The Pump that Couldn't
A large process facility in Central Alberta was having problems with their boiler feed pumps. The original 12 chrome metal wear components caused several pump seizures. Even though the metal components were designed with differential hardness, the original design clearance of 0.016-in (0.406-mm) between rotating and stationary wear components was increased to prevent seizure. The increased clearance led to production problems. When running at full rates, the plant needed to run two pumps in parallel.
Plant reliability and maintenance personnel needed a solution that would allow one pump to meet production needs during full rate operation without increasing the risk of seizure. After consultation with their local repair facility, they decided to overhaul one of their pumps using all composite stationary wear components (case wear rings, inter-stage case bushings, throat bushings, and center-stage bushing) with reduced clearances.
The pump involved is a common design for a process plant boiler feed water pump: 9-stage, horizontally-split, between-bearings design (see Figure 1).
Figure1. The 3-stage boiler feed water pump
This design includes a double-suction first-stage impeller with wear rings, an eye-side wear ring and inter-stage bushing at each of the other eight stages, two throat bushings, and a pressure reducing bushing between the 5th and 9th stages. The impellers are 9.125-in (232-mm) diameter, with 5-in (127-mm) diameter wear rings.
This pump design creates unique challenges which are particularly suited to the application of composite wear components. The rotor configuration - a long, thin shaft (see Figure 2) suspended between bearings - relies heavily upon the wear rings for rotor stability.
While the pump is idle, there is substantial rotor sag (about 0.008-in/0.2-mm) at the center bushing. The rotor sag goes away only when the pump is running and hydrodynamic forces at the wear rings are established. During alignment of the pump to the driver, slow-roll (if coupled to a steam turbine driver), or at start-up, there will be contact at the center stage bushing and other wear components.
During full speed operation off-design conditions like low-flow operation, dry-running, or cavitation can also cause contact between rotating and stationary components, potentially leading to seizure. The service conditions, shown in Table 3, were typical for a process plant boiler feed pump.
Figure 2. Rotor configuration
Pump Retrofit Details
To optimize the performance of this pump during the available repair time, the repair facility selected a composite material for all of the stationary wear components. The existing 12 percent chrome metal rotating components were used in combination with the composite material. Running clearance between rotating and stationary components was reduced to 0.008-in (0.203-mm), 50 percent of the original design values.
Instead of fabricating new components with complex geometry for the stationary composite components, the existing components were used as holders for composite "inserts." The existing metal bores were increased, and thin-radial-wall composite inserts were manufactured and pressed into the metal holders.
Table 3. Process conditions for boiler feed pump
Additional design detail was needed for the split inter-stage bushings, as shown in Figure 3. Using the existing components as "holders" for the composite reduced the required machining time - eliminating the need for milling slots and other details on the outside diameter of the components.
Figure 3. Composite insert in metal case ring holder
To prevent axial movement of the composite, the metal holders incorporated a small shoulder on the low pressure side. There were no pins or screws used for anti-rotation purposes; the composite relying upon a heavy interference fit for anti-rotation. With a very low coefficient of friction and other properties that reduce risk of seizure, this method of installation has proven effective with this composite during the past ten years of field experience.
Turning the Rotor
Retrofitting a pump with composites is a fairly straightforward proposition. However, as the clearance at the wear components decreases, rotor concentricity becomes more important. To ensure success with the new, tighter clearance, the repair facility must pay attention to a few fundamental details.
Before the rotor is installed, the pump should be fully inspected. The split-line should be checked to ensure it is true, the line bore and seal chamber bore should be checked for concentricity (see Figure 4), and the rotating element should have limited run-out (typically 0.002-in max).
Figure 4, below. Line boring the pump case
Individual components should be checked for condition and concentricity. For example, if using the existing case ring as a holder for a composite insert, the metal ring should fit properly within the pump case. Loose or distorted components should be repaired or replaced as needed.
Once the rotor is in place, the case bolted together, and the bearing housings doweled and bolted in place, one final check is needed. The rotor should turn freely, which requires a reasonable understanding of the pump internals.