che_february-2025 - 21

Cover Story
Gas-Flow Calculations
for Sonic Choking -
Revisited
Gas compressibility can lead to choked flow in piping systems. Presented here is an overview of
choked-flow geometries in pipes, and examples of how choked flow arises in different pipe layouts
T
he more things change, the more
they stay the same. At least that
is how the saying goes. What has
changed? The chemical process
industries (CPI), along with industry more
generally, continue to evolve and expand
the importance and breadth of fluid-system
applications. And what has stayed the
same? The phenomenon of sonic choking
in superheated gas flow remains a challenge
in process piping applications.
Sonic choking occurs when gases,
which are compressible, flow at velocities
that approach sonic velocity, the speed of
sound waves in a medium. A typical sonic
velocity for air is 1,000 ft/s (305 m/s).
When a flowing gas at some location in the
pipeline experiences a local velocity equal
to the sonic velocity of the gas at that temperature,
sonic choking occurs.
Twenty-five years ago, an article was
published in Chemical Engineering entitled
" Gas-flow Calculations: Don't Choke " [1]
(Figure 0). This article revisits the material
in the piece from 2000, providing clarifications,
updates, points of emphasis and additional
insights from the author's experiences
over the last quarter century.
Another impetus for this revisit article is
that the author recently wrote a technical
paper that details all the types of sonic
choking [2]. This included discussion of
multiple sonic choking points in series
and in parallel. In doing so, examples of
all these situations were created using
simplified assumptions, such as assuming
perfect gas and adiabatic flow in horizontal
pipes. This allowed the examples
to be solved using closed-form solutions
and applied in Microsoft Excel. The Excel
files are publicly available [3] for those interested
and examples are included here.
The goal of the present article is not to
just repeat discussions from Ref. 1. Rather,
the purpose is to offer some clarifications
and reinforcements to the previous article.
And the Excel-based examples from Ref. 2
will be used to provide a hands-on, accessible
learning tool for those interested in a
deeper dive.
Even though a quarter century has
passed, the author continues to see engineers
trying to use incompressible-flow
methods on compressible-gas systems
and using or promoting the debunked
" 40% pressure drop rule. " Please consult
Ref. 1 for a thorough explanation of why
" rules of thumb " like this and other similar
ones can be highly misleading. Finally,
guidance is provided for what to look for
when evaluating software for modeling
pipe systems where sonic choking might
occur.
Trey Walters
Applied Flow
Technology
IN BRIEF
QUICK REVIEW OF SONIC
CHOKING
GEOMETRY OF SONIC
CHOKING
SOLVABLE BOUNDARY
CONDITIONS
IMPORTANT EQUATIONS
PREFACE TO EXAMPLES
SINGLE CHOKE POINT
EXAMPLES
MULTIPLE CHOKE
POINTS IN SERIES
MULTIPLE CHOKE
POINTS IN A SEQUENCE
OF PIPES
DOWNSTREAM OF
CHOKE POINTS
APPLICATION OF
EQUATION (2)
SIMPLIFIED TO
COMPLICATED
DO YOU KNOW WHAT
k IS?
SOFTWARE FOR SONIC
CHOKING
CORRECTIONS
FIGURE 0. The cover of the Chemical Engineering issue from
January 2000 is shown here, featuring the author's original
article, as cited in Ref. 1
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
FEBRUARY 2025
21

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