)

1. What tasks should be selected out of the wealth of the OEM requirements and the industry's good practices?

2. How should the task be executed and what context and expertise are needed?

3. Who will execute the task? Are the internal resources available? Should it be outsourced?

4. When should the task be executed? What time window to select?

The OEM requirements are the major source of the SM tasks (55%) followed by the functionality and safety checks (25%) and the industry good practices (20%). The breakdown is in order of magnitude as the named categories partially overlap.

The middle group includes such tasks as periodic sampling analysis, pressure-loaded equipment certification, valves exercising, replacement of anodes for corrosion protection, instrument redundancy check, etc.

For the desalination plant of 250 MLD, the list of generic SM tasks includes over 850 entries. When applied to specific equipment pieces they are expanded into over 4500 tasks. Their manual tracking is beyond human capabilities.

The three biggest task groups are given below.

1. Sampling for quality analysis: 24%

2. Instrument calibration: 20%

3. Rotating equipment checks and parts replacement: 28%

Tasks may range from a simple visual check requiring 5 seconds to a replacement of a rotor in such a sophisticated device as a pump lasting 8 hours.

Hence, the task's definition should include the level of required expertise and resources, the workload, the spare parts stock, and tools. The task may be executed at regular intervals or after a finite mileage, or a number of batch processes when the system is in operation or idle.

A conditionally scheduled task is triggered by the first of the competing alarms. Examples of such a task are reverse osmosis membrane replacement, CIP cleaning, cartridge filter replacement, or seawater intake piping chlorination and pigging.

Matching resources expertise and headcount and the tasks’ workload is the problem of task prioritization. Its level should reflect the system criticality defined by the designer during the plant engineering.

As follows from the tasks breakdown and description SM is deeply rooted in plant design and engineering, and stand-alone SM software products not linked to the previous project-plant life cycle phases are utterly useless.

All the above-mentioned factors have been counted upon in developing the framework for automated maintenance planning, scheduling, and execution. (It is part of the crenger.com digital infrastructure for desalination mega-projects.) The framework includes the following foundational elements.

1. The built-in library of generic SM tasks and spare items

2. The user interface for adding the product-specific tasks and spare parts to products quoted during bidding by the vendors

3. The algorithms broadcasting the generic tasks on the P&ID items and elements of the plant wiring diagram

4. Automatic task prioritization

5. The SM resource load balancing

6. The SM broker managing the SM tasks execution

7. The user interface for SM work reporting

The first 4 entries answer the what and how questions mentioned above, and the last 3 - when and who ones. Both are rooted in resource load balancing. It is an optimization problem, which may be defined as follows.

How to distribute the SM tasks in time without breaking the task overdue limits so that the resource requirement reaches its minimum?

When load balancing is applied to the plant of 250 MLD the maintenance personnel headcount drops from 9 to 6 people.

The SM broker (SMB) implements business process management (BPM) discussed in "Business Process Management". When it is time to execute a task, SMB sends an assignee the request to start working on a task. Later, the assignee may get a request to report the progress online, or expedite the work if the progress is insufficient.

After the assignee reports the task completion, SMB may send a request to review and approve the work. Once the task is completed, SMB schedules the next execution time. If a task completion is a condition for payment SMB notifies the procurement department about the work status and due payment.

The SM broker implements an adaptive maintenance strategy. It schedules the next task date based on the previous execution results.

Reprinted from crenger.com