Compressed Air - Maintaining a safe system01 February 2005

Regulation 12 of the Pressure Systems Safety Regulations (PSSR) is very short and simple, as these things go: "The user of an installed system and the owner of a mobile system shall ensure that the system is properly maintained in good repair, so as to prevent danger." That's it. So if you're running a compressed air installation, you have a legal requirement to keep it maintained.

The regulations also define their terms: "danger" means "reasonably foreseeable danger to persons from system failure" (and it is made clear that we are concerned with the pressure of the system fluid, rather than any other hazardous properties); and "system failure" means "unintentional release of stored energy (other than from a pressure relief system) from a pressure system".

Compressed air leaks need not be inherently dangerous, but they may indicate potential sources of an uncontrolled release of stored energy - such as a rusting pipe or a poor joint - so for that reason alone, it is good practice to minimise their occurrence.

One certainty is that leaks are a blatant waste of energy. The table shows the power loss in compressed air systems from different hole sizes. As an example, a 10mm hole (or a set of holes with the same air loss) will use 23.4kW of power. If the system is in operation 60 hours a week, 50 weeks a year, then assuming an electricity cost of 4p/kWh, that equates to £1,020 lost per annum.

Mike Leeson, managing director of Keytime Engineering Services, a provider of maintenance and inspection under PSSR, comments: "If you have a pressure system with 4-inch or 6-inch pipe, I guarantee you will have way over the 10mm equivalent of leakage, unless you've got a very good maintenance regime." Even small-bore systems can leak considerably if they are not maintained, he adds.

In addition to their other downsides, leaks can affect the performance of air tools. This is one of the reasons why total compressor capacity should include an allowance for leakage.

For an installation undergoing regular inspection and maintenance, leakage should not exceed 5% of capacity as an absolute maximum; but as Leeson points out: "If the system was carrying a toxic gas, you wouldn't accept that level of leakage. Most compressed air systems can be totally leak free."

Hoses, couplings and valves account for a large proportion of the total amount of leaks. Poor installation practice is a frequent source of leaks - particularly when insufficient care is taken with the matching of screw threads, says Leeson.
There are a number of methods for detecting leaks. Joints can be checked with soapy water - the escaping air will produce bubbles. Larger leaks can often be heard during shutdowns, but for smaller leaks, and to detect leaks against a background of plant noise, ultrasonic leak detectors are extremely effective. By detecting very high-frequency sounds produced by leaks (but not with machines) they can pinpoint escaping air very efficiently.

There are a couple of warnings to make here. One dangerous practice when looking for a leak is to run a hand over the pipe, find the leak from the cool blast it makes, and then place a thumb over it. This can lead to a dangerous condition known as subcutaneous emphysema. The pressure forces the air (it could also be oil or any pressurised fluid) through the skin till it sits underneath it - producing a crackling sensation, in the case of compressed air. "If you don't get to an A&E department quickly, you can get your hand - even your whole arm - taken off. Air in the blood can get to the brain or the heart," warns Leeson.

Also, once a leak has been found, a maintenance engineer might be sent to tighten the fitting. But if the leak results from a corroded pipe or joint, this could lead to an explosion, and injury. "In maintenance you should never have a regime that tightens fittings when there's pressure in the system - irrespective of what fluid is in the pipes," says Leeson.

Within the compressed air system's preventive maintenance regime, you should be looking for leaks, or for potential future leaks, on a regular basis. For instance, corrosion inside a pipe can be checked using an ultrasonic thickness meter, comparing the reading with the manufacturer's specified dimension. This is of most importance in areas close to where people are working.

Some pressure gauges are classed as protective devices, which are inspected at the same time as the pressure vessel, while others are classed as being for indication only. They should be checked for accuracy periodically. "I would rather have no gauge than one that gives false information," comments Leeson. "The worst case is when the pointer has gone all the way round to the peg, because there is more pressure in the system than the dial is showing." In these cases a maintenance engineer may think it is safe to disconnect the system, which could result in injury.

Mending leaks costs money, in terms of engineer's time, downtime and materials, and at some point the savings will outweigh the costs. However, it's important to consider the implications of a system failure if that leak were to worsen - not only financial.

As well as maintenance, there is also an obligation under PSSR for compressed air systems to be examined regularly, as specified in the written scheme of examination (WSE) that the regulations require. This details what should be examined, how often and how it should be done. The scheme must be drawn up, and the examinations carried out, by a Competent Person.

Another definition within PSSR is important here. "Pipework" does not simply mean lengths of piping: it covers everything that comprises a pressure system apart from the pressure vessel and the protective devices. So a compressor, filters, regulators and lubricators are all pipework.

Why does this matter? It's because some WSEs exclude pipework. If any pressurised item not included in the WSE or in the maintenance scheme causes injury due to the release of stored energy, the company will not have any comeback against the Competent Person that drew up the WSE. "Unless you specifically want it to be excluded, it's worth including pipework in your written scheme," says Leeson.

Finally, records must be kept of your maintenance activities. Contravening PSSR (or other regulations that apply, such as the Management of Health and Safety Regulations, or the Provision and Use of Work Equipment Regulations) will attract the attention of HSE and may even put you in front of a magistrate or judge. But if you documented proof of system maintenance, you will be seen in a more favourable light, as you can show that you have tried to mitigate some of the risks involved.

SOE

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