by Charles S. Goodrich, Centrifugal Technology, LLC
December 17, 2011

In 1992, a pipeline was built in Texas and Oklahoma parallel to an existing line to increase capacity. The design flow rate was 3,600 gpm (818 m3/h). Four stage 8 X 10 X 13 pumps were purchased new when the pipeline was built.

Of 18 pumps purchased for the pipeline, 12 were four stages and six were four stage, destaged to two stages. The four stage pumps were rated at 2,750 ft of head (838 m), and the pumps destaged to two stages were rated 1,250 ft head (381 m). There were six pump stations, each with three pumps, with one pump destaged to two stages and two fully staged four stage pumps. The plan was to operate one two stage pump and one four stage pump at each station. One four stage pump was designated as a spare for both the two and four stage pumps.

In early 2004, more volume was needed in the line. Hydraulic studies had found that the rate could be increased by adding intermediate pump stations between the existing stations and replacing the existing 18 pumps with ones of a higher rate.

The company mentioned to us it was preparing to purchase 18 new pumps to replace all of the mainline pumps on its LPG pipeline. The design conditions for the proposed pumps were 4,000 gpm (908 m3/h) at a head of 2,780 ft (847 m). We suggested that rerating the pumps would be more economical that replacing the pumps.

Rerating the Pumps and Studying the Vibration/Resonance

One of the spare pumps was sent for evaluation. The pump was dismantled and internal measurements taken and documented. It was determined that enough space was available to install a higher capacity impeller in the pump case. The nozzles would need to be chipped approximately 1.5 in (3.8 cm) to open the throat area enough to achieve the higher flow rate.

The new impeller hydraulic design was made and a pattern built for the new impeller. Careful attention was given so the larger impeller would fit in the existing volute without compromising the pump's hydraulic integrity. The impeller's hydraulic width was .094 in wider (2.4 mm) wider than the original impeller, and the impeller eye was 1 in (2.54 cm) larger in diameter to accommodate the higher flow rate of the new high capacity impeller.

The larger eye was an added benefit since the pumps had extensive wear on the wear rings after 12 years of service. The stationary rings could be reused by over-boring the ring to accommodate the larger impeller eye rings, which saved the cost of entirely new stationary wear rings and stage pieces.

The volute cutwater (lips) had to be cut back 1.5 in (3.8 centimeters) to obtain the required nozzle area of 16.25 sq in (104.8 sq cm) for the new impellers. Hand grinding was used to cut back the volute lips and re-contour the volute shape. The new cutwater lips were hand contoured in the shape of an airfoil to minimize turbulence in this high velocity area.

The arrangement of the impeller vanes and stiffening the gland drip pocket under case were also addressed in a vibration study conducted in combination with the rerate. Many multistage pumps exhibit high vibration in the vertical direction when measured on the bearing housings. The standard horizontal multistage pump built more than 10 years ago is fairly strong in the horizontal direction, but because it is open on the top of the drip pocket it is weak in the vertical direction. Installing stiffeners under the drip pocket can improve the vibration. Two ribs were installed on the pump-one on each side of the drain hole in the drip pocket. The ribs were manufactured from 1 in (2.54 cm) steel plate.

Many 8 X 10 X 13 pumps seem to have a resonance problem in the long crossover in the three and four stage configuration with seven vane impellers. When seven vane impellers are used, this produces a high 7X vibration in the vertical direction (three and four stage pumps have the same crossover passage length). The best fix for this acoustical resonance was to install a six vane impeller at the stage driving the long crossover. This simple fix eliminated the resonance problem on more than a dozen pumps of this size from at least three different manufacturers.

Conclusion

As of this writing, the pumps have been in service for two years and are performing fine. The pipeline company is happy for the savings earned from rerating the existing pumps instead of completely rebuilding six pump stations. The vibration is also lower than before. Rerating and using an existing pump saved equipment costs and eliminated the need to build foundations for new pumps and piping.

Most centrifugal pumps can be rerated one size up or down by designing new impellers and modifying the volute area. In some cases a pump can be increased by two sizes. While two sizes is possible, it is not generally recommended because the cut back of the volutes is so aggressive that the diffusion is lost in the volute.