In a previous post we had talked about upcoming changes to the BC and Alberta safety codes, and the effects of pipes bursting with different internal fluids. There was also some talk about developing new restraint systems for these temporary piping runs that would conform to these code changes. At Sparta, we have developed a model that predicts the force required to restrain a pipe burst, and gives an avenue to select the correct restraint.
Other companies have used experimental testing in the past to develop such a system. While impressive, and able to detect uncertainties in the math, a destructive testing regime can be expensive and time-consuming. You wouldn’t want to have to set up a new test for every temporary piping case. A mathematical model can be a much more cost- and time-effective solution to the pipe bursting problem, and an adequate safety factor can be added to account for any uncertainties.
The first step in any engineering solution is to determine the variables at play in the system. For our pipe bursting model we took into account the obvious ones, such as pipe diameter and internal pressure. As well, we also took some less obvious variables into account, such as fluid density and viscosity, pipe roughness, and even temperature. When considering a gas as the working fluid, the speed of sound becomes an important part of the equation as well.
The next step is to make some educated assumptions about the piping system, and how the restraint system will be deployed. Managing assumptions is one of the most difficult part about engineering as a whole. False and un-verified assumptions can result in increased loading on any project, which can lead to failure in an unexpected way.
Perhaps the largest assumption we make in the piping restraint system is this: Whatever restraint system is used, it has no slack between tie points when deployed. This is an extremely important assumption, because slack would allow a dynamic load to develop. To verify this, calculations were performed on a system where slack was present, and the loads on the restraint increased by up to 100x. This points to a universal operational truth: All restraint systems must not have any slack when deployed, as slack will quite probably result in the restraint system failing.
While developing our calculations, an interesting phenomenon was noticed. Increasing density and decreasing viscosity both result in a larger force of burst. While these parameters show a stark difference between liquids and gasses, the differences are very small comparatively between different liquids, or different gasses. Therefore, for a liquid case and a gas case, the worst-case fluid was used in determining the burst force. For liquids, the relatively high density of seawater and its relatively low viscosity resulted in the largest burst force.
From all this information, a set of calculations is developed that adequately describes the piping system. A restraint system can now be devised that can be highly customized to any working fluid, any pipe size, and any internal pressure. As well, the restraint material is not fixed. Wire rope, synthetic slings, or some other material can all be used to develop the restraint.
It was also mentioned in an earlier post, but I think it’s important enough to mention the differences between a liquid burst and a gas burst again. Both types of pipe bursts will produce debris traveling at high speed. Both will impart large energies into the pipe string itself, which can cause the pipe string to whip around. However, only the gas bursting will produce an overpressure wave. Flying debris is a large danger, but an overpressure wave can damage sensitive membranes, like ear drums and lung tissues. In close enough proximity to a gas burst, bodily harm can occur without being hit by flying debris. This goes to emphasize that care must be taken when operating with temporary piping.