A Catechism of the Steam Engine - John Bourne
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468. _Q._--What expedient did you adopt to improve the vacuum in the engine
to which you have referred?
_A._--I put blocks of wood on the air pump piston, which at the end of its
stroke projected between the valve plates and forced the water out. I also
introduced a cock of water at each end of the pump between the valve
plates, to insure the presence of water at each end of the pump to force
the air out. With these ameliorations the pump worked steadily, and the
vacuum obtained became as good as in the old pump. I had previously
introduced an injection cock into each end of the air pump in steam
vessels, from which I had obtained advantageous results; and in all
horizontal air pumps I would recommend the piston and valve plates to be so
constructed that the whole of the water will be expressed by the piston. I
would also recommend an injection cock to be introduced at each end of the
pump.
PUMPS, COCKS, AND PIPES.
469. _Q._--Will you explain the arrangement of the feed pump?
_A._--In steam vessels, the feed pump plunger is generally of brass, and
the barrel of the pump is sometimes of brass, but generally of cast iron.
There should be a considerable clearance between the bottom of the plunger
and the bottom of the barrel, as otherwise the bottom of the barrel may be
knocked out, should coal dust or any other foreign substance gain
admission, as it probably would do if the injection water were drawn at any
time from the bilge of the vessel, as is usually done if the vessel springs
a leak. The valves of the feed pump in marine engines are generally of the
spindle kind, and are most conveniently arranged in a chest, which may be
attached in any accessible position to the side of the hot well. There are
two nozzles upon this chest, of which the lower one leads to the pump, and
the upper one to the boiler. The pipe leading to the pump is a suction pipe
when the plunger ascends, and a forcing pipe when the plunger descends. The
plunger in ascending draws the water out of the hot well through the lowest
of the valves, and in descending forces it through the centre valve into
the space above it, which communicates with the feed pipe. Should the feed
cock be shut so as to prevent any feed water from passing through it, the
water will raise the topmost valve, which is loaded to a pressure
considerably above the pressure of the steam, and escape into the hot well.
This arrangement is neater and less expensive than that of having a
separate loaded valve on the feed pipe with an overflow through the ship's
side, as is the more usual practice.
470. _Q._--Will you describe what precautions are to be observed in the
construction of the cocks used in engines?
_A._--All the cocks about an engine should be provided with bottoms and
stuffing boxes, and reliance should never be placed upon a single bolt
passing through a bottom washer for keeping the plug in its place, in the
case of any cock communicating with the boiler; for a great strain is
thrown upon that bolt if the pressure of the steam be high, and if the plug
be made with much taper; and should the bolt break, or the threads strip,
the plug will fly out, and persons standing near may be scalded to death.
In large cocks, it appears the preferable plan to cast the bottoms in; and
the metal of which all the cocks about a marine engine are made, should be
of the same quality as that used in the composition of the brasses, and
should be without lead, or other deteriorating material. In some cases the
bottoms of cocks are burnt in with hard solder, but this method cannot be
depended upon, as the solder is softened and wasted away by the hot salt
water, and in time the bottom leaks, or is forced out. The stuffing box of
cocks should be made of adequate depth, and the gland should be secured by
means of four strong copper bolts. The taper of blow-off cocks is an
important element in their construction; as, if the taper be too great, the
plugs will have a continual tendency to rise, which, if the packing be
slack, will enable grit to get between the faces, while, if the taper be
too little, the plug will be liable to jam, and a few times grinding will
sink it so far through the shell that the waterways will no longer
correspond. One eighth of an inch deviation from the perpendicular for
every inch in height, is a common angle for the side of the cock, which
corresponds with one quarter of an inch difference of diameter in an inch
of height; but perhaps a somewhat greater taper than this, or one third of
an inch difference in diameter for every inch of height, is a preferable
proportion. The bottom of the plug must be always kept a small distance
above the bottom of the shell, and an adequate surface must be left above
and below the waterway to prevent leakage. Cocks formed according to these
directions will be found to operate satisfactorily in practice, while they
will occasion perpetual trouble if there be any malformation.
