Pumping applications involving cooling water have been especially difficult to solve because of the presence of dissolved air inherent in a cooling tower sump. cc
Water that contains large amounts of dissolved air changes the apparent required net positive suction head (NPSH). In such applications, traditional correction techniques failed because the entire system was not analyzed and the source of the noise generation could not be pinpointed.
This article explains the steps taken to solve this type of problem for Dow Chemical's plant in Freeport, TX, through computational fluid dynamics (CFD) and recounts the results.
Noise problem observed
In 1991, four large 30 x 30-38-in double-suction cooling tower pumps operating at 36,000 gpm and 710 rpm were installed at the Dow plant. Although these pumps met performance specifications on the test stand, they proved to be noisy when installed. Sound power levels greater than 93 dbA were observed approximately 3 ft from the pump casing.
These pumps were operating under duress as indicated by noise as well as other signs. The impeller was removed after 12 months to 18 months of service and some cavitation damage was evident.
Noise is a chronic problem with many cooling water pump installations.
The relatively high vapor pressure of hot water and the presence of dissolved air are both factors that influence the onset and the degree of pump cavitation, which creates noise and damages impeller and casing surfaces.
Apart from the fluid, other items that should be examined when noise is observed are the system installation, which includes piping, sump, valves, elbows, foundation and piping supports; as well as the pump's selection, operating point, mechanical condition and design.
The suction chamber of these pumps wraps around a portion of the discharge volute. As the flow enters the suction chamber, it splits at the discharge volute and undergoes a series of turns as it approaches the impeller (Fig. 1). This is analogous to the flow through a series of elbows. Consequently, a non-uniform velocity/pressure distribution is imposed on the impeller inlet.
Leading to a cure
Because of the relative expense and difficulty of modifying the fluid or the system at the installation at Dow, the pump became the focus of the investigation.
To establish a good understanding of the fluid behavior within the pump, a CFD model was developed.
CFD programs are evolving into useful engineering tools that can predict fluid behavior within almost any geometry. Even if the fluid condition and system effects are not understood, a CFD model can provide a best-case scenario whereby the pump casing design and impeller design can be evaluated.
The process began by creating a 3-D computer-aided drafting (CAD) model of the suction inlet portion of the pump casing and impeller. The CAD model was then imported into the CFD package and a mesh was created within the fluid space. The discharge volute of the pump was not modeled because, according to measurements taken in the field, the source of noise was confined to the suction.
Three flowrates for water at nominal room temperature were examined, based on the typical operating range of the pump [i.e., best efficiency point (BEP), 50% BEP and 120% BEP].
The field installation was relatively simple. A short length of pipe and a diffuser connected the suction flange to a wall of an open sump. A positive head exists at the suction centerline. Because the entrance velocities were relatively low and there was nothing unusual in the suction piping, the boundary condition at the suction flange was specified as a uniform total pressure.
The casing (suction inlet), wearing ring and impeller were intended to be modeled together but, as a preliminary study, the impeller was modeled separately to isolate its influence on the pumped fluid.
Neither backflow nor separation of flow was identified within impeller passages. Recirculation would have been identified by backflow near the blade inlets if it were occurring. However, none was observed. Because the impeller was well behaved in the initial analysis, it was ruled out as a problem source.
The remainder of the analysis focused on the casing suction inlet with the volume between impeller hub and shroud included in the model to provide downstream effects. An angular momentum term was derived from the impeller analysis for the flow entering the impeller eye. Hence, this would be used as a downstream boundary condition in the inlet analysis.
The results of the casing analysis were interpreted graphically by creating plots on several key planes that pass through the model. These plots display static pressure, total pressure, velocity vectors within a plane and magnitude of velocity components perpendicular to a plane.
There are two major planes of interest. The first is perpendicular to the shaft and just outside of the impeller eye and reveals information about the flow as it enters the impeller. The second is parallel to the shaft and passes through its center, while lying at a 51 Degrees angle from the horizontal centerline of the impeller (Fig. 3). This plane displays the profile of the wearing ring, impeller and a nearly central portion of the suction volute and reveals information about how the flow approaches the impeller (Fig. 4).
In an ideal pump, a plot showing the magnitudes of velocity components entering straight into the impeller eye, just upstream of the impeller, should display one uniform flow field. However, this analysis indicates both radial and circumferential variations in inlet velocity (Fig. 6). Radial variations imply that flow entering between impeller blades will have a different speed near the hub than near the shroud. Circumferential variations imply that, at any given instant, one impeller blade will be loaded differently than the next.