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The Practical Pumping Handbook - =- __~ - -.___~__~i..i .....

would if they operate alone on the same system, a steady increase in
numbers of pumps will reduce the flow rate through each pump. This
could result in the final pump adding only a fraction of its capability to
the system output as is indicated in Figure 3.18.

3.6 Pump system analysis

In selecting the pipe sizes to be used in the systems under discussion,
we have limited the velocity in the pump discharge line to a value of 10
ft/second. However, as will be discussed in Chapter 5, the introduction
of computerized pump selection and system design software, allows this
to be taken one step further. It is now possible to balance the higher
cost of larger pipe against the lower velocity in the lines that result in
reduced power costs. This allows the designer to go beyond the
restrictions of a capital cost budget and implement the consideration of
lifetime costs of operation and maintenance.
The friction loss values used in these examples are drawn from the
Friction Loss tables shown in Chapter 13. The values in these tables are
based on the roughness parameter for new Schedule 40 Steel Pipe, with
no allowances for age or abnormal conditions of interior surface.
Consequently it is a fairly common practice to apply a safety factor to
these calculated values, particularly when working with an older system
where the interior surface of the pipes may be scaled or rough, or may
become so very quickly after start-up of the system. A safety factor is
also frequently used if the engineer is working with incomplete
information. The amount of any safety factor must be estimated for
each installation individually and should be based on local conditions
and experience. As the following examples involve fairly short runs of
piping and thus have low levels of friction losses, we will use a safety
factor of 10% to demonstrate its use.
In order to calculate the friction losses for pipes and fittings, two
approaches are possible, depending on the information available. One
option requires the use of a table that shows the resistance of various
valves and fittings in equivalent lengths of pipe. For example, one such
chart identifies the resistance of a 6 inch Standard Elbow as equivalent
to the resistance of a length of 16 feet of 6 inch Standard Pipe. With
such information, all the valves and fittings can be transferred to
equivalent lengths of pipe and the friction losses calculated as shown for
straight pipe.
In the approach used in these examples, it is first necessary to establish
the Resistance Coefficient (K factor) of each valve and fitting. This
information is contained in the charts for Typical Resistance
Coefficients for Valves and Fittings in Chapter 14. The value is then

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