Compressed-air FMEA, risk and criticality analysis

Why compressed air needs risk and criticality analysis

Compressed air is often called the fourth utility, and like power and water it is noticed mainly when it fails. A tripped compressor, a dryer that lets water through, a blocked filter or a failed pressure transmitter can stop a filling line, ruin a coating batch or shut down a pharma clean room. Yet many plants plan their air system as a list of machines, not as a service with consequences.

Risk analysis changes the question from “what equipment do we have?” to “what happens to production if this part of the system stops doing its job, and how likely is that?” The goal is not a long list of every imaginable failure. It is a short, defended list of the failures that matter and a plan for each.

Start with functions, not components

A good study begins by stating what the system must deliver at each process boundary: flow, minimum dynamic pressure, air quality class under ISO 8573-1, availability and the time users can tolerate an interruption. A dryer, for example, is not just an asset. Its function is to deliver air at the required pressure dew point, typically around +3 °C for a refrigerated dryer and −20 °C or −40 °C for a desiccant dryer, at the required flow and with an acceptable pressure drop.

Define the study boundary too: source compressors, treatment, receivers, distribution, controls, utilities such as power and cooling water, end users and emergency supply. Then list the operating states. Normal running, low load at night, peak production, start-up after a power cut, planned maintenance and degraded operation with one machine out can each expose different weaknesses.

The FMEA question set

For each function, the team works through the same questions. The table shows a typical worksheet layout with compressed-air examples.

Describe effects at the next process boundary, not just at the component. “Separator element blocked” is a cause; “line pressure falls below the minimum for the filler and production stops” is the effect that matters.

Typical failure modes in a compressor system

These are the failure modes we meet most often when we attend breakdowns across Telangana and Andhra Pradesh. They are a starting point, not a complete list.

• High discharge temperature trip: normal oil-injected screw discharge is about 75–95 °C. Blocked coolers, low oil, wrong oil, a stuck thermostatic valve or poor room ventilation in 40–45 °C summers push it towards the trip point.
• Loss of capacity: worn airend, intake restriction, inlet valve fault, slipping belts or leaks that grow faster than the plant notices.
• Wet air downstream: dryer overload, failed condensate drain, bypass left open, or dryer sized for a lower ambient than the site sees.
• Oil carry-over: separator failure, scavenge-line blockage or overfilled sump.
• Pressure collapse: too little storage, a sequencer fault, or the standby machine not set to start automatically.
• Control and instrument faults: failed pressure transmitter, communication loss between sequencer and compressors, wrong settings after a controller replacement.
• Utility faults: voltage dips, phase imbalance, cooling-water loss on water-cooled machines.
• Restart failure after a power cut: machines that do not restart automatically or restart all together and trip the supply.

Severity, occurrence and detection without false precision

Most FMEA methods rate severity, occurrence and detection on simple scales, often 1–10, and multiply them into a risk priority number (RPN). The RPN is useful for sorting, but it has known weaknesses. Multiplying ordinal scores can make a rare but catastrophic failure look less important than a frequent nuisance. A severity-10 failure with good detection can end up with the same number as three moderate scores.

Practical rules we recommend:

• Use the site’s own severity criteria for safety, production, quality and environment, so ratings mean something to the people who approve spending.
• Base occurrence on actual breakdown history, service reports and condition data. Where evidence is thin, say so rather than inventing a probability.
• Rate detection honestly. An alarm only counts if the sensor is calibrated, the alarm reaches someone and that person knows what to do.
• Review any item with a top severity score separately, whatever its RPN.

How to classify criticality

Criticality ranks compressed-air users and equipment by consequence and tolerated interruption. A simple three-level model works for most plants.

The same compressor can serve users of different criticality. Where that is the case, consider whether critical users should be protected with local storage, a priority valve or a dedicated supply.

Running an FMEA workshop step by step

1. Gather the team: production, maintenance, utilities, quality and, where useful, the compressor service provider.
2. Collect evidence: P&ID or line sketch, asset list, settings, breakdown history, energy and pressure logs.
3. Agree functions and boundaries for each part of the system.
4. Work through failure modes, effects and causes function by function.
5. Record existing controls and rate severity, occurrence and detection.
6. Rank items, review all high-severity items separately, and agree actions with owners and dates.
7. Re-rate after actions are complete, and schedule a review after any significant incident or change.

