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MYTH: “Pipe Stress makes the alignment difficult.”

TRUTH: Pipe stress does not have any significant influence on the alignment unless you make corrections on the machine with the piping attached. In most alignment the corrections to eliminate misalignment are performed on one machine only, typically the one that is easier to move. In a pump-motor set the corrections are done on the motor. If there is sufficient room to shim and move, excellent alignment can be achieved regardless of how much pipe stress there is on the pump. Of course, pipe stress is undesirable and should be eliminated prior to the alignment.

Watch and learn more about Pipe Stress

by Yolanda Lopez

Guest Post by Ricky Smith, CRL, CMRP, CMRT

Pipe stress is caused by misalignment of the mating surfaces of two pipe flanges creating abnormal internal stress of pump bearings, seals, motor bearings, couplings, and can possibly change the displacement of a pump.

General Rules which must be followed by maintenance personnel and contractors: (if you truly want to stop a long term pump problem)

  1. Pipe flanges attached to pumps must be aligned where the gap does not exceed the thickness of two gaskets or tolerance established by your company’s engineering standards.
  2. Pipe flange bolts must drop in without assistance.
  3. Cable pullers, come-a-longs, or long bars should not be used when aligning a flange which is connected to a pump.
  4. Validate the elimination of pipe stress by following the guidelines listed below.

Failure Modes experienced from Pipe Stress on Bearings:

  • Wear caused by seals leaking
  • Wear caused by static vibration
  • Indentations caused by overloading while static
  • Corrosion caused by inadequate lubrication caused by abnormal loading (seal leaking)
  • Flaking caused by misalignment and excessive loading

WARNING: Ensure your contractors follow the same process to eliminate pipe stress. Pipe stress elimination should be validated during commissioning of new pump.

Follow this process if you want to inspect your pumps which may have pipe stress:

  1. Align the two shafts between your pump and driver (typically an electric motor) to tolerance recommended by the equipment vendor or your company’s engineering standards.
  2. Validate misalignment to insure motor and pumps shafts are aligned to specification.
  3. Disconnect the outlet flange on the pump.
  4. Revalidate laser alignment of shafts.
  5. If alignment has moved then you have pipe stress. Do the same for the inlet flange.
  6. Make corrections as stated in the following procedures to eliminate pipe stress.

Elimination of Pipe Stress – “The Ricky Smith Method for Pipe Stress” as learned from Dan Turner (his maintenance and engineering manager at Exxon during his career in the 1970s)

  1. Bolt flanges to pump and insert blind flange gasket along with two regular flanges between pump and mating flanges (cover the hole between welding area and inside the pump).
  2. Attached welding ground to flange. (do not attach ground lead to pump; welding group must always be attached to flange) WARNING: Failure to accomplish this one task properly will cause bearing failure by “electric arcing” which is a failure mode of bearings.
  3. Tack weld flange into place reverse welding each tack.
  4. Allow to cool for 10 minutes.
  5. Reverse stitch weld on opposite sides on the flange similarly used for cast iron welding.
  6. After initial reverse stitch weld then weld normally using electrode recommended by the American Welding Society (typically E-6010 5P or GTAW)
  7. After root pass; weld in any direction you wish.
  8. Allow to cool and then disconnect flange, replace gaskets and;
    Validate bolts will drop into holes without pry bar.
    Validate gap between flanges is no more than two gaskets thick.

Learn more about the effects of running equipment with pipe stress, watch LUDECA Shaft Alignment Know-How Pipe Stress video.

by Yolanda Lopez

While aligning a Flowserve Booster Pump in Arkansas,  the attempted alignment corrections proved unrepeatable and inconsistent. The centerline of the pump shifted from too high to too low and from too far to the right to too far left. I thought this might be a symptom of pipe stress and suggested to the customer that I check for it. Upon loosening the hold-down bolts,  I noticed that piping lifted the pump right off the base.

Pipe Stress
Figure 1 – Pipe Misalignment

I was authorized to disconnect the piping from the pump. Figure 1 shows that at its worst one pipe was one inch out horizontally and about one and a half inches vertically, and angled to the pump.
We aligned the pump without the pipes connected. The customer was advised to redesign the pipe hangers to provide more support to the piping and reduce the stress on the pump.
It is imperative that once the pipefitting is complete and the piping is reattached to the pump, the alignment and pipe stress measurement with the laser will have to be checked once more.

by Carlos Bienes CRL