During a compressed air audit, 80, 150 or even several hundred individual leaks can be found within just a few hours. A leaking coupling loses 300 Nl/min, a valve 120 Nl/min, while several small leaks on a machine may each release only 20 or 30 Nl/min.
The obvious approach is: repair the largest leak first.
For economic prioritization, however, the leak rate alone is not sufficient. What matters is how long the respective leak is actually pressurized. A leak of 300 Nl/min on a machine that operates only a few hours per week and is completely isolated outside this period can waste less energy over a year than a significantly smaller leak in a main line that remains pressurized for 8,000 hours per year.
In addition, network pressure, the condition of the compressor system, the specific energy demand of compressed air generation and the actual electricity price all influence the economic significance.
The repair itself must also be included in the evaluation. A leak causing an annual loss of 100 euros that can be eliminated within a few minutes by tightening a fitting has a completely different economic profile from a similarly sized leak that requires an entire production area to be shut down for several hours.
Modern ultrasonic leak detectors and acoustic cameras simplify this task because, in addition to locating the leak, they can display an estimated leak rate and in some cases directly calculate annual costs. For reliable prioritization, however, the stored operating parameters must match the actual installation.
The most important rule is therefore: A compressed air leak should not be prioritized solely according to sound level or leak rate. Only the combination of leak rate, actual pressurization time, operating pressure, energy demand of compressed air generation, electricity costs and repair effort shows which leak should be eliminated first from an economic perspective.
Why the largest leak should not automatically be repaired first
A high leak rate is initially a strong indication of substantial savings potential. However, it only describes the current air loss under the conditions at which the measurement was performed.
For annual costs, at least one additional variable is required: time.
A leak only loses compressed air when sufficient pressure is actually present at the relevant point. If the leak is located downstream of an automatically closing machine valve, it may cause practically no loss outside production hours.
A smaller leak in a ring main, on the other hand, may continue losing compressed air at night, over weekends and during non-production periods if the main network remains pressurized.
A smaller continuous leak can therefore be more economically significant than a much larger leak with only a short operating period.
The repair effort must also be considered. An easily accessible leaking push-in coupling may be replaceable within a few minutes. A leaking fitting on a difficult-to-access installation, however, may require scaffolding, production clearance and a planned shutdown.
Prioritization should therefore be based not only on size, but on economic impact.
What the leak rate actually describes
The leak rate describes how much compressed air escapes through a leak under defined conditions.
Typical values include, for example:
20 Nl/min
150 Nl/min
2.5 Nm³/h
The “N” indicates a gas volume referenced to defined standard conditions. This makes it much more meaningful to compare air quantities at different pressure and temperature conditions than using only actual volumetric flow rates.
For an economic assessment, the reference conditions used by the measuring instrument should be clearly documented.
The operating pressure during the leak measurement is also particularly important.
A fixed leak opening does not have a completely constant leak rate independent of the network pressure. As the pressure difference between the line and the surrounding atmosphere increases, more air can escape through the same opening.
A value of 80 Nl/min at 6 bar should therefore not be compared directly with a value of 100 Nl/min at 8 bar.
For economic prioritization, the leak rate should wherever possible be evaluated at the typical actual operating pressure of the respective installation.
Determine the correct operating time
Operating time is one of the most frequently underestimated factors in cost calculation.
Annual production hours are often used as a general assumption. This may be correct, but it does not have to be.
The decisive question is:
How many hours per year is this specific leak actually pressurized?
A central main line may remain pressurized all year round even though production operates only in two shifts.
An individual machine, by contrast, may have an automatic shut-off valve and be completely disconnected from the compressed air network outside its production time.
In this case, plant operating time and leakage time differ significantly.
Weekends must also be included in the assessment. If a compressed air network remains pressurized from Friday evening until Monday morning, leaks can continue during this entire period even though no products are being manufactured.
This production-free leakage time is particularly important economically because the generated compressed air provides no productive benefit at all.
