Optimization of Approach Flow Conditions of Vertical Pumping Systems by Physical Model Investigation

Circulating flow in front of open and covered intake systems in many cases is the initial point of free-surface vortex formation. Depending on the strength of the circulation air pulling vortex formation up to a coherent air core will appear starting at the surface leading directly to an installed pump inside the intake. Thus the mechanical load of the pump will increase and the hydraulic performance will be degraded. Furthermore, pre-rotation of the fluid close to the suction bell can be forced up to limits which are not in compliance to state of the art acceptance criteria. At least non-symmetric velocity distribution at the impeller will arise out of this flow conditions. Within physical model tests intakes together with pumps can be optimized for their best efficiency point of operation. Flow conditions can be achieved generating kinematical affinity at the suction bell which are close to the conditions of the acceptance test. The report shows application oriented solutions for the installation of cost effective flow guiding devices in open and covered intake systems to assure adequate pump performance. Test results of model investigations and experiences by modifying intake structures of existing plants will be presented. Concerning the sensitivity of high specific speed vertical pumps approach flow conditions especially at the intake structures have to be in accordance to state of the art acceptance criteria to assure adequate availability of the pumps. By today it is common practise and state of the art to test the behaviour of intake and outfall structures by physical model tests.