VACUUM ON AC NONCONDENSABLE VENT FIG. 3
condenser AC IC
loop seal during a turnaround. Once
under way, initial turnaround inspections showed that the surface condenser’s tubes were quite clean. Flushing of
the loop seal, however, revealed a large
quantity of muds and solids at the
seal’s bottom. No other major works
were completed on the condenser as
part of the turnaround.
Upon restarting the unit following
the maintenance, the operator noted
a major improvement in the surface
condenser’s performance, lending credence to the theory that the loop seal
was responsible for the condenser’s
previous poor performance.
Before cleaning, the restricted loop
seal was causing water to back into the
IC. This high-water level, combined
with a vertical baffle design that forced
the second-stage ejector suction gases
through a tortuous path, resulted in a
very high pressure drop.
Following the turnaround flushing,
the surface condenser’s performance improved to its highest levels in more than 10 years. Now operating at nearer
to designed rates, the condenser enabled the attached turbine to improve its workload by more than 10%, resulting
in increased throughput for the associated process unit at
Although surface-condenser performance following the
loop-seal flushing exceeded original expectations, the con-
denser still did not perform precisely according to its design,
which could be explained by the following:
• Most of the gap between design and actual perfor-
mance can only be attributed to a problem within the sur-
face condenser. The exact nature of the problem cannot be
determined, as it’s a fixed-tube sheet exchanger. While this
makes it impossible to inspect properly, the result is the
same. Regardless of whether caused by leaking air baffles
or shell-side fouling (possibly due to silicates or salts), the
exchanger must be dismantled to be replaced or redesigned.
• The calculated cooling-water flow to the surface condenser is only 75-80% of design cooling-water flow to the
unit, indicating hydraulic issues with the plant’s cooling-water network. While seemingly a large disparity, it’s only
responsible for a small part of the performance gap.
• This surface condenser was designed for only about
two thirds of the steam flow currently passing through it.
Again, though seemingly a large disparity, it’s responsible for
only a small part of the performance gap. This flaw can only
be resolved by redesigning the exchanger.
showed a negligible improvement in performance, indicating the mechanical condition of the second-stage ejectors
was not the main cause of the poorly performing equipment.
Further field data collected ultimately confirmed the
presence of a slight vacuum on the AC’s noncondensable
vent, which should have been impossible since the AC was
designed to operate at slightly above atmospheric pressure
(Fig. 3). Subsequent field testing revealed the vacuum was
occurring because of a leak in the IC-AC division plate (Fig.
2). This division-plate leak allowed gases to flow from the
AC back into the IC, effectively recycling flow across the
second-stage ejectors. The recycled flow, in turn, severely
impacted the ejectors’ ability to maintain designed suction
pressure because of vapor loading on the ejectors at rates
well above design.
This division-plate leak was resolved online by venting
the second-stage ejectors to the atmosphere. Following this
modification to the IC-AC, the division-plate leak no longer
affected performance of the vacuum system or surface condenser, and second-stage ejector suction pressures returned
to designed operating values.
Despite large improvements in the surface condenser’s
performance, the condenser still was operating well below
design performance. The integrity of the IC drain-loop seal
came into question after additional field testing revealed the
presence of a high pressure drop between first-stage ejector
discharge and second-stage ejector suction.
To address the issue, the authors recommended flushing the