Page 98 - Root Cause Failure Analysis

Pumps 89

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
substantially. 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 4 in. will be greater than for a 1-in.
reduction. 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 previously, 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 if
it adds to the system pressure. If the inlet is on or near the top, the tank will add no
back pressure. However, if the inlet is below the normal liquid level, the total height
of liquid above the inlet must be added to the total system head.

In applications where the liquid is used directly by one or more system components,
the contribution of these components to the total system head may be difficult to cal-
culate. In some cases, the vendor’s manual or the original design documentation will
provide this information. If these data are not available, then the friction losses and
back pressure need to be measured or an overcapacity pump selected for service
based on a conservative estimate.

Operating Methods
Normally, little consideration is given to operating practices for centrifugal pumps.
However, some critical practices must be followed, such as using proper startup pro-
cedures, using proper bypass operations, and operating under stable conditions.

Startup Procedures
Centrifugal pumps always should be started with the discharge valve closed. As soon
as the pump is activated, the valve should be opened slowly to its full-open position.

The only exception to this rule is when there is positive back pressure on the pump at
startup. Without adequate back pressure, the pump will absorb a substantial torsional
load during the initial startup sequence. The normal tendency is to overspeed because
there is no resistance on the impeller.

Bypass Operation

Many pump applications include a bypass loop intended to prevent deadheading (i.e.,
pumping against a closed discharge). Most bypass loops consist of a metered orifice
inserted in the bypass piping to permit a minimal flow of liquid. In many cases, the
flow permitted by these metered orifices is not sufficient to dissipate the heat gener-
ated by the pump or to permit stable pump operation.