DOUBLE-PORTED, MULTIPLE ADMISSION, AND PISTON VALVES.
_Double-ported Valves._—In large marine engines using considerable
quantities of steam per stroke, and consequently requiring a large
amount of port-opening, the ordinary slide valve would require an undue
amount of travel. To obviate this, each end of the cylinder may have two
steam-ports, both ports opening into one passage as shown, and a
double-ported valve. The double-ported valve is virtually a combination
of two slide valves in a single casting (fig. 23), one valve for the
inner set of steam-ports and the exhaust-port, the other valve being
halved, as it were, for each outer steam-port. The exhaust cavity of
each of the outer “half-valves” having a passage leading to the exhaust
cavity of the inner valve, both the valves exhaust into the central
port, which is widened accordingly. The fig. 23 is merely diagrammatic,
to enable the principle of this valve to be easily borne in mind; the
next drawing (fig. 24) shows in section a double-ported valve as
actually constructed, with the exception that the valve-spindle and its
chamber are omitted in order that the form of the valve may be more
clearly shown. The section is taken in two planes, one upon the
longitudinal centre line of the valve, and the other (the left-hand half
of the valve) a little to the near side of the centre line. Fig. 25
shows a sectional view of a double-ported valve in perspective.
[Illustration: FIG. 23.—Diagram showing Flow of Steam and Exhaust
through a Double-ported Slide Valve.]
[Illustration: FIG. 24.—Vertical Section through a Double-ported Slide
Valve.]
[Illustration: FIG. 25.—Perspective view (in Section) of a Double-ported
Slide Valve.]
Now, any movement given to the “inner valve” is given, of course, to the
outer one, so that if the inner valve be moved so as to give, say, 2
ins. of port-opening, the outer valve gives at the same time another 2
ins. of port-opening; therefore, with a given movement of a
double-ported valve we get double the amount of port-opening that we
should have obtained with the ordinary valve. Hence the travel of the
double-ported valve = (lap + half the two port-openings) × 2; whereas
that of the ordinary valve = (lap + the port-opening required) × 2.
The inner and outer valves of the double-ported construction, acting
simultaneously, have each the same amount of lap; but they give the lead
between them, and the eccentric is set with a linear advance equal to
_one_ of the laps and _one_ of the lead-openings. The distribution of
steam with this valve is similar to that of the ordinary valve whose
distribution-diagram was shown in fig. 20. The different periods are of
the same duration in the example chosen, but the operations of cutting
off steam and of opening exhaust are performed by the edges of the inner
and outer valves instead of by a single edge as in the ordinary valve.
A valve very similar to the double-ported valve, shown in fig. 23, is
the Giddings valve used on the Russell engines and some other automatic
high-speed engines in America. By its peculiar construction, this valve
obviates the necessity of double steam ports, yet it is so much like the
double-ported valve that it may be noticed here rather than under the
head of multiple admission valves, where it might be said to properly
belong. This valve accomplishes exactly what the double-ported valve
does, and in pretty much the same way. The Giddings valve in fig. 26
shows steam being admitted to the right-hand end of the cylinder and
exhausted from the left-hand end. The travel of this valve is, as in the
case of the double-ported valve = (lap + half the port opening required)
× 2. The valve should be set with a linear advance equal to the lap plus
one-half the required lead.
[Illustration: FIG. 26.—The Giddings Valve.]
MULTIPLE ADMISSION VALVES.
In America, the type of engine known as the automatic high-speed engine
is very much used. These engines have a wide range of cut-off, from
almost 0 to ¹⁄₂ stroke, and in order to avoid a long valve and a long
travel and to secure a quick admission and cut-off of the steam,
recourse is had to multiple admission valves. These valves are so
constructed that when one of them is moved from midposition, a distance
equal to, say, ¹⁄₁₆ of an inch more than the steam lap, the width of the
opening for the steam to pass through into the cylinder is ¹⁄₁₆
multiplied by the number of admissions of the valve. Thus, if the valve
is a _double_ admission valve, and it is moved ¹⁄₁₆ plus the steam lap
from midposition, the port is opened a distance equal to 2 × ¹⁄₁₆ = ¹⁄₈
of an inch. Double admission valves are quite common and there are also
a number of quadruple admission valves used. They are made as flat
valves and as piston valves, but all are made with a common object in
view, that is, to secure an early cut-off and quick opening and closing
of the ports with a valve whose length and travel shall be short.
One of the best-known forms of double admission valves is the “Straight
Line” or “Sweet” valve, which was invented by Professor John E. Sweet,
formerly of Cornell University, and first used by him on the Straight
Line engines. This valve is now used by a large number of manufacturers
of automatic high-speed engines. The valve is shown in section in fig.
