Alternative arrangement of metered dosing fluid using centrifugal pump

Abstract

Positive displacement dosing pumps are extensively used in various types of process industries in Bangladesh. They are widely used for metering small flow rates of a dosing fluid into a main flow. High head and low controllable flow rates make these pumps suitable for industrial flow metering applications. However their pulsating flow is not very suitable for proper mixing of fluids and they are much more expensive to buy and maintain. Considering such problems some alternative techniques may be suggested to control the fluid flow of a typical centrifugal pumps including - throttling, variable speed drive, impeller geometry control and bypass control. Variable speed drive and impeller geometry control are comparatively costly and the flow control by throttling is not an energy efficient process. Bypass flow control can be an alternative means to control the dosing flow range. An arrangement of creating dosing flow was developed using a typical low cost centrifugal pump with bypass flow technique. A wide range of dosing flow control was attained using fixed pump geometry and drive speed. The returning bulk flow from the pump into the main tank ensured better mixing due to churning effect which may eliminate the need of separate agitators. In this project, performance of a dosing pump was evaluated and compared to an alternative arrangement of similar fluid flow from a centrifugal pump flow arrangement using bypass technique. Water was used as a working fluid of the pump. Experiments were performed in total four phases. The first phase consists of only performance evaluation of the dosing pump and it is found that, the range of the metered flow of the dosing pump is 22.4 -1.5 L/h at corresponding head of 0.7725-71.36 m. The power consumption is almost constant 72 watt. The maximum and minimum efficiency is around 4% and 0.01% respectively. In the second phase, performance study of the centrifugal pump was evaluated. The range of fluid flow rate is almost 2650-11.4 L/h at corresponding head of 0.973-28 m. The power consumption varies from maximum 677.92 watt to minimum 624.4 watt depending on the flow rate changes. The maximum overall efficiency is found around 13.5%. In order to reduce the discharge of the centrifugal pump, bypass flow arrangement has been established and the third phase of experiment was carried out. Similar types of flow rates could be possible to be attained by the arrangement and the new established flow range was found 2.66-23.01 L/h at corresponding head of 13.19-28.5 meter. The power consumption varies from maximum 677.92 to minimum 624.4 watt at this phase. Impeller diameter has been reduced to decrease the power consumption and new operating range has been established due to impeller modification, which varies form 2.83-22.05 L/h at corresponding head of 6.01 -14.0 meter. With the modification, the power consumption reduces by 40% compared to the centrifugal bypass arrangement without impeller diameter reduction. But it slightly decreases the overall efficiency. Operability of the modified system was tested at low voltage, which also showed improvement of overall efficiency and reduction of further power consumption by 32% compared to bypass arrangement with impeller modification only. The capital cost can be saved by this centrifugal bypass flow system is approximately 87.5% of the capital cost of the dosing pump. This alternate system is more cost effective in terms of capital investment.