Chemical Engineering June 2020 - 31

effects of back-pressure on valve operation.
this type of valve can be very useful when a
process requires precise pressure relief upstream
of the valve.
a loud whistling sound is almost always
experienced with a relief valve cracking. of
course, it depends on what pressure magnitudes
are being relieved. with non-hazardous
process gases, relief valves often vent to
atmosphere. that can result in quite a large
drop in pressure from the process line. this
extremely low pressure produces a large
driving force for flow. and that flow moves
out of the valve with a very high velocity.
when venting to such an open space, there
is a major drop in the potential and kinetic
energy of the fluid. as a result, the gas cools,
slows down and drops pressure.
Since energy cannot just disappear, the
question becomes " where does it go? "
Some of the energy goes into the volume
expansion, and some goes into producing
sound. a common safety concern with
valves is sound levels. in fact, this is not
just the case with relief valves, but with any
type of valve. by dropping the pressure and
increasing the velocity, you will hear an accompanying
sound. Some of the pressure
loss gets converted into sound, which is just
the propagation of a pressure wave. the
level of that sound depends on the velocity,
flow area, pressure differential, and the
mitigation efforts in place. this aerodynamic
noise has been the cause for oSha safety
guidelines (Section iii: Chapter 5) to protect
the hearing of those in the field.
Controlling the driving force
there are many factors to keep in mind when
designing for flow or pressure control in gas
handling processes. pressure is the driving
force of the system, even when you are designing
for flow control. by changing the frictional
losses in the system, you change the
operating mass flowrate. mass conservation
is a fundamental law, but volume conservation
is not. volumetric flow, velocity, density,
and temperature will all change according
to the fluid's equation of state even though
mass flow may be constant. down the length
of an adiabatic and constant cross-sectional
area pipe, you will see pressure and density
decrease, volumetric flowrate and velocity
increase, and most often will see temperature
drop.
when pressure is your controlled variable,
you affect both the upstream and
downstream pressure (and flow) with the
same valve operation. Closing a valve will
create larger pressure losses, which implies
a greater upstream pressure, a lower
downstream pressure, and a lower mass
flowrate. a pressure-reducing valve controls
the downstream pressure, while a pressuresustaining
valve controls upstream.
depending on the process application in
question, engineers design a wide range of
flow and pressure control methods, but it
all boils down to changing the pressure to
change the momentum transfer. that is the
beautiful consistency of chemical engineering:
there is always a driving force behind
transport phenomena. Controlling that driving
force is how you ultimately control your
process variables.
n
edited by Scott Jenkins
Author
Walt Prentice is a business applications engineer
at Applied Flow Technology (AFT; Address:
2955 Professional Place, Suite 301,
Colorado Springs, CO 80904; Phone: 719686-1000;
Email:
Prentice holds a B.S.Ch.E. with a minor in
economics
from
the
School
of
Mines. At AFT, he supports engineers around
the world in troubleshooting their piping and
ducting models.
waltprentice@aft.com).
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