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.

Transient pressure waves are emitted throughout a system when there is a change in the fluid velocity. Low pressure transients are emitted from the downstream side of their initiators such as the discharge of a pump that has tripped or the outlet of a valve that has closed. During such an event, a pressure wave moving near the speed of sound travels out. This pressure wave is the transition between the fluid at rest and the fluid that is continuing forward due to momentum. During this transition, vapor pockets will form where the pressure has dropped below the fluid’s vapor pressure. When the low pressure wave reaches the end of the system, it is reflected backwards as a positive wave and there is an eventual reversal in flow risking the sudden collapse of any vapor pockets that have formed. This phenomenon is known as transient cavitation and can produce catastrophic high pressures within a system.

Due to the complexities of interconnected piping networks, computer simulation and high frequency transient monitoring are the only practical means of unveiling critical risks within a liquid piping system. Transient pressure monitoring enables engineers and operators the ability to see low pressure transients normal pressure recording devices cannot capture quickly enough. Further, computer models help identify the most effective mitigation strategies, minimizing complexity and cost. Multiple approaches to accurately modelling this situation are discussed, with specific interest devoted to transient cavitation models.

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.


Addressing Low Pressure Transients

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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