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a process related coating build up can be removed by soaking the pump with the proper solvent (turning by hand) - check with
Tuthill Vacuum & Blower Systems. Inspecting the inside the pump is covered under “DISASSEMBLY.”
PUMP LEAKS
If the pump is suspected of having an air leak, after eliminating oil contamination as the cause of poor performance, use a plastic
sealing compound to seal over suspected areas, such as joints, connections plugs and any penetrations into the vacuum area and
check pump blank off performance. If gasketed connections are suspected, remake the connections. Plastic sealing compound
may be used to make temporary gaskets; these should not be made too thick since the material may be squeezed into the pump.
Check the shaft seal for mechanical defects, such as a cracked carbon washer or hardened rubber components.
For checking leaks, a fast acting total pressure gauge used with acetone or a sensitive freon leak detector can be used. A helium
leak detector is the most convenient to use if available.
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.
1. To read the blank off pressure, close the pump inlet by means of a vacuum valve or blank off plate.
2. Connect a vacuum gauge to the suction side 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 degree elbow, extending upward, to the inlet flange and bolt the blank off plate, with gauge connection,
the open elbow flange.
3. 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. A high reading of both the McLeod and thermocouple gauges indicates that an air leak
is present.
CHECKING PROCESS EQUIPMENT
Attach a vacuum gauge (See Installation, Vacuum Gauges) to the connection on the system side of the isolation valve. 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, 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, leak test the last segment.
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, 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, James Biddle Company or Shell
Company; or Duxseal, Mansville Corp). 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 over 1 torr (1 mbar), pressurize the process equipment with one psig (70 mbar) of clean
compressed air and paint a soap solution on suspected leak areas and bubbles will indicate leaks.
3. If the leak is small causing pressures less than 1 torr (1 mbar), use a fast acting medium such as acetone, alcohol, freon, or
helium. Position the vacuum gauge head downstream from the suspected leak area, between the leak and pump. When the
pressure has been reduced so that the gauge may be used, apply probing medium to suspected leak areas using a squirt
gun 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 leak with plastic sealing compound and continue leak checking until desired pressure is obtained.
4. If leak checking fails, disassemble and remake all demountable joints and connections using new gaskets or vacuum sealing