Why leaks matter more than most plants think
Every compressed-air system leaks. Fittings loosen with vibration, hoses crack in the heat, drain valves stick open, cylinder seals wear and quick couplings are left connected to nothing. Each leak seems small, but together they behave like a permanent user that never stops, even on Sundays.
The cost is not only wasted air. Leakage can keep an extra compressor online, push operators to raise the pressure setting, which costs about 7% more power for every extra bar, increase load and unload cycling, drain receivers faster and hide genuine growth in production demand. Leak control is therefore a reliability task as well as an energy task.
Leaks, legitimate use and artificial demand
Before calling any air “leakage”, separate three things:
- Legitimate continuous use: desiccant dryer purge, instrument air, process users that run during breaks, and timed drains all consume air when production is stopped.
- Physical leakage: unintended escape through fittings, threads, hoses, seals, valves and damaged pipe.
- Artificial and wasteful use: open blow-off pipes, air used for cooling or cleaning where a better method exists, and the extra flow that every user takes because pressure is higher than needed.
Mixing these up leads to wrong conclusions. A plant that switches off a dryer purge and calls it a leak repair has not repaired anything.
How to find leaks
| Method | How it works | Strengths | Limits |
|---|---|---|---|
| Walk-round when quiet | Listening during shutdowns or breaks | Free, finds large leaks | Misses small leaks and noisy areas |
| Soap solution | Bubbles form at the leak | Confirms exact location on fittings | Slow, not for hot or inaccessible points |
| Ultrasonic detector | Picks up the high-frequency sound of turbulent escaping air | Works during production, finds small leaks | Background noise, distance and technique affect results |
| Acoustic imaging camera | Shows leak sound as an image | Fast on large plants, overhead pipes | Sound level is not the same as flow |
An ultrasonic reading can help rank leaks, but converting sound into flow needs a stated method and assumptions. Treat estimates per leak as indicative and prove the total by measuring system flow.
How to measure total leakage
Three practical methods are used, each with conditions that must be recorded.
Off-production flow
With production stopped, measure the residual flow from the compressor room using a flow meter. Subtract the verified legitimate users. Worked example, for illustration: residual flow is 5.0 m³/min and legitimate continuous users take 1.2 m³/min. The unexplained residual is 3.8 m³/min. It is not automatically all physical leakage, but it is the figure to investigate.
Load and unload timing
On a fixed-speed load and unload compressor, with no production running, time the loaded period (T) and unloaded period (t) over several cycles. Leakage is approximately the compressor’s delivered capacity × T ÷ (T + t). For example, if a 10 m³/min machine is loaded for 30 seconds and unloaded for 70 seconds, the estimated leakage is about 10 × 30 ÷ 100 = 3 m³/min. Use the machine’s actual delivered capacity, not just its brochure FAD, and remember that a worn airend delivers less.
Pressure decay
Isolate a known system volume, including receivers and pipework, and record how fast pressure falls between two values. Leakage can be estimated from volume, pressure drop and time. Accuracy depends on knowing the volume, using absolute pressures, stable temperature and making sure no users are active.
Whatever the method, always record the system pressure. A survey at 7.5 bar (108.8 psig) cannot be compared directly with one at 6.5 bar (94.3 psig), because leaks flow more at higher pressure. State the flow basis too: free air delivered at reference conditions, or actual flow.
How to run a leak programme that lasts
- Define the boundary: compressors, headers, zones, machines and operating periods included.
- Measure a baseline: residual demand and pressure before any repairs.
- Survey and tag: give each leak a unique ID, location, component, date, surveyor, severity and pressure, and hang a tag on it.
- Classify repairs: repair now, repair at next stop, needs a spare, or needs shutdown and isolation.
- Repair safely: isolate and depressurise before working on fittings. Never tighten pressurised couplings by force.
- Close each leak: re-check with the detector or soap after repair and record it as closed.
- Verify demand: measure residual demand again under comparable conditions. Illustrative example: residual demand falls from 3.8 to 2.4 m³/min, a measured reduction of 1.4 m³/min.
- Capture the saving: lower the pressure setting if critical users allow, and let surplus compressors stop. A saving that stays in the system as unloaded running is not a saving.
