Chemical Engineering January 2022 - 40

Engineering Practice
The Importance of Steam Quality for
Steam-System Process Operation
Often, operational problems in steam systems are misattributed because the
system's steam quality is not closely monitored. A clear understanding of steam
quality can help to better address these issues
Kelly Paffel
Inveeno Engineering, LLC
S
team quality describes the
proportion of
saturated
steam (vapor) in a saturated
condensate (liquid)/steam
(vapor) mixture. A steam quality of 0
indicates 100% liquid (condensate),
while a steam quality of 100 indicates
100% steam. One pound of
steam with 95% steam and 5% liquid
entrainment has a steam quality
of 0.95, for example.
The parameters needed to obtain
a steam-quality measurement
are temperature, pressure and entrained
liquid content. A high percentage
(88% or more) of industrial
steam systems use saturated steam
for process applications. Saturated
steam (meaning steam that is saturated
with energy) is completely
gaseous and contains no liquid.
The boiler operation uses chemical
energy from a fuel source to
deliver energy to the boiler water.
Inside the boiler, liquid gains energy
from the combustion process and
changes state into saturated steam.
As illustrated in Figure 1, water enters
the boiler at point A, and the
water gains sensible energy (hf) up
to point B. The change of state is
referenced as point B in Figure 1. As
the saturated steam acquires more
energy from the boiler combustion
process, the steam achieves a
high quality (moving left to right), as
represented by points B to C. The
increase in energy gained by the
steam from points C to D goes toward
the superheat of the vapor.
A directly proportional relationship
exists between temperature
and pressure in saturated steam.
This means that as the temperature
increases, so does the pressure. Illustrated
by the lines of constant
pressure in Figure 1, more sensible
energy (hf) is needed for water to
transition from point A to point B
and become a vapor. When steam
enters the process, the energy
level goes from right to left as the
process absorbs the energy from
the steam.
Steam is a vital and critical part in
producing final products in chemical
processing facilities. Therefore,
steam quality should be one of the
main measurable points in creating
a product in today's manufacturing
facilities. Manufacturers of heat
transfer components, such as heat
exchangers and tracing elements,
base their performance calculations
on 100% steam quality, unless the
manufacturer is informed by the end
user that the steam quality is lower
than 100%.
Unfortunately, steam quality is
typically not monitored closely and
is often assumed to be 100% when
that may not be reality. Therefore,
issues that arise from poor steam
quality are frequently blamed on
some other item in the system.
Based on field documentation, a
high percentage of steam systems
are operating below acceptable
steam quality levels.
The impact of steam quality
Low steam quality affects
steam system operations in
many ways. Four examples are
described below.
h
h
FIGURE 1. The relationship between enthalpy and temperature in a boiler is crucial for understanding
steam quality
40
Reduced heat transfer efficiency.
The major problem with low steam
quality is the effect on heat transfer
equipment and the process. In some
cases, low steam quality can reduce
heat transfer efficiency by more than
65%. The liquid entrained in the
steam has sensible energy (16%
estimated; this varies with pressure),
which has a significantly lower
amount of energy than the steam vapor's
latent energy (94%). Therefore,
less usable energy is being delivered
to the process steam equipment.
Also, the additional liquid (low
steam quality) collects on the wet
surface of the heat exchanger, causing
an additional buildup of liquid,
which reduces the ability of the
steam's latent energy to be transferred
to the product.
Premature valve failure. Liquid
passing through steam control
valves will erode the internals of the
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