Compressor condition monitoring: what to measure and how to trend it

What is condition monitoring and what can it tell you?

Every compressor gives early signals before it fails. A cooler fouling with dust shows as a slow rise in discharge temperature. A blocking separator shows as increasing differential pressure and rising current. A bearing starting to wear shows in vibration and oil analysis long before it seizes. Condition monitoring is simply the habit of capturing those signals regularly and reading them in context.

Be clear about its limits. A trend tells you that something is changing; it does not prove why, or exactly when the machine will fail. An alarm or an unusual reading is a prompt to investigate: check the instrument, check the operating conditions, inspect physically, and then decide on maintenance. Manufacturer service intervals still apply. Monitoring helps you catch what happens between them and choose the right time for intervention.

What should you measure on a compressor?

Choose indicators that relate to real failure modes on your machine. For a typical oil-injected rotary screw compressor, the most useful set is:

For reciprocating compressors add interstage pressures and temperatures, valve temperatures and crankcase oil condition. For portable diesel units add engine oil pressure, coolant temperature and fuel consumption against load.

Why a baseline matters more than a single reading

A reading only has meaning against the same machine’s normal state. A discharge temperature of 92 °C might be normal for one compressor in a hot Hyderabad summer and a warning sign for another in a cool, well-ventilated room. Build a baseline before relying on alarms:

1. Record readings when the machine is known to be in good condition, ideally just after a full service or overhaul.
2. Note the conditions alongside each reading: delivery pressure, load percentage, ambient and room temperature, and time of day.
3. Collect enough readings to see normal variation, typically over a few weeks of ordinary production.
4. Re-baseline after any major change: overhaul, new separator, cooler cleaning, oil change to a different grade, or relocation.
5. Keep a seasonal baseline. Summer readings in Telangana and Andhra Pradesh can differ markedly from winter values, so compare like with like.

How to trend readings and spot real problems

Plot each indicator over time and look for patterns rather than single points:

• Slow drift, such as discharge temperature creeping up by a degree or two each week, usually means fouling or gradual wear.
• Step change immediately after a service often points to a fitting error, wrong part, air lock or a sensor disturbed during work.
• Cyclic patterns that follow shifts or ambient temperature are usually operating effects, not faults.
• Intermittent spikes may be a loose connection or a failing sensor rather than the machine itself.
• Correlated changes are the strongest evidence. Rising current together with rising separator dP is far more convincing than either alone.

Before acting on a trend, check the instrument. Controller sensors drift, and a cheap gauge can disagree with a calibrated one by a meaningful margin. An independent reading with a calibrated thermometer, gauge or clamp meter is a good habit before any expensive decision.

Setting alarms that people will respect

Compressor controllers already carry manufacturer warning and shutdown limits. These protect the machine; do not change them without the manufacturer’s guidance. Condition monitoring adds earlier, softer levels based on your baseline.

Too many alarms are as bad as none. If operators see the same warning every day and nothing happens, they stop looking. Review alarm frequency monthly and either fix the cause or adjust the soft limit with a recorded reason.

Vibration and oil analysis in practice

Vibration

Overall vibration velocity readings with a handheld meter, taken at the same marked points on the motor and airend bearings each time, are enough for most plants. Mark the points with paint so readings are repeatable. Compare against your baseline and against general machinery guidance such as ISO 10816 or ISO 20816 for the machine class. A rising trend justifies spectrum analysis to separate misalignment, unbalance and bearing defects. Belts and couplings are frequent sources of vibration on smaller machines.

Oil analysis

Oil is a direct report on the inside of the airend. Take samples from the same point, with the machine warm and running or just stopped, into a clean bottle. Typical laboratory checks include viscosity, total acid number or oxidation, water content, and wear and contaminant elements such as iron and silicon. Rising silicon usually points to dust ingress through the intake, a common issue in dusty industrial estates. Oil analysis can also support extending or shortening drain intervals within the manufacturer’s guidance.

Practical monitoring setups for Indian plants

You do not need an expensive system to start. Choose the level that suits the criticality of the compressor and the skills available.

For remote and SCADA links, agree in advance who owns the data, who can change setpoints, and how remote access is secured. Read-only monitoring is usually the safest starting point.

