Countdown to ATEX

With the second key date in the ATEX calendar fast approaching — ATEX 137, the ‘use’ directive, due on 1 July 2006 — manufacturing and process plants are working hard to put their facilities in order. The main requirement of the new directive is that with a process or manufacturing plant, appropriate risk assessment must be carried out to clearly identify which zone any given area is classified as, and also to ensure installations are ATEX compliant.

The two main considerations for installations are simply the zone classification and the temperature classification. However, these bring with them many technical considerations and implications and should not be applied without some detailed thought.

Traditionally, process plants have had a blanket zone approach, which effectively made everything the same classification (usually zone 1) and this has predominantly applied to external zones only.

Vacuum pump manufacturers have been busy testing and certifying their ranges of vacuum pumps over the last few years as a direct result of the first key date in the ATEX calendar (ATEX 95, the ‘equipment’ directive, 1 July 2003). Up to now, however, they have had to try and second guess the market requirements for Category 1 (zone 0) and Category 2 (zone 1) equipment.

As use and application of ATEX 95 in the process industries has progressed, the manufacturers have adapted their product offerings and testing procedures to accommodate as many processes as possible. ATEX vacuum pump selection is, however, still fraught with hidden dangers which can result in troublesome installations, without some expert advice.

First of all, let us consider the zone requirement. The initial reaction to ATEX was that everything should be zone 0, inside the equipment. In other words, to assume there is a permanent flammable atmosphere inside the relevant equipment and subsequently buy the highest specification of vacuum pump. That way everything will be covered.

The first point here is that Category 1 (zone 0 equipment) inside, has to allow for rare malfunctions, as well as normal operation, and so, comparatively, will have the highest level of instrumentation. The other consideration is that flame arresters will probably be needed (at least for dry vacuum pumps or oil sealed vacuum pumps).

For Category 1 installations there are likely to be two options; dry technology or traditional technology. Traditional technology, such as liquid ring or ejectors, will, of course, result in high operating and servicing costs as well as environmental (IPPC) considerations. As oil-sealed technology is generally not certified to Category 1, it is unlikely to use flame arresters, as exhaust oil mist blocks the elements of larger vacuum pumps.

Over the last ten years, many process industry end-users have moved towards pursuing ATEX-compliant dry technology. If processes are dirty or contain any solid particles, there is the possibility of being faced with operating difficulties with flame arresters.

For applications which require operation on IIC gases, for instance, even tighter flame arresters are needed which can result in regular pump trips, stoppages and lost production due to down time. High levels of instrumentation can aggravate problems because as the flame arresters block, the exhaust temperature and pressure switches quickly reach their set points and shut the vacuum pump down. More aggravation.

So, what if a process was in fact Category 2 (zone 1) inside? With the more advanced dry vacuum pump suppliers, flame arresters would not be required and blockage problems will be eliminated instantly. This means dirty processes can be filtered out using conventional methods instead of the flame arresters acting as filters and causing vacuum pump failure.

The second consideration is the temperature. For many, the initial reaction to ATEX was to have everything as T4 compliant internally and externally. This meant that suddenly vacuum suppliers had to consider reducing very high exhaust temperatures, which had for years helped prevent condensation inside vacuum pumps and, again, the lower temperatures would result in aggravation to the typical process plant operation.

With reduced temperatures comes condensation and this is usually in the exhaust of the vacuum pump as the gas pressure comes back up to atmospheric levels. If the exhaust pipework is not arranged correctly, this can remain inside the vacuum pump exhaust port causing immense amounts of damage.

As many vacuum pump designs are built from standard materials such as cast iron or ductile iron, the end result can often be corrosion or at best severe operational difficulties and different dry vacuum pump suppliers have differing approaches. Some have turned the temperatures back up and tested actual ignition levels. Some have changed the materials of construction. And some have developed coatings to protect the pump internals.

It is a tricky balance, as the answer is not always to turn the temperature up. Going back to the initial scenario where solid material, dust or product is carried through the vacuum pump, the high exhaust temperatures can often result in carbonisation of material. This then changes nature and becomes a hard, blackened material in the final stage of the vacuum pump. If this is then coupled with the corrosive condensation, effectively a built-in acidic grinding paste can result, eroding and corroding the internal components of the vacuum pump simultaneously.

The best advice is to make the decision carefully; the answer lies with the process and which product offering suits it best. For example, would it be suitable to run very hot pumps on chemicals that have low auto ignition temperatures? Is it a viable option to replace major precision pump components that are now manufactured from stainless steel or other exotic materials?

Service and maintenance is the key here and it is wise to consider the options carefully. Make sure a vacuum pump that is easy and cost effective to repair is selected.

Other considerations for trouble-free ATEX vacuum pump operation should be the possible use of oil-sealed technology (no flame arresters), introducing nitrogen to change the pump category, flame arrester bypass systems, frequency control of the motor or gas ballast to prevent condensation. Most importantly, consult an experienced supplier who can advise on all aspects and optimise the installation.

There are many technical aspects that need careful thought and planning. The answers often lie with the end user and the subsequent risk assessments. Consider carefully which category is required and if possible aim for category 2 (or even category 3) equipment, this will mean less complexity and higher reliability. It is also worth considering the chemistry likely to pass through the pump and pay special attention to the gas group requirement and the temperature classification.

The best advice is to consider all the options before installing a vacuum pump that might end up being more trouble than it is worth.

Geoff Jennings is systems manager for vacuum pump manufacturer Busch (UK).