8.6 Constructional Aspects
309
the pressure side of the pump. A mechanical seal consists of two rings with polished
surfaces sliding over each other, one of them being linked to the stator, the other
one to the rotor, pressed together by a spring (applied in the pump of Fig. 8.19). The
rings are normally manufactured from metal, but feature a coating. One ring has a
soft coating (e.g. carbon), the other one a hard one (e.g. silicon carbide or tungsten
carbide). A liquid film should be formed between the rings for lubrication. This liquid mostly evaporates partly. A mechanical seal is not completely tight. Mechanical
seals are applied for sliding speeds up to 60 m/s and pressures up to 450 bar.
8.6.4 Bearings
Rotational speed, load, required life and required rigidity determine the choice of
the type of bearing. Ball bearings and roller bearings of various types are applied,
always with one of the bearings for balancing the axial force (e.g. an angular contact
ball bearing or tapered roller bearing). Oil-lubricated sliding bearings are applied in
case of high rotational speeds, large dimensions and heavy loads.
8.6.5 Axial Force Balancing with Single-Stage Pumps
Rotors with one-sided suction exert an axial force on the shaft, oriented to the suction side. If the force is not very significant, it may be balanced by a ball or a roller
bearing, in other cases by a thrust bearing. Another solution consists in generating a
hydrostatic balancing force, as explained hereafter and in the next section. Fig. 8.21
shows that the rotor is subjected to rotor outlet pressure ( p p ) on the back of the rotor
disc and on the front of the shroud. In the rotor eye, the pressure is suction pressure
( p s ). The resulting axial force is approximately
(8.23)
with A e the eye area and A s the cross section area of the shaft, if the shaft passes
through the suction eye. In case of an overhung rotor, (8.23) becomes:
(8.24)
Both expressions are approximate. The pressure on the back of the rotor is not constant. Dragged by the rotor disc, the liquid outside the rotor is subjected to centrifugal force, which generates a pressure gradient. By the turning of the flow within the
suction eye, suction pressure is not constant, either.
When two rotors are mounted onto the same shaft, with their suction eyes directed into opposite senses, the axial force is completely neutralised when both
rotors operate in parallel. With rotors operating in series, a significant reduction
,
=
−
−
ax
e
s
p
s
F
(A A )(p
p )
.
=
−
−
ax
e
s
p
e s
F
(A A )p
A p
309
the pressure side of the pump. A mechanical seal consists of two rings with polished
surfaces sliding over each other, one of them being linked to the stator, the other
one to the rotor, pressed together by a spring (applied in the pump of Fig. 8.19). The
rings are normally manufactured from metal, but feature a coating. One ring has a
soft coating (e.g. carbon), the other one a hard one (e.g. silicon carbide or tungsten
carbide). A liquid film should be formed between the rings for lubrication. This liquid mostly evaporates partly. A mechanical seal is not completely tight. Mechanical
seals are applied for sliding speeds up to 60 m/s and pressures up to 450 bar.
8.6.4 Bearings
Rotational speed, load, required life and required rigidity determine the choice of
the type of bearing. Ball bearings and roller bearings of various types are applied,
always with one of the bearings for balancing the axial force (e.g. an angular contact
ball bearing or tapered roller bearing). Oil-lubricated sliding bearings are applied in
case of high rotational speeds, large dimensions and heavy loads.
8.6.5 Axial Force Balancing with Single-Stage Pumps
Rotors with one-sided suction exert an axial force on the shaft, oriented to the suction side. If the force is not very significant, it may be balanced by a ball or a roller
bearing, in other cases by a thrust bearing. Another solution consists in generating a
hydrostatic balancing force, as explained hereafter and in the next section. Fig. 8.21
shows that the rotor is subjected to rotor outlet pressure ( p p ) on the back of the rotor
disc and on the front of the shroud. In the rotor eye, the pressure is suction pressure
( p s ). The resulting axial force is approximately
(8.23)
with A e the eye area and A s the cross section area of the shaft, if the shaft passes
through the suction eye. In case of an overhung rotor, (8.23) becomes:
(8.24)
Both expressions are approximate. The pressure on the back of the rotor is not constant. Dragged by the rotor disc, the liquid outside the rotor is subjected to centrifugal force, which generates a pressure gradient. By the turning of the flow within the
suction eye, suction pressure is not constant, either.
When two rotors are mounted onto the same shaft, with their suction eyes directed into opposite senses, the axial force is completely neutralised when both
rotors operate in parallel. With rotors operating in series, a significant reduction
,
=
−
−
ax
e
s
p
s
F
(A A )(p
p )
.
=
−
−
ax
e
s
p
e s
F
(A A )p
A p