Why leaks matter more than most plants think
Leaks are the one compressed-air user that never stops. They run through every shift, every break and often every night and weekend while the system stays pressurised. Because each leak is small and many are inaudible in a running plant, the total is usually underestimated. In many plants that have never had a structured programme, leakage is one of the largest single consumers of compressed air.
Leaks also cause problems beyond energy. They pull down system pressure, which tempts operators to raise the compressor set point; each extra bar costs about 7% more compressor power and makes every leak worse. They add running hours to compressors and dryers, bring forward service intervals and can force a second machine online that would otherwise stay on standby.
Where leaks are usually found
| Location | Typical cause | What to look for |
|---|---|---|
| Quick couplings and hose connections | Worn seals, damaged plugs, couplings left connected without a tool | Hissing at drops, hoses lying on the floor |
| Threaded joints and unions | Vibration, poor sealing, thermal movement | Old joints near machines and at pipe changes |
| Filters, regulators and lubricators | Bowl seals, diaphragm wear, drain valves stuck open | Continuous bleed from FRL units |
| Condensate drains | Timer drains set too long, manual drains cracked open, failed float drains | Air blowing at drain outlets between cycles |
| Pneumatic cylinders and valves | Worn piston and rod seals, solenoid valve exhaust leakage | Exhaust ports bleeding while the actuator is idle |
| Hoses and push-in fittings | Cracked or abraded hose, cut tube ends, heat damage | Kinked or brittle hose near hot equipment |
| Flanges, valves and dead legs | Gasket failure, gland leakage, abandoned branches still pressurised | Unused pipework that was never isolated |
How leaks are detected
Air escaping through a small opening creates turbulence that produces ultrasound, well above the range of human hearing. An ultrasonic leak detector picks up this signal and converts it into an audible tone and a level reading, which lets a technician locate leaks even in a noisy production hall.
- Ultrasonic detectors: the standard tool for a walk-through survey. Directional probes and parabolic attachments help locate leaks at height or behind guards.
- Acoustic imaging cameras: overlay the ultrasound source on a video image, making it faster to scan large areas and document findings.
- Leak-detection fluid: useful to confirm and pinpoint a leak at an accessible joint, where compatible with the equipment and process.
- Pressure-decay and flow tests: measure the total leakage of a zone or the whole system during non-production, giving a figure the walk-through cannot.
Ultrasonic level readings indicate a leak’s likely size but are not a flow measurement. Background ultrasound from other equipment, reflections, distance and probe angle all affect the reading. Any flow estimate needs a stated method, and the total should be cross-checked against a system leakage test.
Estimating how much air is being lost
Two well-established methods give a system or zone leakage figure during a non-production period, with the system pressurised and production users shut off:
- Load/unload timing (fixed-speed compressors): run the compressor with no production demand and time several load and unload periods. Leakage as a fraction of compressor capacity is roughly the loaded time divided by the total of loaded and unloaded time. Multiply by the compressor’s delivery at site conditions to estimate leakage flow.
- Pressure decay: with the compressors stopped, record the time taken for pressure to fall between two values. Leakage flow is approximately the system volume multiplied by the pressure fall (in bar), divided by atmospheric pressure (about 1.013 bar) and by the time taken. The system volume must include receivers and piping.
Where a flow meter is installed at the station outlet, the non-production flow from a demand profile gives a direct measure. Remember that leakage at full operating pressure during production is usually higher than at reduced pressure, and that some leaks only appear when a machine is running.
How a structured leak survey is carried out
- Define scope and zones. Divide the plant into identifiable areas and agree which are included, which are inaccessible and when each can be surveyed.
- Agree safety rules. Follow the site’s permits for access, work at height and machinery. The detector never justifies reaching into moving equipment or loosening a pressurised fitting.
- Record system conditions. Note header pressure, operating state (production, idle or shutdown) and which compressors are running.
- Survey zone by zone. Work along headers, drops, machines and drains in a fixed route so coverage can be checked later.
- Tag and record each leak. Give each a unique number, a physical tag, a photograph, an exact location, the component, a size category and a repair suggestion.
- Prioritise. Rank by safety and process impact first, then by estimated air loss and ease of repair.
- Report. Provide a leak register, zone coverage and any areas not surveyed, with the system leakage test result where carried out.
Some leaks can be fixed during the survey with the site’s permission, such as tightening an accessible coupling or closing a drain valve left open. Anything involving isolation, depressurisation or replacement should go through normal maintenance procedures.
Prioritising repairs
| Priority | Typical findings | Suggested action |
|---|---|---|
| Immediate | Damaged hose under pressure, leak affecting product quality, safety device leaking | Escalate to the site; isolate and repair under procedure |
| High | Large leaks, failed drains, leaks on headers that run all hours | Repair within the next maintenance window |
| Medium | Moderate leaks at fittings, FRLs and cylinders | Group by area and repair in a planned round |
| Low | Small leaks that need a shutdown or a part to be ordered | Plan with the next shutdown; order parts in advance |
Grouping repairs by area and by part type saves time. A list of seals, couplings, drains and regulators needed for the round can go to stores before the work starts.
