Compressor condition monitoring and baselines

What is a baseline, and why one number is not enough

A baseline is not a single figure copied from a commissioning sheet. It is a controlled reference state tied to a specific machine, its configuration, its operating mode and the conditions at the time. A screw compressor running fully loaded at 7 bar on a 25 °C morning behaves differently from the same machine unloading at 8 bar on a 44 °C afternoon. Both can be healthy.

So a useful baseline records the reading together with its context: which machine (by serial), which configuration, what load and speed, what discharge pressure, what ambient and room temperature, which measurement point, which instrument and what date. Machines that run in several states – loaded, unloaded, VSD at low and high speed – need a baseline for each state that matters.

This page covers how to create trustworthy baselines. How condition information is then used to plan maintenance is covered in predictive and condition-based maintenance.

When to take a baseline

• After commissioning and run-in of a new or relocated compressor
• After a major overhaul, airend exchange, motor replacement or cooler cleaning
• After a significant configuration change, such as a new pressure set point or controller replacement
• When monitoring starts on an existing machine – recorded as a “current state” reference, not assumed to be healthy
• Seasonally, where summer and winter conditions differ enough to need separate references

Mark each baseline with a revision. When a new one replaces an old one, keep the old one and the reason for the change.

What to measure

Measurement discipline: making readings comparable

Most “trends” that alarm people are measurement differences, not machine changes. Control the method:

• Fixed points – mark vibration and temperature points physically so every reading is taken in the same place and direction.
• Known instruments – record which sensor or portable meter was used, its range and when it was last verified.
• Stable state – take readings after the machine has reached steady temperature, not just after start-up.
• Same load and pressure – note load percentage, VSD speed and discharge pressure; compare like with like.
• Time stamps – synchronise clocks between controllers and loggers, or note the offset.
• Manual readings – use a standard sheet with units printed on it; a “6.8” with no unit is useless later.
• Data-quality flags – mark readings taken during faults, maintenance or sensor problems.

Portable measurements by a technician are fine for baselines if the method is repeatable. Permanent sensors give continuous data but still need periodic verification.

Context that must be recorded with every baseline

Record ambient and room temperature, humidity where relevant, cooling-water temperature for water-cooled machines, ventilation state, system pressure, production state, and whether dryers and filters were in normal service. In Hyderabad and across Telangana and Andhra Pradesh, compressor-room temperatures in April and May can be far above the design ambient if hot air is not ducted out. A summer reading compared with a December baseline will always look worse; that is a ventilation and seasonal effect, not necessarily a failing machine.

How to set up a baseline: step by step

1. Identify the machine by make, model, serial and site tag; record its role (base, trim, standby).
2. Confirm it is in a known good state: recent service, clean coolers, correct oil level, no active alarms.
3. Record the configuration: set points, unload band, VSD limits, controller version.
4. Mark measurement points and list the instruments to be used.
5. Run to steady state in each operating mode you want to baseline.
6. Take readings with full context: load, speed, pressure, ambient, time.
7. Calculate derived values such as specific power and approach temperature, stating the formula.
8. Store the baseline with a revision number, date and the name of the person who approved it.
9. Set review limits for change from baseline, based on manufacturer guidance and engineering judgement, not generic charts.

Using the baseline: reading change correctly

The goal is a trend, not a single alarming reading. One high value should prompt a repeat measurement under controlled conditions before any work is ordered.

Baselines for different compressor types

The principles are the same across technologies, but the emphasis changes.

• Oil-injected screw – discharge temperature, separator differential pressure, oil condition, specific power and airend bearing vibration are the core set.
• Oil-free screw – stage temperatures and pressures, intercooler and aftercooler approach, and bearing vibration matter most; record each stage separately.
• Reciprocating – valve temperatures, stage pressures, crankcase oil condition, vibration and noise, and running time per start.
• VSD machines – baselines at several defined speeds, plus drive temperature and motor current at each speed.
• Portable diesel compressors – engine oil pressure, coolant temperature, discharge temperature, fuel use at a stated load and chassis vibration; on hire jobs, readings at each 300-hour service build a useful history.
• Dryers – PDP, inlet temperature, pressure drop and, for desiccant units, purge behaviour and tower switching.

