4.6 Preventive, Corrective and Predictive Maintenance, Work Orders & Asset Management
Key Takeaways
- Corrective maintenance is planned work arising from a defect found during inspection, while reactive maintenance is unplanned repair after a failure has already occurred.
- Vibration signatures identify faults: imbalance dominates at one times running speed, misalignment at two times with axial vibration, and cavitation appears as broadband random energy.
- Business risk exposure equals the probability of failure multiplied by the consequence of failure, so criticality rather than age drives replacement priority.
- Spare parts are stocked by criticality and lead time rather than by cost, so an inexpensive part with a long lead time can be the most critical spare in the plant.
- Energy typically dominates the life-cycle cost of a pumping asset, which is why trimmed impellers, variable frequency drives, and off-peak pumping usually pay back quickly.
Four maintenance strategies, and when each is right
WPI lists three job tasks that this section covers directly: "perform corrective, preventative, and predictive maintenance," "complete equipment maintenance and repair records, including work orders," and "conduct asset management." The Water Distribution outline adds "understand elements of an asset management program," and the Wastewater Collection outline adds "maintain all equipment in accordance with SOP/OEM specifications."
| Strategy | Trigger | Best for | Cost profile |
|---|---|---|---|
| Reactive (run to failure) | The equipment breaks | Cheap, redundant, non-critical items: a spare shop fan, a light fixture | Lowest planned cost, highest failure cost |
| Preventive (PM) | Elapsed time or runtime hours | Most rotating equipment; anything with a manufacturer schedule | Moderate; may replace parts with life left |
| Predictive (PdM) | Measured condition crossing a threshold | Critical, expensive, instrumented machines | Highest tooling cost, lowest failure cost |
| Corrective (CM) | A defect found during inspection or PM | Everything; this is the work PM generates | Planned, so far cheaper than emergency repair |
The most misunderstood pair is corrective versus reactive. Corrective maintenance is planned work arising from a discovered defect: the PM inspection finds a seal weeping, so a work order is written and the seal is changed on a scheduled outage. Reactive maintenance is unplanned work after a failure. A mature program converts failures into findings.
Building a preventive maintenance program
- Inventory the assets. Every pump, motor, valve, blower, analyzer, generator, and structure gets a unique asset identification number that appears on the equipment, in the records, and on the work order.
- Collect the manufacturer's O&M manuals and extract the recommended tasks and intervals. The OEM interval is the starting point, adjusted for actual duty and environment.
- Set intervals by calendar (monthly, quarterly, annually), by runtime hours, or by cycles. Runtime-based intervals are more honest for duty and standby pumps that alternate.
- Write the task list so it is specific and verifiable: not "check pump," but "record suction and discharge pressure, motor amps on all three phases, bearing temperature, and packing leak-off rate; grease per lubrication chart."
- Schedule and assign through a computerized maintenance management system (CMMS) that generates work orders automatically.
- Close the loop. The completed work order records what was found, what was done, parts used, labor hours, and the follow-up corrective work order if a defect was found.
Predictive techniques
- Vibration analysis. The single most powerful tool on rotating equipment. Characteristic frequencies distinguish imbalance (dominant at 1x running speed), misalignment (strong 2x with axial vibration), looseness (multiple harmonics), bearing defects (high-frequency bearing fault frequencies), and cavitation (broadband random energy).
- Infrared thermography. Finds hot spots in motor control centers, loose electrical connections, overloaded conductors, failing bearings, and bad breaker contacts, all without an outage.
- Oil analysis. Wear metals, viscosity, water content, and particle counts on gearboxes and large bearings.
- Ultrasonic testing. Detects bearing lubrication problems, compressed air and steam leaks, valve pass-by, and electrical arcing.
- Motor circuit analysis for winding insulation condition, and megger testing for insulation resistance.
- Performance trending. The cheapest predictive tool of all: rising amperage at constant flow, falling discharge pressure at constant speed, and rising bearing temperature are all early warnings available on any plant with instrumentation.