Consider network pressure in the evaluation
The network pressure influences both the leak rate and the energy required to generate compressed air.
A leak inspected at a low network pressure may lose a different quantity of air under the later normal operating pressure.
For a fair comparison of several leaks, all measurements should therefore either be carried out under comparable pressure conditions or the values should be referenced to a defined common condition.
This is particularly important if an audit is carried out over several days and the network pressure varies between shifts or plant areas.
Without this information, a leak measured at 8 bar could appear larger simply because of the higher pressure than a technically similar leak measured at only 6 bar.
For reliable prioritization, the measured or assumed operating pressure should therefore be included in every leak data record.
Calculate annual loss from leak rate and operating time
The first step in the economic assessment is to extrapolate the instantaneous leak rate to the actual operating time.
If the leak rate is stated in Nl/min, it can first be converted to Nm³/h:
qN [Nm³/h] = qN [Nl/min] × 60 / 1,000
A leak of 100 Nl/min therefore corresponds to:
100 × 60 / 1,000 = 6 Nm³/h
If this point is pressurized for 6,000 hours per year, the annual air loss is:
6 Nm³/h × 6,000 h = 36,000 Nm³/a
This value is already much more illustrative. Instead of a seemingly small leak of only 100 litres per minute, it becomes clear that tens of thousands of standard cubic metres of compressed air can be lost over the course of a year.
For the economic assessment, however, the energy required to generate this quantity of air is still missing.
From air loss to energy consumption
Compressed air is not an independent energy source. It is generated by compressors using electrical energy and is then cooled, dried, filtered and distributed through the network.
The electrical energy required per standard cubic metre generated is therefore a key parameter for cost calculation.
Wherever possible, a general internet benchmark should not be used. Instead, the actual specific energy demand of the existing compressor system should be applied.
This can be determined from energy measurement and the generated compressed air quantity or derived from the existing compressed air energy monitoring system.
In simplified form:
specific energy demand = electrical energy / generated standard volume
The unit can, for example, be:
kWh/Nm³
The less efficiently the compressed air generation system operates, the more expensive every lost standard cubic metre of compressed air becomes.
The same leak can therefore cause different energy costs in two different plants.
Use the actual electricity price
The final essential factor for calculating pure energy costs is the electricity price.
Here too, a general value is only suitable for an initial estimate.
For internal economic decisions, the energy price that is actually relevant to the company should be used wherever possible.
Depending on the company’s costing method, this may be the pure energy charge, an internal transfer price or a complete calculated electricity price.
Above all, the same calculation basis should be used for all leaks.
Otherwise, a ranking may result that varies not because of technical differences, but because different assumptions were used.
For an energy audit, the electricity price and specific energy demand used should therefore be documented.
Calculate annual leakage costs
These parameters allow the economic significance of a compressed air leak to be calculated transparently.
In simplified form:
Annual loss [Nm³/a] = leak rate [Nm³/h] × pressurization time [h/a]
Then:
Energy loss [kWh/a] = annual loss [Nm³/a] × specific energy demand [kWh/Nm³]
and finally:
Leakage costs [€/a] = energy loss [kWh/a] × electricity price [€/kWh]
This calculation is more transparent than a general euro value because every assumption remains traceable.
A leak detection instrument may already perform these steps automatically. Nevertheless, it should still be known which values for operating time, pressure and energy costs are stored in the instrument or evaluation software.
Only then does the displayed annual amount have real meaning for the specific installation.
Practical example: large leak or continuous leak?
Two leaks are found during an audit.
Leak A loses 300 Nl/min on a production machine. The machine is pressurized for only 1,000 hours per year.
Leak B loses only 100 Nl/min, but is located in a permanently pressurized main distribution line and is active for 8,000 hours per year.
