Short answer: A 5 m³ system that loses 1 bar in 10 minutes with the compressor off is leaking about 0.49 m³/min, which costs roughly 1,400 USD a year (per 0.08 USD/kWh · 6,000 h/yr · 6.0 kW per m³/min). Three hours inside your own plant — a decay test, an ultrasonic walk-round and a repair list — normally finds and prices most of it without a survey contractor.
By Johnny Wayne, General Manager, AirSpace Machinery Co., Ltd. — 20 years in industrial compressed air systems. Reviewed by our engineering team.
We started running leak surveys in-house because the quotes we saw priced a two-day contractor visit at more than the leaks were worth on a 45 kW system. The method below is what we now use on site: one decay test for the total, one walk-round for the individual leaks, one hour to fix the easy ones. It is no substitute for a full ISO 11011 assessment on a 400 kW plant, but on 30–150 kW systems it pays for itself the same afternoon.
What does a compressed air leak actually cost per cubic metre?
Every cubic metre per minute of leak is a cubic metre per minute the compressor must make, so it is charged at the compressor’s specific power rather than at the price of a repair kit. At 6.0 kW per m³/min on a 7 bar system, a 0.5 m³/min leak draws about 3 kW continuously — 18,000 kWh a year at 6,000 hours, or 1,440 USD.
That is the number that changes behaviour. A leaking push-fit fitting looks like a five-dollar part; it is really a 1,400 USD a year subscription, and it renews every year until someone tags it.
How do you measure total leak rate with a pressure decay test?
Shut production down, let the system stabilise, stop the compressor, and time the pressure drop against known storage volume. The leak rate is Q = V × Δp / (t × 1.013), with V in m³, Δp in bar, t in minutes, and 1.013 bar as atmospheric pressure.
The storage volume is the part most plants get wrong: it is the receiver plus the pipework, and pipework is often 20–30% of the total. If the nameplate says 2 m³ and the header runs 200 metres of 100 mm pipe, the effective volume is closer to 3.5 m³, and using the wrong figure understates the leak by a third.
What does that look like on a worked example?
Take a 75 kW plant at 7 bar with 5 m³ of effective storage. With production stopped, the header falls from 7.0 to 6.0 bar in 10 minutes:
- Pressure drop: 7.0 − 6.0 = 1.0 bar.
- Stored free air released: 5 m³ × 1.0 / 1.013 = 4.94 m³.
- Leak rate: 4.94 / 10 = 0.49 m³/min.
- Share of a 12.8 m³/min machine: 0.49 / 12.8 = 3.8% of output.
- Cost: 0.49 × 6.0 kW per m³/min = 2.9 kW, which is 17,600 kWh and 1,410 USD a year at 0.08 USD/kWh and 6,000 hours.
A result under 5% is a healthy plant; sites that have never been surveyed typically come back at 10–25%.
How do you find the individual leaks in one hour?
Walk the header with an ultrasonic detector, in this order: compressor room first, then the main header, then the distribution drops, then the point-of-use hoses and tools. Ultrasound ignores the background hiss of a working factory, which is why it finds leaks a soap test never will on a running line.
Log every hit with four fields: location, size class, repair type, and whether it needs a shutdown. The log matters more than the detector — a survey that produces 60 tags and no list is a walk.
What does the three hours actually look like?
| Hour | What you do | Output |
|---|---|---|
| 1 (20 min) | Stop production, stabilise, run the decay test | Total leak rate in m³/min and USD/yr |
| 1 (40 min) | Ultrasonic walk-round: compressor room, header, drops, tools | Tagged leaks with location and size class |
| 2 | Quantify each tag, photograph it, price the repair | Ranked list by annual cost, not by size |
| 3 | Fix the top 20 non-shutdown repairs, then re-run the decay test | Verified reduction, and a repeatable baseline |
The third hour is the one most surveys skip. Fixing twenty joints while the plant is still down turns a report into a measured saving, and the second decay test proves it in a number finance can accept.
Which leaks are worth fixing first?
Rank by annual cost, which follows orifice area, not by how loud the leak sounds. At 7 bar and 6.0 kW per m³/min, the arithmetic is brutal at the top end of the size range:
| Orifice at 7 bar | Free air lost | Continuous power | Annual cost |
|---|---|---|---|
| 1 mm | about 0.04 m³/min | 0.24 kW | about 115 USD |
| 3 mm | about 0.36 m³/min | 2.2 kW | about 1,050 USD |
| 6 mm | about 1.45 m³/min | 8.7 kW | about 4,180 USD |
Costs are per 0.08 USD/kWh · 6,000 h/yr · 6.0 kW per m³/min, scaling by orifice area as the standard orifice tables do. One 6 mm hole is worth thirty-six 1 mm holes, which is why the first pass should be the largest openings — usually a cracked hose, a missing cap on an unused drop, or a condensate trap stuck open.
What does a repair actually pay back?
Twenty tagged leaks averaging 0.05 m³/min add up to 1.0 m³/min, or 6 kW, or about 2,880 USD a year. The repairs are typically 12 labour-hours at 25 USD/h plus 150 USD of fittings — 450 USD — so the payback is under two months and the work is done inside the same shift.
That ratio is why leak repair sits ahead of almost every capital project on a compressed air system, including a new compressor: capacity you already paid for is the cheapest capacity there is.
What did the numbers look like in a real installation?
Our documented 75 kW PMV VSD installation case study in Vietnam is an iron processing plant in Ho Chi Minh City: 15,000 m², more than 200 employees, 16 hours a day, compressed air costing over 3,200 USD a month in electricity, maintenance at 850 USD a month, and pressure swinging ±0.5 bar before the change.
