8 Pumps
312
8.7.1 Borehole Pumps
The main requirement is a small external diameter in order to lower the pump into
a borehole pipe. Therefore, the pump is always of the multistage type, even with
relatively low head. Mixed-flow stages according to Fig. 8.12 are connected in series. The pump may be driven by a motor mounted on top of the borehole pipe,
through a long shaft, borne in rubber bearings, lubricated and cooled by the pumped
liquid. When this liquid contains too many solid particles, the bearings may be
mounted in a separate tube, filled with pressurized water or oil. Mostly, the motor is
submersible, connected directly to the pump. The motor is traditionally a wet rotor
squirrel cage motor (see below: canned pump). The smallest diameter amounts to
about 60 mm.
8.7.2 High-Pressure Pumps
High-pressure pumps are used, for instance, as boiler feed pumps (pressure up to
300 bar) or in chemical plants. Avoiding leakage of ring-section pumps, composed
of rings tightened with external bolts may then be difficult, especially when the
fluid is also hot. For such applications, the ring sections are often mounted in a
second cylindrical tube casing. Inlet and outlet parts are connected to the envelope
of the second casing and the rings are pressed by end plates fitted to the cylindrical
casing. These pumps may be opened without disconnecting the pipes. Another possibility for a high pressure pump is an axially split casing.
8.7.3 Sealless Pumps: Circulation Pumps, Chemical Pumps
Circulation pumps are used with steam generators with forced circulation and domestic heating boilers. The head is generally low, but pressure and temperature may
be very high. Pumps with the shaft passing to the exterior and conventional shaft
seals may lead to large leakage. With some applications as circulation pumps for
domestic use and chemical pumps, leakage is just not allowed. Sealless pumps are
commonly applied, with liquid penetrating around the rotor of the driving electric
motor (Fig. 8.23). The traditional motor is a squirrel cage induction motor with stator windings isolated from the rotor with a non-magnetic can. The pumped liquid
lubricates the sliding bearings. Due to the can, the gap between the stator winding
and the rotor is rather large, which impairs efficiency. A modern evolution is application of permanent-magnet synchronous motors. These have higher efficiency as
the gap size is then not very critical.
Another kind of sealless pump is realised with a magnetic coupling formed by
permanent magnets on both ends of a split shaft (Fig. 8.24). Canned motor pumps
and magnetic drive pumps are often applied for pumping chemicals, as complete
avoidance of leakage is then necessary.
312
8.7.1 Borehole Pumps
The main requirement is a small external diameter in order to lower the pump into
a borehole pipe. Therefore, the pump is always of the multistage type, even with
relatively low head. Mixed-flow stages according to Fig. 8.12 are connected in series. The pump may be driven by a motor mounted on top of the borehole pipe,
through a long shaft, borne in rubber bearings, lubricated and cooled by the pumped
liquid. When this liquid contains too many solid particles, the bearings may be
mounted in a separate tube, filled with pressurized water or oil. Mostly, the motor is
submersible, connected directly to the pump. The motor is traditionally a wet rotor
squirrel cage motor (see below: canned pump). The smallest diameter amounts to
about 60 mm.
8.7.2 High-Pressure Pumps
High-pressure pumps are used, for instance, as boiler feed pumps (pressure up to
300 bar) or in chemical plants. Avoiding leakage of ring-section pumps, composed
of rings tightened with external bolts may then be difficult, especially when the
fluid is also hot. For such applications, the ring sections are often mounted in a
second cylindrical tube casing. Inlet and outlet parts are connected to the envelope
of the second casing and the rings are pressed by end plates fitted to the cylindrical
casing. These pumps may be opened without disconnecting the pipes. Another possibility for a high pressure pump is an axially split casing.
8.7.3 Sealless Pumps: Circulation Pumps, Chemical Pumps
Circulation pumps are used with steam generators with forced circulation and domestic heating boilers. The head is generally low, but pressure and temperature may
be very high. Pumps with the shaft passing to the exterior and conventional shaft
seals may lead to large leakage. With some applications as circulation pumps for
domestic use and chemical pumps, leakage is just not allowed. Sealless pumps are
commonly applied, with liquid penetrating around the rotor of the driving electric
motor (Fig. 8.23). The traditional motor is a squirrel cage induction motor with stator windings isolated from the rotor with a non-magnetic can. The pumped liquid
lubricates the sliding bearings. Due to the can, the gap between the stator winding
and the rotor is rather large, which impairs efficiency. A modern evolution is application of permanent-magnet synchronous motors. These have higher efficiency as
the gap size is then not very critical.
Another kind of sealless pump is realised with a magnetic coupling formed by
permanent magnets on both ends of a split shaft (Fig. 8.24). Canned motor pumps
and magnetic drive pumps are often applied for pumping chemicals, as complete
avoidance of leakage is then necessary.