Engineered Software, Inc. and KCF Technologies Inc. recently presented a case study at the 2015 Chem Show in New York City. What made this project so much fun is that it was a Capstone Project conducted by a group of engineering students at Penn State. The students were evaluating the cause of a major cavitation problem at the University Area Joint Authority (UAJA) pumping station in State College, PA.
Figure 1 shows the general layout of the pumping station at the UAJA. The system consisted of a single storage tank that supplied fluid to a main and backup system only one system operated at a time. The piping system fed by the main pumps had small elevation changes before discharging to a holding tank. The piping system supplied by the backup pumps took a different route and crested two hills prior to discharge to its tank.
The system operated without problems when the main pumps were on line, but when the backup system was placed online the pumps experienced extensive vibration and it sounded like cavitation was occurring. The vibration was troubling enough that the UAJA did not operate the backup system. The objective of the students’ capstone project was to determine what was causing the vibration and cavitation in the backup pumps and what can be done to correct the problem.
Figure 1 – Piping Schematic of UAJA waste collection systems.
The first step was to determine what was causing the vibration in the backup pumps. The students worked with KCF Technologies to install vibration sensors on the backup pumps and took extended vibration readings. Using the analysis capabilities of their vibration monitoring software they were able to determine that the backup pump was, in fact, cavitating.
The second step was to determine the cause of the cavitation. The students then modeled the system using PIPE-FLO® and discovered that the program calculated that the Net Positive Suction Head available at the pump suction was greater than the NPSH required by the pump manufacturer.
After examining the physical system, they had two groups of pumps pumping from the same sump with roughly the same flow rate (approximately 320,000 gal/hr). When the main pumps were operating there was no indication of excessive pump vibration or cavitation, but when the backup pumps were operating there was excessive vibration and cavitation. Since the suction piping on all pumps was identical it was determined that the suction piping was not the cause of the problems associated with the backup pumps.
It was suggested that the students model the remainder of the backup system using PIPE-FLO® (See Figure 2). When the model was created and the calculations performed it was discovered that the pressure at the system’s high point (elevation 100 ft) was -37.06 psig. This is well below the vapor pressure of the process fluid, and even a negative absolute pressure value which is a physical impossibility. In other words, PIPE-FLO® was pointing out that the system cannot operate properly.
The excessive negative pressure at the high point is because the energy of the fluid at the high point is much greater than the head loss in the pipeline going from the high point to the outlet tank. When looking at the model results, we came to the understanding that the pump should not experience cavitation until the pipeline to the high point is full. Once the fluid starts flowing from the high point to the outlet tank the pipeline down pipe is not properly vented.
To validate this assumption more operating data was required. We wanted to start up the backup pumps and determine when cavitation started, and what was happening to the vent and vacuum breakers located at the high points in the system.
Once one backup pump was started, cavitation did not occur for over 45 seconds. The 45 second delay corresponded to the length of time needed to fill the pipeline to the high point. Once the pipeline to the high point was full then the vacuum breakers started allowing air into the pipeline and the vibration and cavitation occurred on the backup pumps.
Based upon the results of the PIPE-FLO® model and the plant observation, the Penn State students were able to determine that the cause of the pump cavitation was a rapid change in the system static head as caused by the pressure variation at the high point. These wild pressure variations were caused by the fact that the down pipe from the high point to the outlet tank was not running full and was not properly vented. With this information the students were able to determine the cause of the high vibration and cavitation and were able to make a recommendation on how to correct the problem.