Selected Case Histories
Of Venting System Problems
The following case histories are drawn from among the field
problems which have been experienced by the operators of condenser vacuum systems. The approach previously described was
utilized to identify and correct the problem.
CASE HISTORY I:
IMPROPER PIPING LAYOUT
BETWEEN STEAM SURFACE CONDENSER AND
VENTING SYSTEM
Problem:
A combined cycle cogeneration facility could not maintain design
vacuum at the turbine discharge. The turbine was supported by a
steam surface condenser with twin 100% liquid ring vacuum
pump condenser exhausters. Even while operating under part
load conditions, the vacuum level could not be reduced below 3.1
inches HgA. This was the case even when the venting system was
functioning with both 100% vacuum trains in operation. When
one of the pumps was shut down, an increase in operating pressure of only 0.12 inch Hg was measured.
Solution:
A 1.45 inch Hg pressure drop (3.1 inch HgA - 1.65 inch HgA)
between the steam surface condenser and the venting system
seemed excessively high and quite uncustomary. Shutting down
the second liquid ring vacuum pump and achieving only a slight
increase in operating pressure suggested that air inleakage was not
the problem.
for several feet and then went vertically downward. After a drop of
approximately 6 ft., the piping traveled horizontally for another 6
ft. and then went vertically upward to the venting system.
The loop in the noncondensible gas extraction piping was immediately suspected. When systems are shut down, vapor within the
piping condenses and liquid resides in low horizontal sections of
the piping loop. Upon startup, a liquid slug restricts extraction
flow to the venting system. If liquid entirely fills the horizontal
run, a pressure differential must be established to overcome the
difference between the operating pressure of the condenser and
that of the venting system in order to force the liquid slug to pass
through to the venting system. When the horizontal portion of
the loop is partially filled, a significant pressure within the condenser increases to create a driving force of sufficient magnitude
to overcome any pressure resistance associated with the restriction.
To remedy the problem, it was determined that it was too costly
to modify the piping. The installation of a drain connection in
the horizontal section of the piping was all that was necessary.
This drain connection permitted removal of residual water from
the loop prior to startup. Once the water was removed, the system was restarted and the condenser pressure was brought to
design levels with only one liquid ring vacuum pump on operation. Plant operators were given instructions to open the drain
connection prior to each startup.
As with any troubleshooting exercise, it is always appropriate to
examine the equipment layout. This should include the motive
steam piping to the
ejector system, condensate drain to the
hotwell, all miscellaneous connections
entering the condenser,
cooling water piping,
and vent piping from
the steam surface condenser to the venting
equipment. While
conducting the layout
survey, it was noticed
that piping between
the steam surface condenser and venting
equipment was not
properly routed.
Between the condenser
and the venting system
the noncondensible gas
extraction piping exited the condenser shell,
traveled horizontally
The American Society of Mechanical Engineers
8