Alignment on a Hot Water Pump After a Motor Change
A 75 kW end-suction pump handling 120 C water is being recommissioned after a motor replacement. The coupling is a 6 in spacer elastomer type. The casing reaches 120 C in service, and the OEM's cold alignment target is 0.005 in motor low at the drive end and 0.002 in motor high at the non-drive end. With the motor bolts snug, a laser shows the motor 0.006 in low at the coupling in the vertical plane, and vertical angular misalignment of 0.010 in per 10 in with the motor sloping up toward the pump. The coupling face is 12 in from the DE foot and 36 in from the NDE foot. A dial test with each motor foot bolt loosened in turn shows 0.005, 0.001, 0.000 and 0.002 in. The suction and discharge flange bolts were pulled up tight before the alignment check.
1. What do the foot readings tell you, and in what order do you deal with them?
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Those are soft foot readings, taken with each bolt loosened in turn while the dial watches the foot. Three feet are at or under 0.001–0.002 in, which is acceptable; one foot at 0.005 in is not (target is 0.002 in / 0.05 mm or less).
Sequence: soft foot first, alignment second. A 0.005 in soft foot distorts the motor frame when the bolts are torqued, and that distortion moves the shaft - it can eat most of the 0.006 in offset you are trying to correct, so any alignment done before it is meaningless. You would be chasing the reading around the machine.
How to correct it: lift that corner, pull the shim pack and inspect for burrs, paint, dirt, crushed or rusty shims and a cracked baseplate; shim under the full foot pad rather than stacking washers at the bolt hole; use the fewest, largest shims possible. Then re-torque and re-check all four feet in turn. Iterate until every foot is 0.002 in or less, then re-zero and start the alignment. Record the soft foot readings - they are the first thing to look at if the alignment will not repeat.
2. Calculate the vertical shim correction at each motor foot, including the thermal target.
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Convert the angular reading: 0.010 in per 10 in = 0.0010 in/in, with the motor sloping up toward the pump - so going back from the coupling toward the feet, the motor centreline is progressively lower.
Current motor-low position at each foot plane:
- DE (12 in from coupling): 0.006 + (0.0010 x 12) = 0.006 + 0.012 = 0.018 in low
- NDE (36 in from coupling): 0.006 + (0.0010 x 36) = 0.006 + 0.036 = 0.042 in low
Apply the cold targets (required shim = current low minus target low; a target of "high" is a negative low):
- DE: required low = 0.005, so shim = 0.018 - 0.005 = add 0.013 in
- NDE: required low = -0.002, so shim = 0.042 - (-0.002) = add 0.044 in
Sanity check on the differential: 0.044 - 0.013 = 0.031 in over the 24 in foot span, which equals the 0.024 in of angular correction plus the 0.007 in difference between the two thermal targets. Consistent.
Add the shims, torque to spec in a cross pattern, then re-measure and iterate - one pass is rarely exact. Correct horizontal offset by moving the motor, not by shimming.
3. What must be dealt with regarding the piping and the hot/cold condition before this alignment is signed off?
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Pipe strain. The flanges were pulled up tight before the alignment check, so the pump may be sprung into position by its own piping. On a 120 C casing that grows on warm-up, the pipe loads get worse. Check it: put a dial indicator on the coupling or the pump shaft, loosen the suction and discharge flange bolts, and watch for movement. More than about 0.003 in means the piping is straining the casing - fix the supports, re-fit the spool piece, and do not compensate with alignment. Then align with the flanges made up but not straining, and tighten them evenly afterwards.
Hot versus cold. The 0.005 / 0.002 targets are only valid for a cold machine; they exist because the casing grows upward while the motor does not. Do not "improve" them - set exactly what the OEM specifies, and record the ambient temperature at the time.
Verify hot. Once at operating temperature, re-check with the laser (or at least with a dial and feeler) and compare to the coupling manufacturer's allowable offset and angular limits for a 6 in spacer coupling. Also confirm coupling runout and gap before the guard goes back on, and check the motor's driven-equipment alignment after the first heat cycle.