What compressed air leaks really cost (and why the usual guess is low)

Ask a plant what its air leaks cost and you will usually get one of two answers: "not much," or a number from a leak survey that covers the shifts the plant runs. Both are usually low. Leaks don't keep shift hours, a survey never finds every leak, and a leaking system costs more than the air that escapes.

Start with one leak

The U.S. Department of Energy publishes how much air escapes through a hole of a given size. A 1/8-inch hole at 100 psig passes about 26 scfm if it is a clean, round opening. A real leak, a cracked fitting or a worn hose coupling, is rarely that clean. Expect around 16 scfm.

A typical plant uses about 20 kW of compressor power to make 100 scfm. So that one leak takes about 3.2 kW whenever the system holds pressure. At $0.12 per kWh:

• Counted over two shifts (4,160 hours a year): about $1,600 a year.
• Counted over every hour the system holds pressure (8,760 hours): about $3,300 a year.

That is one hole you can barely see, costing as much as a small motor running all year.

Now a whole plant

A leak survey in a plant that hasn't had one in a while might find ten small leaks (1/16 inch), five medium (1/8 inch) and one large one (1/4 inch). Together, that is about 180 scfm, and about 36 kW of compressor power.

• Over two shifts: about $18,000 a year.
• Over every hour the system holds pressure: about $38,300 a year.

The Department of Energy puts leaks in a poorly maintained system at 20 to 30% of what the compressors make. A well-run system keeps them under 10%. Most plants are somewhere in between, and few know where.

Try your own numbers: Compressed air leak cost calculator

Why the usual guess is low

1. Leaks run on nights and weekends

The biggest miss is the hours. Estimates often count only production hours, because that is when the air is "being used". But leaks blow whenever the system is pressurized. If the compressors stay on over a weekend to keep the plant ready, or to feed a few small loads, much of what they make in those hours is leaks. In a plant that runs two shifts, the hours with no production outnumber the hours with it.

2. A survey finds most leaks, not all

An ultrasonic survey finds the leaks you can reach and hear: in the open, at fittings, hoses and drops. It misses leaks inside machines, behind guards, overhead, and in areas that are too noisy to survey. So the leaks found are a floor, not the total.

3. Leaks push the pressure up

When leaks pull the system pressure down, the usual fix is to raise the compressor setpoint. Higher pressure makes every leak worse, and it raises the air every unregulated use draws. That added load is often called artificial demand. Each 2 psi of added pressure costs about 1% more compressor power.

4. Leaks buy compressors

When demand creeps up to the limit of the compressors, the answer is often another compressor. If a fifth of the air is leaks, part of that new compressor, its installation and its maintenance is paid for to feed holes. A compressor that runs more hours also needs service sooner.

The other side: what fixing them actually saves

The cost of a leak and the savings from fixing it are not always the same number. When a leak is fixed, demand drops, and the savings depend on how the compressors respond.

• A variable speed compressor slows down, and the power falls close to in step with the air.
• A load/unload compressor with good storage spends more time unloaded, which still saves a fair share.
• A compressor throttling its inlet (modulating) saves only a little. It can still draw most of its full power at well below full output.
• If fixing leaks lets a compressor shut off for whole shifts, the savings can be larger than the leak's share of the air.

So a leak program can pay off well or disappoint, with the same leaks fixed. The difference is the supply side, and that is worth knowing before you promise a number to management.

Getting to the real number

• Log the system over a full week, including a weekend. What the compressors make with production stopped is mostly leaks and the few loads that run all the time.
• Count every hour the system holds pressure, not just the production hours.
• Tag and fix in order of size. A few large leaks are often half the total.
• Check the savings against the compressors' controls. Know how they will respond before you count the dollars.
• Lower the pressure after the fixes. Fixing leaks often lets the setpoint come down, and that saves on every use, not just the leaks.
• Repeat. Leaks come back. A survey once or twice a year keeps the gains.

Leaks are rarely the whole story, and rarely as small as they look. Measure them over every hour the system runs, and judge the savings by how the compressors will respond.

Source: U.S. Department of Energy, Improving Compressed Air System Performance: A Sourcebook for Industry, leak rates by orifice size and typical leak levels. The examples assume 100 psig, 20 kW per 100 scfm and $0.12 per kWh, as in our calculator.