For Leak A:
300 Nl/min = 18 Nm³/h
18 Nm³/h × 1,000 h/a = 18,000 Nm³/a
For Leak B:
100 Nl/min = 6 Nm³/h
6 Nm³/h × 8,000 h/a = 48,000 Nm³/a
Although Leak B has only one third of the instantaneous leak rate, it loses more than two and a half times as much compressed air over the course of a year.
For illustration purposes, assume a specific energy demand of 0.11 kWh/Nm³ and an electricity price of €0.20/kWh.
Leak A then causes:
18,000 × 0.11 × 0.20 = €396/a
Leak B causes:
48,000 × 0.11 × 0.20 = €1,056/a
These figures are for illustration only. For a real installation, the actual energy demand and operating costs must be used.
However, the example demonstrates the decisive point: For economic prioritization, the annual impact is more important than the leak rate alone.
Include repair costs and payback period
The annual energy loss does not yet fully answer the question of which leak should be repaired first.
The repair effort must also be considered.
A leaking coupling may be repairable using a replacement part costing only a few euros and ten minutes of labour.
Another leak may be located on a line six metres above ground and can only be repaired during a planned production shutdown.
The payback period is therefore useful for economic decisions:
Payback period = repair costs / annual savings
If repair costs and savings are stated in euros, the result can, for example, be converted into months:
Payback period [months] = repair costs / annual savings × 12
A repair costing €100 with annual savings of €1,000 pays back in approximately 1.2 months.
A repair costing €2,000 with the same annual savings, by contrast, requires around two years.
Both leaks have the same energy loss, but a completely different economic repair priority.
Distinguish between absolute savings and fast payback
Two different economic indicators can be useful in a leak list.
The first is the absolute annual saving. It shows where the most money is being lost overall.
The second is the payback period. It shows which repair produces a positive economic effect particularly quickly.
A very small, freely accessible leak can have an excellent payback period even though its annual loss is relatively low.
A large leak on a difficult-to-access main line may offer much greater absolute savings, but require a planned plant shutdown.
Good prioritization therefore considers both perspectives.
| Situation | Economic significance | Typical planning |
|---|---|---|
| High annual costs, very simple repair | Very high priority | Repair as soon as possible |
| High annual costs, repair possible only during shutdown | High absolute savings | Schedule firmly for the next suitable shutdown |
| Low annual costs, repair possible within a few minutes | Very short payback possible | Repair directly or promptly during the inspection round |
| Low annual costs, high repair effort | Low economic priority | Combine with a larger maintenance activity |
However, this economic classification should never result in safety- or process-critical leaks deliberately being left unrepaired. Technical risks take priority regardless of pure energy costs.
Machine operating time is not automatically compressed air operating time
For accurate cost calculation, a distinction must be made between production time and pressurization time.
A machine may, for example, produce for only 2,000 hours per year but remain connected to the compressed air network for 5,000 hours.
If the machine valve remains open outside production periods, internal leaks continue during setup times, breaks and non-production shifts.
Conversely, a machine may operate in three shifts but use an automatic shut-off valve to pressurize individual pneumatic circuits only during certain process steps.
The actual leakage time can therefore be significantly shorter than the machine operating time.
For larger economic savings opportunities, it is therefore worthwhile to consider not only the shift schedule but also the actual compressed air supply of the respective machine.
This analysis alone can reveal additional savings opportunities. If a machine is automatically isolated from the network during breaks, all unrepaired leaks downstream of this valve are also reduced during these periods.
Give special consideration to leaks in the main network
Leaks in main and ring lines often have particular economic significance.
They are typically located upstream of local machine shut-off valves and therefore lose compressed air for as long as the central network remains pressurized.
Even a comparatively small leak rate can therefore accumulate a very high number of annual operating hours.
In addition, such leaks often cannot be assigned directly to one production area and may therefore remain unnoticed for a long time.
A systematic leak survey should therefore not investigate only machines, couplings and tools.
Distributors, wall and ceiling lines, filter stations, condensate drains and main valves should also be included in the audit.
