The 3/4-interstage purge and 2/3-interstage purge flows are switched on and off by the
solenoid-valves (17 and 18). The inlet-purge, shaft-seals purge and exhaust-purge flows are
switched on and off by the solenoid-valves (22, 19 and 24). The 2/3-interstage and
3/4-interstage purges solenoid-valve (21) operates in conjunction with the inlet-purge
solenoid-valve (22), to provide reduced interstage purge flow when inlet-purge is switched
on.
Variable restrictors (12 to 16 and 20) limit the purge flow rates.
A pressure transducer (23) monitors the pressure of the shaft-seals purge flow, and another
pressure-transducer (9) monitors the pressure in the exhaust pipeline. The outputs of the
pressure transducers are used to display the shaft-seals purge and exhaust pressures on the
Pump Display Terminal.
If there is a pressure rise in the exhaust pipeline, the outputs of the exhaust pressure
transducer (9) will bring up warning and alarm messages before causing the iH system to be
shut down via the relay interlock circuit.
The dashboard has three pressure status LEDs (Figure 1-7, items 3, 5 and 6). These LEDs
show whether the nitrogen supply pressure is too high, too low or is in the acceptable range:
refer to Section 5.7.
1.7 The HCMB mechanical booster pump (iH600/iH1000 only)
The HCMB pump is a positive displacement roots vacuum pump. The pump mechanism is
driven directly by a three-phase electric motor. The HMCB is fitted with a pressure relief
valve which decreases the differential pressure between the inlet and outlet of the booster
during inlet transients, thereby enhancing the robustness of the iH system. The HCMB1000
pump in the iH1000 system is driven through an electrical inverter, which increases the speed
of the motor to 6000 r.min
-1
.
The HCMB pump is backed by the HCDP pump:
•
The HCMB600 pump in an iH600 system operates at inlet pressures of 5 mbar (5 x 10
2
Pa) and
below; when you start the HCDP pump, the iH system delays the start of the HCMB pump for
a preset time, to allow the HCDP pump to reduce the pressure in the process system to below
the maximum inlet pressure for the HCMB pump.
•
The HCMB pump in an iH1000 system operates between atmospheric pressure and ultimate
vacuum; when you start the HCDP pump, the HCMB pump is also started.
1-6 iH Dry Pumping Systems
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