POWER January 2021 - 31

OPERATIONS & MAINTENANCE
■ Saturated steam temperature at condenser
pressure.
■ ∆T between seal water exchanger inlet
temperature and saturated steam
temperature. (This is a critical performance
indicator. If the seal water temperature
exceeds the saturated steam
temperature, the vacuum pump will
begin cavitating, and its capacity will
be reduced.)
4. This image shows an example of significant
fouling on the seal water side of a seal
water heat exchanger. Courtesy: Kurt Bayburt
should be in order.
By comparison, seal water-side fouling
will be isolated to the vacuum pump/heat
exchanger in question. To the untrained
eye, however, the situation may appear
as if there is a fouling/scaling issue in the
condenser related to improper chemistry
control, as the condenser performance
indications will show increased backpressure
and TTD (terminal temperature
difference), as well as a reduced
condenser cleanliness factor (the comparison
of the actual heat transfer to the
design heat transfer of the condenser). If
the troubleshooting efforts go down this
path, a significant amount of time and
cleaning effort will be consumed with no
net gain in efficiency.
Key Performance Indicators
To track efficiency losses over time from
the vacuum pump/heat exchanger, a performance
monitoring program should be
implemented. In such a program, key
readings should be taken and recorded
at least seasonally. Parameters to monitor
include:
■ Seal water inlet and outlet temperatures
from the heat exchanger.
■ Cooling water inlet and outlet temperatures.
■
Vacuum pump air flowrate.
■ Condenser absolute pressure.
■ Steam turbine load.
Using this data, the following indicators
are calculated:
■ Seal water differential temperature
(∆T).
■ Cooling water ∆T.
■ Approach temperature, that is, the
difference between seal water outlet
temperature and cooling water inlet
temperature.
January 2021 | POWER
■ Normalized vacuum pump air flowrate
(flowrate/turbine MW). A common
industry standard is to control air inleakage
to no more than 1 cubic foot
per minute (cfm)/100 MW.
Additionally, the vacuum pump will
have specifications that aid in determining
the root area of performance loss
(although they aren't always listed on
documentation, and may require distemperature
and the cooling water
outlet from the condenser. This effect
is a result of the air being trapped in
the condenser and causing an insulating
effect around the condenser tubes,
essentially preventing them from condensing
steam and passing the latent
heat to the cooling water.
EPRI studies suggest that for every
1.0 in-Hg increase in condenser pressure,
the unit experiences a resultant
2.5% efficiency loss. The fuel costs can
increase very quickly from this seemingly
innocuous issue. Even a simple mechanical
cleaning and/or high-pressure water
wash of the seal water heat exchanger
can yield significant cost savings.
In another testing scenario, the seal
water ∆T was found to be above design.
This is an unwanted situation, as
this is a sign that the seal water recirStudies
suggest that for every 1.0
in-Hg increase in condenser pressure,
the unit experiences a resultant 2.5%
efficiency loss.
cussion with the OEM). A few of these
to gather are design heat exchanger
approach, seal water ∆T, seal water
flowrate, and maximum rated airflow.
It is also important to know the design
seal water temperature for the vacuum
pump inlet, as the maximum rated airflow
is based off this temperature. If
the actual seal water temperature is
higher than design (which is a common
problem), the pump capacity will be reduced.
For example, a pump rated at
8 cfm at 68F seal water temperature,
may only be able to flow 6 cfm at 80F
seal water.
If during performance testing, the
heat exchanger approach temperature
is found to be considerably higher than
design, this is a tell-tale sign that the
exchanger is fouled and needs to be inspected.
If the seal water is unable to
be cooled sufficiently, it will likely be
higher than the saturated steam temperature
at the condenser pressure, and
the pump will begin cavitating and have
reduced capacity.
If air in-leakage into the condenser is
sufficient enough, the condenser pressure
will begin increasing and there
will be a noted increase in TTD, which
is the difference between the hotwell
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culation pump's flow has decreased
below design. Although the seal water
may have sufficient residence time in
the heat exchanger for cooling, there
may not be enough flow as it is sprayed
into the inlet chambers of the vacuum
pump to effectively form the seal ring.
These small pumps are notorious for
being neglected, and do require periodic
attention (rebuild or replace).
Should the pump require maintenance/
replacement, a rotameter should be
added in order to allow quick visual
checks on the pump's health.
Naturally, not all of these examples
pertain to each and every steam generating
unit. The overarching theme, however,
is that regardless of configuration,
there are many mechanical systems
that dovetail into steam cycle chemistry
control, and ultimately affect efficiency
and reliability. Understanding these relationships,
as well as corrective actions,
should be made a priority among engineering,
operations,
and maintenance
personnel. Ultimately, this will lead to
more effective troubleshooting and fewer
long-term cost implications. ■
-Kurt Bayburt (kbayburt@azgt.coop) is
the chemistry manager for Arizona
Generation & Transmission Cooperatives.
31
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POWER January 2021

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