27, where _a_ is the “cover plate,” and _b_ is the valve, proper. The
valve, _b_, moves back and forth between the cover plate, _a_, and the
valve seat. The cover plate rests on pieces called “distance pieces,”
placed at the sides of the valve. These distance pieces are made of such
a thickness that while the cover plate cannot touch the valve, it is so
close to it as to make a steam tight joint between the plate and the
valve. Therefore, the valve acts as a piston of rectangular
cross-section, moving back and forth between the cover plate and the
valve seat. The result of its peculiar construction is that a balanced
flat valve is secured; a valve which works with very little friction, as
there is no surface upon which the steam can act so as to press the
valve against the valve seat. The cover plate is kept in place by the
pressure of the steam against its back, aided by one or two small
springs inserted between the plate and the steam-chest cover.
[Illustration: FIG. 27.—The Straight Line Valve.]
This valve should be set so that the distance the valve is from
midposition when the piston is at the end of its stroke, or the linear
advance, shall be equal to the steam lap plus one-half the desired lead.
The travel of the valve is equal to twice the sum of the steam lap and
one-half the maximum opening of the port for steam, or 2 × (steam lap +
¹⁄₂ maximum opening of port for steam).
In the case of a quadruple admission valve, such as the Woodbury valve
shown in fig. 28, the travel is equal to twice the sum of the steam lap
and one-fourth the maximum opening of the port for steam, or 2 × (steam
lap + ¹⁄₄ maximum opening of port for steam).
[Illustration: FIG. 28.—The Woodbury Valve.]
The different phases of the movement of a multiple admission valve are
just the same as for the ordinary slide valve, the only difference being
in the suddenness with which the ports are opened and closed.
_Piston Valves._—It may be readily seen that between the ordinary flat
slide valve and the valve-seat considerable friction is set up, because
the steam pressure on the back of the valve is only partly balanced by
the varying pressure in the cylinder steam-passages acting upon small
portions of the area of the “rubbing” or under-side of the valve; but by
making the port-faces cylindrical and making the faces of the valve in
the form of pistons, the pressure is given no surface upon which to
press in the direction of the valve-seat; or, in the case of hollow
piston valves (an example of which will be presently illustrated),
although the steam has access to the interior of the tube, pressure on
parts thereof is balanced by an equal pressure in an opposite direction
upon exactly opposite parts, and therefore is confined to stressing the
valve and has no tendency to force any part of it on to the valve-seat.
In the plain piston valve shown (fig. 29) the faces are formed by the
two valve pistons, which work in a cylinder wherein steam and exhaust
openings are made.
[Illustration: FIG. 29.]
Sometimes the steam-supply enters _between_ the two pistons, as in figs.
30 and 31, exhaust taking place into the spaces at the ends of the
valve, which spaces are connected by passages formed in the valve, which
is tubular, as shown. In such valves the steam-lap must be put upon the
_inner_ edges of the valve pistons, for the steam and exhaust edges have
their positions reversed; and the lead will also be given by the inner
edges.
[Illustration: FIG. 30.]
[Illustration: FIG. 31.—The Ide Valve.]
The eccentrics for a valve with internal steam-supply must be placed 180
degs. in advance of the positions which they would occupy if the valve
were of similar proportions, but with _external_ steam-supply, for the
reason that the internally supplied valve must be moving _up_, for
instance, when the ordinary valve would be moving _down_, and _vice
versa_.
If the valve be an externally supplied valve, but driven from one end of
a two-armed rocking-shaft, the valve gear actuating the other end, as in
American locomotive practice, the eccentrics must, for obvious reasons,
be set 180 degs. in advance of their usual position.
Fig. 30 is given by the kind permission of Messrs. W. Denny & Company,
the well-known Clyde ship-builders, and it possesses many points of
interest, one of which is that the packing in the valve-spindle
stuffing-box below the valve, and not shown in the drawing, is subject
only to the influence of the temperature and pressure of exhaust steam,
instead of to that of the higher temperature and pressure of the supply
steam; this is a matter of importance, having in view the high pressures
of steam which are now used.
Further, it may be seen that the upper piston of the valve is of greater
area than the lower one, and that the high-pressure steam entering
between the two pistons will consequently exert a total pressure upward
upon the larger piston greater in amount than the downward pressure
exerted upon the lower and smaller one; thus the valve has a tendency to
rise, so long as steam is on. This upward tendency balances wholly or in
part the downward tendency due to the weight of the valve, which has not
to be lifted by the valve gear, which may be of a lighter character than
usual, and give less trouble in working, owing to diminution of wear and
tear. The valve-spindle of the ordinary flat valve is sometimes
prolonged (where the valve works vertically) to end in a small piston
exposed to steam pressure, and acting so as to balance the weight of the
valve.
In the “Joy” assistant-cylinder, a balance-piston is provided with
special passages which, in the movement of the valve-spindle, are
brought opposite ports in the assistant-cylinder wall, so that steam is
let in to propel the balance-piston up or down, and thereby assist the
valve gear in each stroke. The balance-piston acts as its own exhaust
valve in sliding over exhaust-ports in the assistant-cylinder wall.
Fig. 31 shows the piston valve used in America on the well-known Ide and
Ideal engines. It is very similar to the valve shown in fig. 30, and has
the same advantages which have been pointed out as belonging to that
valve. As, however, it is principally used on horizontal engines, the
two pistons are made of the same diameter, in order to secure a
perfectly balanced valve.
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