Turning the analysis into action

An FMEA that sits in a folder achieves nothing. Typical actions that follow a compressed-air study are practical and often modest in cost:

• adding a standby compressor or setting an existing one to start automatically on low pressure;
• increasing receiver storage near critical users;
• adding dew-point, differential-pressure and discharge-temperature alarms with clear responses;
• stocking critical spares identified by serial number, such as separators, sensors, drain kits and solenoid valves;
• revising preventive maintenance tasks and intervals where causes are maintenance-related;
• agreeing a rental or breakdown-cover plan in advance, with a known connection point and power supply;
• writing a restart procedure for power failures.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka in Secunderabad and serving industry since 2002, works on compressors of all major brands as an independent specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. Our more than 20 service engineers and technicians see real failures every week, which gives a practical basis for risk work.

• We take part in FMEA and criticality reviews, bringing failure-mode knowledge across ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes.
• We carry out air audits and logging to provide evidence for occurrence and detection ratings.
• We identify and supply critical spares, confirming fit from the nameplate and serial number.
• We provide standby and breakdown rental from our own fleet of diesel and electric compressors, 50–2,000 cfm at 2–30 bar. We have supplied compressors on breakdown to pharmaceutical manufacturers in Hyderabad and Telangana.
• We offer service and AMC to put the agreed maintenance actions into practice.

Our service teams cover all of Telangana and Andhra Pradesh, and we supply equipment and spares across India.

What to send us

• A simple line sketch of the compressed-air system, from compressors to main users.
• Nameplate photos of compressors, dryers and receivers.
• Your list of critical users and how long they can tolerate low pressure.
• Breakdown and service history for the last two or three years, if available.
• Current pressure settings and any alarm or trend data.

Frequently asked questions

What is FMEA for compressed-air systems?

FMEA, or failure mode and effects analysis, is a structured way to ask how each part of the air system can fail, what that would do to production, why it might happen and how it would be detected. For compressed air it covers compressors, dryers, filters, receivers, controls, pipework and utilities, and ends with a ranked list of actions such as standby, monitoring or spares.

How do I decide which compressor is critical?

Look at the users it supplies, not at the machine itself. Ask what happens if air pressure falls below the minimum for each user and how quickly production, quality or safety is affected. A compressor that feeds a sterile process or a continuous line is critical even if it is small, while a larger machine feeding only workshop tools may not be.

Is a risk priority number enough to rank compressor failures?

It is a useful sorting tool but not enough on its own. Multiplying severity, occurrence and detection scores can hide a rare but very serious failure behind a frequent minor one. Always review every item with the highest severity rating separately, and base occurrence ratings on real breakdown history and condition data rather than guesses.

What are the most common causes of compressed-air outages?

In our service experience the frequent causes are high discharge temperature trips from blocked coolers or poor ventilation, failed condensate drains, dryer overload in summer heat, sensor or controller faults, power disturbances and standby machines that do not start automatically. Many of these can be detected early with simple trends on temperature, pressure, dew point and filter differential pressure.

Should we keep a rental compressor plan for critical air?

For critical duties it is good practice. Agree in advance the capacity, pressure, air quality, connection point, hose route, power or fuel arrangement and who calls whom. A plan written before a breakdown saves hours of confusion during one. Oil-free or treated air for sensitive processes needs particular attention, because a standard site compressor may not meet the required class.

How often should a compressed-air FMEA be reviewed?

Review it after any significant incident, after changes to equipment, controls or production, and at a regular interval agreed by the site, often once a year. Each review should check that actions were completed, re-rate the risks that changed and add any new failure modes found from service reports or breakdowns since the last review.

Related pages

• Criticality classification and service levels
• Reliability, availability and resilience
• Failure analysis and defect elimination
• Business continuity and emergency air supply
• Breakdown diagnostics and recovery