THE THROTTLE-VALVE AND STEAM PIPES.
QUESTION 155. _How is the steam admitted to and the supply regulated or
shut off from the cylinders?_
_Answer._ By a valve, _h_, fig. 41, called a _throttle-valve_, which is
usually placed at the end of the pipe _I_, near the top of the dome.
Throttle-valves are sometimes placed in the smoke-box at the front end,
_n_, fig. 41, of the dry-pipe. Until within a few years they consisted
of plain slide-valves which covered openings similar in form to the
steam-ports, but smaller in size. The pressure on such valves is of
course greatest when there is no steam underneath, which is the case
when the valves are closed. It is then very difficult to open them, and
as it is important that the supply of steam admitted to the cylinders
when the locomotive is started should be easily regulated, such valves
are objectionable, and therefore the form has been introduced which is
illustrated in fig. 41, and also on a larger scale in fig. 85, which
represents a longitudinal section of the throttle-pipe and valve. This
is what is called a _double-poppet valve_, and consists of two circular
discs, _a_ and _b_, which cover two corresponding openings in the end
of the pipe _I_. When these discs are raised up, as shown in fig. 85,
steam flows in around their edges, as represented by the darts. It will
be observed that the steam pressure in the boiler comes on top of the
disc _a_ and against the under side of _b_. The pressure on the one
thus neutralizes or balances that on the other. If the two discs were
of the same size, the pressure of the one would be exactly the same
as on the other; but as they are joined together and are made to fit
steam-tight on their seats by what are called _countersunk joints_,
their diameters must be somewhat larger than the openings they cover.
The only practicable way, therefore, by which the lower disc _b_ can
be introduced into the end _h_ of the pipe _I_ so as to cover the
lower opening is through the upper opening _a_. For this reason the
lower disc must be made smaller than the upper one, and therefore
the pressure on the upper one, being in proportion to its size, has
a constant tendency to close the valve. As it is of the greatest
importance that a throttle-valve should remain closed after steam is
shut off, and never be opened at any time accidentally, the arrangement
described accomplishes just what is needed--that is, makes the valve
work comparatively easily, and at the same time keeps it closed after
the steam has been shut off.
[Illustration: Fig. 85. Scale ³⁄₄ in. = 1 foot.]
[Illustration: Fig. 86. Scale ³⁄₄ in. = 1 foot.]
QUESTION 156. _How is the valve opened and closed?_
_Answer._ By a lever, _O′ O_, called a _throttle-lever_, figs. 85,
86[41] and 71. This lever is connected by a rod, _d_, called the
_throttle-stem_, with a bell crank, _i_, the other arm of which works
the rod _c_, to which the throttle-valve is attached. The rod _d_
works through a steam-tight stuffing-box, _f_, in the back end of the
boiler. The end of the throttle-lever is attached to two links, _g_,
fig. 86, which are fastened by a pin to the stud _h_. These links have
a slight vibratory motion, which enables the pin _k_, by which the
lever _O′ O_, is fastened to the rod _d_, to move in a straight line,
which is necessary in order that the rod _d_ may work steam-tight in
the stuffing-box, _f_. The throttle-lever has a latch, _l_, which gears
into a curved rack, _n_, so as to hold the lever and valve in any
required position. This latch is operated by a trigger, _m_. Various
other devices are used to fasten the throttle-lever and thus hold it in
any position required.
[41] Fig. 86 is a plan, showing the _top_ of the valve and lever.
QUESTION 157. _How are the steam pipes constructed?_
_Answer._ The steam, after it is admitted by the throttle-valve, as
was explained in answer to Question 155, passes into the throttle-pipe
_I_ and the dry-pipe _m m_, fig. 41. At the front end of the dry-pipe
a pipe, _n_, figs. 40 and 41, which divides into two branches like the
top of the letter ~T~ and is therefore called a T-pipe, is attached.
The steam-pipes _o_, _o_, fig. 40, are connected to each of the two
branches of the T-pipe at one end and to the cylinders at the other.[42]
[42] In fig. 40 the right-hand side represents a section through the
steam-pipe _o o_, and the left a section through the exhaust pipe _e
e_.
[Illustration: Fig. 87. Scale 1¹⁄₂ in. = 1 foot.]
[Illustration: Fig. 88. Scale 1¹⁄₂ in. = 1 foot.]
