Chemical Engineering May 2020 - 48

Condensate
collection bottles
Steam to FCC unit
FIGURE 9. A turbine installation with properly drained and separated steam supply promotes
reliable operation
from the top of the distribution line,
then a combination drainage and
separation design may resemble the
example shown in Figure 9. Note that
after the initial separation, the trip and
throttle valve is drained, as well as the
turbine casing(s), and outlet riser.
Sufficient collecting legs and their
steam traps do not always exist in
the distribution line supplying steam
to the turbine. This can create major
issues in operating units, such as a
fluid catalytic cracker (FCC). An oversized
collecting leg (condensate collection
bottle or CCB) and steam trap
installation may provide some slug
mitigation, which should be followed
by a high-velocity mechanical separator
to remove entrained moisture
(Figure 10) [9].
Seal steam Per API 682 Plan 62.
Seal steam maintains pressure on the
Isolation
Strainer
Drainage and
dirt pocket
Quench inlet
Check
valve
Seal
Isolation
Strainer
Discharge
TLV FS
model trap
48
Quench outlet
(open to atmoshpere)
Isolation
Pump
FIGURE 10. Poor quality steam to an FCC unit may require
extraordinary measures
pump seal while providing cooling to
it, but wet steam can cause premature
seal failure. So, moisture should
not be allowed to enter and should
be removed beforehand. API 682
Plan 62 is intended to prevent water
in the seal, but it is commonly misunderstood,
with incorrect installation
as the result. An important consideration
is to make sure that the seal
steam supply is trapped prior to the
pressure regulating valve and that the
trap does not discharge condensate
into the seal itself (Figure 12). Normally,
seal steam lines have small diameter,
but if wet steam is being supplied
to multiple pumps in a compact
area - the installation of a separator
before branching off to the individual
pumps can disentrain condensate
and optimize steam quality.
Flares. Flares can experience a
Pressure
regulator
FIGURE 11. Trap condensate
from seal steam before pressure
relief valve (PRV)
myriad of issues, some of which are
shown in Figure 12. The cause of
each issue can usually be identified
as either water slugs or entrained
moisture in the steam supply. Again,
the mitigation efforts are the same:
drain condensate from the distribution
line to the flare control valve
station (and after), and separate wetness
out of the steam supply prior to
the control valve station (and steam
meter) (Figure 13).
The same basic recommendations
can be made for all steam controlvalve
stations because steam traveling
through the valve trim can reach
incredibly high velocity. It is important
to remove condensate and separate
wetness out before the valve. Otherwise,
moisture can erode the valve
and vaporize during the pressure drop
through the valve - causing cavitation,
premature wear, and loss of control.
Ejectors and vacuum systems. The
situation regarding steam quality is
similar for ejectors used in vapor removal
and column vacuum systems.
High-velocity steam travels through
the ejector at speeds of 3,000-4,000
ft/s [11], and entrained wetness creates
severe erosion of the nozzle, diffuser
and outlet elbow. " If moisture is
present in the steam, a separator and
steam trap should be used to improve
steam quality to better than 99.5% "
[12, 13]. Additionally, if an ejector nozzle
or diffuser throat is enlarged by just
7%, the vacuum can be diminished,
erratic, or broken, and the discharge
temperature lowered - requiring reCHEMICAL
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