Night-time or weekend measurements in particular can reveal main-network losses that disappear within normal production consumption during operating hours.
Correctly assess acoustically estimated leak rates
Acoustic leak detectors and ultrasonic cameras are particularly powerful because a leak can be located during normal operation without opening or modifying the line.
Modern systems can additionally estimate the leak rate from the ultrasonic level, distance, operating pressure and device-specific models.
This quantification is extremely valuable for prioritization.
However, it should not be confused with direct, billing-grade flow measurement.
The acoustic signature of a leak depends, among other things, on opening geometry, distance, pressure, viewing angle, reflections and ambient conditions.
The resulting l/min value should therefore primarily be understood as a tool for comparative assessment and maintenance planning.
If 100 leaks are investigated under conditions that are as similar as possible, the locations with the greatest savings potential can be identified very efficiently.
For economic prioritization, this relative information is often much more useful than a supposedly exact individual measurement to the last percent.
Plausibility-check total leakage using flow measurement
Acoustic localization and permanently installed flow measurement answer different questions.
The ultrasonic instrument shows where a leak is located and approximately how large it is.
A flow meter in a main or area supply line, by contrast, shows how much compressed air the complete plant section is actually consuming.
The two methods can therefore complement each other effectively.
If, during a defined production shutdown, a standard volumetric flow of 120 Nm³/h is still being measured into a plant area even though all intended consumers have been switched off, there is significant base consumption.
This base consumption can consist of leaks, intentionally operated auxiliary consumers, purge air or incorrectly switched consumers.
Acoustic leak detection can then be used to identify the individual causes.
After the repair campaign, the area flow measurement shows whether the total consumption has actually decreased.
This creates a very reliable link between local leak detection and overall energy consumption.
Use idle consumption as a key performance indicator
A particularly informative time for compressed air analysis is a period without regular production.
When machines are stopped but the network remains pressurized, a large proportion of normal useful consumption disappears.
The remaining flow then consists mainly of leaks and deliberately operating base-load consumers.
This value can be used as an operational key performance indicator.
If idle consumption falls, for example, from 80 to 35 Nm³/h after a leak repair campaign, the effect becomes directly visible in the network regardless of the sum of individual leak reports.
For fair trend assessment, network pressure, plant condition and consumers that remain active should be as comparable as possible.
Such a measurement does not replace leak detection, but provides valuable overall verification.
Do not confuse pressure reduction with leak repair
Reducing the network pressure can reduce the air loss from existing leaks.
However, the leaks themselves are not eliminated.
If the pressure is later increased again, the loss also increases again.
Pressure optimization and leak management are therefore two separate measures that complement each other.
The network pressure should be set as low as technically possible for the process. At the same time, technically unnecessary leaks should be systematically eliminated.
Changes in network pressure should be recorded in the documentation.
If the measured total leakage decreases after a repair campaign while the network pressure has simultaneously been reduced from 8 to 6 bar, the improvement does not result solely from the repairs carried out.
Reliable before-and-after comparisons therefore require comparable operating conditions or appropriate consideration of the pressure differences.
Add production and safety relevance
Not every priority can be expressed in euros per year.
A small leak in a pneumatic line can, for example, cause a gradual pressure drop that makes a gripper or actuator unreliable.
The immediate energy loss may be small, while an unplanned production shutdown could be significantly more expensive.
Leaks in other compressed gases can also be relevant to safety or process quality.
An economic leak list should therefore include technical criteria in addition to energy cost assessment.
These can include process criticality, possible consequential damage, accessibility and planned maintenance windows.
This turns a pure energy list into a maintenance plan that can actually be used.
Document leaks so that they become a work plan
A photo with the note “Leak on machine 4” is usually not sufficient for a larger repair campaign.
Good documentation should allow the location to be found again later and at the same time support economic prioritization.
Useful information therefore includes plant area, machine, exact component, photo, leak rate, operating pressure, assumed operating hours and calculated annual costs.