Work orders and records
A work order should capture, at minimum:
- Asset ID, description, and location
- Work type: PM, CM, reactive, or project
- Priority and required completion date
- Description of the problem or the PM task list
- Findings — what the technician actually observed
- Corrective action taken and parts and quantities used
- Labor hours and personnel
- Any safety permits used: lockout/tagout, confined space entry, hot work
- Signature or electronic sign-off and date
The records serve three masters: they prove compliance to a regulator, they build the failure history that justifies capital replacement, and they tell the next operator what has already been tried.
Asset management
Asset management is the discipline of delivering the required level of service at the lowest life-cycle cost with an acceptable level of risk. The classic framing is five questions:
- What do I own? — the asset inventory and register, with installation date, material, size, and location.
- What is it worth? — replacement cost, usually current-dollar replacement value rather than book value.
- What is its condition and remaining useful life? — from inspection, condition scoring, and failure history.
- What is the risk of failure? — the business risk exposure, which is probability of failure multiplied by consequence of failure.
- What do I do about it, and how do I pay for it? — the capital improvement plan, the renewal and replacement schedule, and the rate structure that funds them.
The fourth question does the real work. Two assets can have identical condition scores and completely different priorities. A 60-year-old cast iron main under a residential cul-de-sac and a 60-year-old cast iron main under a hospital access road may fail with the same probability, but the consequences differ by an order of magnitude. Criticality analysis ranks by risk, not by age.
Level of service is the promise the utility makes to its customers and regulator: pressure maintained between defined limits, water quality compliance, response time to a main break, number of service interruptions per thousand customers, and taste and odor complaints per year. Asset management works backward from those commitments.
Life-cycle cost counts acquisition, installation, energy, operation, maintenance, rehabilitation, and disposal. It is the reason the cheapest pump is often the most expensive choice: over twenty years, energy typically dominates the life-cycle cost of a pumping asset, so a premium-efficiency motor and a properly sized pump pay for themselves many times over.
Spare parts and inventory
WPI lists "track and maintain inventory (e.g., equipment, chemical, and general supplies)" as a job task. Practical principles:
- Stock spares by criticality and lead time, not by cost. A $40 gasket for a chlorinator with a six-week lead time is a critical spare; a $4,000 pump with a next-day replacement available locally may not be.
- Maintain min-max levels with reorder points that account for lead time and usage rate.
- Keep a critical spares list for equipment with no redundancy, and review it after every emergency repair that could not be completed because a part was unavailable.
- Rotate stock so that gaskets, O-rings, belts, and chemicals with shelf lives are used before they age out.
- Store spares in a controlled, labeled location with the asset ID cross-referenced.
Energy and chemical optimization
WPI lists "optimize the use of energy and chemicals" explicitly. Concrete levers available to an operator:
- Pump at off-peak times where the utility has time-of-use rates, filling storage overnight.
- Trim excess head. A throttled discharge valve is money converted to heat; a trimmed impeller or a variable frequency drive is usually cheaper.
- Correct the power factor where the electric utility charges for it.
- Right-size the dose. Jar testing to find the true coagulant optimum routinely cuts alum use by 10 to 20 percent and reduces sludge production at the same time.
- Eliminate unnecessary recirculation and idle equipment.
- Track pounds per million gallons and kilowatt-hours per million gallons as the standing efficiency benchmarks.
For the exam, the connecting idea is simple: maintenance, records, asset management, and optimization are one system. The PM generates the record, the record builds the failure history, the failure history plus criticality drives the renewal plan, and the renewal plan protects the level of service the operator is legally responsible for delivering.
What distinguishes corrective maintenance from reactive maintenance?
Vibration analysis on a centrifugal pump shows a dominant vibration peak at twice the running speed with significant vibration in the axial direction. What condition does this signature indicate?
In an asset management program, how is business risk exposure determined?