Page 380 - Centrifugal Pumps 2E

Shaft Design and Axial Thrust 345

Double-Suction Single-Stage Pumps

Theoretically double-suction pumps with double-suction impellers are
in axial balance due to the symmetrical mechanical arrangement. If the
flow approach to the impeller is also symmetrical, there is no need for
thrust bearings. However, as minor mechanical or casting variations will
cause axial unbalance, small capacity thrust bearings are normally sup-
plied,
If symmetry of this type of pump is violated by relocating the impeller
away from volute center, the developed pressure above the impeller rings
on either side of the impeller will not be the same and substantial axial
thrust will be developed. The amount of thrust will vary directly with
axial displacement.

Single-Suction Single-Stage Overhung Pumps

Single-suction closed or open impellers have different geometries on
either side of the impeller and thus develop substantial axial thrust. Cal-
culating this thrust requires analysis of the different design variations
normally used in commercial pumps.

Closed impeller with rings on both sides and balancing holes in back
shroud. Balance holes through the impeller allow liquid leaking across
the wear ring clearance to flow back to the suction side of the impeller.
This aids in reducing thrust as the pressure behind the balance holes will
be less than the pressure above the wear ring but greater than the suction
pressure. Many simplified axial thrust calculations assume the pressure
behind the balance holes to be the same as suction pressure; however,
testing on small process pumps has shown this pressure to range from 2%
to 10% pump differential pressure, depending on balance hole geometry.
The calculation method shown in Figure 16-5 assumes liberal clearance
holes with the hole area being a minimum of eight times the area between
the wear rings. In the event that jetting action of leakage behind the back
ring clearance creates a wear pattern on the back cover, it is often neces-
sary to install a hardened ring on the pump cover,

Closed impeller with front ring only. All things being symmetrical, this
impeller is axially balanced from the impeller outside diameter down to
the suction ring diameter. Below this, there is an unbalance force acting
on the area between the suction ring and the shaft sleeve. For the calcula-
tion method see Figure 16-6.