A repair recommendation, required spare part, expected effort and responsible person can also be stored.
These data can then be used to create a prioritized action list.
Especially when several hundred leaks have been found, this is considerably more efficient than later determining the repair sequence solely on the basis of photos or subjective assessment.
After repair, the data record should not be deleted, but supplemented with the repair date and verification measurement.
This creates a traceable long-term history of compressed air losses.
Measure again after repair
A leak marked as repaired does not yet represent proven energy savings.
A new seal may have been installed incorrectly, a fitting may still leak slightly, or there may be a second leak immediately next to it.
Verification is therefore part of the complete process.
Ideally, the location is checked again at an operating pressure as similar as possible to the original condition.
This allows verification that the leak rate has actually been significantly reduced or completely eliminated.
For large repair campaigns, idle consumption or area consumption can also be compared before and after the measures.
This turns theoretical savings potential into a demonstrable improvement in the compressed air system.
Suitable leak measurement technology at ICS Schneider
Within the category leak detection / leak measurement equipment, ICS Schneider Messtechnik offers various ultrasonic leak detectors and acoustic cameras for compressed air and gas networks.
The LeakCam 600 visually locates multiple ultrasonic sources and can estimate the leak quantity in l/min or cfm. Annual leakage costs can also be calculated using the stored economic parameters. The integrated distance measurement supports more reproducible quantification.
The HIKMICRO AI56L is also suitable for regular audits. The camera uses an array of 64 MEMS microphones and, in addition to locating sound sources, enables real-time estimation of leak rate, leakage costs and leak level.
The HIKMICRO AI76 uses 136 MEMS microphones and is designed for rapid inspection of larger industrial plant areas. In addition to gas leak detection, it can also be used to detect electrical partial discharge.
With the ILD 500 / ILD 510, ultrasonic leaks can be located, documented photographically and evaluated according to leak rate or annual savings potential. The data can then be processed further for reports and repair planning.
For higher-level consumption analysis, a permanently installed flow sensor can also be useful. The IVA520 measures the standard volumetric flow of compressed air and gases and can, for example, be installed in distribution stations or upstream of individual production areas.
The combination of permanently installed consumption measurement and mobile ultrasonic leak detection enables a particularly comprehensive strategy: the consumption measurement shows which area is losing an unusually large amount of compressed air, while the mobile leak detector identifies the individual leaks and allows them to be prioritized economically.
Leak detection and leak measurement equipment at ICS Schneider
Further reading: Comparing compressed air leak rates at different network pressures
Further reading: Evaluating compressed air leakage after repair
Conclusion
Evaluating a compressed air leak economically means more than simply looking for the highest l/min value in a list.
The leak rate describes only the instantaneous loss. The economic impact results only from the combination of leak rate, pressurization time, energy demand of compressed air generation and electricity price.
Operating time in particular can completely change the ranking. A small leak in a permanently pressurized main line can lose significantly more compressed air over a year than a much larger leak on a rarely operated machine.
For a traceable evaluation, the lost annual volume should therefore be calculated first. Using the actual specific energy demand of the compressor system and the internal electricity price, a realistic savings potential can then be derived.
Repair costs add the payback period to this assessment. This makes both the largest absolute savings opportunities and the particularly quick-to-implement measures visible.
Acoustic cameras and ultrasonic leak detectors simplify this process considerably because localization, documentation and leak-rate estimation can be combined directly. However, the resulting euro value should be understood as a prioritization parameter and based on realistic operating conditions.
Permanently installed flow measurements can additionally verify the local leak survey. They show the actual overall consumption of a plant area and make idle consumption particularly visible.
By evaluating leak rate, operating time, pressure, energy costs and repair effort together, a long list of detected leaks can be turned into a clear action plan – allowing maintenance resources to be used where they generate the greatest technical and economic benefit.
