|1. Definition : what
is parallel and series pumping ?
|2. Operation, flow
and head calculation
It is sometimes necessary to use pumps in parallel or in series.
Parallel operation is the fact to put in operation 2 pumps besides each other on 2 parallel pipes that are merging after the pumps. It allows to add the pump capacities.
Series operation is the fact to put in operation 2 pumps one after the other on a single circuit. It allows to add the pump heads.
Some precautions are however necessary when operating pumps in parallel or series (see corresponding section)
All calculations are performed thanks to the pump characteristics established by the supplier. This document is necessary for design parallel and series pumping systems. A typical pump characteristic curve is represented below, for a centrifugal pump.
Figure 1 : Pump NPSH, required and available
Reading and undertanding the pump characteristic curve is a very important 1st test for pipe circuit calculation.
In pump parallel operation, both pump will deliver the flow they have been designed for, at the given head of the combined flow. The characteristic of the parallel pumping system can be calculated by adding the capacity of each pump at a given head.
Figure 2 : Building resulting characteristic of 2 pumps in parallel
However, it must be noted the following : the combined flow is requiring a higher head for a given system curve, thus the combined capacity Q3 of the system is not equal to Q1 + Q2 when the pumps are running one by one. This is illustrated on the graph below by the system curve.
Figure 3 : Flow in a system with pump in parallel
In pump series operation, at a given capacity, the 1st pump will deliver its head and send the fluid to the next pump, then the 2nd pump will add its own head. The characteristic of the series pumping system can be calculated by adding the head of each pump at a given capacity./p>
Figure 4 : Building resulting characteristic of 2 pumps in series
To understand which head and flow the system will deliver, the system curve must be added to the pump characteristic.
Figure 5 : Flow in a system with pump in series
Precautions must be taken especially for parallel operations.
Indeed, if the head of both pump is very different, when the system
is put in a system, the following can happen : the head delivered by
one of the pump exceed the head delivered by the other one. In such
a case, a recirculation will happen where the flow in the pump with
the smaller head will go backwards. To avoid such issue, it is
important to draw the characteristics and system curve as explained
above, and make sure that both pump don't have performances too far
one from the other.