Page 99 - Root Cause Failure Analysis

90 Root Cause Failure Analysis

If a bypass loop is used, it must provide sufficient flow to assure reliable pump opera-
tion. The bypass should provide sufficient volume to permit the pump to operate
within its designed operating envelope. This envelope is bound by the efficiency
curves that are included on the pump’s hydraulic curve, which provides the minimum
flow required to meet this requirement.

Stable Operating Conditions
Centrifugal pumps cannot absorb constant, rapid changes in operating environment.
For example, frequent cycling between full-flow and no-flow assures premature fail-
ure of any centrifugal pump. The radical surge of back pressure generated by rapidly
closing a discharge valve, referred to as hydraulic hummer, generates an instanta-
neous shock load that actually can tear the pump from its piping and foundation.

In applications where frequent changes in flow demand are required, the pump system
must be protected from such transients. Two methods can be used to protect the system:
Slow down the transient. Instead of instant valve closing, throttle the system
over a longer time interval. This will reduce the potential for hydraulic ham-
mer and prolong pump life.
. Install proportioning valves. For applications where frequent radical flow
swings are necessary, the best protection is to install a pair of proportioning
valves that have inverse logic. The primary valve controls flow to the pro-
cess. The second controls flow to a full-flow bypass. Because of their
inverse logic, the second valve will open in direct proportion as the primary
valve closes, keeping the flow from the pump nearly constant.

POSITIVE DISPLACEMENT
Centrifugal and positive-displacement pumps share some basic design requirements.
Both require an adequate, constant suction volume to deliver designed fluid volumes
and liquid pressures to their installed systems. In addition, both are affected by varia-
tions in the liquid’s physical properties (e.g.. specific gravity, viscosity) and flow char-
acteristics through the pump.

Unlike centrifugal pumps, positive-displacement pumps are designed to displace a
specific volume of liquid each time they complete one cycle of operation. As a result,
they are less prone to variations in performance as a direct result of changes in the
downstream system. However, there are exceptions to this. Some types of positive-
displacement pumps, such as screw-types, are extremely sensitive to variations in sys-
tem back pressure. Causes of this sensitivity were discussed previously in this chapter.
When positive-displacement pumps are used, the system must be protected from
excessive pressures. This type of pump will deliver whatever discharge pressure is
required to overcome the system’s total head. The only restrictions on its maximum