Hydrocarbon Processing - November 2022 - 43

Maintenance and Reliability
ing turbine driving a gas compressor.
Those turbines mounted directly on
top of a condenser typically do not experience
issues with condensate removal
from the turbine casing. However, turbines
mounted on a pedestal that exhaust
steam upward to the condenser
can experience significant difficulties in
getting proper condensate drainage from
the turbine casing.
Consider the desired discharge of condensate
from a turbine casing through a
steam trap to atmosphere. Depending on
the elevation of the turbine over the trap
and because the turbine drain/trap inlet
is at vacuum, approximately 35 ft of head
is needed to discharge to atmosphere.
Typically, this is not available and, therefore,
the system cannot drain (FIG. 19).6
An alternative might be to discharge
the casing drain condensate to the hotwell,
but unless the condensate can
drain down by gravity, this system cannot
work. Condensate will try to flood
the turbine casing by manometer effect
up until the horizontal entry level into
the hotwell. However, it is likely that the
water is spun by the turbine blade itself
before this higher condensate level is
reached. This can cause damage, loss of
power, erosion and precipitate buildup
issues (FIG. 20).
Since the turbine casing is in vacuum,
a pumping method is needed where
condensate gravity drains down into the
pump's reservoir to be pumped into the
hotwell (FIG. 21).
Air
removal and steam ejectors.
Condenser systems condense steam, but
not non-condensable vapors such as air.
As a result, an air exhauster/removal system
is needed. FIG. 22 shows basic details
of such a system, incorporating first- and
second-stage ejectors as well as a hogger
jet system for large volume air removal
on startup.
Steam ejectors are critical to create
the necessary suction to pull air from
the condenser, but the resulting mixture
contains both some of the motive steam
and air in the entrained flow. The multiple
stages of the exhauster system are designed
to condense that motive steam as
it passes through, and eventually enable
the air to be vented to atmosphere at the
end of the second stage in the example
shown in FIG. 22.
A condenser system can be expected
FIG. 15. Once steam enters the turbine, condensate can pool from work and radiation, and must
be drained.
FIG. 16. If an exhaust side trap is not
properly installed or maintained, condensate
must pool to rise.
to have significant amounts of air present
on startup, so an additional " hogger " jet
is often used initially to pull substantial
amounts of air out of the condenser and
exhaust it. Hogger systems do not have
additional condensers, so a lot of steam
can also be exhausted. For this reason,
hogger systems should only be used on
startup, but many are commonly used
during normal operation to supplement
the rest of a deficient suction system.
A hogger used in normal operation
typically indicates that the main air exhauster
system is not pulling enough air
out of the condenser and the hogger is
used to compensate. In many cases of
proper ejector sizing, it indicates that the
ejector nozzles and/or throats may have
become enlarged due to erosion, and this
FIG. 17. A correctly selected float trap
can discharge condensate immediately,
while maintaining a water seal.
FIG. 18. The low temperature indicates a
trap or other blockage failure, reducing
separator effectiveness.
can be an indication of poor steam supply
to the ejectors. There may also be a
precipitate buildup on the diffuser that
further weakens the suction produced.
Hydrocarbon Processing | NOVEMBER 2022 43

Hydrocarbon Processing - November 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - November 2022