Protective covers such as textile/nylon sleeving and flat plastic or metal-spring guards have long been used to prevent cuts and abrasive wear of hydraulic hose. But the covers typically offer little or no protection to workers from bursts and pinhole leaks. With the demand for more-compact,higher-pressure systems, design engineers are increasingly looking for ways to safely protect personnel from catastrophic hydraulic hose failures.
Who, What, Where
Authored by Yelena Gray
Edited by Kenneth J. Korane,
And make no mistake, high-pressure failures can be dangerous, and there are lots of ways things can go wrong — particularly when hose is misapplied, routed improperly, or abused. Pressure surges that exceed the hose’s maximum operating pressure rating can cause the reinforcement to fail and the hose to burst. Excessive flexing, kinking, and crushing leads to reinforcement fatigue and eventual failure. Bending hose tighter than recommended can excessively stress the reinforcement, open large gaps between reinforcing strands, and severely reduce the hose’s ability to withstand pressure.
Although a hose’s high-tensile wire-reinforcement structure bears the brunt of internal forces, the inner tube itself has to bridge gaps in the braid formed when the hose bends and stresses. If the tube compound is not strong enough, it blows through gaps in the braid and cuts a hole in the tube — resulting in a pinhole leak.
Pinhole leaks in pressurized hose can release toxic fluid at more than 600 fps — close to the muzzle velocity of a gun. The fine jet of hydraulic fluid at high pressure acts like a hypodermic needle that can penetrate protective clothing and skin. And hydraulic fluid “injected” into muscle tissue can lead to gangrene and amputation.
To protect equipment operators from the hazards of sudden hydraulic failure, several hose manufacturers have developed “safe” nylon sleeving. For instance, Gates has engineered a sleeving system, called LifeGuard, that:
• Contains catastrophic hose bursts up to 6,000 psi.
• Protects against 3,000-psi pinhole leaks at 212°F for up to 5 min.
• Handles hydraulic fluids and biodiesel fuel.
• Creates a safe, yet noticeable, spill.
• Correlates to ISO 3457 and meets MSHA flame-resistance requirements.
The sleeve consists of two fiber layers, continuously woven one inside the other during manufacture— but not attached to each other. The inner, load-bearing layer is a tightly woven filament nylon that elongates or stretches up to 20%. Stretching lets it absorb the energy of a burst or pinhole leak.
The textured-nylon outer layer contains and routes escaping fluid to the hose ends. The outer sleeve also resists abrasion, but is not specifically designed for abrasive environments.
Channel clamps secure the sleeve at the hose ends. The clamps let leaking fluid escape, preventing the fluid from collecting and subsequently bursting the sleeve. Fluid leaking past the clamps lets operators know that a hose has failed.
Design engineers need to understand that current standards and specifications relating to hydraulic-hose protection are vague. ISO 3457 and EN474-1, for example, state, “Hydraulic hoses containing fluid with a pressure of more than 5 MPa (50 bar) and/or having a temperature over 50°C, and which are located within 1.0 m of the operator, shall be guarded.”
Mining equipment must conform to Mine Safety and Health Administration (MSHA) standards and be certified. But MSHA’s Accident Prevention Program simply says, “Install abrasion protection such as spiral wrapping, sleeves, and guards.” And neither ISO nor European standards specify test methods for protective sleeves.
Nonetheless, Gates has tested LifeGuard sleeving both in the lab and field to verify performance at typical fluid pressures and temperatures. The product fits –4, –6, and –8 size hoses; is compatible with a wide range of fluids, including environmentally friendly oils; and is rated for temperatures to 212°F, the SAE standard for most industrialhydraulic applications.