Page 158 - Centrifugal Pumps Design and Application

136 Centrifugal Pumps: Design and Application

• Incorporate a discharge nozzle to guide the flow from the outer column
to the system pipe. For barrel-mounted pumps, a suction nozzle may
also be located in the discharge head.

The discharge head must house a shaft sealing device suitable for the
maximum pressure the pump can be subjected to. The sealing device is
located in a stuffing box that can be placed either in the discharge stream
for flushing or mounted externally for cooling and flushing. The actual
sealing can be done with packing or a mechanical face seal. A pressure
breakdown bushing, with bleed-back to pump suction, can also be in-
cluded in the sealing device for high pressure applications.
The standard drive coupling for vertical pumps with solid shaft drivers
is of a rigid design, capable of transmitting the maximum torque from the
driver and the combined axial force from hydraulic thrust plus rotating
element weight. The coupling typically incorporates a disc threaded on to
the top end of the column shaft, clamped between the two coupling
halves, that permits adjustment of the impeller setting within the bowls
(see Figure 9-14). For drivers with limited thrust carrying capability, a
thrust bearing must be incorporated into the discharge head design.
For pumps using hollow shaft drivers, torque is transmitted to the top
column shaft or head shaft through a keyed clutch at the top of the motor,
and impeller adjustment is made by a nut seated on top of the clutch (see
Figure 9-3).
Except for the smaller well pumps and barrel-mounted pumps, most
vertical pumps are of a structurally flexible design. This means that the
structural, natural frequency of the first order is of the same magnitude
as the operating speed. A careful analysis must therefore be made of the
discharge head design in relation to its foundation and the connected
driver and system piping to ensure that the combined natural frequency
does not coincide with the pump operating speed. Similarly, deflection
calculations for the unit must be made to ensure that pump alignment is
not impaired when it is subjected to nozzle loads and the liquid filled
weight,

Pump Vibration

The vibration pattern of a vertical pump is an inherent characteristic of
its configuration, manufacture, and physical condition. Vibration results
from factors such as rotating element unbalance, misalignment, loose-
ness in the assembly, bent shafting, or bad driver bearings. Also, the op-