STEAM TURBINE-DRIVEN COMPRESSOR FIG. 1
Shell Oil Products US
Norman P. Lieberman
Process Improvement Engineering
Declining performance of a large compressor-driven steam
turbine’s surface condenser at Shell Chemical LP’s petrochemical complex in Norco, La., was constraining an associated process unit to a maximum 80% of its design capacity,
significantly affecting the unit’s operating margin and energy efficiency.
Working with the surface condenser’s original manufac-
turer to troubleshoot the issue, the authors executed several
online field tests that revealed a critical component of the
steam-jet ejector system—the combined intercondenser-
aftercondenser (IC-AC)—had developed an internal divi-
sion-plate leak. To remedy the malfunction, the permanent
steam-jet ejector system was modified so that the second-
stage ejectors were routed directly to the atmosphere, elimi-
nating performance losses associated with the leak.
With the unit still not performing to the operator’s de-
sired level, however, the authors conducted further field
tests and discovered restriction of the IC loop seal. Based
on the authors’ recommendation, the loop seal was cleaned
during a planned unit turnaround. The cleaning resulted in
improved condenser performance, increasing both operat-
ing margin and efficiency of the associated process unit.
This article discusses the unit system and equipment, system deficiencies discovered during the field investigation,
and recommendations undertaken to address those deficiencies.
The surface condenser at Norco is part of a compressor-driven steam turbine critical to an associated process unit at the
plant. Equipped with a two-stage steam ejector, the condenser consists of a fixed-tube heat exchanger that uses cooling
water to condense turbine-exhaust steam which flows back
into the condenser. Because it’s desirable to condense steam
at the lowest-achievable pressure to maximize work developed by or energy extracted from each pound, surface condensers are designed to operate at the
highest quality vacuum possible. The
effects of improved surface-condenser vacuum can be approximated as
an isentropic expansion on a Mollier
diagram, or enthalpy–entropy chart,
which plots the functional relationship
between enthalpy, entropy, temperature, pressure, and steam quality.
Fig. 1 shows a diagram of the compressor-driven steam turbine. Fig. 2
shows a detailed diagram of the surface condenser.
The vacuum generated in a surface condenser by the condensation of
steam cannot be maintained without
a continuous, noncondensable gas-re-moval device to remove air and other
noncondensable gas from turbine-seal leaks. It’s also possible for minor
noncondensable contaminants in the
steam, like carbon dioxide, to enter the
This surface condenser uses steam
Field troubleshooting improves
steam surface-condenser performance