Page 288 - Intro Predictive Maintenance

Operating Dynamics Analysis 279

deviations from good engineering practices result in turbulent suction flow and cause
hydraulic instability that severely restricts pump performance.

The restrictions on discharge piping are not as critical as for suction piping, but using
good engineering practices ensures longer life and trouble-free operation of the pump.
The primary considerations that govern discharge piping design are friction losses and
total vertical lift or elevation change. The combination of these two factors is called
TSH, which represents the total force that the pump must overcome to perform prop-
erly. If the system is designed properly, the discharge pressure of the pump will be
slightly higher than the TSH at the desired flowrate.

In most applications, it is relatively straightforward to confirm the total elevation
change of the pumped liquid. Measure all vertical rises and drops in the discharge
piping, then calculate the total difference between the pump’s centerline and the final
delivery point.

Determining the total friction loss, however, is not as simple. Friction loss is caused
by several factors, all of which depend on the flow velocity generated by the pump.
The major sources of friction loss include:

• Friction between the pumped liquid and the sidewalls of the pipe
• Valves, elbows, and other mechanical flow restrictions
• Other flow restrictions, such as back-pressure created by the weight of liquid
in the delivery storage tank or resistance within the system component that
uses the pumped liquid

Several reference books, like Ingersoll-Rand’s Cameron’s Hydraulics Databook,
provide the pipe-friction losses for common pipes under various flow conditions.
Generally, data tables define the approximate losses in terms of specific pipe lengths
or runs. Friction loss can be approximated by measuring the total run length of each
pipe size used in the discharge system, dividing the total by the equivalent length used
in the table, and multiplying the result by the friction loss given in the table.

Each time the flow is interrupted by a change of direction, a restriction caused by
valving, or a change in pipe diameter, the flow resistance of the piping increases sub-
stantially. The actual amount of this increase depends on the nature of the restriction.
For example, a short-radius elbow creates much more resistance than a long-radius
elbow; a ball valve’s resistance is much greater than a gate valve’s; and the resistance
from a pipe-size reduction of four inches will be greater than for a one-inch reduc-
tion. Reference tables are available in hydraulics handbooks that provide the relative
values for each of the major sources of friction loss. As in the friction tables
mentioned earlier, these tables often provide the friction loss as equivalent runs of
straight pipe.

In some cases, friction losses are difficult to quantify. If the pumped liquid is deliv-
ered to an intermediate storage tank, the configuration of the tank’s inlet determines