Xylem 8100 Series Centrifugal Pumps AC2515 REV.C - Manual (Page 15)

SUCTION AND DISCHARGE PIPING
The introduction of a pump into a piping system
which is not well designed or adjusted may cause
strain on the pump, leading to misalignment or
even impeller rubbing. Since slight strain may go
unnoticed, final alignment should be done with the
system full.
Pipe flanges should not impose any strain on the
pump. This can be checked by a dial indicator.
Any strain must be corrected by adjustments in the
piping system.
When installing the pump piping, be sure to
observe the following precautions:
Piping should always be run to the pump.
Do not move pump to pipe. This could make final
alignment impossible.
Both the suction and discharge piping should be
independently anchored near the pump and
properly aligned so that no strain is transmitted to
the pump when the flange bolts are tightened. Use
pipe hangers or other supports at necessary
intervals to provide support. When expansion
joints are used in the piping system they must be
installed beyond the piping supports closest to the
pump. Tie bolts and spacer sleeves should be
used with expansion joints to prevent pipe strain.
Do not install expansion joints next to the pump or
in any way that would cause a strain on the pump
resulting from system pressure changes. When
using rubber expansion joints, follow the
recommendations of the Technical Handbook on
Rubber Expansion Joints and Flexible Pipe
Connectors. It is usually advisable to increase the
size of both suction and discharge pipes at the
pump connections to decrease the loss of head
from friction.
Install piping as straight as possible, avoiding
unnecessary bends. Where necessary, use 45° or
long radius 90° fittings to decrease friction losses.
Make sure that all piping joints are air-tight.
Where flanged joints are used, assure that inside
diameters match properly.
Remove burrs and sharp edges when making up
joints.
Do not “spring” piping when making any
connections.
Provide for pipe expansion when hot fluids are to
be pumped.
Suction Piping
When installing the suction piping, observe
the following precautions (See Figure 13).
The sizing and installation of the suction
piping is extremely important. It must be
selected and installed so that pressure losses
are minimized and sufficient liquid will flow
into the pump when started and operated.
Many NPSH (Net Positive Suction Head)
problems can be directly attributed to
improper suction piping systems.
Suction piping should be short in length, as
direct as possible, and never smaller in
diameter than the pump suction opening. A
minimum of ten (10) pipe diameters between
any elbow or tee and the pump should be
allowed. If a long suction pipe is required, it
should be one or two sizes larger than the
suction opening, depending on its length.
CAUTION:
An elbow should not be used directly
before the suction of a double suction pump if its
plane is parallel to the pump shaft. This can cause
an excessive axial load or NPSH problems in the
pump due to an uneven flow distribution (See
Figure 12). If there is no other choice, the elbow
should have straightening vanes to help evenly
distribute the flow.
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