To Achieve Ultimate Vacuum (If Gas Ballast Fitted); To Pump Condensable Vapours (If Gas Ballast Fitted); Implication Of Atex Directive; Flammable/Pyrophoric Materials - Edwards XDS35i Instruction Manual

A730-01-880 Issue J
5. Open the vacuum system isolation valve (if fitted).
4.3

To achieve ultimate vacuum (if gas ballast fitted)

In order to achieve the best possible vacuum, the pump should be operated with the gas ballast control on position '0'.
However, if the pump, or elements of the vacuum system it is attached to, are new or have been newly fitted, some
atmospheric moisture may be present. If atmospheric moisture is present, run the pump with the gas ballast control
in position 'I' or 'II' for 20 minutes before switching to position '0'. If moisture is allowed to remain, the performance
of the pump will be impaired.
4.4

To pump condensable vapours (if gas ballast fitted)

Select a suitable gas ballast setting (gas ballast control in position 'I' or 'II') when there is a high proportion of
condensable vapours in the process gases. This will assist the vapours to pass through the pump without condensing
and keep the pump performance from degrading.
4.5

Implication of ATEX directive

4.5.1

Flammable/pyrophoric materials

When flammable or pyrophoric materials are present within the equipment:
Do not allow air to enter the equipment.

Ensure the system is leak tight.

Dilute any flammable gases or vapours by using an inert purge, for example nitrogen, to the pump inlet

and/or gas ballast (if fitted) to reduce the concentration of flammable gases or vapours in the pump and
exhaust line to less than one quarter of the published lower explosive limits (LEL).
Prevent the condensation of flammable vapours within the pump mechanism and exhaust line by using an

inert gas purge to the pump gas ballast connection. Note that this option is not available on the No Gas
Ballast versions. In these pumps only inlet purge can be used.
4.5.2

Gas purges

If nitrogen purges are used to dilute dangerous gases to safe levels, ensure that the system shuts
down if the nitrogen supply to the pump fails.
The following actions must be taken to ensure that the gas being pumped stays out of the flammable range:
The inert gas purge should be switched on to remove air from the pump and exhaust before the process

starts. The purge flow can be switched off at the end of the process only after any remaining flammable
gases or vapours have been purged from the pump and exhaust line.
If liquids that produce flammable vapours could be present in the pump foreline then the inert gas purge to

the pump should be left on all the time this liquid is present. Flammable liquids could be present in the
foreline as a result of condensation or may be carried over from the process.
When calculating the flow rate of inert gas required for dilution, consider the maximum flow rate for the

flammable gases/vapours that could occur. For example, if a mass flow controller is being used to supply
flammable gases to the process, assume that a flow rate for flammable gases that could arise if the mass
flow controller is fully open. The inert gas purge flow rate should be continually measured and if the flow
rate falls below that required, then the flow of flammable gases or vapours to the pump must be stopped.
The Vacuum Pump and Vacuum System Safety publication P400-40-100 is available from Edwards or the supplier.
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