2000 Hook-up Book - Spirax Sarco

Steam Quality
vs. Steam Purity
It is important to define the difference
between steam quality and
purity.
Steam Quality— “The ratio of the
weight of dry steam to the weight
of dry saturated steam and
entrained water. For example, if
the quality of the steam has been
determined to be 95%, the wetsteam
mixture delivered from the
boiler is composed of 5 parts by
weight of water, usually in the
form of a fine mist, and 95 parts
by weight of dry saturated steam.
Likewise, if the quality of the
steam has been determined to be
100%, there is no wet steam
delivered from the boiler, 100% of
the steam delivered from the boiler
is dry saturated steam.”
Steam Purity— “A quantitative
measure of contamination of
steam caused by dissolved solids,
volatiles, or particles in vapour or
by tiny droplets that may remain in
the steam following primary separation
in the boiler”.
Plant
Steam
Condensate
Pure/
Clean
Steam
Generator
Main drip
traps for
distribution
system
condensate
removal
Condensate
Figure 56
Clean/Pure Steam Generator and Distribution System
Thus, the three different
types of “clean” steam (filtered,
clean and pure) can, and will,
have different characteristics,
summarized in Table 14.
Note in particular that:
1. The quality of filtered steam
will normally be high because
water droplets larger than the
filter element rating will be
removed. Clean and pure
steam systems will have a
quality related to the design
and operating characteristics
of the generator, length and
installation details of distribution
system, insulation of
system, number and effectiveness
of mains, drainage
points, etc.
Clean Steam
Table 14: Differences in Steam Characteristics
Quality Purity
Particles Boiler Additives
Filtered High Typically 2.8 microns Normally present
Clean Varies on System Design Varies Limited to process
Pure Varies on System Design Varies None
Clean Steam
Distribution Main
Separators and
filters for efficient
conditioning of
steam.
Condensate
Pure Water
Hygienic ball valves
for isolation on
distribution systems.
Regulators
for accurate
pressure control
2. Boiler additives may well be
present in filtered steam and
also possibly in clean steam,
but often this will be limited by
process requirements. For
example, the FDA restricts
the use of certain additives,
including amines, in any
steam which comes into
direct contact with foods or
dairy products.
3. Assuming the generating and
distribution system have
been designed and installed
correctly, the particles present
in a pure steam system
will be water only. Dependent
on feed water type, the same
may also apply to clean
steam systems.
Process steam traps for
effective contaminated
condensate drainage.
Process
Vessel
Condensate
Diaphragm
Valves
51
SYSTEM DESIGN