POWER September 2017 - 47

OPERATIONS & MAINTENANCE
2. Fort St. Vrain boiler feedwater pump. Each of the plant's three heat recovery
steam generators has two 10-stage, axially split boiler feedwater pumps. Courtesy: Leif Bothun/
Xcel Energy
The Importance of the
Coefficient of Thermal
some challenges. For example, the pumps in
the boiler feedwater system are often forced
to cycle multiple times per day. Because of
the rotor sag, metal-to-metal contact can occur
at each start or stop of the pump, creating
the risk of a pump seizure. Adding to the
challenge, the original equipment manufacturer
(OEM) informed plant personnel that
the pump was not originally designed for
cycling operation. Rather, it was designed
for steady-state operation-being online for
weeks or months at a time.
Therefore, during a recent rebuild of the
31A pump, plant engineering and management
requested that the repair shop evaluate
and provide upgrade options that would allow
the pump to better handle the constant
cycling and avoid galling. Furthermore, if the
pumps were going to be modified, the repair
3. A marriage by design. To account
for the difference in the coefficient of thermal
expansion between Vespel CR-6100 and
pump base metal components, the composite
material is press fit into metal holders.
Courtesy: Boulden Co.
shop was also requested to consider potential
performance upgrades.
One option was to continue using metal
parts and standard clearances, but increase
the difference in hardness between the rotating
and stationary parts to reduce the
risk of galling. Another option was to upgrade
the stationary wear components to a
composite material called DuPont Vespel
CR-6100 and continue running stainless
steel rotating parts. That option would
eliminate the metal-to-metal contact points
in the pump and effectively eliminate the
galling and seizure potential.
With the risk of seizure minimized, the
clearance between wear parts could also be
reduced, providing an additional benefit of
higher efficiency and lower operating costs.
Due to the frequent cycling of the boiler feed
pumps and the potential for performance
gains, the plant selected the composite material
for the stationary wear parts.
Accounting for Thermal Expansion
Differences
In order for the application to be successful,
important design details needed to be addressed.
The first issue was accounting for
the difference in thermal expansion between
the metal components and the composite material.
The coefficient of thermal expansion
of Vespel CR-6100 is approximately 60%
less than the coefficient of thermal expansion
of the base metal components in the pump
(see sidebar).
The way to design for the difference is to
install Vespel CR-6100 inserts with a press
fit into metal holders (Figure 3). The press fit
allows the composite insert to " follow " the
metal part as it thermally expands, and it also
September 2017 | POWER
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Expansion
A very low coefficient of thermal expansion
is required for a reliable
upgrade to composites in boiler feedwater
service because of the operating
temperature and potential contact between
rotating and stationary parts in
the pump. The low coefficient of thermal
expansion allows for dimensional
stability as the pump heats up to operating
temperature.
More importantly, when contact occurs
between rotating and stationary
parts, frictional heat is generated. A
typical plastic with a high coefficient
of thermal expansion will grow, reducing
the clearance between the parts,
increasing the contact, and generating
more heat in a rapid cycle toward
component failure. A truly engineered
composite with a very low coefficient
of thermal expansion can withstand this
frictionally generated heat, allowing the
pump to survive through transient conditions
without galling or seizure.
prevents rotation of the insert. No additional
anti-rotation pins or screws are needed.
The inserts follow the metal part because
the press fit creates a residual compression
stress in the insert. The inside diameter of the
insert will decrease at very close to a 1:1 ratio
with the press fit. For the Fort St. Vrain
pumps, the press fit on the case rings was
0.026 inch; therefore, the inside diameter of
the insert decreased by 0.026 inch during the
press fit.
As the pump is brought up to the 260F
operating temperature and the metal holder
thermally expands, the Vespel CR-6100 effectively
mirrors the thermal expansion
properties of the metal holder because the
compression stress created by the press fit is
reduced. The end result is that the composite
insert expands at the same rate as the metal
part and maintains the installed clearance at
normal operating temperatures.
Addressing Differential Pressure
Requirements
A second design requirement is for the composite
insert to be able to withstand the differential
pressure across the component. To
account for that necessity, the metal holders
were machined with a shoulder on the
47

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POWER September 2017

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