Hydraulic Support Internal Leakage Testing: Methods, Standards, and Acceptance Criteria

Hydraulic support internal leakage testing is a critical quality and safety control measure for hydraulic roof supports used in underground longwall mining, verifying the sealing integrity of hydraulic legs, jacks, and control valves under rated pressure. Because internal leakage is invisible from the outside yet directly causes loss of support resistance and gradual support sinking, specialized pressure-holding, flow measurement, and in-situ diagnostic methods are required. This article explains the principles, sample types, test items, standard methods, and applications of internal leakage detection for hydraulic supports in accordance with GB/T 25974, EN 1804, and related standards.

Technical Overview

Hydraulic supports (hydraulic powered roof supports) are self-advancing steel structures that shield miners and equipment in longwall coal faces, powered by a high-pressure water-in-oil emulsion supplied from a pump station, typically at 31.5 MPa. Each support unit combines hydraulic legs (columns) that carry the roof load, hydraulic jacks that perform advancement, lifting, balancing, and shielding motions, and a hydraulic control circuit comprising manipulator valves, hydraulic-controlled check valves, and safety valves.

Internal leakage is defined as the unintended transfer of pressurized fluid across internal sealing boundaries within the hydraulic circuit, most commonly past piston seals between the piston-side chamber and rod-side chamber of a cylinder, or past the seats and spools of control valves. Unlike external leakage, which produces visible fluid loss, internal leakage is completely hidden from observation and can only be detected through purpose-designed testing.

The consequences of undetected internal leakage are severe: progressive decay of setting force and working resistance, gradual sinking of the canopy against the roof, uncontrolled lowering speed, roof deterioration, and ultimately loss of strata control that endangers the entire longwall face. For this reason, internal leakage tests are mandated at multiple life-cycle stages of hydraulic supports and are performed by manufacturers, accredited third-party laboratories, overhaul workshops, and mine maintenance departments in accordance with the GB/T 25974 series, the EN 1804 series, and Chinese coal industry standards (MT/T series).

Testing Samples

Hydraulic Legs (Columns)

Single-telescoping and double-telescoping hydraulic legs removed from shield, chock, or frame supports. Each telescoping stage, piston seal assembly, and gland seal configuration is evaluated individually, since a leak in any stage degrades the resistance of the entire leg.

Hydraulic Jacks

Advance jacks, lifting jacks, balance jacks, base-lifting jacks, side-shield jacks, and other auxiliary cylinders. These cylinders operate at lower loads than the legs but rely on the same sealing principles and are tested for cross-chamber leakage in both the extended and retracted positions.

Hydraulic Control Valves

Manipulator (multi-port control) valve groups, hydraulic-controlled check valves, safety (relief) valves, alternation valves, and isolation valves. Valve seat leakage is a dominant internal leakage source because a check valve that fails to hold pressure will allow the entire leg circuit to bleed down.

Complete Support Assemblies

Fully assembled supports placed on reaction test rigs, where the interaction of legs, valves, and piping is evaluated under simulated roof loading to confirm system-level leak-tightness rather than only component-level performance.

Testing Items

The scope of internal leakage testing typically covers the following items, with acceptance limits defined by the applicable product standard or the agreed technical specification:

• Piston seal internal (cross-chamber) leakage of hydraulic legs and jacks
• Pressure-holding performance of piston-side chambers at rated working pressure
• Pressure-holding performance of rod-side chambers at rated working pressure
• Seat leakage of hydraulic-controlled check valves under sustained rated pressure
• Seat tightness and reseat tightness of safety valves below their set pressure
• Inter-channel internal leakage of manipulator and control valve blocks
• Working resistance decay (load retention) of the complete support under sustained loading
• Automatic lowering speed (sink rate) of the canopy and legs
• Sealing integrity re-verification after endurance and fatigue cycling

Testing Methods

Static Pressure-Holding (Pressure Decay) Test

The chamber under evaluation is pressurized with filtered emulsion to its rated working pressure, then isolated by closing the supply and locking valves. A calibrated pressure transducer or precision gauge monitors the pressure over a specified holding period, commonly several minutes for routine production acceptance and extended holds of up to several hours for type testing where the standard requires. The measured pressure decay is compared against the allowable limit, which is typically expressed as an absolute pressure drop or as a percentage of the test pressure. Temperature stabilization is ensured before recording, since thermal contraction of the fluid can mimic leakage.

