Page 250 - Know and Understand Centrifugal Pumps

Avoiding Wear in Centrifugal Pumps

resistant materials. Materials such as carbon steel, high chrome iron,
harden stainless steel or hard coatings like ceramic or tungsten alloy are
some of the most used.

Corrosion

Corrosion is caused by a chemical or electrochemical attack on the
surface of the metals. It is increased when there is an increase in
temperature and/or presence of oxygen in the fluid or the surface of
the fluid.
We can aggravate the corrosion effect if misaligned parts have relative
movement, such as loose fit bearings or rapid changes in the system.
Cavitation, erosion and high fluid velocity advance the corrosion
process.
Cast Iron is a widely used material for centrifugal pump housings. It is
used when the fluid PH is 6 or higher (not acid). Cast Iron corrodes
and forms a protective coating on the surface of the metal. This
graphitized surface protects the metal fi-om further corrosion as long as
the coating is intact. High velocity fluids, cavitation, metal to metal
contact and erosion can affect this protective coating.
If corrosion exists, the pump-wet parts can be changed for other
materials such as stainless steel or composite material. Impellers can be
replaced by bronze cast impellers or other materials.

Wear rings

Wear rings provide for a close running, renewable clearance, which
reduces the amount of liquid leaking from the high pressure zones to
the low pressure zones in the pump. They are commonly fitted in the
pump casing and on the impeller (Figure 16-1, next page).

These wear rings are lubricated with the fluid being pumped.
Eventually they will wear. Tolerances open and more liquid passes from
the discharge end back to the suction end of the pump. The rate of
wear is a function of the pumped liquid’s lubricity. When the wear is
excessive, the pump suffers degradation in its performance. This is
particularly true with small pumps running at high speed. The strict
tolerance in the replaceable wear rings governs the efficiency of the
pump. When the pump goes to the shop, these wear rings should be
changed.
You can expect the pump to loose 1.5 to 2% efficiency points for each
one thousandths (0.001 inch) wear in a wear ring beyond the original