Process engineering: Understanding pipe strain

March 14, 2005
Pipe strain is a condition that is closely related to equipment reliability. Learn about misaligned piping and what to do if its a problem in your plant.

Pipe strain is one of these conditions that the unwary may find difficult to relate to equipment reliability. How could seal and bearing failure have anything to do with the pump piping? The reality is that suction and discharge piping that is incorrectly or inaccurately installed and aligned will create stresses on the pump casings that will detrimentally affect the reliable life of the seal and bearings.

In theory, pumps built to the API (American Petroleum Institute) standard are supposed to be able to absorb specified levels of Moments of Forces on the suction and discharge flanges. Pumps that are built and supplied to any other industry standard (or none at all) require that the pumps be installed in such a way that there are no stresses imposed by the piping on the pump flanges.

So much for theory! Now let’s go back to the real world where force from a variety of equipment is used to bring the pipe flanges sufficiently into line with the pump flanges that the bolts can be slipped through the holes in both. (An uncle of mine tells me he used to use a fork-lift for this job!)

So when it comes to the strain created by misaligned piping, the big question is how much is too much and how do you know it’s there.

Let me offer you a rule of thumb that has proved effective for many years and with all pipe sizes. It is this.

If you can personally push the pipe into place to where the two flanges are in full face contact and aligned, then the pump casing can usually handle the stress, and there shouldn’t be a problem. It must be stressed that no outside equipment should be used, and you must depend only on your own strength.

If you can’t do that, the stress on the pump casing will be excessive and will result in frequent and regular seal and bearing failure. So keep the flanges properly aligned, and keep the pump running!

Author of “The Practical Pumping Handbook” and a specialist in Pumping Reliability, Ross Mackay can be reached at or at 1-800-465-6260

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