Keeping leakage down after the survey
- Run the repair round promptly while tags and photographs are fresh.
- Recheck each repaired point and record the result against the leak number.
- Repeat the system leakage test after a repair round to confirm the total has fallen.
- Isolate idle zones and machines with a valve when they are not in use.
- Replace timer drains with level-sensing, zero-air-loss drains where practical.
- Lower header pressure once leaks are fixed, because it reduces the remaining leakage too.
- Repeat surveys at regular intervals, as new leaks appear with wear and plant changes.
- Train operators to report hissing couplings and damaged hoses, and to disconnect tools at the end of a shift.
- Include leak checks in the commissioning of any new machine or pipe extension before it is handed over to production.
In Telangana and Andhra Pradesh, summer heat and dust speed up the ageing of hoses, seals and plastic tubing, especially where lines run near roofs, furnaces or outdoor areas. Plants in these conditions often see new leaks appear faster and benefit from shorter survey intervals.
How Reddy Compressors helps
We carry out ultrasonic leak surveys for factories, process plants and workshops across Telangana and Andhra Pradesh. Our engineers work zone by zone with your team, tag and photograph each leak, estimate its size, run a load/unload or pressure-decay leakage test where the plant allows and hand over a leak register your maintenance staff can work through.
We can also carry out the repairs, supply couplings, hoses, regulators, drains and compressor spare parts, and follow up with a recheck. As an independent multi-brand specialist that sells, rents and services ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes, we can then review whether your compressor set-up, pressure band and controls should change now that demand has fallen. Leak detection fits naturally into an AMC or a wider compressed-air audit.
Our office is at Tarnaka, Secunderabad. Call +91 99590 60718 or +91 92468 88812.
What to send us
- Plant location, approximate floor area and number of buildings or bays.
- Compressor makes, models and running pressure.
- Whether a non-production window (night, weekend, shutdown) is available for a leakage test.
- A layout sketch of the main headers, if you have one.
- Access restrictions, permit rules and areas that need escorts.
- Any previous leak survey reports or tag lists.
Frequently asked questions
How do I find compressed-air leaks in my factory?
Small leaks are often inaudible in a running plant, so the most reliable method is an ultrasonic detector, which picks up the high-frequency sound of escaping air. Work zone by zone along headers, drops, hoses, FRL units, drains and machines. During a quiet period you may hear larger leaks, and leak-detection fluid can confirm an accessible joint. Tag and record every leak so it can be repaired and rechecked.
How much compressed air is lost through leaks?
It varies widely from plant to plant, which is why it should be measured rather than assumed. A load/unload timing test or a pressure-decay test during non-production gives a system figure. A flow meter at the compressor outlet shows non-production flow directly. Plants without a regular leak programme often find leakage is a large share of total demand, so measuring it is worth the effort.
How often should a compressed-air leak survey be done?
Leaks return as seals wear, hoses age and equipment is moved, so a one-off survey has limited lasting value. Most plants benefit from a full survey followed by regular repeat surveys, with the interval set by how quickly new leaks appear and how critical air supply is. Combining surveys with routine maintenance or an AMC keeps the programme going without extra planning.
Can leaks be detected while the plant is running?
Yes. Ultrasonic detectors are designed to work in noisy production areas, and many leaks only show when a machine is operating. However, measuring total system leakage needs a non-production period, because production use would mask the leak flow. A good programme combines a walk-through survey during operation with a leakage test during a planned quiet window.
Why does my compressor run at night when the plant is closed?
If the system stays pressurised, the compressor is supplying leaks, open drains, dryer purge and any equipment left valved in. A load/unload pattern at night is a clear sign of leakage. Where some air is genuinely needed at night, a small dedicated supply may help.
Does lowering system pressure reduce leakage?
