A typical compressed air system leaks between 20% and 30% of the air it generates. On a site running a 55kW compressor continuously, that works out somewhere between £8,000 and £12,000 a year in wasted electricity — before you factor in the cost of running the compressor harder to maintain pressure.
I've done leak audits on dozens of sites. The range is remarkable. A well-maintained food production facility I visited last year had leakage below 8%. A fabrication shop I walked around the following month was losing over 40%. Both were running the same age of equipment.
Why Leaks Get Ignored
Part of the problem is visibility. Unlike a water leak, a compressed air leak is invisible and largely silent at normal background noise levels. The only immediate symptom is that the compressor runs more. If you're not tracking your specific energy consumption — kilowatts per cubic metre of free air delivered — you probably won't notice.
The other issue is that leaks develop gradually. A fitting that seals perfectly when installed starts to weep after a few years of vibration, pressure cycling and temperature variation. By the time it's significant, it's been there long enough to feel normal.
Where the Air Goes
In my experience the biggest culprits are:
Threaded connections — particularly where BSP fittings have been assembled without proper thread sealant, or where PTFE tape has been applied incorrectly. One thread applied with tape in the wrong direction will leak from day one.
Push-fit fittings — the collet-style connections used in most pneumatic circuits are reliable when new, but the collet and O-ring wear over time. On a system that's ten years old, a quarter of the push-fits will be weeping at least slightly.
Flexible hose connections — the crimped ferrules on air hoses are a common failure point, especially on hoses that get coiled and uncoiled repeatedly.
Condensate drains — float-operated condensate drains that have failed open are the single highest-flow leaks I find. A stuck-open float drain can dump several litres per minute of compressed air. On a busy site with ten drains, two or three failed ones are common.
Quantifying the Cost
The calculation isn't complicated. Take your compressor's power consumption (kW), multiply by your electricity unit cost (pence per kWh), multiply by annual running hours, and multiply by your estimated leak percentage. Divide by 100 to get pounds per year.
A 75kW compressor running 6,000 hours a year at 12p/kWh costs roughly £54,000 in electricity annually. If 25% of that air is leaked, you're spending £13,500 a year on air that goes nowhere.
That makes even a £3,000 ultrasonic leak audit — which is what a thorough job typically costs — look like an obvious decision.
Fixing What You Find
Ultrasonic detection equipment finds leaks quickly and accurately, even in noisy environments. A competent engineer with a good ultrasonic detector can survey a reasonably sized site in a day and produce a prioritised repair list.
Most repairs are straightforward. Thread sealant on a leaking compression fitting takes ten minutes. Replacing a worn push-fit takes five. The investment isn't in the repair itself — it's in systematically finding and logging every leak rather than just fixing the obvious ones.
One thing worth noting: fix leaks before you consider downsizing your compressor. Sites that fix leaks often find their existing compressor now has excess capacity for the first time in years, which changes the calculation for capital investment significantly.