Local conditions should shape the programme. Compressor rooms in Telangana and Andhra Pradesh often run very hot from April to June, so ventilation and cooler cleanliness deserve extra attention, and discharge temperature trends become most valuable then. Dust from construction, cement, mining and agricultural processing blocks intake filters and coolers quickly. Voltage fluctuation can show up as current swings that are not mechanical at all, so check supply voltage before blaming the machine.

Closing the loop: from reading to maintenance decision

1. Detect: a reading or trend moves outside its normal range.
2. Verify: confirm with a second instrument and check operating conditions.
3. Diagnose: inspect physically, correlate with other indicators, review service history.
4. Decide: repair now, plan for the next shutdown, or continue watching with a closer interval.
5. Act and record: carry out the work and record what was found.
6. Re-baseline: take new readings after the work so future trends start from the correct reference.

Recording what was actually found matters. Over time it shows which indicators really predicted faults on your machines and which only created noise.

How Reddy Compressors helps

We are an independent multi-brand compressor specialist at Tarnaka, Secunderabad, serving industry since 2002. We service Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic, Gardner Denver, Sullair, Doosan, Kirloskar, Bobcat and many other makes, with service teams covering the whole of Telangana and Andhra Pradesh.

• Set up practical log sheets and baseline readings for your compressors and dryers.
• Take readings during service visits and help you interpret trends.
• Investigate rising temperatures, currents, dP or vibration and recommend corrective work.
• Include condition checks within service and AMC visits, scoped to how critical each machine is.
• Carry out air audits with power, flow and pressure logging when a wider system view is needed.
• Supply spare parts identified from your machine’s make, model, serial number and nameplate, with fit confirmed before quoting.

On our own hire fleet we service and change oil every 300 running hours, and routine readings on those machines are part of how we keep them running on continuous project duty.

What to send us

• Nameplate photos of each compressor and dryer (make, model, serial, pressure, FAD).
• Running hours, load hours and the date of the last service.
• Recent controller readings or photos of the display, plus any alarm history.
• The symptom you are seeing, such as high temperature, rising current or oil carry-over.
• Compressor room conditions: ventilation, ambient temperature and dust levels.
• Any existing log sheets, oil reports or vibration readings.

Frequently asked questions

What parameters should be monitored on a screw compressor?

The most useful are discharge temperature, delivery pressure, separator and filter differential pressure, motor current or power, running and load hours, and alarm history. Add vibration readings on motor and airend bearings and periodic oil analysis for important machines. If a dryer is installed, monitor pressure dew point too, because poor air quality can be as damaging as low pressure.

What is the normal discharge temperature of a screw compressor?

For a typical oil-injected rotary screw compressor, normal discharge temperature is about 75–95 °C, depending on design, load and ambient conditions. What matters most is the trend against the machine’s own baseline. A steady rise usually points to a fouled oil cooler, low oil level, a faulty thermostatic valve or poor compressor-room ventilation, especially during summer.

Why is my compressor drawing more current than before?

Higher current at the same pressure and load often means a blocked oil separator, a higher delivery pressure setting, mechanical drag from worn bearings, or a motor or supply problem such as low or unbalanced voltage. Check the pressure setting and separator differential pressure first, then measure supply voltage on all three phases before suspecting the airend itself.

How often should compressor oil be analysed?

Many plants sample critical compressors every few months or at a fixed running-hour interval agreed with the oil supplier, and before deciding to extend a drain interval. Always sample from the same point with the oil warm. More frequent samples make sense after an overhaul, after contamination, or when wear metals or silicon start to rise.

Can condition monitoring replace scheduled compressor servicing?

No. Condition monitoring complements scheduled servicing but does not replace the manufacturer’s maintenance requirements. It helps you spot faults that develop between services and decide when extra work is needed. Some intervals, such as oil changes, may be adjusted using oil analysis evidence, but filters, separators and safety checks still need regular attention.

Is remote monitoring worth it for a small plant?

For a single non-critical compressor, a disciplined daily log sheet and periodic checks usually give most of the benefit at very low cost. Remote monitoring becomes worthwhile when air is critical to production, when there are several compressors to coordinate, or when the compressor room is not regularly attended. Start with read-only access and clear data ownership.

Related pages

• Compressor condition monitoring and baselines
• Compressor vibration monitoring
• Lubricant and oil analysis
• Alarms, trends and performance data
• Remote compressor monitoring
• Preventive maintenance planning