Keep the same reading sheet format across brands so a mixed compressor house can be compared machine by machine.

Common baseline mistakes

• Treating the first reading as healthy – a machine already running hot with fouled coolers gives a false reference. Service it first, or label the reading as current state.
• Losing the context – readings without load, pressure or ambient cannot be compared later.
• Changing the measurement point – a vibration reading taken on the cover instead of the bearing housing is a different number, not a trend.
• Overwriting the old baseline – after an overhaul, keep the previous reference so the improvement can be shown and the next decline measured.
• Ignoring the system – a rise in compressor power may come from a raised pressure set point or new leaks; each extra bar costs roughly 7% more power.

Avoiding these costs nothing and makes every later reading worth more.

How Reddy Compressors helps

Reddy Compressors, Tarnaka, Secunderabad, is an independent multi-brand compressed-air specialist serving industry since 2002. We sell new and used compressors, rent from our own fleet, and provide service, AMC, overhauls, audits and spares for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and many other brands. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.

• Record baseline readings after commissioning, overhaul or airend work, with full operating context
• Take consistent readings at every visit under our compressor AMC so trends are comparable
• Log power and flow to establish specific power and demand baselines
• Investigate deviations: coolers, thermostatic valves, separators, bearings, alignment and controls
• Supply spares identified from make, model, serial or the old part, with fit confirmed before quoting

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

What to send us

• Make, model, serial and nameplate photo of each compressor
• Running hours, last service date and parts changed
• Current pressure set points and typical load pattern
• Any readings or controller logs you already have
• Photos of the compressor room showing ventilation and layout

Frequently asked questions

What is a baseline in compressor condition monitoring?

It is a set of readings taken when the compressor is known to be healthy, recorded with its load, speed, pressure, ambient temperature, measurement points and instruments. Later readings are compared with this reference to spot deterioration. Without the context, a reading cannot tell you whether the machine has worsened or is simply operating in a different state or season.

When should I take a new baseline for my compressor?

Take one after commissioning and run-in, after any major overhaul, airend exchange, motor replacement or cooler cleaning, and after significant configuration changes such as new set points. Keep the previous baseline with its revision and reason for replacement. For existing machines starting monitoring, record a current-state reference rather than assuming the machine is healthy.

What is a normal discharge temperature for a screw compressor?

For oil-injected screw compressors it is typically about 75–95 °C in steady operation, depending on design, load and ambient. The machine’s own baseline matters more than a general range. A steady rise over weeks usually points to cooler fouling, a thermostatic valve fault, low oil or ventilation problems, and should be checked before it reaches the trip level.

Why does my compressor show higher readings in summer?

Higher intake and room temperatures raise discharge and oil temperatures and reduce cooler and dryer performance. In a Telangana or Andhra Pradesh summer, room temperatures can exceed design ambient if hot air is not ducted away. Compare summer readings with a summer baseline, or correct for ambient, and improve ventilation before assuming the compressor itself is deteriorating.

Can I use controller readings for a baseline or do I need separate instruments?

Controller readings are a good starting point and are often all that is available. Verify key values, such as discharge temperature and pressure, with a calibrated portable instrument at least once, and note any offset. Vibration and power usually need separate instruments. Record which source was used so future comparisons use the same basis.

Does Reddy Compressors record baselines for all compressor brands?

Yes. As an independent multi-brand specialist we take and record baseline readings on ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and many other brands, using the same format across a mixed fleet. This is done after commissioning, overhaul or airend work, and as part of routine service visits under an AMC.

Related pages

• Compressor vibration monitoring
• Predictive and condition-based maintenance
• Condition monitoring indicators and trending
• Compressor asset data and lifecycle records
• Rotary screw compressor service
• Specific-power assessment