Fluid Mechanics Solutions
Rising Main
This scenario consists of two pumps draw fluid from a common feed reservoir. A non-return valve is located on
the output of each pump. Both pumps feed a common rising delivery pipe which then branches to feed two
discharge reservoirs. The feed reservoir, pumps and non-return valves are all at datum level. The higher of the
two discharge reservoirs is 50m above datum, while the lower is at 40m above the datum. The flow to the lower
discharge reservoir is controlled by a valve. The effect of a pipe fitting is modeled at the discharge to the higher
reservoir by modeling a smaller pipe diameter at the entrance to the reservoir. The image below is a Flowmaster
network that represents the system as described.
The network will be analyzed over a 10 second period. Initially both pumps are running and the control valve is
fully open. The valve is then progressively closed over the first two seconds. After a further two seconds, one of
the pumps trips out.
It is suspected that cavitation will occur downstream of the control valve - therefore a ‘vapor cavity’ is built into
the network at this point using Auto-vaporization. The effects of the cavity collapse are quite serious and a
number of techniques can be used to overcome the problem:
o adding an Air Admission Valve downstream of the control valve
o adding a Surge Tank or Air Vessel downstream of the control valve
o adding a swing check valve and a finite area reservoir.
The red line tracks the size of the cavity bubble. We can see that the bubble is the largest when pressure is thelowest and that the bubble collapses as soon as the pressure rises above the vapor pressure.
