Page 153 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Rarely do design engineers request the L/D factor in their quotes.
Some engineers don’t know they have the option. Most pumps are
bought based on price, and because a high deflection resistance (low
L/D Factor) indicates a larger diameter shaft with oversized bearings;
these type pumps don’t normally win a competitive bid process.
If you suspect, or know, that you have a deflected shaft, or know that
standard operating procedure in your plant requires controlling the
flow in the pipes by opening and closing valves, then you have three
options to reduce shaft deflection:
Use a larger diameter shaft.
rn Use a shorter shaft (this may affect the motor mounts, and/or
piping mounts).

rn Change the shaft metallurgy (this will change the elasticity modulus
and may even start a round of galvanic corrosion).
Increasing the shaft diameter is the most logical solution. This can be
done with some pump models by simply replacing sleeved shafts with
solid shafts, or by increasing the diameter of the solid shaft with a small
modification to the seal chamber bore. With the pump disassembled on
the shop table, the mechanic can identifjr the source of the problem in
the pump.

Signs of shaft deflection
~~
Most pumps have tight tolerances in the following rotary elements:
rn The OD of the blades on open and semi open impellers.
rn The wear bands on pumps with enclosed impellers.

rn The shaft under the restriction bushing at the bottom of the
stuffing box or seal chamber.
rn The OD of the posterior end of the internally mounted mechanical
seal.
These tight tolerance rotary elements have corresponding tight
tolerance stationary elements. These are:
rn The internal volute wall and/or back plate on pumps with open and
semi open impellers.

Stationary wear band bores on enclosed impellers.
rn The restriction bore at the bottom of the stuffing box or seal
chamber where the shaft passes through.