Chemical Engineering January 2022 - 43

quantity of heat in the steam will
be exactly the same after passing
the orifice as before passing or the
valve-inlet heat quantity.
Consider the following parameters
for a sample steam quality
determination for the system shown
in Figure 10:
* Inlet to pressure reducing valve:
150 psig
* Total energy at 150 psig: 1,196
Btu/lb
* Outlet of pressure-reducing
valve: 15 psig
* Outlet temperature (saturated
conditions) 15 psig: 249.7ºF
* Total energy at 15 psig: 1,164
Btu/lb
* Difference in Btu/lb from high to
low pressure: 32 Btu/lb
There is a difference of 32 Btu/
after
lb
existing
the
pressurereducing
station at the lower
steam pressure (15 psig) due to
the fact that no external work
was accomplished.
The 32 Btu/lb specific energy will
create the effect of superheat. Assuming
the specific heat of superheated
steam to be 0.52, each pound passing
through the station will be superheated
to 61.5°F (32/0.52). Therefore,
the downstream temperature,
if 100% steam quality exists, would
be 311.2°F.
For the example, if the steam had
contained 1% moisture, it would
have contained less heat units per
pound than if it were dry steam.
Since the latent heat of steam at
150 psig is 857.4 Btu/lb, it follows
that the 1% of moisture would have
required 8.5 Btu/lb to evaporate it,
leaving only 23.5 Btu/lb (32 - 8.5)
available for superheating. Hence,
the superheat would be 45.1°F
(23.5/0.52), as against 61.5°F for
dry steam.
The degree of superheat for
other percentages of moisture
may be determined. The action
of
the throttling
based upon several parameters,
as follows:
* H = total heat of one lb of steam
(inlet steam pressure) to the valve
station
* L = latent heat of steam at the
inlet to the valve station
* h = total heat of steam at the reduced
steam pressure or outlet of
the valve station
* t1 = temperature of saturated
steam at the outlet of the pressure
reducing station or the reduced
steam pressure
* t2 = temperature of saturated
steam pressure at the inlet to the
pressure reducing valve station
* 0.52 = specific heat of saturated
steam at the outlet steam
pressure at the outlet of the
valve station
* x = proportion by weight of moisture
in steam
The difference in Btu/lb of steam
at the inlet valve station pressure
and after passing the orifice (valve
internals) is the heat available for
evaporating the moisture content
and superheating the steam. Therefore,
the following expressions
are given:
H - h = x L + 0.52 (t2 -t1)
or
x = [H - h - 0.52 (t2 -t1)]/L
Almost invariably, the lower prescalorimeter
is
sure is taken as that of the atmosphere.
Under such conditions, h
= 1,163.9 and t1 = 249.5°F.
A slight error may arise from the
value used as the specific heat of
superheated steam at the example
lower steam pressure of 15 psig:
0.52. However, any error resulting
from its use will be negligible. ■
Edited by Mary Page Bailey
Reference
1. Ganapathy, V., " Steam Plant Calculations Manual, " 2nd
Edition, CRC Press, 1993.
Author
Kelly Paffel is the technical manager
at steam-engineering firm
Inveno Engineering, LLC (Unit
320, 16215 Marsilea Pl., Naples,
FL, 34110; Phone: 239-2893667;
Website: www.invenoeng.
com; Email: kelly.paffel@invenoeng.com).
Paffel has 42 years of
experience in steam and power
operations, and is an experienced
lecturer who has published many technical papers
on the topics of steam system design and operation.
He is known for writing " Steam System Best
Practices, " which are used by plants and engineers
globally to ensure proper operation of steam and
condensate systems.
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