Chemical Engineering January 2022 - 41

FIGURE 5. Steam calorimeters are used to determine the moisture content of steam
FIGURE 2. Visual inspection can be used to show
that steam is of acceptable quality. The steam
should be nearly invisible
FIGURE 3. This highly visible steam is not of
acceptable quality
FIGURE 4. Visual inspection shows that there is
clearly liquid being discharged with the steam,
indicating poor quality
valves, causing premature failure.
Internal turbine component failures.
Liquid introduced with the
steam in a saturated turbine operation
will reduce the life expectancy of
the internal components.
Waterhammer. Steam systems are
usually not designed to accommodate
the additional liquid in steam.
Additional liquid creates the chance
for waterhammer to occur. Waterhammer
poses a safety risk and
may cause premature failure in the
steam system.
Visually measure steam quality
A true measurement of steam
quality can be obtained from the
use of a throttling calorimeter and
Ganapathy's steam plant calculations
[1]. Unfortunately, most
industrial plants do not have the
luxury or capability of doing such
rigorous steam quality testing.
Another way to measure steam
quality is by relying on the basics
of steam. Saturated steam
is a dry invisible gas and only becomes
visible with the presence
of entrained air or liquid. Therefore,
opening a steam valve and
allowing steam to be released
into the atmosphere provides a
visual manner of estimating the
steam quality in the system. To
learn more about steam valves,
read Proper Sizing and Installation
for Steam System Safety
Valves, Chem. Eng., July 2017,
pp. 48-51.
This is a steam quality test that
all plants can and should conduct
on a routine basis. Figure 2 indicates
an acceptable steam quality.
The discharge from the valve
through the tube is almost invisible.
Figure 3 shows the discharge
from the valve off the steam line
to be very visible, with liquid being
discharged with the steam vapor.
This steam quality is not acceptable
for the process. Figure 4
shows the discharge from the valve
off the steam line to be very visible,
with liquid being discharged
with the steam vapor. The steam
quality is also not acceptable for
the process.
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Steam calorimeters
The measuring device used to determine
the moisture content of
steam is called a steam calorimeter
(Figure 5). However, the device really
does not measure the heat in
the steam. The first known name
used was the " barrel calorimeter, "
but the liability of error was so great
that the device was totally abandoned.
Modern calorimeters are
generally either the throttling variety
or a separator measuring device.
All steam-quality measuring devices
use the same principle, which
is described in the pressure-reducing
section of this article.
Throttling calorimeter. Figure 6
shows the typical form of a throttling
steam calorimeter. Steam is
flowing from a vertical main steam
line through the sampling nipple.
The steam then flows around the
first measuring thermometer cup,
then passes through a 1/8-in. orifice
in a disk between two flanges,
around the second measuring thermometer
cup, and then is released
to the atmosphere. The instrument
and all pipes and fittings leading to
it should be thoroughly insulated
to diminish the energy loss that
can affect the measurement. The
small orifice can have issues with
corrosive material flowing through
the measuring device. Therefore,
proper steam filtration needs to be
part of the measuring system. The
discharge steam piping needs to
be short to prevent any backpressure
below the disk area and causing
an error in the measurement.
Compact throttling calorimeter.
41
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