Determination of Air Leakage in Compressed-Air Self-Rescue Device Systems: Testing Methods and Techn

Determination of air leakage in compressed-air self-rescue device systems is a critical safety verification procedure for underground coal mines, ensuring that breathable air can be reliably delivered to personnel during gas outbursts, fires, and other emergencies. This article explains the testing samples, measurable parameters, standard test methods, calculation formulas, and acceptance criteria used by accredited third-party laboratories and mine safety inspectors to quantify system leakage in accordance with relevant mining industry standards.

Technical Overview

A compressed-air self-rescue device system is a life-safety installation branching from a mine's underground pneumatic pipeline network. At designated self-rescue stations, terminal devices—typically comprising a shut-off valve, pressure-reducing valve, filter, pressure gauge, flexible hose, and breathing outlet—deliver filtered, pressure-regulated air so that miners can breathe safely while evacuating or awaiting rescue when the roadway atmosphere becomes contaminated.

Because the protective function of the entire system depends on pneumatic integrity, excessive air leakage through joints, seals, valve seats, hoses, and fittings directly undermines the guaranteed air supply at the terminal devices and wastes compressor capacity. Determination of system leakage therefore forms part of product type testing, installation acceptance, and periodic statutory inspection. Relevant references include the Chinese mining industry standard MT 390 for compressed-air self-rescue devices used in coal mines, the Coal Mine Safety Regulations, GB/T pressure-test and flow-measurement standards, and the general pneumatic system practices of ISO 4414.

Leakage is normally expressed as the volume flow rate of free air escaping per unit of time (L/min or m³/min) when the system or a section is held at its rated working pressure, and it is compared against the allowable limit defined by the applicable product standard or the mine's approved technical requirements.

Testing Samples

Complete Self-Rescue Device Units

Finished terminal devices of the types installed at underground self-rescue stations, including combined valve-and-regulator assemblies, filter housings, and hose-and-outlet kits, submitted for type testing, certification, or post-repair verification. Samples should be in as-delivered condition with factory-sealed connections intact.

System Components and Subassemblies

Individual pneumatic elements whose internal leakage behavior governs system performance, such as pressure-reducing valves, shut-off valves, quick couplings, check valves, filter elements and housings, pressure gauges, orifice plates, flexible hoses, and threaded or flanged fittings.

Underground Pipeline Network Sections

In-situ sections of the compressed-air delivery network serving the self-rescue system, including main headers, branch lines, manifold boxes, and the connecting pipework between the mine pneumatic supply and the terminal devices, tested as installed during acceptance or periodic inspection.

Testing Items

Depending on the purpose of the test (type approval, installation acceptance, or in-service inspection), the following items are typically determined:

• Overall system leakage rate, expressed as free-air volume flow (L/min or m³/min) at rated working pressure
• Device-level airtightness, determined as pressure drop over a specified observation period with the outlet closed
• Internal leakage across pressure-reducing valves and shut-off valves under both static and set-pressure conditions
• Seal integrity of threaded joints, flanges, quick couplings, and hose connections
• Leakage at terminal outlets under closed-valve (lock-up) conditions
• Segment-by-segment leakage distribution to locate and quantify major leak sources
• Temperature-corrected leakage values for comparison with standard limits and trend analysis across inspection cycles

Testing Methods

Pressure-Decay Method

The tested section is isolated from the supply, all outlets are blanked or sealed, and the section is charged with compressed air to the specified test pressure—commonly the rated working pressure of the pipeline (typically in the range of 0.3 to 0.8 MPa gauge, or as required by the applicable standard). After a stabilization period to allow air temperature to equilibrate, the initial pressure P1 and temperature are recorded, and the pressure is monitored over a defined interval t until a final pressure P2 is reached. The leakage rate as free air is calculated from the ideal gas relationship:

Q = V × (P1 − P2) / (t × P0)

where Q is the leakage rate (L/min), V is the internal volume of the pressurized section (L), P1 and P2 are the absolute pressures at the start and end of the observation period, t is the test duration (min), and P0 is atmospheric pressure. If the ambient or air temperature changes measurably during the test, pressures are corrected to a common temperature before computing the result. For finished self-rescue device units, the acceptance form of this test is often a simple hold test: the outlet is closed, the specified test pressure is applied to the inlet, and the pressure drop within the observation period defined by the product standard must not exceed the allowable value.

