In some instances suppliers are fitting hose to equipment installations that are rated at a lower working pressure than required by the equipment. Positive set systems are an example, with 420 bar the standard working pressure for such systems. In several recent installations, suppliers have fitted 350 bar rated and marked hose to these types of installations, claiming that a factor of safety of 3:1 would suffice.

In November, Longwall Hydraulics, a company involved in the repair and overhaul of hydraulics in longwall mines sent a letter to hose and fitting suppliers. The letter described an instance where adaptors were supplied for a high-pressure positive set system which operates at a working pressure 420 bar. The adaptors supplied were rated at 414 bar but were stamped with a working pressure 350 bar.

Acting on behalf of BHP Illawarra, Longwall Hydraulics said this was not acceptable to BHP and that “all hose assemblies and fittings supplied to Longwall Hydraulics for use in BHP underground coalmines will be required to meet the required system specifications.”

This follows a meeting BHP Illawarra held a meeting in September 2000 with hose and fitting suppliers to develop minimum requirements for the group’s mines. As a result of this BHP developed requirements for hydraulic fittings (adaptors and couplers) but not hose assemblies.

These include a minimum factor of safety of 3:1, with a preference for 4:1 where available. What has confused some mine operators and suppliers is that BHP’s minimum requirements appear to reflect an acceptance of reduced safety factors.

The problem is that few international or Australian standards exist to govern fittings commonly used in longwall installations. One of the issues is how these hoses are rated. For example, a 25mm staple adaptor has a range of published working pressures from 165 to 420 bar, a variation of more than 2.4:1, (German DIN standards recommend 400 bar static, 165 bar dynamic while some suppliers publish a working pressure of 420 bar).

Another difficulty for component suppliers is the anomaly of BSP thread ratings, commonly used in longwall systems, where working pressures are limited to 2 ratings across the entire range: 250 bar from 2” to 1-1/4”, 350 bar from 1” down. Clearly the factor of safety must vary on each size, as it does not relate to basic hydraulic principles of pressure x area.

Another problem highlighted by these recent events is the highly competitive nature of supply, combined with increasing pressures and flow demand to improve the performance of roof supports.

One hose supplier, Pirtek, said it was considering options to prove the superiority of the products it supplied.

“These are products that look similar to our competitors, but are manufactured with unleaded steels of higher UTS (ultimate tensile strength),” a Pirtek spokesperson said. “Perhaps it is about time the Mines Departments in Australia should consider implementing standards or guidelines to ensure the rule of ‘near enough is close enough’ doesn’t become the standard,” he added.

These words echo the feelings of many suppliers who argue that legislation is putting the onus on them to self-regulate the maintenance of standards. At the heart of the issue is the question: who is culpable if a small component failure causes injury? Is it the mine owner who has accepted a lower factor of safety? The hose supplier? The OEM?

No-one seems to have a clear answer for these questions. A draft ISO standard currently being developed may move closer to providing a solution.