Chemical Engineering June 2020 - 26

Cover Story
gas handling:
process Control for
Compressible flow
Controlling flow in gas-handling applications can present challenges because of the fluids'
compressibility. This article presents an overview of how compessible gases behave in response
to interacting variables
Walt Prentice
applied flow
technology
In BrIef
CompreSSible flow
preSSure aS driving
forCe
flow meaSurement
and Control
preSSure Control
Controlling the
driving forCe
g
as-handling
operations are
common
almost
in
every
sector of the chemical
process industries (Cpi).
Some examples are obvious,
and others not as
much so. process plants
may have heating, ventilation
and air conditioning
(hvaC) systems for
recirculating air; auxiliary
steam systems for
heating unit operations;
or even compressed air
for pneumatic valve and
tool operation, among
others. while controlling
Part 1
Figure 1. Since gases are compressible, variable densities alter volumetric flowrate and
velocity. This compressible flow presents unique challenges to facilities in the chemical
process industries
parameters such as flowrate and pressure in
a gas-handling process may be an intuitive
goal, the methods of achieving it may not be.
there are certainly physiochemical reasons
to control flow and pressure, but there are
regulatory reasons as well. for example, controlling
pressure drop to keep the sound levels
from getting too high in a process plant is
a major topic in the occupational Safety and
health administration's (oSha; washington,
d.C.; www.osha.gov) technical manual.
Compressible flow
process control involving gases has many
different aspects, but this article presents a
high-level discussion of process control from
the fluid system perspective. Chemical engineers
need to understand how process
fluids behave in response to changing variables.
however, the details of process con26
trol
become especially cloudy when working
with process gases because of their compressibility
(figure 1).
Compressible flow presents unique challenges
and requires special considerations
because density is a function of temperature
and pressure. this relationship is represented
by equations of state, which are often not
considered with liquid systems. with gases,
this changing density changes volumetric
flowrate and velocity. in a theoretical adiabatic
(no heat transfer to or from surroundings) pipe
of constant cross-sectional area, the fluid
velocity increases along with dropping pressure
down the line - even though the mass
flowrate is steady. in compressor systems,
the heat of compression must be considered.
this changes the volumetric flowrate - although
the mass flow is constant.
because gas handling operations typically
ChemiCal engineering www.Chemengonline.Com June 2020

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Chemical Engineering June 2020

Table of Contents for the Digital Edition of Chemical Engineering June 2020

Contents
Chemical Engineering June 2020 - Cover1
Chemical Engineering June 2020 - Cover2
Chemical Engineering June 2020 - Contents
Chemical Engineering June 2020 - 2
Chemical Engineering June 2020 - 3
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Chemical Engineering June 2020 - Cover3
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