Chemical Engineering January 2022 - 42

PRV (TWO STAGE VALVE ARRANGEMENT) SYSTEM
PRESSURE
TRANSMITTER
TEMPERATURE
TRANSMITTER
PRESSURE
TRANSMITTER
PRESSURE
TRANSMITTER
PRV (TWO STAGE VALVE ARRANGEMENT) SYSTEM
Thermometer
Thermometer
Disk
TEMPERATURE
TRANSMITTER
To atmosphere
FIGURE 6. A throttling calorimeter's discharge
piping should be short to minimize backpressure
C
B
A
E
F
Section E-F
PRESSURE
TRANSMITTER
FIGURE 9. In a steam pressure-reducing valve (PRV) station, temperature and pressure readings can be
either from indicators or transmitters
FIGURE 7. A compact throttling calorimeter employs
two concentric cylinders
Steam ➔
FIGURE 10. Reducing pressure across the station results in a reduction in specific energy, as well as a
superheating effect
FIGURE 8. A separating calorimeter mechanically
separates entrained water from steam for measurement
purposes
There are many forms of throttling
calorimeters, each of which work
upon the same principle. A compact
throttling calorimeter (Figure
7) consists of two concentric metal
cylinders connected to a cap containing
a thermometer well. The
steam pressure is measured by a
gage placed in the steam supply
pipe or other convenient location.
Steam passes through the orifice
(point A in Figure 7) and expands to
atmospheric pressure. The steam
temperature at this pressure is
42
being measured by a thermometer
placed in the cup (point C). To prevent
radiation losses, the annular
space between the two cylinders is
used as an insulating jacket, since
steam is being supplied to this
space through the hole (point B).
Separating calorimeter. A separating
calorimeter (Figure 8) mechanically
separates the entrained
water from the steam and collects
it in a reservoir, where its amount is
either indicated by a gage glass or
where it is drained off and weighed.
The steam passes out of the calorimeter
through an orifice of known
size so that its total amount can be
calculated, or it can be weighed. A
gage is ordinarily provided with this
type of calorimeter, which shows
the pressure in its inner chamber
and the flow of steam for a given
period, this latter scale being graduated
by trial. The instrument, like
a throttling calorimeter, should be
well insulated to prevent losses
from radiation.
Pressure-reducing stations
A steam-pressure-reducing valve
(PRV) station (Figure 9) will work
the same way as a throttling calorimeter.
In a typical installation of a
steam-pressure-reducing station,
upstream and downstream pressure
measurements, with the addition
of a temperature measurement
downstream, will provide the continuous
online " steam calorimeter "
functionality.
When steam passes through an
orifice (within the valve internals)
from a high steam pressure to a
lower steam pressure, as is the
case with the throttling calorimeter,
no external work has to be done in
overcoming a resistance. Hence, if
there is no loss from radiation, the
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