This problem occurs quite frequently in discharge piping arrangements where the piping goes to a higher level at one stage of the system and then comes back down to a lower level at discharge. This can, in fact, create a siphoning effect on the discharge system that can be quite confusing, until you go back to basics and understand the interplay between pump and system.

The highest point of the system piping is 6.0 m. elevation, at which point the discharge static head is 6.0 – 0.7 = 5.3 m.

However, the destination elevation is only 2.0 m, at which point the discharge static head is 2.0 – 0.7 = 1.3 m.

Under these conditions, the system demand will include a static head of 5.3 m, which the pump must supply.

However, once the piping in that system is full of liquid, a siphoning effect will kick in, and the system will only require a static head of 1.3 m. This means that the entire system curve will fall, and intersect the pump curve at a higher flow rate than it will when the static head is 5.3 m. Consequently, it is essential that the pump be selected so that it can operate satisfactorily under both conditions.