gives an indication of the appropriate orifice size to be fitted to the vent valve for given
vacuum system volumes in order that the rate of pressure rise remains within the limits
shown in Vent gas specification and vent control data on page 34.
Note:
If a vent restrictor is used, the time required to vent the vacuum system may be
unacceptably long. The time may be reduced by using a vent valve without a vent
restrictor and waiting until the pump speed has fallen to 50% of full rotational speed
before opening the vent valve.
7.2.3 Vent valve control
The TAV solenoid valve can be controlled by the nEXT motor controller or by an Edwards
TIC Turbo Instrument Controller.
The nEXT controller can control the rate of venting, using the vent valve options in Table:
Vent valve options. Using this feature the pump can be vented from full rotational speed
in a controlled manner that will not damage the pump bearings. Once the pump
rotational speed has dropped to below 50% of maximum speed it is safe to hard vent
(open the vent valve fully) provided the backing pump is left on and the backing valve is
open.
To use the nEXT controller function, the nEXT turbo pump controller has a 4-pin auxiliary
connector socket on the side of the pump, as circled in , which the vent valve with the
auxiliary connector fitted is plugged into.
The controller is capable of controlling a number of different venting options. These are
defined in Table: Vent valve options.
The controller is capable of accommodating both normally open and normally closed
vent valve types. These are defined in Table: Valve types. A power cycle is required after
making a change to the valve type, before the auxiliary output circuitry adjusts to
accommodate the requested change.
Table 19
Vent valve options
If a stop command is received, vent valve fully opens immediately. In a
fail condition, controlled venting from 100% to 50% full rotational
speed and then vent valve opens fully below 50%.
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