471. _Q._--What is the best arrangement and configuration of the blow-off
cocks?
_A._--The blow-off cocks of a boiler are generally placed some distance
from the boiler; but it appears preferable that they should be placed quite
close to it, as there are no means of shutting off the water from the pipe
between the blow-off cock and the boiler, should fracture or leakage there
arise. Every boiler must be furnished with a blow-off cock of its own,
independently of the main blow-off cocks on the ship's sides, so that the
boilers may be blown off separately, and may be shut off from one another.
The preferable arrangement appears to be, to cast upon each blow-off cock a
bend for attaching the cock to the bottom of the boiler, and the plug
should stand about an inch in advance of the front of the boiler, so that
it may be removed, or re-ground, with facility. The general arrangement of
the blow-off pipes is to run a main blow-off pipe beneath the floor plates,
across the ship, at the end of the engines, and into this pipe to lead a
separate pipe, furnished with a cock, from each boiler. The main blow-off
pipe, where it penetrates the ship's side, is furnished with a cock: and in
modern steam vessels Kingston's valves are also used, which consist of a
spindle or plate valve, fitted to the exterior of the ship, so that if the
internal pipe or cock breaks, the external valve will still be operative.
Some expedient of this kind is almost necessary, as the blow-off cocks
require occasional regrinding, and the sea cocks cannot be re-ground
without putting the vessel into dock, except by the use of Kingston's
valves, or some equivalent expedient.
472. Q.--What is the proper construction and situation of the injection
cocks, and waste water valves?
A.--The sea injection cocks are usually made in the same fashion as the sea
blow-off cocks, and of about the same size, or rather larger. The injection
water is generally admitted to the condenser by means of a slide valve, but
a cock appears to be preferable, as it is more easily opened, and has not
any disposition to shut of its own accord. In paddle vessels the sea
injection pipes should be put through the ship's sides in advance of the
paddles, so that the water drawn in may not be injuriously charged with
air. The waste water pipe passing from the hot well through the vessel's
side is provided with a stop valve, called the discharge valve, which is
usually made of the spindle kind, so as to open when the water coming from
the air pump presses against it. In some cases this valve is a sluice
valve, but the hot well is then almost sure to be split, if the engine be
set on without the valve having been opened. The opening of the waste water
pipe should always be above the load water line, as it will otherwise be
difficult to prevent leakage through the engine into the ship when the
vessel is lying in harbor.
473. Q.--What is the best arrangement of gauge cocks and glass gauges?
A.--Gauge cocks are generally very inartificially made, and occasion
needless annoyance. They are rarely made with bottoms, or with stuffing
boxes, and are consequently, for the most part, adorned with stalactites of
salt after a short period of service. The water discharged from them, too,
from the want of a proper conduit, disfigures the front of the boiler, and
adds to the corrosion in the ash pits. It would be preferable to combine
the gauge cocks appertaining to each boiler into a single upright tube,
connected suitably with the boiler, and the water flowing from them could
be directed downward into a funnel tube communicating with the bilge. The
cocks of the glass tubes, as well as of the gauge cocks, should be
furnished with stuffing boxes and with bottoms, unless the water enters
through the bottom of the plug, which in gauge cocks is sometimes the case.
The glass gauge tubes should always be fitted with a cock at each neck
communicating with the boiler, so that the water and steam may be shut off
if the tube breaks; and the cocks should be so made as to admit of the
tubes being blown through with steam to clear them, as in muddy water they
will become so soiled that the water cannot be seen. The gauge cocks
frequently have pipes running up within the boiler, to the end that a high
water level may be made consistent with an easily accessible position of
the gauge cocks themselves. With the glass tubes, however, this species of
arrangement is not possible, and the glass tubes must always be placed in
the position of the water level.