Yes. A leak through a fixed opening passes more air at higher pressure, so reducing header pressure reduces the leak flow and the compressor power used to supply it. Each extra bar of generation pressure costs about 7% more compressor power. Lower pressure only after checking that critical users still receive the pressure they need at the point of use.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
System leakage test: Load/unload timing, residual flow or pressure decay
Turn a non-production leakage test into a leakage flow, a share of compressor delivery, leak power and annual energy. Choose the method you used; only that method’s inputs are read. Optionally enter the result after repairs, or the measured station power before and after, to state the reduction.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Test method: Load/unload timing (fixed-speed compressor)
- Compressor actual delivered air, FAD (m³/min): 10
- Load/unload: loaded time per cycle, averaged (s): 30
- Load/unload: unloaded time per cycle, averaged (s): 70
- Residual: measured flow with production stopped (m³/min): 5
- Residual: verified legitimate continuous users (m³/min): 1.2
- Decay: system volume, receivers plus pipework (m³): 5
- Decay: start pressure (bar g): 7
- Decay: end pressure (bar g): 6
- Decay: time for the pressure fall (s): 60
- Atmospheric pressure (bar a): 1.013
- System pressure during the test (bar g): 7.5
- Normal operating pressure to restate the result at (bar g): 7.5
- Leakage after repairs, same method and pressure (m³/min; 0 if not yet measured): 0
- Station specific power (kW per m³/min): 6.5
- Pressurised hours per year: 6000
- Optional: station power in the idle test before repairs (kW; 0 to skip): 0
- Optional: station power in the idle test after repairs (kW): 0
- Idle (non-production) hours per year for the measured power: 2000
Result
- Loaded share of the cycle: 30% T ÷ (T + t) = 30 ÷ 100 s
- Estimated leakage at the test pressure: 3 m³/min free air = 50 L/s = 105.9 cfm (load/unload)
- Leakage as a share of compressor delivery: 30% of 10 m³/min
- Leak power at the operating pressure: 19.5 kW at 6.5 kW per m³/min
- Leak energy per year: 1,17,000 kWh over 6,000 pressurised hours
- Leak energy cost per year: Enter your electricity tariff (cost per kWh) above
Method. Load/unload timing (fixed-speed load/unload compressor, production stopped, timed over several cycles): leakage ≈ FAD × T ÷ (T + t), with T the loaded and t the unloaded time per cycle. Residual flow (flow meter, production stopped): unexplained residual = measured residual flow − verified legitimate users (dryer purge, instrument air, drains); it is the figure to investigate, not automatically all leakage. Pressure decay (compressors stopped and isolated, no users): leakage in free air ≈ V × (p_start − p_end) ÷ (p_atm × time), with V the receiver plus pipework volume in m³, pressures in bar and time in minutes; the pressure difference is the same in gauge or absolute terms. Leak flow through an orifice rises roughly in proportion to upstream absolute pressure, so a result taken at one pressure is restated at the normal operating pressure by × (p_op + p_atm) ÷ (p_test + p_atm). Leak power = leakage × station specific power (kW per m³/min); annual energy = power × pressurised hours. The after-repair figure must come from the same method at the same pressure. The measured-power option gives (kW before − kW after) × idle hours, which is a measured saving, not an estimate. Money is shown only when you enter your own tariff.
What to take from it. Record the method, the pressure and the plant state with every result, so the next test is comparable. A lower leak flow saves energy only if the compressors then unload, slow down or stop.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Load/unload cycle energy
Turn measured load and unload times (or the logged percentage of time in each state) and measured package kW into average power, annual energy, the energy spent running unloaded, and specific power based on the air actually delivered. An optional second set of times compares a change.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Times entered as: Measured cycle times (seconds)
- Loaded package power (kW): 82
- Unloaded package power (kW): 28
- Loaded time (s per cycle, or percent): 40
- Unloaded time (s per cycle, or percent): 60
- Stopped time, motor off, 0 if none (s per cycle, or percent): 0
- Power while stopped, controls only (kW): 0
- Running hours per year with this cycle pattern: 6000
- Free air delivered when loaded, FAD (m³/min): 12.5
- After change, optional: loaded time, all three 0 = not used (s or percent): 0
- After change, optional: unloaded time (s or percent): 0
- After change, optional: stopped time (s or percent): 0
Result
- Cycle time: 100 s (36 cycles per hour)
- Loaded fraction: 40 %
- Unloaded fraction: 60 %
- Unloaded power as a share of loaded power: 34.1 %
- Average package power: 49.6 kW
- Annual energy: 2,97,600 kWh per year
- Unload energy (buys no air): 1,00,800 kWh per year = 33.9% of the annual energy
- Loaded specific power: 6.56 kW per m³/min
- Average air delivered: 5 m³/min
- Effective specific power over the cycle: 9.92 kW per m³/min
- Energy cost per year: Enter your electricity tariff (cost per kWh) above
Method. Two input modes. Cycle timing: enter measured seconds per cycle. Fraction: enter the share of time loaded, unloaded and stopped (for example 40, 60, 0, from controller loaded/running hours); no cycle time is reported in this mode. Average kW = (P_load × t_load + P_unload × t_unload + P_stopped × t_stopped) ÷ (t_load + t_unload + t_stopped); the states must not overlap. Annual kWh = average kW × the hours this pattern runs. Unload energy = P_unload × unloaded fraction × hours. Average air delivered = loaded fraction × loaded FAD; effective specific power = average kW ÷ average air delivered. Unloaded power is never assumed to be zero: enter measured package kW (unloaded read after blowdown has settled). Scenario comparison: enter the predicted times after a change (bigger receiver, wider band, auto-stop) in the “after change” fields; the same powers, FAD and hours are used and the kWh difference is shown. An auto-stop timer is modelled by moving unload time into the stopped state (check the motor’s permitted starts per hour). For different day and night patterns, run the tool once per pattern and add the results. The rapid-cycling warning below 30 s per cycle is a screening value used by this tool. Money is shown only when you enter your own tariff.
What to take from it. A large unload-energy share, or an effective specific power well above the loaded figure, means a fixed-speed machine is running lightly loaded: look at storage, pressure band, sequencing, auto-stop or a smaller/VSD trim machine.