Addressing Low Pressure Transients
AFT Impulse™ Technical Paper
Authors: Amy Marroquin, BLACOH Surge Control; Scott Lang, Applied Flow Technology
Presented at the ASME 2020 Pressure Vessels and Piping Conference (Virtual)
Abstract
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.
CONCLUSION
Low pressure transients when unmitigated can lead to a multitude of problems. The abrupt and often extreme pressure fluctuations within a pipeline can further fatigue an already aged system. Sensitive systems are exposed to contaminants with negative pressures and pipe breaks. Unpredictable high pressures are generated with the collapse of vapor pockets that have formed with low pressures transients.
With the right tools and practices such as transient modeling and monitoring, appropriate and effective solutions can be incorporated into the design of piping systems adding years of design life, reducing safety risks and significantly reducing cost.
Below is an excerpt. Use the link above to view the full paper.
INTRODUCTION
Negative pressures often do not get the focus they deserve when evaluating a piping system. It is more natural to be concerned with the maximum pressure ratings of a pipe. In fact, when determining the root cause of a system failure, it’s all too common to seek out and settle upon what circumstance could have caused such catastrophic high pressures. In reality however, negative pressures are just as likely to occur in nearly every system, and quite often they are the unknown root cause of piping failures. With the right tools, engineers and operators can not only uncover such root causes, they can also design effective mitigation solutions adding years of life to a pipeline.
BACKGROUND
A transient pressure wave is essentially a slice of fluid where the pressure on one side is different than the pressure on the other side (Thorley [1]). This “slice” is communicated through a pipeline as a wave and moves with a speed near the speed of sound. Its behavior is much like a sound wave communicated through air. The cause of these pressure waves is quite simple. They are induced by a change in fluid velocity. While it is quite intuitive that a change in velocity would cause a change in pressure, the magnitude and behavior of a transient pressure wave as it moves through a system is not nearly as intuitive.
Wavespeed
How quickly a transient pressure wave moves through a pipeline is largely a function of a fluid and pipe system’s elasticity. In general, the wavespeed will be higher for more rigid systems. In fact, in a steel piping system, the wavespeed will quite commonly exceed 4,000ft/sec. The following factors increase wavespeeds:
- Higher Young’s Modulus (lower pipe elasticity)
- Higher Bulk Modulus (lower fluid elasticity)
- Thicker pipe walls
- Stronger or additional pipe restraints
- Lower fluid density (at the same bulk modulus)
Another contributor to wavespeed is gas or air dispersed within the fluid. While typically unfavorable, air entrainment can greatly reduce the wavespeed