Chemical Engineering May 2020 - 46

tion, %
Do = Steam dryness before separation,
%
S = Moisture separation, %
Steam in
FIGURE 6. High-velocity cyclone separators can
remove 98% wetness from steam provided that the
flow velocity and wetness percentage are within
the separator's required operating specifications
percentage of wetness - up to a
15% wet steam mixture. Some applications
may have a wet steam
slurry of 30% condensate, so 15%
wet steam is a real possibility to be
considered, particularly at the " wet
end " of a plant.
The brown line represents the
total heat of steam, and the two
lighter lines directly beneath it
show how the total heat changes
when steam is wet. Heat added
above the brown line shows a superheat
region [6]. While total heat
is required to produce steam, only
latent heat is used in most steamequipment
applications.
Is letdown steam saturated?
A common belief is that letting
down steam from a higher pressure
to a lower pressure through
valves can create saturated or even
superheated steam. While relatively
easy to demonstrate from a theoretical
standpoint, such calculations
may be unrealistic. Consider
the two tables shown as Figure 5.
The table on the left lists the total
heat of steam at 200 psig for 5, 10
and 12% wetness (95, 90 and 88%
dryness), as well as the required
total heat for saturated steam at 15
psig. Even the 200 psig steam with
only 5% wetness (1,158 Btu) is not
sufficient to create saturated steam
at 15 psig (1,164 Btu). In real applications,
steam can have much
more wetness due to poor trapping
that does not remove sufficient
disentrained water, insulation
inefficiency and other heat-robbing
aspects that naturally occur in pipe
systems, such as flanges acting as
a heat sink or normal convection.
This is why plant steam - if not superheated
- is wet.
46
FIGURE 7. Steam turbines are crucial to plant
operations
How to improve steam quality
The right-hand chart of Figure 5
shows the effects of 98% moisture
separation from a steam flow with
entrained moisture. All of the steam
quality values exiting the separation
stage approach dry steam territory,
with estimated values of 99.7 to
99.9% dryness.
There are several key requirements
to achieve high dryness, such
as proper trapping, high percentage
of " good " condition steam traps,
and operation within a separator's
specified required range for flow,
velocity and pressure. However,
when properly designed and installed,
separation can significantly
enhance equipment operational reliability.
It is important to understand
that moisture removal does not increase
energy efficiency, but rather
improves heat transfer and equipment
reliability.
The final dryness of steam, after
moisture removal, can be calculated
using Equation (2):
De = Do/[1 - S(1 - Do)]
(2)
Where:
De = Steam dryness after separaMechanical
separation
Collecting legs and their steam traps
remove condensate that has already
been disentrained from flowing
steam. A useful way to remove
condensate that is still entrained in
steam flow is to install mechanical
separation. There are four key characteristics
of separator design: physical
impediments, impingement, flow
velocity and directional changes.
Physical impediments can consist
of ridges, walls or baffles. Impingement
may be aided by including
rough surfaces, flow velocity can be
increased by pushing steam through
small openings and incorporating
a cyclonic design, and directional
changes should twist and ideally reverse
the flow path [7].
An example of a high-velocity
steam separator incorporating these
four characteristics and their interactions
to accomplish efficient separation
is shown in Figure 6.
Some important considerations
when selecting separators and
achieving the separation efficiency
include the steam flowrate, its velocity
- to make sure the flow stays
within the required range for the
separator to perform to specifications,
the expected condensate load
to be discharged (for the purpose
of selecting the steam trap to drain
the condensate), and the maximum
pressure drop allowed. Selecting
a separator with minimal pressure
drop is especially important to maintain
steam pressure and its corresponding
temperature. Lowering
temperature unnecessarily can deFIGURE
8. Steam
blow in turbine areas
can be avoided when
using improved
steam quality
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 2020
http://WWW.CHEMENGONLINE.COM
Chemical Engineering May 2020

Table of Contents for the Digital Edition of Chemical Engineering May 2020