- Repeat: new leaks appear constantly. Schedule surveys and track whether repaired areas stay repaired.
Where leaks usually hide
- Quick couplings, push-in fittings and hose clamps on tools and machines.
- Threaded joints on older galvanised pipework.
- Filter, regulator and lubricator units, especially bowls and drains.
- Condensate drains stuck open or timer drains set too long.
- Cylinder rod seals and valve exhausts on pneumatic machines.
- Flexible hoses cracked by heat and sunlight in outdoor runs.
- Abandoned branches and idle machines left pressurised.
Turning leak flow into energy
To express leakage as energy, multiply the leak flow by the compressor station’s specific power, in kW per m³/min at the stated pressure, and by the hours the system is pressurised. Illustrative example: 1.4 m³/min reduced, at a station specific power of about 6.5 kW per m³/min, is roughly 9 kW. Over 6,000 pressurised hours that is around 55,000 kWh a year. Use your own measured specific power and hours. The saving is only real if compressors actually unload or stop as a result.
Who should own the leak programme?
Leak programmes fade when nobody owns them. Name one person, usually in maintenance or utilities, who holds the leak register, sets survey dates and reports results. Production teams help by reporting audible leaks and closing isolation valves on idle machines, and stores help by keeping common fittings, hoses and drain kits in stock.
Report a small set of figures each month: leaks found, leaks repaired, leaks still open by age, residual demand at a stated pressure and whether a compressor was able to stop as a result. These numbers show quickly whether repairs are keeping pace with new leaks.
Design choices matter too. Fit isolation valves on each zone and each machine so idle areas can be shut off outside production hours. Prevention costs less than repeated surveys.
Safety when repairing leaks
Compressed air stores energy, and leak repairs are a common cause of minor injuries. Isolate and vent the section before loosening any fitting, and confirm with a gauge that it is at zero pressure. Never use compressed air to clean skin or clothing, and keep hoses restrained so a failed coupling cannot whip. Follow the plant’s permit-to-work and isolation rules for any work on receivers or main headers.
How Reddy Compressors helps
Reddy Compressors has served industry from Tarnaka, Secunderabad since 2002 as an independent multi-brand compressor specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. Our more than 20 service engineers and technicians work on ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands.
- We carry out leak surveys and tagging as part of our air audits.
- We measure residual demand and load and unload behaviour to estimate total leakage.
- We repair leaks in compressor rooms, treatment and main distribution, and supply drains, fittings and spares.
- We check compressor delivery so leakage estimates are based on real capacity.
- We review pressure settings and sequencing after repairs so the saving is captured.
- We include periodic leak checks within service and AMC arrangements where agreed.
Our service teams cover all of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India.
What to send us
- Number, make and model of compressors, with nameplate photos.
- Running and loaded hours from each controller.
- Working pressure setting and the pressure at critical users.
- Production hours and when the plant is fully stopped.
- A layout or sketch showing main headers and zones.
- Any past leak survey or audit reports.
Frequently asked questions
How do I check my compressed-air system for leaks?
Start by listening around the plant when production is stopped, then use an ultrasonic leak detector during normal running to find smaller leaks. Tag each one with an ID and location. To measure total leakage, record compressor load and unload times or measure residual flow with production off, after subtracting legitimate users such as dryer purge and instrument air.
How do I calculate air leakage from load and unload time?
With production stopped and a fixed-speed compressor cycling, time the loaded period and the unloaded period over several cycles. Leakage is roughly the compressor’s actual delivered capacity multiplied by loaded time divided by total cycle time. For example, 30 seconds loaded out of 100 seconds on a 10 m³/min machine suggests about 3 m³/min of leakage.
Why does my compressor run all night when the plant is closed?
Usually because of leaks and continuous users that remain pressurised. Desiccant dryer purge, timed drains, instrument air and idle machines left connected all take air. Measure the residual flow, separate legitimate users, then find and repair leaks. Isolating unused zones with valves outside production hours also reduces the load considerably.
Does reducing pressure reduce leakage?
Yes. Leaks flow more at higher pressure, so lowering the system pressure reduces leak flow as well as the power needed for compression, which is about 7% for each bar. Pressure should only be lowered to a level that still meets the minimum needs of critical users, measured at those users rather than at the compressor.
How often should a leak survey be done?
