8.6 Constructional Aspects
307
opposing sense. For instance, a six-stage pump may be divided into two parts with
three rotors in the same sense. After a first group of rotors, the liquid is then collected by a volute and led by a conduit to the suction eye of the first rotor of the
second group. Such pumps are made with an axial split casing and very often, the
connecting conduit is integrated in a casing part (realised by a connection in the cast
piece). In some pumps, more than two groups of rotors are used.
8.6 Constructional Aspects
8.6.1 Rotor
The rotor is mostly cast, exceptionally welded. Small rotors for pure water are manufactured from bronze (CuSn) because of the small allowed wall thickness (minimum 3 mm). Plastics are sometimes applied. Larger rotors are mostly manufactured
from cast iron. Cast iron and bronze may be applied for peripheral speeds up to
about 50 m/s. Beyond this speed, alloyed steels are applied and chromium steel
up to about 90 m/s (for comparison: fans 110 m/s and compressors 450 m/s max.).
The choice of the material is often determined by the characteristics of the liquid
to be pumped, temperature, pressure, presence of solid materials and corrosion resistance. Manufacturing the moulds may be complex with doubly curved blades or
narrow channels. Sometimes the lost wax process has to be applied, or casting must
be performed in dies in order to get a smooth finish. The surface condition is very
important in view of rotor efficiency. It is often impossible to finish the surfaces,
as they are inaccessible to tools. Axial pump blades can be finished with a digital
milling machine.
8.6.2 Stator
The stator of a single-stage single-suction pump mostly consists of a rear part with a
shaft passage and a volute and a front part with an internal suction tube. Single-stage
pumps with double suction often feature a stator with a horizontal split (Fig. 8.19).
Dependent on the fluid pressure, stators are manufactured from cast iron, cast steel
or alloyed cast steel (chrome steel). Forged steel is appropriate for high pressures
and temperatures.
8.6.3 Shaft Sealing
Shaft sealing may be done with a stuffing box or a mechanical seal. In a stuffing
box, packing rings are pressed around the shaft with a gland (Fig. 8.20). A packing
307
opposing sense. For instance, a six-stage pump may be divided into two parts with
three rotors in the same sense. After a first group of rotors, the liquid is then collected by a volute and led by a conduit to the suction eye of the first rotor of the
second group. Such pumps are made with an axial split casing and very often, the
connecting conduit is integrated in a casing part (realised by a connection in the cast
piece). In some pumps, more than two groups of rotors are used.
8.6 Constructional Aspects
8.6.1 Rotor
The rotor is mostly cast, exceptionally welded. Small rotors for pure water are manufactured from bronze (CuSn) because of the small allowed wall thickness (minimum 3 mm). Plastics are sometimes applied. Larger rotors are mostly manufactured
from cast iron. Cast iron and bronze may be applied for peripheral speeds up to
about 50 m/s. Beyond this speed, alloyed steels are applied and chromium steel
up to about 90 m/s (for comparison: fans 110 m/s and compressors 450 m/s max.).
The choice of the material is often determined by the characteristics of the liquid
to be pumped, temperature, pressure, presence of solid materials and corrosion resistance. Manufacturing the moulds may be complex with doubly curved blades or
narrow channels. Sometimes the lost wax process has to be applied, or casting must
be performed in dies in order to get a smooth finish. The surface condition is very
important in view of rotor efficiency. It is often impossible to finish the surfaces,
as they are inaccessible to tools. Axial pump blades can be finished with a digital
milling machine.
8.6.2 Stator
The stator of a single-stage single-suction pump mostly consists of a rear part with a
shaft passage and a volute and a front part with an internal suction tube. Single-stage
pumps with double suction often feature a stator with a horizontal split (Fig. 8.19).
Dependent on the fluid pressure, stators are manufactured from cast iron, cast steel
or alloyed cast steel (chrome steel). Forged steel is appropriate for high pressures
and temperatures.
8.6.3 Shaft Sealing
Shaft sealing may be done with a stuffing box or a mechanical seal. In a stuffing
box, packing rings are pressed around the shaft with a gland (Fig. 8.20). A packing