FAQ on the economic prioritization of compressed air leaks
Is the largest compressed air leak automatically the most expensive?
No. In addition to the leak rate, the number of hours per year during which the specific location is actually pressurized is decisive. A smaller continuous leak can lose more air over a year than a larger leak with a short operating time.
How is the annual lost compressed air volume calculated?
The leak rate converted to standard cubic metres per hour is multiplied by the annual number of pressurized operating hours.
How are energy costs calculated from Nm³ of compressed air?
The lost standard volume is multiplied by the specific energy demand of compressed air generation in kWh/Nm³ and then by the electricity price.
Which specific energy demand should I use?
Ideally, use the value actually measured or determined for the existing compressor station. General benchmark values are mainly suitable for initial estimates.
Which operating hours are correct for the calculation?
The relevant hours are those during which the specific leak location is under pressure. These can differ significantly from production hours or machine operating hours.
Why is a leak in the main network often particularly expensive?
Main lines frequently remain pressurized for much longer than individual machines. Even a comparatively small leak can therefore accumulate a very high number of annual operating hours.
Does network pressure influence the leak rate?
Yes. A leak generally loses more air at a higher pressure difference to the surrounding atmosphere. Leak measurements should therefore be documented together with the operating pressure.
Can I directly compare leaks measured at 6 and 8 bar?
Not without further consideration. The different pressure influences the leak rate. For a reliable comparison, the same conditions should be used or the values should be referenced to a common condition.
How accurate are the leak rates from an acoustic camera?
The instruments estimate the leak rate from the measured ultrasonic signal and additional parameters. These values are particularly useful for prioritization and comparison, but should not be confused with direct billing-grade flow measurement.
Why must the measuring distance be correct?
The ultrasonic level received from the leak also depends on distance. Instruments with distance measurement can take this influence into account when estimating the leak rate.
Can an acoustic camera display annual costs directly?
Depending on the instrument, yes. However, the stored operating time, pressure and other calculation parameters as well as the energy costs must match the actual installation.
What is more important for prioritization: annual costs or payback period?
Both parameters answer different questions. Annual costs show the absolute savings potential, while the payback period shows how quickly the repair pays for itself economically.
Can a small leak have a high priority?
Yes. If it can be eliminated with very little effort or is process-critical, immediate repair can make sense despite relatively low annual costs.
Should safety-critical leaks also be assessed only economically?
No. Safety, quality and process risks must be considered independently of pure energy savings and can require a higher priority.
What is idle consumption?
This is the compressed air consumption during a period without regular production. After subtracting intentionally operating consumers, it can provide a good indication of the total leakage of a plant area.
Can permanently installed flow measurement replace leak detection?
No. It shows the total consumption or loss of an area but does not locate the individual leaking point. Ultrasonic leak detection and flow measurement therefore complement each other very well.
Why should another measurement be carried out after repair?
Only a verification measurement can confirm that the leak has actually been eliminated or significantly reduced. It can also reveal remaining leaks in the immediate vicinity.
Should I simply reduce network pressure instead of repairing leaks?
A sensible pressure reduction can reduce leakage loss but does not eliminate the leaks. Pressure optimization and leak repair should therefore be used together.
What data should a leak report contain?
Useful information includes at least the exact location, photo, plant or machine, leak rate, operating pressure, assumed pressurization time, calculated annual costs, repair recommendation, status and result of the later verification measurement.
Which instruments are suitable for regular compressed air audits?
Acoustic cameras such as the LeakCam 600, HIKMICRO AI56L or AI76 as well as ultrasonic leak detectors such as the ILD 500/510 enable localization, quantification and documentation. For higher-level consumption assessment, a permanently installed standard volumetric flow sensor such as the IVA520 can also be used.
What information is required for a reliable economic calculation?
The required data include, in particular, leak rate, operating pressure, annual pressurization time, specific energy demand of compressed air generation, the electricity price used and, for complete prioritization, the expected repair effort.