The shaft (2100) is made of Siemens-Martin steel, or in certain cases, of special quality steel. The impellers (2300), shaft
protecting sleeves (5240.1, 5240.2), spacer sleeves (5251, 5210) and the balance device can be supplied in cast iron, bronze, carbon
steel or chrome steel according to the nature of the fluid pumped and operating conditions.
2.1.3. Bearings
The rotor assembly in MML pumps sizes 40 to 150 inclusive runs in two anti-friction bearings; from MML 200 and upwards two
plain bearings are used. The bearing brackets (3500) flanged to the suction and discharge casings are identical in dimensions. The
rotor assembly is afforded a small amount of axial float by the plain bearings and also by the anti-friction bearings, which are NU series
taper lock sleeve cylindrical roller bearings (3220).
All bearings are lubricated with oil, the plain bearings having oil rings. The over-flow hole drilled in the bearing end covers
(3600,3610) provides a means of checking the oil level (see section 4.4.4.) .
The table in section 7, (page 29) gives a summary of the various types of anti-friction bearings fitted in these pumps.
2.1.4. Stuffing boxes
The purpose of stuffing boxes is to prevent high-pressure liquid from escaping through the gap between shaft and casing, or the
entry of air into the pump, if the pump suction is at less than atmospheric pressure.
Whether conventional soft-packed stuffing boxes or mechanical seals in one of the many types available are used, will depend
on the pressure, temperature and nature of the liquid pumped.
2.1.4.1. Soft-packed stuffing-boxes
Soft-packed stuffing boxes rely on rings of soft-packing as the sealing element; successive packing rings are inserted in the
annular space between the stuffing box housing (4510) and the shaft protecting sleeve (5240.1 or 5240.2), and are lightly pressed in
position by the stuffing box gland (4520). The standard stuffing box in fig.3 top left, is quite adequate for sealing clean water under
pressure at temperatures not exceeding 220°F (105 °C). When the temperature exceeds 220 °F, the so-called hot water execution HW,
with hot water stuffing boxes is used (fig. 3). With this type, an end cover (304) is mounted between the bearing bracket (3500) and the
stuffing box housing (4510).
The stuffing box housing (4510) surrounds the shaft protecting sleeve (5240.1 or 5240.2) and can be flushed with cooling water,
thus ensuring that the hot water from the pump is effectively cooled in the narrow annulus up-stream of the packing rings, and its
temperature considerably reduced before it reaches the packing rings. A prerequisite for effective cooling of the water which reaches
the stuffing box is that the packing rings them selves are in good condition and provide an effective seal. The pressure at the stuffing
box is not affected by this gap or annulus.
A part from these standard stuffing boxes, there are SPECIAL TYPES recognizable from the fact that the space for the packing
is extra long, to provide room for an increased number of packing rings, and usually also a lantern ring, which may be located at the
bottom of the box, or amid the packing rings, according to requirements. This lantern ring forms a means of preventing the ingress of
liquid from the interior of the pump; this is effected by drilling a passage in the housing (4510) in register with the ring, and passing
through it a sealing liquid (e. g. water from the discharge side of the pump) ; in certain cases, leakage fluid may be led off by means of
this lantern ring.
If the temperature of the liquid handled by pumps with special stuffing boxes exceeds 220 °F, a cooling chamber 301 (fig. 3) is
fitted up stream of the stuffing box to cool the liquid flowing from pump to stuffing box, the action being similar to that of the HW
execution described above.
Fig.3 illustrates the various special stuffing boxes V, VSH and VSM with or without cooling chamber (301) for a variety of liquids
pumped.
The maintenance of soft-packed stuffing boxes is described in section 4. 4. 2.
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