Pumping flammable vapours with dry vacuum pumps

Author : Dr Don Collins, Market Development Manager, Edwards

20 September 2010

Chemical and pharmaceutical processing usually requires high efficiency and reliability. Vacuum is needed in many processes, including distillation, evaporation, crystallisation, drying, solvent recovery, house or general vacuum, deodorization and filtration.

Dry vacuum pumping offers major benefits for these applications and it is a viable and practical alternative to traditional wet pumping technologies. However, as a mechanical device, there are safety issues that need to be considered when using an all-dry, close tolerance machine to pump flammable vapours.

The ATEX directive requires process operators to generate an "explosion protection" document, based on a risk analysis of the whole process, and then to identify protection strategies to mitigate the risks.

Dry pumps include dry backing pumps, which discharge to atmosphere, and mechanical vacuum boosters, which discharge to backing pumps (they can be wet or dry). Dry pumps have none of the problems associated with the oil or water in wet pumps; they are clean, reliable, provide high vacuum and require minimal maintenance. They do not produce waste, or contaminate valuable solvents, products or the environment. They can offer clear savings in running costs and installation space compared with liquid-based water ring pumps, oil-sealed pumps and steam ejector systems.

Dry pumps have contact-free pumping mechanisms and no ignition sources in normal operation. However, they are often required to pump potentially explosive atmospheres and it is necessary to consider the possibility of ignition sources resulting from failure conditions. When installing pumps, process managers developing a safety strategy should include the following steps:

Identify explosive atmospheres
Avoid explosive atmospheres
Eliminate ignition sources
Limit the effects of a potential ignition

　
1. Identifying explosive atmospheres
Understanding where the explosive atmosphere is and where an ignition might take place is a vital first step in considering explosion safety. There are three areas to consider:

1 – the external atmosphere surrounding the pump
2 – Internal 1 – the atmosphere from the process interface to the inlet flange of the dry vacuum pump
3 – Internal 2 – the atmosphere from the inlet flange on the dry vacuum pump to the exhaust interface.

2. Avoiding explosive atmospheres and the flammable range
Operation outside the flammable range ensures that even if an ignition source is present there will not be an explosion. Pressure, temperature, gas mixture, size of equipment, direction of flame propagation and turbulence all affect the flammability of a vapour. There are various ways to avoid flammable atmospheres, although not always practical, so that in some cases it is possible to pump a flammable vapour or gas continuously without ever being in the flammable zone.

Low pressure operation < 60 mbar

As the pressure of the system reduces, eventually a point is reached where flammability is not supported. Edwards' research has proven that it is not possible to sustain a burn in a potentially explosive atmosphere below 60 mbar.

Operating above the upper explosion limit (UEL) or below the lower explosion limit (LEL)

By controlling the relative concentrations of fuel and air it is possible to keep the concentration of the vapours above the UEL or below the LEL, (see fig. 1), therefore eliminating the risk of an explosion. Suitable safety margins should be used and it is common practice to use levels of 60 per cent above the UEL and below 25 per cent of the LEL.

Controlling the leak tightness of the process system

It is good practice to run a pressure rise test on a process vessel before operation. This is normally conducted to ensure air leakage does not affect the quality of the product. It can also be used to show that the air leakage is above or below the calculated limit for flammability.

Inerting

It is advisable to inert the process vacuum chamber back to atmospheric pressure at the end of the cycle. This ensures that the subsequent evacuation for the pressure rise test is conducted out of the flammable range. This inerting policy can also be applied at the end of a process to flush out flammable residues.

Excluding or limiting oxidants, typically oxygen, from the process vessel

The limiting oxidant concentration (LOC) depends on the vapour and the type of inert gas used. In chemical, pharmaceutical and similar plants, the inert gas is usually nitrogen. If the concentration of oxygen is kept below five per cent, by using nitrogen, the atmosphere will be out of the flammable range for all but a few vapours such as ethylene oxide and carbon disulphide. You should apply a safety margin of typically 40 – 60 per cent of the LOC as the maximum oxidant concentration.

Adding an inert gas to flammable gases reduces the flammability limits until they meet.
The relative effectiveness of inert gases on narrowing the flammability limits follows the series CCl₄ > CO₂ > N₂ > He > Ar.

3. Eliminating ignition sources
If pumping in the flammable zone is unavoidable, the third step is to eliminate possible ignition sources. It is impossible to eliminate all ignition sources with dry vacuum pumps and other rotating equipment, because they contain due to bearings which might fail, there is the possibility of metal to metal contact and debris or solids build-up.

Most flammable vapours have auto-ignition temperatures above 200°C. A vacuum pump is a compressor and as such has the potential to increase the temperature of the gas that is being pumped. Internal gas temperatures near the pump exhaust are in the region of 200°C and have traditionally been used to handle solvents with auto-ignition temperatures above this.

When pumping materials with lower auto-ignition temperatures it is common practice to reduce the internal gas and surface temperatures by slowing the rotational speed of the pump and lowering the external case temperature (and therefore the internal temperatures)
Dry vacuum pumps are non-contacting mechanisms that have internal clearances of typically less that 0.2 mm. If the gas that is being pumped contains particles or resins, the material can fill the internal clearances, potentially creating a running contact and hot spot. To avoid trapping any particles or resins in the pump, operate the pump when it is at its recommended working temperature, make sure that the exhaust does not block and flush the pump with a suitable non-corrosive solvent or purge at high inlet pressure with air or nitrogen when required or at suitable intervals and before stopping.

4. Limiting the effect of an explosion
If, despite the above precautions, an ignition does occur, it is important that it does not cause a major incident or any damage. The following considerations will limit the effect of an explosion.
Containment in a vacuum system requires the prevention of flame propagation to the external atmosphere, the process chamber and the exhaust pipe-work.

Independent testing has shown that the construction of the Edwards chemical dry vacuum pumps prevents flame propagation from inside the pump out to the atmosphere regardless of the zone. Preventing propagation along the pipe-work requires individual consideration.

Flame arresters have been used effectively for many years in the chemical, pharmaceutical and associated industries to prevent the propagation of internal explosions. However, their performance is dependent on the geometry of the connecting pipework and pump. It is therefore essential that the performance of the flame arresters are tested with the pumps to prove that as a combination they will contain any explosion.

The protection principle of constructional safety is defined in European standard EN13463-5. This specifies the way in which mechanical equipment should be designed to avoid active ignition sources. This is particularly applicable to the internals of dry vacuum pumps that commonly operate with small internal clearances. It is possible to use this alone and safely pump flammable vapours.

Dry pumping has many benefits in the fine chemical and pharmaceutical industry and virtually all dry pumps (with the exception of dry running rotary vane pumps) do not have ignition sources that are active during normal operation.

However, as they often pump flammable vapours, it is important for process managers to fully understand the risks associated with the use of these pumps and assess if an explosion is possible. If an explosion is possible, remove or reduce the risk by following the straightforward guideline of avoiding, eliminating, containing and limiting the effects of an explosion.