By Colin Rice
Colin Rice Exploration and Training (Pty) Ltd
High pressure air is a potentially lethal hazard and so it is essential that the hazards and associated risks are clearly understood by all involved.
This is the third article of Part 1 of our Technical Series on Compressor and Air Hose Safety. Click here for an outline of the entire Compressor and Air Hose Safety Series.
In preparing this article I have borrowed liberally from a number of sources – while these have not been mentioned in the body of the document, I would like to acknowledge the contributions of all of these sources.
In the course of the past few years I have done many audits of rotary percussion and reverse circulation drill rigs and I have seen many variations in the application of restraints on high-pressure air hoses. Many installations fell far short of what is required by the law or by good common sense and so exposed the drill crew to unnecessary risk.
Chapter 23 of the Regulations of the Minerals Act No 50 of 1991 (which is a part of the Mine Health and Safety Act) deals with compressors and various aspects around their installation and inspection. The regulations do not however consider the distribution of air through flexible hoses and so a significant gap exists in the legislation.
High pressure air is a potentially lethal hazard and so it is essential that the hazards and associated risks are clearly understood by all involved in these types of drilling operations. The whipping motion of a failed high-pressure air hose can be extremely violent and can result in serious injury, property damage or death. It has been common practice in the industry to fit a cable or sling type ‘whip-check’ to air hoses on rotary percussion and dual-tube reverse circulation drill rigs. The picture below illustrates a device of this type fitted to a discharge hose of a 24 bar compressor.
While these single line whip-checks may prevent a failed hose from travelling a significant distance they do not prevent the hose from whipping and so they are an inadequate method of restraining high pressure air hoses. In fact, a major international manufacturer of these hose restraints includes the following statement in their literature:
“This safety device is intended only for use on air hoses 4" ID and under, operating at pressures of no more than 200 PSI. We disclaim any other uses and accept no liability for the misapplication of this product”.
Research on a wide range of alternative hose restraining devices has shown that a stocking type hose restraint, commonly called a “hose sock”, as pictured below, provides the highest level of safety.
Hose socks are manufactured from a number of wire ropes woven into a helical structure that shrinks and tightens when it is slid onto a hose. The key to the hose sock’s ability to prevent whipping lies with the two anchor points and the long gripping area provided by the hose sock. In the event of a failed connection, the force of the escaping air forces the hose backwards and as it moves backwards the woven sock tightens and grips the hose.
Ideally, the hose sock should be securely attached to the drill rig at two anchor points which are at 180 degrees apart. In practice, it is sometimes impossible to create two anchor points 180 degrees apart and so a suitable way of ensuring that the hose sock is adequately secured must be found.
Typically, the hose sock will be secured to the anchor point/s by shackles which must be rated at a greater strength than the anticipated burst force of the hose. If correctly specified, fitted and anchored, the hose sock will prevent any travel or whipping in the air hose in the event of a hose or connection failure and so most mining companies specify that hose restraints of this type must be used on all high-pressure air hoses on all drill rigs. For evidence of the effectiveness of the hose-sock restraint and the inadequacy of the whip-check restraint, have a look at the video – “Hose socks vs whip checks”!
Hose sock design
Two types of hose sock are commonly used on drill rigs; two eye types and double eye types, respectively in the pictures below.
The double eye type of hose sock has anchor points at both ends and is used particularly when a short air hose has to be restrained at both ends.
Joints in air hoses must also be restrained and it is recommended that it is best to fit a hose restraint to each hose with the eyes pointing towards the join, and shackled together with appropriately rated shackles as shown in the figure to the left.
Despite the wide spread requirement from mining companies for hose sock type restraints there are no standards for the strength or length of socks for different hose sizes at different pressures or for the strength of anchor points. In Part 2 of this series, coming in February 2018, I will examine some of these issues and provide some recommended standards that can be applied to these essential safety devices.
In Part 2 of this series, coming in February 2018, we examine some of these issues and recommend standards that can be applied.