Additionally, today’s pumps are not as tolerant to off-design inlet conditions as in the past.

Both flow velocity and impeller speed have increased. The result is a lower equipment cost for a given operating point, but the tradeoff is a pump that is more sensitive to inlet conditions.

For example, consider a
double-suction pump with a close-coupled elbow at the inlet. This type of pump is designed for flow to be split evenly between each side the impeller. The elbow creates turbulence that splits
the flow unevenly across the impeller (Figure 1a). The pump performance can be degraded, and the flow can create an axial imbalance on the shaft which would result in shortened seal, bearing and impeller life.

*CAVITATION
Cavitation is the formation of and subsequent violent collapse of vapor bubbles in liquid. Bubbles or cavities of vapor form in the liquid when static pressure drops below vapor pressure. These bubbles implode when the static pressure rises above vapor pressure, creating a highpressure shock. This implosion can be extremely damaging if it is in the vicinity of a pump component, and creates a distinct noise.
Elbows close-coupled to pumps cause cavitation in two ways: First,
as liquid passes through an elbow it experiences a drop in pressure
on the inside radius. Vapor bubbles form in this area and can be carried into the impeller. Second, non-uniformity of the velocity exiting the elbow can cause local acceleration at the impeller vanes, resulting in low-pressure areas and cavitation.

Consider the feed piping configuration of a 3550 rpm double suction pump utilizing an 8-inch Suction Diffuser Vane, shown in
Figure 2. Prior to installing the Suction Diffuser Vanes, operation had
been noisy and the pump required
an overhaul every four months. A vibration test of the pump prior to the Suction Diffuser Vane installation is shown in Figure 3(a), and just after Suction Diffuser Vane installation is shown
in Figure 3(b). The dramatic reduction in measured cavitation (6-14 range on the frequency scale) also resulted in a reduction in impeller imbalance (1 on frequency scale) which reduced noise, extended seal, bearing and impeller life, and increased head and flow. Prior to Suction Diffuser Flex installation,
three identical pumps in parallel operation were required to deliver the required flow rate.

After the Suction Diffuser Flex was installed on each pump, two pumps met the flow rate requirement and the third pump
serves as an installed spare.

The Indiana University Cyclotron Facility (IUCF) located in Bloomington Indiana is a state of the art multidisciplinary facility performing research, development and application of accelerator physics, nuclear physics, nuclear science, and material science. Research activities from NASA and around the world are carried out at the IUCF.

Generating subatomic particle beams traveling at almost the speed of light, the accelerator at the IUCF is a life-saving marvel. The proton beam from the accelerator is used for medical treatments, especially the irradiation of cancerous tumors. In addition, two beam lines were built with support from NASA and are used to test components that will be flown into outer space as part of the Radiation Effects Research Program (RERP) that simulates years of exposure to the environment of space.

At peak operation the IUCF requires 4.0 megawatts of power to operate. The accelerator alone requires 1.5 megawatts to operate. With this power load, cooling of the equipment is critical, making the chilled water system a very important part of the operation.

“When the facility was first built,” comments Bill Starks, Mechanical Designer at IUCF, “we needed to install two, 200 HP, 3,700 GPM chilled water pumps in an area with inadequate overhead clearance. Vertical split case double suction pumps were selected.

“When we first started up the pumps,” he continued, “we were going through bearings and wearing out the impellers in 3 months time. The pump manufacturers confirmed it was a piping issue. It’s very critical, especially on a split case double suction pump. It has to be equal flow to each side. Otherwise, if the flow is concentrated to one side, it will result in an uneven thrust load on the impeller. The uneven load would wear out the bearings and also cause the impeller to shift to the side. Eventually the impeller will start hitting the housing. The bearings go bad, the housing goes bad and the impeller goes bad.”

Starks was able to improve that situation by doing a more critical alignment of the motor in the pump, yet bearings and impellers were still wearing out after just six months.

When the Suction Diffuser Flex was installed the results were immediately evident. “The Suction Diffuser Flex increased the suction pressure by about 25% from 3.5 pounds to 4.5 pounds,” detailed Bill Starks.” And it increased our output pressure by 10 PSI at the same flow rate.

“So this is a real big boost for us,” he continued, “and we’ve been running six months without bearing failure or any increase in noise. Usually the bearings would start to get noisier or you start wearing the impeller and you get some type of cavitation noise. Right now it’s still the same as day one. We’re actually running on what was our standby pump and reserving our main pump, which has the old style diffuser on it.”

IUCF is so pleased with the performance they are planning to replace their diffuser on the main pump with the Suction Diffuser Flex.