Power pump performance improved with redesign of the first-stage, double-suction impeller and twin volute.
by Dave Allard & Dr. Gary Dyson

In the aftermarket business, part replication is not enough. Precision engineering combined with the latest technology are essential for manufacturing high-quality parts. A main boiler feed pump at a Midwestern United States power plant was built in 1967 using sand casting and wooden patterns, now considered outdated technology. Even though the pumps received refurbishment every six to eight years, the pumps continued to have low performance as well as vibration issues.

Using all its resources—including casting simulations, 3-D models, up-to-date foundry casting techniques and considerable engineering data—an engineering and repair services company fully manufactured a complete element, performed sophisticated testing in the Pumps Test Lab Approved Program (PTLA) certified test lab, and returned the pump to operation within just 12 weeks.

This project involved the manufacture of a complete first stage twin volute and a description of the latent defects.

The pump suffered from ongoing vibration issues which were caused by pressure pulsations at vane frequency. To improve the vibration levels, hydraulic analysis and redesign were required to develop a new, improved design.

This project has been divided into two articles. The first is the manufacturing of the twin volute and the second is the design of a new impeller.

Rusted volute pumpImage 1. A received bundle showing failure in the twin volute stage piece. The aftermarket company received the internal element and casing (pump bundle, or element) of the pump. (Images and graphics courtesy of Hydro, Inc.)

The first-stage twin volute is a complicated casting, which failed during operation as a result of poor design.

The engineering and repair services company re-engineered the casting by using sophisticated engineering and 3-D modeling, along with simulation software and 3-D sand printing.

In addition, the company identified the opportunity to improve the performance of the pump by redesigning the first-stage double-suction impeller. To improve vane passing frequency, the first-stage double suction impeller was redesigned with staggered and split vanes.

The company’s aftermarket services capability provided a completely new replacement element for this high-energy boiler feed pump and also redesigned the castings to eliminate the original latent defect in the casting design.

The company provided sophisticated hydraulic engineering improvements to increase the mean time between repairs (MTBR) of the newly manufactured element.

Background

The end user came experienced an issue when the life expectancy of a pump case was not meeting expectations. The pump was removed from service due to performance deterioration manifested by increased vibration. The first-stage double suction volute had failed. A crack had developed on the volute initiating from the interface between a welded core plug and the volute casting.

At the foundry, images were taken with an articulating bore scope and processed “as received.” The results showed burnt sand on the wall of the volute crossover, minor flashing in the fillet area of the crossover and acceptable surface finish in the clean passage. Reverse engineering of the pump uncovered a latent defect in the original casting.

Root Cause of Failure

Traditional foundry tooling sometimes requires support methods to hold the cores in place inside the mold. Cores are pieces of sand held together by a binder that has been packed into the tooling to create a specific geometry. Cores form most internal geometries for pump parts.

The challenge is to secure the cores once the metal begins to fill the mold. The use of chaplets is a common method practiced in foundries when there is a need to support a core inside a mold. Chaplets are small metal spacers built to the wall thickness of a casting to minimize any shift or lift of the core. The negative impact is that they typically need to be excavated and welded during the cleaning process.

Metal chaplet failureImage 2. Root cause analysis showed that metal chaplets used during manufacturing initiated the failure mechanism.

Chaplets are usually comprised of different metallurgy from the metal being poured and can create a weak area if left in place. Image 2 shows areas that were either utilized for chaplets or are core sand clean-out plugs, which were excavated and welded.

The engineers eliminated the need for chaplets during the pouring process by using 3-D technology, which allows for the manufacturing of the cores in a hybrid design that will lock them in place inside the mold. By using this method, the sophisticated technology averages the wall thickness, minimizes core shift and minimizes welding. The volute areas were examined by the bore scope and properly cleaned.

Graphic illustration of voluteFigure 1. Data was captured using scanning techniques and the twin volute was modeled in CAD.

Casting Defects

In the volute crossover areas, the engineering and repair services company noticed a significant amount of flash on both sides. This is created by core lift where two cores are jointed together. While the 3-D printing method will not eliminate the need for separate cores, it allowed the engineering and repair services company to move the core split to an area that is accessible for grinding. The 3-D printed cores were 0.005-0.20-inch accurate dimensionally in the x, y, and z axis. This gave the engineers precision core filament during the mold building process.

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