Pipe strain happens when the piping is not adequately supported, either by the use of pipe supports underneath (stiff legs), hangers, or both. It can cause problems when bolting up flanges and can contribute to premature seal failure. Pipe strain can also move the pump or base. For those performing precision shaft alignment, this can be especially troublesome.
Pipe Strain in the Field
Recently, while helping a training class perform a precision shaft alignment on a small pump, we noticed things weren’t going as planned. Every time they tightened the hold down bolts on the motor, the motor would move up vertically by 10 to 20 mils (0.010″ to 0.020″) not down, as one might expect. After a couple of times shimming and moving, we decided to dig deeper.
- Their first guess as to why this was happening was to check for soft foot. There was only about 2 mils of soft foot, so that was ruled out.
- The base was checked to make sure it was properly bolted down. No visible problems were detected with the base.
- The coupling was inspected. While it was a very stiff coupling, it appeared to be normal.
- Out of curiosity, I stepped on the motor, and the vertical measurement did indeed change by greater than 10 mils (0.010”).
Pipe Support System Exposed
After some conversation with the group, we decided to look not just at the machine, but the entire system. A pipe support (stiff leg) was underneath the suction piping, about 4 feet away from the pump inlet. One of the mechanics grabbed the pipe support, and it moved freely. He could pull the pipe support out from underneath the pipe with almost no effort.
We estimated the weight of the piping attached to the pump (and the fluid within it) at 6000 pounds. All of this was being held in place by (3) pump feet, and (3) 1⁄2” bolts. The pump was holding up the piping – not the pipe support. We all agreed that a new pipe support should be made, and placed under the piping before any further precision alignment could be attempted. They will also replace the coupling, inspect the base, ensure the foundation bolts are tight, and inspect the grout.
I don’t know the final answer, but I suspect that coupling stiffness, pipe strain (due to a non-working pipe support), and either a loose, or insufficiently stiff base plate may yield the answer.
When performing alignment, it is best to look at the entire machine, including the base and support structures, not just the coupling.
