Page 90 - Root Cause Failure Analysis

Pumps 81

tion loss, velocity head, and static lift must be enough to provide the minimum NPSH
requirements of the pump. The requirements vary with the volume of liquid trans-
ferred by the pump.

Most pump curves provide the minimum NPSH required for various flow conditions.
This information, generally labeled NPSHR, usually is presented as a rising curve
located near the bottom of the hydraulic curve. The data usually are expressed in feet
ofhead rather than psi.

To convert from psi to feet of head for water, multiply by 2.31. For example, 14.7 psi
is 14.7 x 2.3 1 or 33.957 feet of head. To convert feet of head to psi, multiply the total
feet of head by 0.433 1.

SuctionVolume The pump’s supply system must provide a consistent volume of
single-phase liquid equal to or greater than the volume delivered by the pump. To
accomplish this, the suction supply should have relatively constant volume and prop-
erties (e.g., pressure, temperature, specific gravity). Special attention must be paid in
applications where the liquid has variable physical properties (e.g., specific gravity,
density, viscosity). As the suction supply’s properties vary, effective pump perfor-
mance and reliability is adversely affected.

In applications where two or more pumps operate within the same system, special
attention must be given to the suction flow requirements. Generally, these applications
can be divided into two classifications: pumps in series and pumps in parallel.

Pumps in Series The suction conditions of two or more pumps in series are extremely
critical (see Figure 7-3). Since each pump depends on the flow and pressure of the pre-
ceding pump, the flow characteristics must match. Both the flow and pressure must be
matched to the required suction conditions of the next pump in the series.

For example, the first pump in the series may deliver 1,OOO gpm and 100 ft of total
dynamic head. The next pump in the series then will have an inlet volume of 1,OOO
gpm, but the inlet pressure will be 100 ft minus the pressure losses created by the total
vertical lift between the two pumps’ centerlines and all friction losses caused by the
piping, valves, and the like.

This pressure at the suction of the second pump must be at least equal to its minimum
NPSH operating requirements. If too low, the pump will cavitate and not generate suf-
ficient volume and pressure for the process to operate properly.

Pumps in Parallel Pumps that operate in parallel normally share a common suction
supply or discharge (or both). This is illustrated in Figure 7-4. Typically, a common
manifold (i.e., pipe) or vessel is used to supply suction volume and pressure. The
manifold’s configuration must be such that all pumps receive adequate volume and
net positive suction head. Special consideration must be given to flow patterns, fric-
tion losses. and restrictions.