POWER September 2017 - 48

OPERATIONS & MAINTENANCE
4. Lean on me, when you're not
strong. A shoulder machined on the lowpressure
side (right side of this cross-sectional
view) of the metal holder supports
the composite insert (shown in blue on the
drawing), holding it in place against differential
pressure (∆P). Source: Boulden Co.
Table 1. Original design clearance versus upgraded design. By reducing
the clearance, significant efficiency gains were achieved (mm = millimeter). Source: Boulden
Co.
Component
Wear rings
Center bushing
Throttle bushing
Original Clearance
0.020 inch (0.50 mm)
0.011 inch (0.27 mm)
0.011 inch (0.27 mm)
Upgraded Clearance
0.012 inch (0.30 mm)
0.009 inch (0.22 mm)
0.009 inch (0.22 mm)
Reduction
40%
18%
18%
Table 2. Comparing performance. Fort St. Vrain rebuilt pump 41B to original equipment
manufacturer specifications and upgraded pump 31A with composite seals, which offered
tighter clearances. Testing at maximum flow confirmed that the upgraded pump was nearly
10% more efficient while providing the same flow and higher head. Source: Boulden Co.
Pump 41B
Parameter
Capacity
Head
Brake horsepower
5.
PERF-Seal design applied to
the center-stage bushing. In some
instances, high differential pressure can force
fluid into the press-fit interface between the
composite insert and the metal housing. The
PERF-Seal design uses a pattern of holes
drilled through the composite insert to eliminate
the problem. Courtesy: Boulden Co.
Efficiency
Units
gallons/minute
ft
HP
%
the composite insert (Figure 5).
The holes balance the pressure between
the outside diameter and inside diameter of
the insert. This essentially eliminates radial
deformation of the composite insert, allowing
the reliable use of composites at much
higher differential pressures.
The design has been validated using finite
element analysis and in subsequent field trials
up to 2,320 psi differential pressure. Further
laboratory testing has also shown the
design to reduce leakage across the seals by
an additional 25% compared to a plain seal
and to increase the rotor-dynamic damping
coefficient of the seals by a factor of three to
four times when compared to a plain seal at
typical running speeds.
How Reduced Clearances Affect
low-pressure side of the Vespel CR-6100 insert
to prevent the composite material from
being extruded by the differential pressure
(Figure 4).
In the case of the center and throttle bushings
in the Fort St. Vrain pump, the maximum
differential pressure across each component
is 1,000 psi. Experience with composite materials
over the years has revealed the possibility
that high-pressure fluid can penetrate
the press fit interface between the composite
and the metal housing. Even though this is a
rare occurrence, if it happens, it can create radial
deformation of the composite insert and
lead to premature failure.
To add a safety factor against this potential
failure mode, the principals from the material
supplier, Boulden Co., invented a patentpending
solution called the PERF-Seal,
which uses a pattern of holes drilled through
48
Pump Efficiency
The internal seals in the pump separate the
high-pressure from the low-pressure regions.
Due to the differential pressure, there is significant
leakage across these parts, which decreases
pump efficiency.
By upgrading to composite materials
and eliminating the metal-to-metal contact
inside the Fort St. Vrain pumps, it was possible
to reduce the clearance at all of the internal
seals-the wear rings, center bushing,
and throttle bushing. Original and upgraded
clearances are shown in Table 1.
As noted earlier, testing of the PERF-Seal
design has shown an additional 25% reduction
in leakage across the components compared
to plain, smoothbore components. The
combination of reduced clearance and the
PERF-Seal design in the upgraded pump led
to a significant efficiency gain.
www.powermag.com
(original design)
1,210
6,099
2,923
59.30
Pump 31A
(upgraded design)
1,210
6,290
2,608
68.50
Before the results of the upgrade were
clear, the site rebuilt another boiler feed
pump, opting to return it to OEM specifications.
Having two newly rebuilt pumps
offered a great opportunity to compare performance
between the original model and
the upgraded design. The results are shown
in Table 2.
Benefits Continue
Since the upgrade, plant operators have been
happy with the pump. There have been no issues
starting and stopping the pump repeatedly
when the plant is asked to cycle. After
two years of operation, there has been no
noted increase in power consumption or decrease
in performance of the upgraded pump.
Furthermore, Fort St. Vrain has experienced
at least one added benefit. Previously,
boiler feedwater pump motors would bump
into and sometimes surpass their full-load
rated amperage during hot summer days.
Occasionally, the motors would overheat
and trip. However, with the upgraded pump
drawing less current, it has added a significant
safety factor between full load and a
tripped pump when it is needed most.
The savings in house power shouldn't be
discounted either. For a 3 x 1 combined cycle
facility, if the operating pump in each of the
three units is upgraded, the savings in electric
house power alone is nearly 1 MW. For
a utility to add this power to the grid instead
of consuming it adds to plant profitability. ■
-Leif Bothun is senior operations
manager at Xcel Energy's Fort St. Vrain
Generation Station and Robert Aronen
is managing director with Boulden
International.
POWER | September 2017

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POWER September 2017

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