Instrument Air Leak Audit: Survey Methods, Cost Calculation, and Repair Prioritization

How to Conduct a Leak Survey

There are three primary methods for finding instrument air leaks: ultrasonic detection, soapy water testing, and audible/visual inspection. Each has its place, and a thorough survey typically uses all three.

Ultrasonic leak detectors are the primary tool for systematic surveys. These handheld instruments detect the ultrasonic noise (20-100 kHz) generated by air escaping through a small orifice. They work in noisy environments because the detector filters out audible frequencies. Point the detector at tubing connections, fittings, actuator housings, and positioner exhausts while watching the signal level on the display. Most units have a directional cone attachment that narrows the detection angle for pinpointing leak locations in congested areas.

Soapy water (or commercial leak detection fluid) is applied to suspected leak points. Bubbles indicate a leak. This is the most positive identification method — if it bubbles, it leaks. The limitation is that it requires physical access to each connection and is slow for large surveys. Use soapy water to confirm leaks found by ultrasonic detection and to pinpoint exact locations (which fitting, which thread, which O-ring).

Audible and visual inspection catches the large leaks that are obvious: hissing sounds from tubing connections, ice formation on large leaks (from Joule-Thomson cooling), and actuators that visibly cycle or bleed air continuously. These are the high-priority leaks that waste the most air and should be repaired immediately.

Organize the survey by area (units, buildings, pipe racks) and tag each leak with a unique number, location description, estimated severity, and date found. Use a standard leak tag that can be attached to the tubing or fitting near the leak. This creates a repair list that maintenance can work through systematically.

Orifice-Based Leak Rate Estimation

The leak rate through a small hole depends on the hole size, the upstream pressure, and whether the flow is choked (sonic). For instrument air at 80 PSI, the pressure ratio across the leak (atmospheric to supply) is 14.7/94.7 = 0.155. Since this is well below the critical pressure ratio of 0.528, virtually all instrument air leaks are choked (sonic flow). This simplifies the calculation because downstream pressure does not affect the flow rate.

For choked flow through a sharp-edged orifice: Q = 11.3 × Cd × A × P1 where Q is leak rate in SCFM, Cd is the discharge coefficient (0.65 for sharp-edged, 0.85 for rounded), A is the hole area in square inches, and P1 is the upstream absolute pressure in PSIA. For a 1/32-inch diameter hole at 80 PSI: A = π × (1/32)² / 4 = 0.000767 in². Q = 11.3 × 0.65 × 0.000767 × 94.7 = 0.53 SCFM.

Standard leak size categories used by most survey programs: small leak (equivalent to 1/64-inch hole, ~0.13 SCFM), medium leak (1/32-inch, ~0.53 SCFM), large leak (1/16-inch, ~2.1 SCFM), and very large leak (1/8-inch, ~8.5 SCFM). Most instrument air leaks fall in the small to medium category. Ultrasonic detectors with quantification capability can estimate the dB level, which correlates to leak rate through manufacturer calibration charts.

A more accurate but time-consuming method is to bag the leak. Place a calibrated bag or inverted container over the leak point and time how long it takes to inflate to a known volume. This directly measures the leak rate without estimating hole size. For example, a 1-gallon bag that fills in 30 seconds represents a leak rate of approximately 0.27 SCFM at 80 PSI (correcting for compression).

Cost Calculation: From SCFM to Dollars

The cost of a compressed air leak is: Annual cost = SCFM × kW_per_CFM × 8,760 hours × $/kWh × load_factor. The key variable is kW per CFM, which depends on compressor efficiency. A typical industrial compressor requires 0.18-0.25 kW per CFM at 80-100 PSI. A well-maintained rotary screw compressor at full load runs about 0.20 kW/CFM. Older or poorly maintained compressors may use 0.25-0.30 kW/CFM.

The load factor accounts for the fact that not all leaks are active 24/7. Leaks on headers and main distribution lines are continuous (load factor = 1.0). Leaks on branch lines that are isolated during shutdowns have a lower load factor (0.85-0.95 depending on the operating schedule). Leaks on equipment that is only pressurized during operation may have a load factor of 0.5 or less.

Example: A medium leak (0.53 SCFM) on a continuous header, with a compressor at 0.22 kW/CFM and electricity at $0.08/kWh: Annual cost = 0.53 × 0.22 × 8,760 × 0.08 = $81.70. That single fitting leak costs $82 per year. A plant with 200 such leaks wastes $16,340 per year. At typical repair costs of $20-$50 per leak (fitting replacement plus labor), the payback period for a comprehensive leak repair campaign is a few months.

The indirect costs are harder to quantify but often more important. When leaks consume 25% of compressor capacity, the plant is effectively running an extra compressor to supply leaks. That compressor requires maintenance, has a finite lifespan, and produces waste heat that must be removed by the cooling system. Eliminating leaks may allow the plant to shut down a standby compressor, saving not just energy but capital maintenance costs.

Reliability Impact for Instrument Air

Instrument air leaks have a reliability dimension that plant air leaks do not. Plant air (for tools, cleaning, general use) can tolerate some pressure fluctuation without consequence. Instrument air feeds safety systems, control valves, and process analyzers that require stable, adequate pressure to function correctly.

When instrument air header pressure drops below the minimum required by the instruments (typically 18-20 PSI for I/P converters, 35-60 PSI for valve actuators), the consequences include: control valves responding slowly or stalling, I/P converters unable to deliver full output pressure, positioners losing calibration accuracy, and safety shutdown valves that may not meet their required stroke time.

A pressure excursion study involves measuring the header pressure at multiple points during peak demand periods (startup, large compressor trip, simultaneous valve stroking during a process upset). If the pressure drops below the minimum instrument requirement at any point, the leak repair priority increases because the leaks are directly compromising process control and safety system reliability.

Some plants prioritize leak repairs based on proximity to critical instruments. A leak on a branch line feeding an ESD valve or a critical control loop gets a higher priority than the same size leak on a branch feeding a non-critical gauge or utility control valve. This risk-based approach focuses the limited maintenance resources on the leaks that have the greatest potential for causing a process event.

Prioritizing Repairs by ROI

Not all leaks are worth repairing immediately. A systematic prioritization approach ranks leaks by a combination of energy cost, reliability impact, and repair difficulty. The simplest metric is annual cost divided by repair cost, which gives the payback period in years. Any leak with a payback under 1 year should be repaired. Leaks with payback under 3 months should be repaired immediately.

Priority 1 (repair immediately): Large leaks (over 2 SCFM) on any line, any leak on an ESD valve or critical control loop air supply, leaks that are causing audible noise or visible icing, and leaks that contribute to measured header pressure drops during peak demand. Typical repair: replace fitting, tighten compression nut, replace tubing section.

Priority 2 (repair during next scheduled maintenance): Medium leaks (0.5-2 SCFM) on non-critical lines, leaks on filter-regulator drain ports, and leaks on gauge connections. These can usually be repaired during routine rounds without a specific work order.

Priority 3 (repair during next turnaround): Small leaks (under 0.5 SCFM) on non-critical lines, leaks inside actuator housings that require valve isolation and partial disassembly, and leaks in inaccessible locations (inside insulation, behind cable trays) that require scaffolding or insulation removal to access.

Track leak repairs and verify effectiveness. A common mistake is tightening a compression fitting to stop a leak, only to have it leak again in a few weeks because the ferrule was damaged. If a fitting leaks again after tightening, cut the tubing and install a new fitting. The cost of a new fitting ($5-$10) is trivial compared to the ongoing leak cost and the labor of repeated repair attempts.