Under Pressure: Pressure Buildup in Pipelines
In the third issue of a series on piping hydraulics, Hixson’s August From Experience newsletter looks at pressure buildup in pipelines.
While leaks and ruptures in pipelines may be due to a number of factors, one culprit can be temperature change: Increasing temperatures resulting from the transfer from an outdoor tank to a warm production room or solar heating on an external line will cause liquids to expand and increase the internal pipe pressure, resulting in leaks or ruptures at the weakest point(s) in the line.
To prevent unsafe pressure rises, it is wise to monitor the temperature variations to which pipes are exposed. It is also important to recognize the relationship between temperature and specific liquids that commonly exist within a GMP facility, since different liquids will exhibit different degrees of expansion (see Experience in Brief).
Excessive pressure buildup can be managed by either blowing lines clear using air or nitrogen, draining liquid from the pipes, or leaving lines open to a production or storage tank. For example, the quantity of a typical vegetable oil that would need to be drained from a pipe can be estimated using the graph below. The graph depicts the final pressure in a pipeline (starting from atmospheric pressure) depending on the fullness of the piping, and degree of temperature rise. In essence, the smaller the void in the pipe, the higher the final pressure will be.
As shown, more void space within a pipe provides room for typical vegetable oils to expand. While a drastic pressure rise could be experienced with only a 20°F increase were there very little void space (1%), a relatively small pressure increase would be realized if the void space were 5% instead.
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