by Van der Schrieck
Understanding the world of sand and water
This book is derived from chapter 5.3 from the book `Dredging Technology Book 1`. In order to keep track of things the section and formula numbers are kept the same and start with number 5. This book combines the knowledge from chapter 5.1 and 5.2 and adds necessary new knowledge to it about the Suction Process and the Interaction between Pump and Pipeline. These will define the Work Point of the Pump Installation! This will all be explained in the first part of this book starting analysing the suction side of the pump. This results in the so called “Vacuum Formula or Suction Formula”.
The suction process is very important since it determines the maximum possible suction production, no matter how many pump capacity you have installed beyond the suction pipe. “Cavitation” in the pump must be prevented, since that causes mayor problems with the drive engines. The sudden change in mass inside the pump (water turns into vapour!) and the extreme and sudden drop in torque demand thereof will cause “tripping” ( = over speed) of the diesel engines and causing them to suddenly stop! Often such stops will cause the lower parts of the long pipeline to be blogged (silted)!
This book explains how to prevent such nasty scenarios. The second part of the book explains how to optimize the use of the pump and drive and how to benefit from a small change of the impeller diameter or a different number of blades in the impeller (3 or 5). This enables to keep the flow value of your working point ahead of the minimum “critical” flow velocity necessary to prevent siltation of the pipeline. It also enables to keep your flow velocity below the max flow when pumping through a too short pipeline.
Finally the book describes how to optimize the vacuum production using the vacuum production graph in which the suction production is given as a function of the mixture flow velocity. This graph clearly shows that the operator should use about half the max velocity one can reach when pumping clear water only. This is a very simple rule. This handy concept is further expanded by adding more limits in this “Production – Flow” graph or “P-Q graph” such as the production in m3/s as a function of the pipeline distance, the so called “P-L graph”, and the production as function of density, and also the lines of the rotation speed of the engine so one can clearly identify when the engine runs full speed and when in the full torque mode.