POWER January 2021 - 30

OPERATIONS & MAINTENANCE
2. A typical non-return valve is shown here
with seal water supplied to the shaft packing.
Courtesy: Kurt Bayburt
pipe connections. A discussion with the
pump original equipment manufacturer
(OEM) would be in order in this case.
For older conventional boiler units with
copper-alloy feedwater heaters, the lowpressure
heater drain pumps need to be
addressed in the same manner as condensate
pumps. Typically, this sub-circuit
has little, if any, online monitoring equipment.
While the vacuum on the suction
of these pumps is not as high as condensate
pumps, air in-leakage through
the mechanical seals can introduce DO
that is undetected by normal chemistry
monitoring equipment.
Typically, the pump discharges back
into the condensate circuit
after the
gland steam condenser or first feedwater
heater, which is downstream of
the condensate DO sample point. At
the deaerator, the remaining DO has
been mechanically removed. The only
chemistry-related feedback that would
indicate an abnormal condition would
be a reduction in hydrazine residual, if
hydrazine or carbohydrazide were the
reductants of choice, or a change in @T
ORP (at-temperature oxidation-reduction
potential), if this instrumentation is
utilized. A DO analyzer monitoring the
heater drain pump discharge would provide
valuable feedback as to the purity
of the sample, and is a highly recommended
addition to the array of sample
analysis equipment.
Non-Return Valve Shaft Packing and
Seal Water
Used in conventional boiler systems,
non-return valves (NRVs) installed in
feedwater heater extraction lines prevent
flashing of shell-side condensate
and subsequent return into the steam
turbine in the event of a turbine trip.
While high-pressure extractions are wellabove
atmospheric pressure throughout
the turbine load range, the low-pressure
extractions can swing into the sub-atmospheric
pressure range at lower loads.
30
This being the case, the NRV swing
arm shaft must be packed differently in
the location where the shaft protrudes
through the body. High-pressure NRVs
can simply have multiple rows of packing
rings in the stuffing box, which prevent
extraction steam from escaping. Lowpressure
NRVs, however, must have seal
water piped into the stuffing box (Figure
2), as well as a lantern ring installed with
the packing rings. This prevents air inleakage
into the extraction piping during
low-load conditions.
This seemingly small source of air ingress
can have a significant impact on
the DO of the heater drain pump discharge.
Although a majority of the air
will be removed through the feedwater
heater vent piping back to the main condenser
(provided the vent line is open),
there will be some fraction that will dissolve
in the condensate. There will also
be a cumulative effect, if there are multiple
feedwater heaters operating at subatmospheric
pressure.
The preceding two examples of uncontrolled
air in-leakage have long-term
operational effects related to corrosion
and metal transport. Operating outside
of the EPRI DO guidelines will likely
result in increased metal transport,
leading to equipment damage (copper
removal from feedwater heaters,
causing tube leaks) or premature boiler
cleanings (increased iron/copper deposition
on steam generator tubes, causing
reduced heat transfer).
Condenser Liquid Ring Vacuum Pump
Performance
The liquid ring vacuum pump is a relatively
simple piece of equipment, but
plays a major role in the efficiency of
the steam turbine. Typically, the skid
(Figure 3) consists of the vacuum pump
proper along with a heat exchanger,
which is used to cool the internal compressant
seal water (condensate or another
high-purity water source) using
an external open- or closed-loop cooling
system. The seal water gains heat
as it is pumped/sprayed into the inlet
of the vacuum pump and compressed,
creating the internal seal ring. Also, the
moisture-saturated air that is removed
from the condenser contains heat that
increases the water temperature.
In order for the vacuum pump to
function properly, the seal water temperature
must be below the saturation
temperature at the condenser operating
pressure. If it is not, the seal water will
flash inside the vacuum pump, causing
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loss of the liquid seal ring and ultimately
loss of vacuum. A tell-tale sign of this
occurrence is a " knocking " noise coming
from the vacuum pump.
To ensure the seal water remains
below the saturation temperature, two
main components must be present:
a cooling water source of sufficiently
low temperature and high-enough flowrate,
and a heat exchanger capable of
efficiently transferring heat from the
seal water to the cooling water. Aside
from an extremely abnormal condition,
the first component should have been
specified during the engineering, procurement,
and construction period, and
always available and in-service. The second
component, however, can change
over time.
If the cooling water source is from
an open recirculating system or oncethrough
system, the heat exchanger
can be susceptible to scale formation
or micro- and/or macro-biological fouling
(such as bacterial slime or mollusks).
These heat exchangers should be included
in the list of components to be
cleaned during scheduled outages.
Unfortunately, the cooling water side
of the heat exchanger is typically the
only side that sees regular attention.
Loss of exchanger efficiency can also
occur on the seal water side, showing
up as iron fouling as a result of internal
vacuum pump corrosion/degradation.
When this occurs (Figure 4), a mechanical
or chemical cleaning is in order.
It is important to differentiate the situations
related to performance losses
rooted in cooling water-side fouling versus
seal water-side fouling. Cooling water-side
issues indicate a larger scope
problem due to inadequate chemistry
control, whether it be improper/insufficient
scale inhibitors or a lacking biocontrol
program. Other heat exchangers
will likely be suffering the same issues,
including the main condenser. Consultation
with a water treatment vendor
3. A typical vacuum pump and seal water
heat exchanger skid is shown here. Courtesy:
Kurt Bayburt
POWER | January 2021
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POWER January 2021

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