Thermal Stress Affects 
Pipe Forces on Pumps


Written by:
Dr. Lev Nelik, P.E., P&S Editorial Advisory Board
Published:
February 22, 2012

Second of Two Parts
 

To view all of the articles in the Thermal Stress Affects Pipe Forces on Pump series, click here. 

Two readers responded to “Thermal Stress Affects Pipe Forces on Pumps,” from the January 2012 issue of Pumps & Systems. The conversation was interesting and worth reviewing this month.

I read your article, “Thermal Stress Affects Pipe Forces on Pumps,” in Pumps & Systems, January 2012. I am not an engineer, just an “old rubber hose salesman.” Maybe I am simplifying the situation, but wouldn’t the logical answer for the thermal forces encountered with the expanding and contracting pipe be the use of an expansion joint…either rubber or stainless steel? We recommend expansion joints for solutions to such situations on a daily basis.

The last spools shown in Figure 2 could very well be replaced with expansion joints to handle the pipe growth and contraction. Thank you very much, Dr. Nelik. I look forward to reading articles authored by you in the future.

Rand Goldstein
Goldstein-Schwartz Inc.

With regard to your calculation of axial load on the 2-inch long pipe, the force will not be 60,000 pounds because the calculation assumes that the pipe is not allowed to expand, or each end is infinitely rigid. Obviously, there will be some expansion at both ends of the pipe.

Lyn Greenhill, PE
DynaTech Engineering, Inc.

Lev Nelik responds:

Lyn, the pipe is either welded to the tank or bolted to it. It is also bolted to the pump inlet flange. How can it expand?

Lyn Greenhill responds:

If you conducted a finite element analysis (FEA) of the pipe and included a representative section of the pump and tank and constrained the pump and tank as per the drawing, you would find that the pump would be pushed to the left, and the pipe joint at the tank would deform into the tank. The stress in the pipe would not be all that high. However, if you anchored the ends of the pipe, say by bolting the flanges into thick, rigid plates, then the pipe would see the kind of compressive stress and internal load that you calculated. The other comment I guess I could make about your example is that you assumed that the pipe would be the same temperature as the water. Of course, there is the thermal mass of the system to consider, which probably means that the pipe would see nothing close to the water temp.

The bottom line to any pipe installation, whether to a pump, turbine, compressor, etc., is if you do not have expansion joints, you will get pipe strain and/or cracked pipes. I am working on a steam turbine that moves 0.4 inch from cold to hot due to pressure and temperature. Large diameter, hot pipes can generate enormous loads. That is why you see all the pipes in a power plant hanging from springs.

Lev Nelik responds:

You properly mentioned two mitigating effects: 

Non-rigid constraints

Temperature of the pipe lower than water

Both are true. However:

Movement of the pipe at the tank will not be too significant because tanks are usually much more rigid than the connecting pipe effects. Perhaps the FEA would show about a 10 percent reduction, not much more. On the other side (at the pump), your comment is much more applicable. Indeed, most pumps are “softer” to resist the movement of the expanding pipe. That, however, is also a point of the article, although not stated explicitly. The good aspect of load reduction due to pump movement is completely negated by the bigger problem of the pump movement causing misalignment and, with it, leaky seals, worn or broken couplings and overloaded bearings.

Assume the pipe temperature is half that of the water, so that the 60,000 pounds is only 30,000 pounds, still a very significant load.

However, there is still a much more dramatic, but fortunately helpful, mitigating effect that you have not yet touched on. Take a closer look at the picture in the article. It has a clue.

Lyn Greenhill responds:

I have looked at the picture, and I do not see anything obvious that would significantly change the pipe restraints. The pump volute appears to be firmly anchored to the baseplate and is attached to a vertical riser. That would tend to serve as an anchoring point.

Quite frankly, if I were looking at this application for pipe strain and knew that the tank only held water that had some sun exposure, I would not be concerned about thermal expansion and load on the pump inlet. I would be much more concerned about the vertical, offset load on the volute from the discharge piping, particularly the isolation and check valves. Those things are heavy and need to be supported. 

Lev Nelik responds:

I will take this opportunity to respond to two readers of the article—Lyn and Rand (the first email)—as their feedback touches on a similar aspect.

Lyn, the third missing aspect is gaskets. If the pipe were entirely continual, the high stresses would be as high—or close to as high, if considering Lyn’s point on the mitigating effects of the (somewhat) non-pinned end supports—as the stresses shown in the article. This is because the nearly-rigid supports would not allow the movement, and it will all be absorbed into stress.

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