Constant-Pressure Flow-Compensation Method

In this method the isolated, sealed section is fed through a calibrated mass flow meter or precision rotameter while a regulator holds the pressure constant at the test value. Once the flow stabilizes, the feed flow consists solely of the air escaping through leaks, so the steady-state reading directly gives the total leakage rate. This approach avoids the volume-estimation uncertainty of the pressure-decay method and is well suited to sections whose internal volume is difficult to determine, such as complex in-situ pipeline runs.

Localized Leak Detection and Segmentation

When the measured leakage exceeds the allowable limit, leaks are localized by segmenting the network with isolation valves and repeating the measurement section by section to bracket the leaking zone, by applying soap solution to suspect joints and observing bubble formation, or by using ultrasonic leak detectors that sense the high-frequency noise generated by escaping air. Identified defects—worn seals, loose fittings, corroded pipe walls, or damaged hoses—are corrected, after which the leakage determination is repeated for verification.

Instrumentation and Accuracy Requirements

Testing should employ calibrated instruments traceable to national standards, including calibrated pressure gauges or digital manometers of suitable accuracy class, calibrated flow meters covering the expected leakage range, calibrated temperature sensors and a barometer for corrections, a regulator and a stable air source, blanking fittings and sealing plugs, and a stopwatch or data logger for timing. Ambient conditions, test pressure, and observation duration are recorded in the test report so results are reproducible and auditable.

Applications

Determination of system air leakage supports mine safety management and equipment quality control in several contexts:

• Type testing and conformity assessment of new compressed-air self-rescue devices by accredited third-party laboratories prior to market entry
• Installation acceptance of newly built or modified underground self-rescue pipeline systems before they are placed into service
• Periodic statutory inspections mandated by mine safety regulations to confirm continued pneumatic integrity
• Post-repair and post-maintenance verification after replacement of valves, hoses, filters, or fittings
• Compressed-air energy audits, since pipeline leakage is a major source of wasted compressor power in underground networks
• Risk assessments and emergency-preparedness audits verifying that design air delivery can be maintained at all terminal stations under accident conditions

FAQ

Why is leakage determination important for compressed-air self-rescue systems?

During an emergency, the system must deliver breathable air at adequate pressure and flow to every terminal device. Excessive leakage reduces delivered pressure and flow at distant stations, delays or prevents effective protection, and indicates degraded components that could fail completely when needed. Leakage determination quantifies this degradation objectively and triggers maintenance before the system's protective function is compromised.

What are common acceptance criteria?

Acceptance limits are set by the applicable product standard, the manufacturer's specification, or the mine's approved technical requirements. Typical practice requires that the pressure drop of a sealed device within the standard observation period not exceed the specified allowable value, and that total system leakage remain a small fraction of the rated compressor or pipeline delivery capacity. The governing standard's limit always takes precedence over general guidelines.

How often should leakage testing be performed?

Testing is performed at installation acceptance, at the periodic interval required by the mine's safety management system or applicable regulations, and after any repair or component replacement that disturbs pneumatic seals. Many operators also test opportunistically when pressure loss, audible hissing, or elevated compressor duty suggests increased leakage.

Can testing be carried out while the mine is operating?

Yes. By isolating individual sections with shut-off valves and coordinating the test window with the working schedule, leakage determination can be completed without shutting down the entire compressed-air network, provided the isolated section is clearly flagged and the affected self-rescue stations are temporarily covered by alternative arrangements during the test.

What are the most common causes of excessive leakage?

Frequent causes include aged or hardened rubber seals, insufficient thread sealant or loose fittings, corroded or damaged pipeline walls, split flexible hoses, worn valve seats in reducing and shut-off valves, and poorly matched quick couplings. Routine tightening, seal replacement, and corrosion control keep leakage within acceptable limits.

Which standards govern this testing?

Testing is generally conducted with reference to the Chinese mining industry standard MT 390 for coal-mine compressed-air self-rescue devices, the Coal Mine Safety Regulations, relevant GB/T standards for pressure and flow measurement, and general pneumatic system practices such as ISO 4414. Where certification is required, tests are performed by accredited third-party laboratories under documented quality management systems.