This clearly shows that the difference between the shutoff (SO) head and the total dynamic head (TDH) at the operating point stands at only 2.0 m of slurry column.
To charge the siphon, the pump must be pumping at least 15% or 20% of the design flow rate (refer to the discussion in Part 1 of this blog article), in which case the corresponding TDH will be at about 35 m, thus the said difference goes further down to 1.7 m, which is clearly below DH=3.0 m in the basic option and barely passes DH=2.0 m in the alternative layout.
This proves that if the pump were sized for the system static head, measured between the downcomer discharge elevation and slurry level in the pump box (SH2), the pump would not be able to start pumping, provided it had a fixed speed pump motor, or if it were not designed to override motor speed rated at 100% with a variable speed motor option.
To ensure stability and problem-free pumping operation, it is obvious that AVR valves must be installed on the top pipe. In which case, the system static head is always measured between the centreline of the feed pipe top section and the level of slurry in the pump box (SH1).
Once the AVR valve is included in the concept, if the top pipe was lowered by 1 m, as in the alternative piping layout, pump energy consumption would also be reduced and would be an additional benefit to this routing. A rough estimate of energy savings, in this case, was calculated as follows (with respect to 1 m of static head and some friction loss reductions).