Transactions A.S.M.E.

ALLEN—STEAM-TURBINE BLADING 697
modified by welding the shrouding in groups of two blades.
Fig. 13 shows a group of standard impulse blades for a tworow
wheel of the type used for the high-pressure element of 3600-
rpm condensing turbines up to about 75,000 kw or for 3600-rpm
top-turbine service up to about 25,000 kw. One of the important
features is the adoption of radial planes on the fronts and backs
of the root and shroud sections of the moving blades. In the
short first-row blades, the center portion of the root and shroud
on each side is in a plane passing through the axis of the spindle.
The overhanging edges have corresponding projections on the
root and shroud to reinforce them. The cooperating fit between
pairs of blades is on the central radial-plane surfaces. Note
that between the unwelded shroud sections, a clearance of approximately
0.005 in. is allowed during the cold assembly.
F i q . 14
A s s e m b l e d Sin g l e - R o w I m p u l s e W h e e l
F i q . 13
I n d iv id u a l I m p u l s e B l a d e s; D e s M o in e s
driving force F, and the moment arm L\. The stability factor
without amplification is, therefore, expressed as FeK L/F ,Li.
L im i t i n g C a p a c it y o f C i r c u m f e r e n t ia l - G r o o v e B l a d in g
Fig. 12 shows the high-pressure impulse wheel for the 35,000-
kw 3600-rpm condensing turbine at Des Moines, Iowa. The
first-row blades are IV 2 in- wide, and the steam-port height is
1 in. The steam load is 238 lb per blade at the '/a load point
on the turbine. This condition exists when the first two nozzle
blocks, to which steam is admitted simultaneously, are
operated at full inlet pressure. These blades are of Cyclops 17-A
alloy, with the integral shrouding welded together in groups of
two blades. The second-row impulse blades are shown welded
in groups of three. These second-row blades have since been
The limiting of the number of blades per group to two for hightemperature
service is believed important, as severe bending
stresses may be set up if a greater number of blades are joined
together. These stresses may be caused by the more rapid heating
and cooling of the blades with respect to the massive spindle.
Fig. 14 shows a single-row impulse wheel with the circumferential-groove-type
blading. This impulse wheel represents the
standard high-pressure element of large Allis-Chalmers highpressure
condensing turbines.
A x i a l -S l o t B l a d in g f o r H ig h e s t -C a p a c it y P a r t ia l -A d m is s io n
I m p u l s e W h e e l s
For maximum-capacity top-turbine service, the design analysis
briefly described calls for extremely wide blades. Such blades
have a large circumferential pitch, which makes it undesirable to
leave out a blade for assembly purposes. The centrifugal force
of one of these large blades is such that a practical locking device,
whereby the last blade can be secured in a circumferential groove
of the conventional design does not appear available. An extensive
survey has indicated the necessity for roots of the axialslot
type. Fig. 15 shows the impulse-wheel assembly for the
50,000-kw Northwest top turbine. The blades are inserted in
axial slots which are milled and broached in the spindle body. In
this way, the full complement of blades is inserted in the wheel
without the need of a special entry slot or locking blade.