Differential Chamber Pressurization Test

To isolate piston seal leakage specifically, one chamber of the cylinder is pressurized to the rated pressure while the opposite chamber is vented to a low-pressure return line fitted with a flow indicator or pressure gauge. Any pressure rise or measurable return flow in the vented chamber demonstrates fluid migrating across the piston seal. The test is repeated in both directions, and in both the extended and retracted positions, because seal performance varies with stroke position and guide wear.

Flow Compensation (Make-Up Flow) Method

For valve bench testing, the test piece is connected to a constant-pressure supply and the steady-state make-up flow required to maintain pressure is measured with a flow meter or by timed volumetric collection. At equilibrium, the make-up flow equals the total internal leakage rate, providing a quantitative figure suitable for comparing valves against defined leakage classes.

Full-Scale Load Retention Test

A complete support is installed on a reaction frame or test rig and loaded to its rated working resistance using the actual hydraulic legs and control circuit. Load cells monitor the resistance while displacement sensors track the lowering of the canopy over time. A decay of resistance, or a lowering speed exceeding the specified maximum, indicates internal leakage somewhere in the leg, check valve, or safety valve circuit, and component-level tests are then used to localize the fault.

Test Conditions and Instrumentation

Testing is performed with the same medium used underground, normally a water-in-oil emulsion with a concentration of approximately 3% to 5%, held within a controlled temperature range. Pressure gauges, transducers, and flow meters must be calibrated and traceable, and test pressures are verified at the cylinder port rather than only at the pump, to exclude hose and fitting losses from the measurement.

In-Situ Diagnostic Techniques

For supports already installed in the longwall face, non-intrusive methods supplement bench tests: portable ultrasonic leak detectors sense the high-frequency noise generated by fluid throttling across a leaking seal; infrared thermography reveals localized temperature rise at leakage points; pressure logging of individual supports identifies abnormal bleed-down trends; and systematic measurement of lowering speed under constant roof load flags legs with degraded seals for replacement during scheduled maintenance windows.

Applications

Internal leakage testing supports safety and reliability across the entire life cycle of hydraulic supports:

• Factory acceptance testing and type testing of new support designs prior to mandatory mine product safety marking in China
• Serial production quality control of hydraulic legs, jacks, and valve groups
• Verification testing after overhaul or seal replacement, before supports return to service underground
• Periodic in-service inspection and condition monitoring during scheduled longwall maintenance
• Root-cause analysis of support sinking incidents and roof control failures
• Research and development validation of new cylinder coatings, seal geometries, and emulsion formulations

FAQ

What is the difference between internal and external leakage?

External leakage is fluid escaping visibly from the circuit, such as dripping at glands, fittings, or hoses. Internal leakage is fluid bypassing seals or valve seats inside the circuit, so no fluid is lost to the environment; the symptom is pressure or load decay rather than visible fluid loss, which is why dedicated testing is essential.

Which standards govern hydraulic support internal leakage testing?

The principal references are the GB/T 25974 series for coal mine hydraulic supports, including the parts covering general technical conditions, hydraulic legs and jacks, and hydraulic control systems, together with the EN 1804 series on safety requirements for hydraulic powered roof supports and relevant Chinese coal industry MT/T standards. Accredited third-party laboratories conduct conformity testing to these specifications.

Can internal leakage be detected without disassembling the support?

Yes. In-situ methods such as ultrasonic detection, infrared thermography, pressure-trend logging, and automatic lowering-speed measurement can identify and often localize internal leakage underground. Bench pressure-holding tests on extracted cylinders and valves remain necessary to quantify leakage precisely and to verify repair quality.

Why is emulsion used as the test medium instead of mineral oil?

Hydraulic supports operate on fire-resistant water-in-oil emulsion, and seal behavior, viscosity effects, and leakage rates differ significantly between emulsion and oil. Testing with the actual service medium ensures the measured leakage characteristics are representative of underground conditions.

How often should internal leakage testing be performed?

Testing is required at manufacture, after every major overhaul, and whenever a support exhibits abnormal sinking or pressure loss in service. Mines typically also apply risk-based periodic checks, for example at each scheduled face-end maintenance, so that degraded legs and valves are removed and repaired before resistance loss compromises roof control.