Addressing Low Pressure Transients

Low transient pressures in piping systems are different in many ways to high transient pressures. While high pressures can obviously burst pipes or damage components, low pressures can collapse pipes, pull in environmental contaminants, bring components out of solution, or induce transient cavitation, a particular concern for hydrocarbon liquids. This paper will use examples of computer modeling to reveal how common system events such as pump trips or valve closures induce low pressure transient waves that have potential to be just as destructive as more intuitive high pressure waves. Fluid transient studies and literature often focus on high pressures, or do not clearly demonstrate how liquids with low vapor pressures (such as many hydrocarbons) can be affected. Even discerning a pipe’s negative pressure rating through codes and standards can be a challenge. It is shown that low pressure transients are a potential issue in any liquid system. It is further demonstrated that “Rule of Thumb” or typical simplified calculations are not sufficient to capture these effects, and cannot be used to properly locate and size equipment.

When the Joukowsky Equation Does Not Predict Maximum Water Hammer Pressures

The Joukowsky equation has been used as a first approximation for more than a century to estimate water hammer pressure surges. However, this practice may provide incorrect, nonconservative, pressure calculations under several conditions. These conditions are typically described throughout fluid transient text books, but a consolidation of these issues in a brief paper seems warranted to prevent calculation errors in practice and to also provide a brief understanding of the limits and complexities of water hammer equations. To this end, various issues are discussed here that result in the calculation of pressures greater than those predicted by the Joukowsky equation. These conditions include reflected waves at tees, changes in piping diameter, and changes in pipe wall material, as well as frictional effects referred to as line pack, and the effects due to the collapse of vapor pockets. In short, the fundamental goal here is to alert practicing engineers of the cautions that should be applied when using the Joukowsky equation as a first approximation of fluid transient pressures.

When the Joukowsky Equation Does Not Predict Maximum Water Hammer Pressures

The Joukowsky equation has been used as a first approximation for more than a century to estimate water hammer pressure surges. However, this practice may provide incorrect, non-conservative, pressure calculations under several conditions. These conditions are typically described throughout fluid transient text books, but a consolidation of these issues in a brief paper seems warranted to prevent calculation errors in practice and to also provide a brief understanding of the limits and complexities of water hammer equations. To this end, various issues are discussed here that result in the calculation of pressures greater than those predicted by the Joukowsky equation. These conditions include reflected waves at tees, changes in piping diameter, and changes in pipe wall material, as well as frictional effects referred to as line pack, and the effects due to the collapse of vapor pockets. In short, the fundamental goal here is to alert practicing engineers of the cautions that should be applied when using the Joukowsky equation as a first approximation of fluid transient pressures.