an Engineering & Service Bulletin

Technical Topics

J.H. WRIGHT & ASSOCIATES

EXPANSION JOINTS

Most engineers quite normally assume that a pump isolated with expansion joints both in the suction and discharge piping, cannot be subjected to piping strains. This is true, however, the use of expansion joints can sometimes cause an entirely new problem with sufficient force to pull the pump right off its foundation.

This pump has a suction pressure of 50 psi, and a 10" diameter suction pipe. Considering the forces at section A-A, it is apparent that a downward force of 50 psi is directed on the pipe cross sectional area of 78.5 square inches. Almost two tons is acting to pry the pump up off its foundation! Also, assuming that the center line of the vertical suction pipe is three or four feet from the pump, we have a real fine lever to assist our efforts.

Where does this force come from? Well, let's remember Sir Isaac Newton's third law, "For every acting force, there is a reacting force that is equal in magnitude, but opposite in direction." If it were not for the expansion joint, this reaction to the line pressure would be supplied by the pipe itself. Since it now cannot do this, the pump is called upon to carry the load.

Actually, we have over simplified the problem slightly. Since a bellows type expansion joint has a maximum diameter considerably over the 10" pipe diameter and an effective area almost twice the 10" pipe area, the real loading on the pump would be between three and four tons.

The same type problem can arise with expansion joints in the pump discharge. Here the problem is often worse due to the higher pressures involved.

The solution, of course, is to examine each expansion joint installation with care. Remember that most pump casings are cast iron and will deform under continued strain. High pump maintenance costs will surely result.

One of the greatest exercises a maintenance man can perform is to disconnect all the suction and discharge flanges on his pumps. In every case where he has to pry them back together with pry bars and the bolting, he can be certain the pumps will sooner or later start giving bearing and other problems.

THOUGHT FOR THE MONTH:

"Wealthy people miss one of life's greatest satisfactions - paying the last installment.''

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