Session: FSI-01-01 Thermal Hydraulic Phenomena with Vessels, Piping and Components

Paper Number: 108499

108499 - How Fast Is a Finite Gas Transient Wave and Why Does It Steepen? 

The topic of transient compressible flow has many important applications in industry. Among these are the prediction of:

o pipe forces in high pressure steam turbine piping in nuclear and fossil power stations

o the rate of pressure change in gas turbine supply conditions

o the disruption of flow conditions to air and gas compressors

o pipe forces during pressure relief events

o pipe flow during tank blowdown and charging events

The complications of accurately simulating such behavior have been noted by many authors over many decades. As a result, it is often the case that simplified calculation methods have been adopted to assist engineers in making practical design decisions. However, when evaluating finite magnitude and finite length waves, the authors note frequent misconceptions in published methodologies. Some of these misconceptions can result in significantly unconservative predictions used for design purposes.

The purpose of this paper is to untangle some of these misconceptions as they relate to wave speed in steam and gas piping. More specifically, it is typical in industrial systems that waves have a finite magnitude (they cannot be accurately treated with infinitesimal wave methodology) and, perhaps more importantly, they have finite length. In other words, they are not instantaneous. They have a starting time (e.g., when a valve begins to close) and an ending time (e.g., when the valve finally closes). As is commonly known, this finite process over time generates a family of waves. The length of this family of waves can change with time. Why? Because the wave speed at the front of the wave family is not the same as the wave speed at the back of the wave family.

Properly understanding why and how this length changes over time leads to a better understanding of why and how fast a family of waves can steepen (i.e., the back of the wave catching up with the front). This has immensely important applications (for example) in predicting forces on pipe sections bounded by direction changes (e.g., elbows). If a wave can steepen more quickly, it can exert a greater imbalanced force when it passes through a given pipe section.

It is often assumed that the effects summarized above are of lesser concern in low Mach number conditions. This assumption is misleading and potentially dangerous. A practical example will be given for pipes where all Mach numbers in the piping (steady and transient) never exceed 0.1. It will be shown how the wave steepening effect is significant and not accounted for in commonly used design methods.

Presenting Author: Trey Walters Applied Flow Technology

Presenting Author Biography: Trey Walters, P.E., is the Founder and President of Applied Flow Technology in Colorado Springs, Colorado. AFT develops simulation software for fluid transfer systems. At AFT Mr. Walters has developed software in the areas of incompressible and compressible pipe flow, waterhammer, slurry systems, and pump system optimization. He has performed and managed thermal/fluid system consulting projects for numerous industrial applications including power, municipal water, oil and gas, chemicals, mining and HVAC . He actively teaches training seminars around the world. Mr. Walters founded AFT in 1993. He has 36 years of experience in thermal/fluid system engineering. He has published 30 papers and articles.

Mr. Walters’ previous experience was with General Dynamics in cryogenic rocket design and Babcock & Wilcox in steam/water equipment design. Mr. Walters holds a BSME (1985) and MSME (1986), both from the University of California, Santa Barbara. He is an ASME Fellow.

Authors:

Trey Walters Applied Flow Technology
Scott Lang Applied Flow Technology