Page 289 - Intro Predictive Maintenance

280 An Introduction to Predictive Maintenance

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 should always be started with the discharge
valve closed. As soon as the pump is activated, the valve should be slowly opened 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 into the bypass piping to permit a minimal flow of liquid.
In many cases, the flow permitted by these metered orifices is not sufficient to dissi-
pate the heat generated by the pump or to permit stable pump operation.

If a bypass loop is used, it must provide sufficient flow to ensure reliable pump oper-
ation. 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 needed 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 ensures premature failure of any centrifugal pump. The radical surge of
back-pressure generated by rapidly closing a discharge valve, referred to as hydraulic
hammer, generates an instantaneous shock load that can literally 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.