In practice the leak survey was not the project — the machine replacement was. But a plant spending 3,200 USD a month on air is spending it on cubic metres, and every leak is billed at the same rate as production air. That is the argument to make internally: leaks are not a maintenance item, they are a line in the energy budget.
How do leaks behave in textile, ceramic, glass and cement plants?
A low-pressure air compressor for the textile / ceramic / glass / cement industry runs at 3–5 bar, so a given orifice passes less air than at 7 bar — but these plants compensate with far more connections, and that is where the leaks live.
| Industry | Where leaks concentrate | Typical pressure | What to check first |
|---|---|---|---|
| Textile | loom drop hoses, quick-couplers, solenoid banks | 3–5 bar on low-pressure systems | couplings and hose barbs at each loom |
| Ceramic | press blow-off, spray-dryer atomisation, glaze lines | 3–5 bar on low-pressure systems | valve seats and flexible connectors |
| Glass | mould cooling, blow-off nozzles, conveyor drops | 3–5 bar on low-pressure systems | nozzle threads and manifold joints |
| Cement | outdoor conveying lines, pulse-jet valves, silo aeration | 4–6 bar typically | diaphragm valves and outdoor pipe joints |
Those pressures are typical for the process and should be confirmed against the machine datasheet. The low-pressure series is rated 15–300 HP for 3–5 bar duty, and on those systems a leak that would be minor at 7 bar can be a measurable share of output.
Why does a leak survey beat buying a bigger compressor?
Because the leak is already in the system and the capacity to feed it is already installed. A plant leaking 15% of 12.8 m³/min is throwing away 1.9 m³/min, which is roughly 11 kW — more than the difference between two adjacent machine sizes, and available for the cost of a week of maintenance time.
Buying capacity to cover leaks also buys the energy to run it, every hour, for the life of the machine. Fixing the leaks buys almost nothing and saves the same energy.
Key takeaways
- Measure the total first: a decay test on known storage gives the leak rate in ten minutes, with no contractor and no production loss.
- 0.49 m³/min costs about 1,410 USD a year at 6.0 kW per m³/min · 0.08 USD/kWh · 6,000 h/yr.
- Rank by orifice, not by noise: one 6 mm leak is worth about 4,180 USD a year, thirty-six times a 1 mm leak.
- Fix inside the same shift: twenty small repairs typically cost 450 USD and return about 2,880 USD a year.
- Re-test after repair so the saving is a measured number rather than a claimed one.
Frequently asked questions
Can I do a leak survey while production is running?
Partly. The decay test needs the compressor off and demand at zero, so it has to happen during a shutdown, a lunch break or a weekend. The ultrasonic walk-round can run alongside production, because ultrasound is directional and the detector filters factory noise.
How accurate is a pressure decay test?
Within about 10% when storage volume is known and the system is stable — enough to decide whether to act. The error comes from unmeasured pipework volume and from temperature drift, so test at a steady temperature.
Do I need to buy an ultrasonic detector?
For occasional use, a rental for a week typically costs less than 300 USD and covers a full plant. For plants above 100 kW, buying one pays back after the second survey, and it also finds bearing and trap faults between surveys.
How often should a plant repeat the survey?
Every 12 months with a maintenance team, every 6 months where hoses and quick-couplers sit at every workstation. Leaks regrow, and a repeat survey is faster because the baseline and tag list already exist.
What is a realistic leak rate for a well-maintained plant?
Typically 5–10% of compressor output, against 20–30% on sites that have never been surveyed. Anything above 10% is worth a targeted repair programme before any capacity decision is made.
Sources and standards
- ISO 11011:2013 — Compressed air — Energy efficiency — Assessment. The method behind specific power in kW per m³/min and the supply-side measurement approach used here.
- ISO 8573-1:2010 — air purity classes; leaks also change dew point behaviour downstream, which matters once a dryer is sized on measured flow.
- ISO 1217:2009 — acceptance testing of displacement compressors, the basis for any FAD figure quoted at 7 bar.
- ISO 9001:2015 and CE marking under Machinery Directive 2006/42/EC — held by AirSpace Machinery Co., Ltd., with certificate scans on our downloads page.
- Our published references — the guide to running a simple compressed air leak audit this week, and the low-pressure compressor series rated 15–300 HP at 3–5 bar.
About the author
Johnny Wayne (Wei Zhuang) is General Manager of AirSpace Machinery Co., Ltd., the Shanghai manufacturer behind ChinaCompressor.org. The company builds oil-injected and Class 0 oil-free screw compressors from 2 to 540 HP (1.5–400 kW), including the low-pressure series at 15–300 HP for 3–5 bar duty, from a 4,000 m² production facility with 20 years of engineering history. Products carry CE marking and ISO 9001:2015 certification, and we sell through distributors rather than around them.
Send your flow, discharge pressure and running hours to sales@chinacompressor.org and we will size the survey against your own storage volume, or start with the free compressed air kWh forecast and system review.
Related guides
- How Do You Find the Best Air Compressor for Your Textile Business in China?
- How Do You Convert CFM to kW and FAD to Actual Output? A Buyer’s Sizing Worksheet for 50–500 CFM Systems
- How do you Verify a TÜV Class 0 Certified Chinese Air Compressor Supplier , 5 Red Flags Your Procurement Team Can’t Ignore?
⚡ Is Your Factory Bleeding Cash?
Most fixed-speed compressors waste $3,600/year in "Unload Tax." Test your waste level in 30 seconds.
RUN THE ROI TEST →Ready to See How Much Your Facility Can Save?
Fill out the form below for a Free Energy Assessment.
