Page 97 - Root Cause Failure Analysis

88 Root Cause Failure Analysis

strain. In addition, flexible isolator connectors should be used on both suction and dis-
charge pipes to ensure proper operation.

Inlet- Piping Configuration
Centrifugal pumps are highly susceptible to turbulent flow. The Hydraulic Institute
provides guidelines for piping configurations that are specifically designed to ensure
laminar flow of the liquid as it enters the pump. As a general rule, the suction pipe
should provide a straight, unrestricted run that is six times the inlet diameter of the
Pump.
Installations that have sharp tsuns, shutoff or flow-control valves, or undersized pipe
on the suction-side of the pump are prone to chronic performance problems. Such
deviations from good engineering practices result in turbulent suction flow and cause
hydraulic instability that severely restricts pump performance.

Discharge-Piping Configuration
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, discussed in the section earlier in this chapter, which represents the total
force that the pump must overcome to perform properly. If the system is designed
properly, the TDH of the pump will equal the TSH at the desired flow rate.

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 pip-
ing, 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 a number of factors, and all 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 liq-
uid in the delivery storage tank or resistance within the system component
that uses the pumped liquid.

A number of reference books, like Ingersoll-Rand’s Cameron 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.