Leaks reappear continuously, so one survey is never permanent. Many plants survey every few months, with smaller checks in high-risk areas such as tool points and machines. The right interval depends on how quickly leakage returns, which you can see by tracking residual demand or load and unload behaviour between surveys.
Can ultrasonic detectors measure how much air a leak wastes?
They are best for finding and ranking leaks. Some instruments estimate flow from sound level, but the result depends on distance, pressure, leak shape and background noise, so treat it as indicative. The total saving should be proved by measuring system demand before and after repairs under comparable pressure and operating conditions.
Related pages
Field guide
Leakage is a load, a capacity loss and a capital question, not only a maintenance defect
A leak is air that has already been paid for: electricity into the motor, compression, cooling, drying, filtration, storage and distribution, then released to atmosphere before it does any work. The compressor cannot tell 100 cfm going to production from 100 cfm escaping through fittings, so every leak becomes extra loaded running hours.
Leakage is also a capacity problem
Take a station that can make 1,000 cfm, with 200 cfm lost to leaks. Production effectively has 800 cfm. Before another compressor is bought, ask how much of the capacity already installed reaches productive users. A worked split of a measured 1,000 cfm might be 750 cfm of legitimate demand, 180 cfm of leakage and 70 cfm of avoidable demand. Sizing a new machine from the full 1,000 cfm would build capital into waste. The numbers here are an illustration of the method; measure your own with the demand calculator.
Leakage and redundancy
With two 500 cfm machines and 900 cfm of total demand (700 productive plus 200 leakage), both must run. Removing the 200 cfm of leakage reduces demand from 900 to 700 cfm, but 700 cfm is still more than one 500 cfm machine can deliver, so both machines are still required. Leak repair reduces loading, running hours and energy, and leaves more margin on each machine, but it does not create full standby. True N+1 standby needs the verified critical demand at the required pressure to be met with the largest machine out of service. A small rise in demand can also be enough to start an extra machine, which changes the efficiency of the whole station, so the penalty is not always proportional to the leak size.
Pressure and leakage feed each other
Low pressure at the machine is often answered by raising the compressor setpoint. Leak flow rises with pressure, so demand rises, compressors load longer and the pressure complaint can remain. Treat leakage and pressure together: find where pressure is actually lost (see the pressure-drop audit and pressure optimisation), repair leaks, then check whether the setpoint can come down.
The leak management loop
- Detect. Ultrasonic instruments pick up the high-frequency signature of turbulent escape that ears miss in a noisy plant.
- Tag. Every leak gets an ID, a location and equipment reference.
- Quantify. Estimated flow, operating pressure, hours per year and annual cost. A small leak on a 24×7 main can cost more than a larger one on an occasional machine.
- Prioritise. Rank by flow × hours × pressure × energy cost, adjusted for repair difficulty and process criticality.
- Repair the physical defect: couplings, hoses, threaded joints, drains, regulators, seals, valves, abandoned branches.
- Verify under operating pressure. A closed work order is not proof the leak is gone.
- Trend and re-survey. Leaks come back as seals age and fittings loosen.
Keep the findings in a leak repair register and read the cost side with the leakage cost calculator.
Use the off-shift baseline
Ask how much air the plant uses when it is not producing. If compressors keep running or cycling with machines idle, the remainder is leakage, drains passing air, open blow-offs, unisolated machines or purges. This often shows hidden waste more clearly than daytime observation.
Leakage is not the only non-productive demand
Open blow guns, oversized air knives, continuous enclosure purging, air used to cool people, excessive blow-off pressure, inefficient vacuum generation and abandoned equipment still connected all belong in the same conversation. The question for each is whether the airflow creates useful value.
Better measures than “leaks found”
| Weak measure | Better measure |
|---|---|
| Number of leaks found | Estimated leakage in cfm or Nm³/h, and as % of total demand |
| Number of leaks repaired | Verified repair rate and flow recovered |
| Power fell | Specific power before and after correction |
| Need another compressor? | Capacity recovered from the system already owned |
The realistic target is not zero leakage. It is controlled leakage at the lowest level that is technically and economically sustainable, and a useful-air ratio (useful air ÷ total generated air) that you track over time.
Next step: request an audit and measurement visit.
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.