Author : Dr Don Collins, Market Development Manager, Edwards
20 September 2010Chemical and pharmaceutical processing generally requires high perexplos info formance and reliability. Vacuum is needed in lots of methods, together with distillation, evaporation, crystallisation, drying, solvent restoration, residence or preferred vacuum, deodorization and filtration.
Pumping flammable vapours with dry vacuum pumps
Dry vacuum pumping offers fundamental advantages for these programs and it’s miles a possible and realistic opportunity to conventional moist pumping technology. However, as a mechanical tool, there are safety issues that need to be considered while the usage of an all-dry, near tolerance machine to pump flammable vapours.
The ATEX directive requires system operators to generate an “explosion safety” record, based on a chance evaluation of the complete technique, and then to become aware of protection techniques to mitigate the dangers.
Dry pumps consist of dry backing pumps, which discharge to surroundings, and mechanical vacuum boosters, which discharge to backing pumps (they may be moist or dry). Dry pumps have not one of the problems related to the oil or water in moist pumps; they’re clean, dependable, provide excessive vacuum and require minimum renovation. They do not produce waste, or contaminate valuable solvents, merchandise or the environment. They can offer clear financial savings in strolling expenses and installation area as compared with liquid-primarily based water ring pumps, oil-sealed pumps and steam ejector structures.
Dry pumps have contact-free pumping mechanisms and no ignition resources in ordinary operation. However, they are often required to pump doubtlessly explosive atmospheres and it’s far essential to consider the possibility of ignition assets due to failure conditions. When putting in pumps, procedure managers developing a safety approach should consist of the following steps:
1. Identifying explosive atmospheres
Understanding wherein the explosive environment is and in which an ignition might take vicinity is a important first step in considering explosion protection. There are 3 regions to do not forget:
1 – the outside environment surrounding the pump
2 – Internal 1 – the ecosystem from the process interface to the inlet flange of the dry vacuum pump
three – Internal 2 – the ecosystem from the inlet flange at the dry vacuum pump to the exhaust interface.
2. Avoiding explosive atmospheres and the flammable variety
Operation out of doors the flammable variety guarantees that even if an ignition supply is present there will now not be an explosion. Pressure, temperature, fuel combination, size of equipment, route of flame propagation and turbulence all affect the flammability of a vapour. There are diverse ways to keep away from flammable atmospheres, even though now not continually realistic, so that during some instances it is possible to pump a flammable vapour or gasoline constantly without ever being within the flammable region.
- Low stress operation < 60 mbar
As the strain of the device reduces, finally a point is reached where flammability isn’t always supported. Edwards’ studies has established that it is not possible to preserve a burn in a potentially explosive environment beneath 60 mbar.
- Operating above the top explosion restriction (UEL) or beneath the decrease explosion restriction (LEL)
By controlling the relative concentrations of gas and air it’s far feasible to hold the attention of the vapours above the UEL or under the LEL, (see fig. 1), therefore getting rid of the chance of an explosion. Suitable protection margins have to be used and it’s far common exercise to apply stages of 60 in step with cent above the UEL and under 25 consistent with cent of the LEL.
- Controlling the leak tightness of the system machine
It is right exercise to run a strain upward thrust test on a process vessel before operation. This is usually carried out to ensure air leakage does no longer have an effect on the excellent of the product. It can also be used to show that the air leakage is above or beneath the calculated restrict for flammability.
- Inerting
It is beneficial to inert the system vacuum chamber lower back to atmospheric stress at the stop of the cycle. This ensures that the subsequent evacuation for the pressure upward push take a look at is performed out of the flammable variety. This inerting coverage can also be applied at the give up of a manner to flush out flammable residues.
- Excluding or limiting oxidants, usually oxygen, from the system vessel
The restricting oxidant awareness (LOC) relies upon on the vapour and the kind of inert gasoline used. In chemical, pharmaceutical and comparable flowers, the inert gas is normally nitrogen. If the awareness of oxygen is saved underneath five in keeping with cent, by using nitrogen, the ecosystem can be out of the flammable variety for all however a few vapours together with ethylene oxide and carbon disulphide. You must apply a protection margin of commonly 40 – 60 in step with cent of the LOC because the most oxidant awareness.
- Adding an inert gasoline to flammable gases reduces the flammability limits until they meet.
- The relative effectiveness of inert gases on narrowing the flammability limits follows the collection CCl4 > CO2 > N2 > He > Ar.