21
06
Operation
Manual 1843 Rev B p/n 001843 0000
Checking Pump Performance
If the processing time or the ultimate pressure
becomes poor with no recent changes in the
process or in system configuration, test the
pump to determine if the trouble is in the pump
or the connected process equipment. To check
the condition of the pump, measure the blank-off
pressure as directed below using a McLeod gauge.
If possible, also read the blank-off pressure with
a thermistor or thermocouple gauge. To read the
blank-off pressure, close the pump inlet by means of
a vacuum valve or blank-off plate.
Connect a vacuum gauge to the suction inlet
and position the gauge tube, facing downward
in the higher area of the manifolding so that the
tube will not become flooded and blocked by
splashing pump oil. If the pump is disconnected
from the process equipment, connect a 90° elbow,
extending upward, to the inlet flange and bolt the
blank-off plate, with gauge connection to the open
elbow flange.
Operate the pump for a minimum of 15 minutes
and record the lowest pressure reached. Average
blank-off readings are 5 to 25 microns with a
McLeod gauge, and 10 to 100 microns with a
thermocouple gauge. The specification pressure is
10 microns, McLeod gauge reading. If the McLeod
gauge reading is low and the thermocouple gauge
reading is high, the pump oil is contaminated.
See
Oil Contamination on page 24
. A high reading
of both the McLeod and thermocouple gauges
indicates that an air leak is present.
Checking Process Equipment
Attach a vacuum gauge to the connection on the
system side (upstream) of the isolation valve.
See
Vacuum Gauges on page 16
. For this test,
the system should be clear of any process work
which might give off vapors and change the reading.
Run the pump to obtain the best vacuum possible
with the valve open, and then close the valve
and observe the pressure rise. If the pressure
rise is greater than desired, the leaks should be
eliminated. Check the system carefully for loose
joints and obvious leaks. Use a leak detector if
available. The trouble can be isolated further by
applying the following procedures:
Isolate each segment by valves or blank-off plate at
convenient locations. Pump down each segment of
the process equipment individually, starting at the
segment closest to the vacuum pump.
Check the lowest pressure attainable when each
segment is added. If the pressure is close to that
obtained previously, add the next segment. If the
pressure is not close to the previous pressure,
leak-test the last segment added.
When leak-checking process chambers, start at
the air and gas inlet valves, doors, sight ports,
electrical and mechanical feedthroughs, gauge
tube fittings, and any other gasketed penetrations
and O-ring connections. After a suspected leak has
been found, cover it with plastic sealing compound,
such as Apiezon Q or Duxseal, and check the
equipment performance before sealing the leak
permanently. Thus, all permanent repairs can be
made at the same time.
If a leak detector is not available, use the following
methods to locate leaks:
1. Cover suspected leaks with a low vapor
pressure sealing compound such as
Apiezon-Q or Duxseal. Do this while pumping
on the equipment and monitoring the pressure.
A sudden decrease in pressure indicates
that a leak has been covered. Repair leaks
permanently as necessary.
2. If the leak is large, causing pressures greater
than 1 Torr (1.3 mbar), pressurize the process
equipment internally with 1 PSIG (70 mbar) of
clean compressed air and paint a soap solution
on suspected leak areas. Bubbles will indicate
leaks.
3. If the leak is small, causing pressures less than
1 Torr (1.3 mbar), use a fast-acting medium
such as helium. Position the vacuum gauge
head downstream from the suspected leak
area, between the leak and pump. When the
pressure has been reduced so the gauge may
be used, apply probing medium to suspected
leak areas using a spray bottle or brush. If
the probing fluid is directed at the leak or an
area close to it, a sudden change in pressure
will occur. Cover suspected leaks with plastic
sealing compound and continue leak-checking
until desired pressure is obtained.