These pipes, being in the smoke-box, are exposed to great changes of
temperature, and are therefore subjected to expansion by heat and
contraction by cold. The joints are therefore constantly subject to
disturbance by the contraction and expansion of the pipes and so are
difficult to keep tight. It is also practically impossible to construct
the boiler, the cylinders and the pipes with perfect accuracy, and
therefore a small amount of adjustability and flexibility is necessary
in the joints of the pipes. If, for example, the opening _x_ in the
cylinder, fig. 40, were either too near or too far from the cylinder
of the engine, it would be necessary to move the end of the pipe _o_
either to the right or to the left in order to connect it with _x_.
If the joint of the upper end of the steam-pipe were attached to the
T-pipe with a flat joint like that shown at _a b_, fig. 87, it would
be impossible to move the lower end of the steam-pipe either to the
right or to the left without disturbing the joint and causing it to
leak. For this reason these pipes are connected with what are called
_ball joints_, fig. 88, that is, the end _a b_ of one of the pipes is
turned into the form of a part of a sphere,[43] and the other end into
a corresponding concave form. It is known that a sphere will fit into a
corresponding socket in any position; for example, an acorn in its cup
or the bones at the hip or shoulder joints. If, therefore, the pipes
are joined with such spherical or _ball-joints_ as they are called,
the lower end can be moved sideways several inches either way, and the
joint will still be steam-tight if it is then firmly bolted together.
Even after it is bolted together it will have so much flexibility that
the expansion and contraction of the pipes will not cause it to leak.
[43] The dotted lines indicate what would be the form of the sphere
if the pipe was solid instead of hollow.
There is, however, still another difficulty. Although the lower end of
the pipe, _o o_, fig. 40, can, with a ball-joint above, be moved in any
direction horizontally, yet if the pipe is too long or too short it
is obvious such a joint will not permit it to be moved up or down. A
joint with a flat surface, like that shown in fig. 87, would, however,
permit such motion in the pipe without leaking. If, for example, the
steam-pipe were ¹⁄₈ of an inch too short, it might be drawn down that
distance, and if the upper joint were then screwed up it would still
be steam-tight. In order, then, to get both vertical and lateral
flexibility in the joints of the steam-pipes, a ring, _a b_, fig. 89,
is interposed between the pipes. One side of this ring is spherical and
the other flat, so that the pipes can move either around the spherical
part or slip up or down or sideways on the flat surface of the ring.
In this way the pipes are flexible and adjustable in every direction,
and for all kinds of motion caused by expansion, or which may be needed
when the parts are put together. Sometimes the joints at one end only
of the steam-pipes are made in this way, and the other is connected
with a simple ball-joint.
[Illustration: Fig. 89. Scale 1¹⁄₂ in. = 1 foot.]
In designing these joints their form should be drawn with a radius, _c
d_, fig. 88, from one centre, _c_, so that the surface of the joint
will form a part of a sphere. If they are drawn from two centres, as
is sometimes done, it is obvious that the surface of the joint will
not be a part of a sphere, and therefore will not have the requisite
flexibility. The surfaces of the joints are carefully turned to the
proper form, and then made steam-tight by scraping or grinding them
with emery and oil, and the pipes are then fastened together with
bolts, _g_, _g_, fig. 89, and flanges, _f_, _f_, cast on the pipes.
QUESTION 158. _How are the exhaust pipes constructed?_
[Illustration: Fig. 90. Scale 1¹⁄₂ in. = 1 foot.]
_Answer._ They are made of cast iron. When two nozzles are used they
are generally cast together, as shown in fig. 90. When only one is
used, the form of the pipes resembles somewhat that of an inverted
letter ~⅄~, as shown in fig. 91, so as to cover the two openings which
connect with the cylinders. The tops of these pipes have rings or
bushings, _a a_, fitted into them, which are held by set screws, _b_,
so that they can easily be removed and others with larger or smaller
openings be substituted. If the openings in the exhaust-nozzles are
small, the steam must be discharged at a higher rate of speed, in
order to exhaust all that is in the cylinders, than if the blast
orifices are larger. Therefore, if the latter are reduced in size, the
draft becomes more violent, but at the same time the _back-pressure_
in the cylinder (which will be explained hereafter) is increased. It
therefore becomes necessary to adjust the size of the blast orifices
with the greatest care, so as to have them just small enough to
produce the required draft and yet leave them as large as possible,
so as to reduce the back-pressure. For these reasons what are called
_variable exhausts_ are sometimes used. In these the blast orifice can
be increased or diminished at pleasure, and thus regulated to suit the
conditions under which an engine is working. A great variety of such
devices has been used, but now nearly all have been abandoned for the
simpler arrangement described, which is not variable when the engine is
working.
[Illustration: Fig. 91. Scale 1¹⁄₂ in. = 1 foot.]