I’ve heard that planning for pipe expansion and contraction is important for long hydro penstocks. How long does the pipe need to be before this is a consideration? What are the best approaches for dealing with pipe expansion and contraction to avoid joint blowouts?
John Hibner • Denton, Texas
With any length of pipeline, expansion from heat and contraction from cold is cause for concern. For instance, PVC pipe will lengthen about 1/3 of an inch per 100 feet per 10°F, so a 250-foot-long PVC pipeline would lengthen about 4 1/2 inches with a temperature change from 40°F to 90°F.
Temperature changes are much less with buried pipeline compared to aboveground pipeline. But most designs necessitate some length of aboveground pipeline, such as stream crossings on bridges, trestles across low areas, entrances to the powerhouse, and exits from the intake structure. If expansion and contraction are not factored in to the design, the pipeline may experience joint blowouts, buckling, support pull-out, or turbines and valves pushed off their supports.
Pipelines that are aboveground can shrink or expand dramatically over their length. From baking in the sun during the day to below-freezing temperatures at night, an aboveground pipeline can change temperature a great deal. If these temperature swings take place when the pipe is empty, the effect is even more dramatic. Keeping the pipeline full of water greatly limits the temperature change, but the one sunny day you need to drain the pipeline is the one that can cause you grief if you haven’t accounted for the expansion.
Pipe material plays an important role in expansion and contraction. Plastic expands a lot more than steel pipe, and steel expands more than concrete pipe. Flexible pipe like polyethylene can absorb expansion by bowing, if there is enough room, but stiff pipe like steel is more likely to expand in a direct line against the thrust blocks or supports that hold it. For instance, a long plastic pipeline that lies straight and taut between two anchors when it is cold may need room to move back and forth between the anchors when it is warm. A straight steel pipe in the same situation may simply push apart the two end anchors when it expands.
The most common way to deal with expansion and contraction is to install expansion joints (an HDPE expansion joint is shown in the photo at left). These flexible couplings allow some movement of the pipe at the joint. The pipe must be securely fastened on either side of the joint to keep the joint from coming apart. There are formulas, published in the American Water Works Association’s M series manuals on pipelines: steel, PVC, and HDPE, to help you figure out how much a pipe will expand or contract with given changes in temperature.
There are also some simple tricks. Put some unanchored bends in your pipe, since they will absorb a small amount of expansion and contraction. In some situations, you can anchor the pipe well enough to resist the effects of expansion and contraction. With plastic pipe, another option is to secure both ends and allow the pipe to bow or snake freely when it gets warm. However, make sure you secure the ends when the pipe is cold, not when it is warm, or it will shrink and pull the anchors together.
Keeping the concept of expansion and contraction in mind while designing your pipeline may help avoid some simple, but costly mistakes. Check out the AWWA (www.awwa.org) and other pipe manuals for basics.
Lee Tavenner, Solar Plexus • Missoula, Montana