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Full text of "Modern Mechanical Engineering Vol-I"

252

HEAVY  MACHINERY

be carefully examined, especially the nozzle-box or space behind the nozzles,
which appears to be a favourite place for foreign matter to collect. Any
foreign matter not removed will be blown through by the steam, and may
seriously damage the turbine blading. As a rule, the joints between the
steam-chest and the turbine cylinder are dowelled, in the manner shown
herewith, in order to definitely fix the position of the steam-chest. The
dowels are put in at the joint in order that they can be easily withdrawn or
knocked out when the joint is broken. If the dowels are put in at right
angles to the joint, as in the other sketch, there is a danger of the dowel
being " burned in " by the prolonged action of the heat of the steam, more
particularly when the steam is superheated. On the other hand, great care
should be taken to see that the dowel used in the joint is made an easy tapping
fit after the joint is bolted up tight. If the dowel is too tight, there is a danger

of the joint being forced open at the
dowel-pin, and thus causing serious
steam leakage. A note will be added
later, on the making of joints for
steam-, water-, and oil-pipes.

Assuming, then, that the nozzles
and steam-chest have been carefully
examined and cleaned out and jointed
up to the bottom half of the cylinder,
the next step is to put in position

headed dowel-pins at right angles to joint.   On high-    the   bottom   halves    of   the    Stationary

temperature steam-pipes these dowels are liable to      «.      ,                    r>   r         1 •      •      j           1

"burn in", and have to be drilled out, unless they are    uiapnragmS.       JoeiOre  tillS   IS  UOne  the

made a fairly easy fit initially.                                  outside    Surface    of    the    diaphragms,

which  fit   into   the  grooves   in  the

cylinder, should be carefully rubbed over with a little flake graphite to
prevent them rusting in in course of time. After the diaphragm bottom
halves are in position, the drainage of the cylinder should be tried in order
to see that water, condensed steam, &c., cannot collect in the cylinder,
and not only cause rapid deterioration of the blading, but also be the
cause of the turbine shaft " whipping ", due to the wheels running in
water at the bottom. The effects produced in this way are sometimes
very serious, and have been disastrous. The best way to test the drainage
is to open all drain-cocks and run water from a hose in between the dia-
phragms, and into all pockets where water may lodge. As a rule, the dia-
phragms are so arranged that any water in the cylinder automatically drains
away to the exhaust end, and thence into the condenser. This is accomplished
by the design, or, where necessary, a small hole, say f in. diameter, is drilled
through the bottom of the diaphragms in an axial direction, the hole being
increased at the L.P. stages to | or f in. diameter. A small quantity of
steam blows through this drain hole and keeps the cylinder clear of water,
the loss of steam being quite insignificant. Previous to putting the bearings
in position, the bottom halves should be carefully scraped and bedded on
the journals of the shaft they have to carry. This is done by smearing a

Fig. 7.—Dowel-pins in Joints

i, Dowel-pins drilled radially.   Joint can easily be
split and dowel-pins removed.    2, Holes for hex-