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Page 15
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Installation
B777-50-880 Issue D
3.3.3
Backing connection
Use suitable vacuum tubing and connectors to connect the NW flange of the backing port to the backing pump.
If necessary use flexible pipe or bellows to reduce the transmission of vibration from the backing pump to the
EXT556H pump.
Edwards recommends using an Edwards RV backing pump. The minimum size of the backing pump required is given
in
. A larger backing pump may be needed if running the pump at a high inlet pressure or high throughput, or
if purging the pump.
CAUTION
If the EXT pump is to be backed by the interstage port of another turbomolecular pump, make sure that the
backing pressure does not fall below 5 x 10
-4
mbar (5 x 10
-2
Pa). Lower backing pressures will increase the
evaporation rate of the lubricant and so will reduce the life of the bearings.
3.4
Vent options, vent valve connection and control
CAUTION
If venting the pump when it is at full rotational speed and the rate of pressure rise is too high the pump bearing
life may be reduced.
To maintain the cleanliness of the vacuum system Edwards recommends that, whenever switching the pump off,
venting the pump or the vacuum system when the speed of the EXT556H pump is between full rotational speed and
50% of full rotational speed. At and above 50% of full rotational speed the rotor spins fast enough to suppress any
hydrocarbon oil from the backing pump. Venting may be accomplished by one of the following methods:
Use a TAV5 or TAV6 solenoid vent valve accessory (see
) in place of the manual vent valve.
Use a TAV5 or TAV6 solenoid vent valve connected to a convenient flange on the vacuum system upstream of
the turbo pump.
Use an alternative valve connected to the vacuum system upstream of the turbomolecular pump.
The maximum rate of pressure rise is given in
When using the manual vent valve supplied, the vent valve should only be opened when the rotational speed of the
EXT556H pump has fallen to 50% of full rotational speed.
If using a TAV vent valve or other solenoid vent valve, the vent valve should only be used to vent the pump from full
speed if the vacuum system has a volume of 20 litres or more. If the system has a volume of less than 20 litres,
incorporate a suitable restrictor (see below and
). Alternatively, if using an EXC controller to operate the vent
valve, the valve can be configured so that it opens once the pump has reached 50% of full rotational speed.
Do not vent from the backing line as this may lead to contamination entering the turbo pump and hence reduced
bearing life. If venting into the vacuum system, select a point upstream of the pump to prevent oil backstreaming
from the backing line.
Table 7 - Vent restrictor orifice diameter (with atmospheric pressure at the inlet)
Vacuum system volume (litres)
Orifice diameter (mm)
< 20
< 1.0
< 10
< 0.7
< 5
< 0.5
< 2
< 0.35