Flow duration diagrams
When evaluating a pump system it is helpful to be able to identify the system effectiveness easily and to compare different solutions . In order to make an intelligent choice , some basic facts will need to be established. The first is: What are the Process Demands?
The Process Demands will generally indicate a certain direction, i.e. will the process require varying flow rate and, if so, should it be continuously variable or can flow rate be varied in steps? Can on-off batch pumping be used? What is the peak flow rate and how is the flow rate distributed over time?
The answers to these questions will determine if, and how, to regulate the flow. It will also give some guidance regarding the design of the pumping system .
An easy way of showing the flow demand is to use a duration diagram.
A duration diagram (see Figure 5.1) in its simplest form shows how many hours (t) during a year that a given flow rate (Q) is needed, the blue line. The red curve in the same diagram is interpreted differently. Each point on the red curve tells how many hours during a year the flow rate exceeds the value on the y-axis .
This diagram is instrumental in understanding the pumping needs. The system must be able to deliver the peak flow but, from an economic point of view, it is also important to know at what flow rates the system is going to operate most of the time .
Given this information, the piping system can be designed. If, for example, the maximum flow rate occurs only for short periods of time , it may not pay to install a large diameter pipe. On the other hand, if the
system is operated at peak flow rates for extended periods of time, this fact should be taken into account when specifying the pipe diameter . When the piping system has been designed , it is time to calculate the system curve.
The system curve (see Figures 2.4 and 2.5) tells how much head, or pressure, is needed from a pump to push a given flow rate through the pipe system.
The duration curve is used to determine where on the system curve the pump will operate and for how long. If a considerable length of time is spent high up on the system curve, a larger pipe diameter, or other modifi cations, should be considered, so that a pump of lower head and power can be used.
The pump, or pumps , can now be specified, plus suitable means for regulating the flow – if needed. Section 4 .2.2.2 explains how a pump performs in conjunction with a variable speed drive .