474. Q.--What is the proper material of the pipes in steam vessels?
A.--Most of the pipes of marine engines should be made of copper. The steam
pipes may be of cast iron, if made very strong, but the waste water pipes
should be of copper. Cast iron blow-off pipes have in some cases been
employed, but they are liable to fracture, and are dangerous. The blow-off
and feed pipes should be of copper, but the waste steam pipe may be of
galvanized iron. Every pipe passing through the ship's side, and every pipe
fixed at both ends, and liable to be heated and cooled, should be furnished
with a faucet or expansive joint; and in the case of the cast iron pipes,
the part of the pipe fitting into the faucet should be turned. In the
distribution of the faucets of the pipes exposed to pressure, care must be
taken that they be so placed that the parts of the pipe cannot be forced
asunder, or turned round by the strain, as serious accidents have occurred
from the neglect of this precaution.
475. _Q._--What is the best mode of making pipes tight where they penetrate
the ship's side?
_A._--In wooden vessels the pipes where they pierce the ship's side, should
be made tight, as follows:--the hole being cut, a short piece of lead pipe,
with a broad flange at one end, should be fitted into it, the place having
been previously smeared with white lead, and the pipe should then be beaten
on the inside, until it comes into close contact all around with the wood.
A loose flange should next be slipped over the projecting end of the lead
pipe, to which it should be soldered, and the flanges should both be nailed
to the timber with scupper nails, white lead having been previously spread
underneath. This method of procedure, it is clear, prevents the possibility
of leakage down through the timbers; and all, therefore, that has to be
guarded against after this precaution, is to prevent leakage into the ship.
To accomplish this object, let the pipe which it is desired to attach be
put through the leaden hause, and let the space between the pipe and the
lead be packed with gasket and white lead, to which a little olive oil has
been added. The pipe must have a flange upon it to close the hole in the
ship's side; the packing must then be driven in from the outside, and be
kept in by means of a gland secured with bolts passing through the ship's
side. If the pipe is below the water line the gland must be of brass, but
for the waste water pipe a cast iron gland will answer. This method of
securing pipes penetrating the side, however, though the best for wooden
vessels, will, it is clear, fail to apply to iron ones. In the case of iron
vessels, it appears to be the best practice to attach a short iron nozzle,
projecting inward from the skin, for the attachment of every pipe below the
water line, as the copper or brass would waste the iron of the skin if the
attachment were made in the usual way.
DETAILS OF THE SCREW AND SCREW SHAFT.
476. _Q._--What is the best method of fixing the screw upon the shaft?
_A._--The best way is to cut two large grooves in the shaft coming up to a
square end, and two corresponding grooves or key seats in the screw boss
opposite the arms. Fit into the grooves on the shaft keys with heads, the
length of which is equal to half the depth of the boss, and with the ends
of the keys bearing against the ends of the grooves in the shaft. Then ship
on the propeller, and drive other keys of an equal length from the other
side of the boss, so that the points of the keys will nearly meet in the
middle; next burr up the edge of the grooves upon the heads of the keys, to
prevent them from working back; and finally tap a bolt into the side of the
boss to penetrate the shaft. Propellers so fitted will never get slack.
477. _Q._--What is the best way of fitting in the screw pipe at the stern?
_A._--It should have projecting rings, which should be turned; and cast
iron pieces with holes in them, bored out to the sizes of these rings,
should be secured to the stern frames, and the pipe be then shipped through
all. Before this is done, however, the stern post must be bored out by a
template to fit the pipe, and the pipe is to be secured at the end to the
stern post either by a great external nut of cast iron, or by bolts passing
through the stern post and through lugs on the pipe. The pipe should be
bored throughout its entire length, and the shaft should be turned so as to
afford a very long bearing which will prevent rapid wear.
478. _Q._--How is the hole formed in the deadwood of the ship in which the
screw works?
_A._--A great frame of malleable iron, the size of the hole, is first set
up, and the plating of the ship is brought to the edge of this hole, and is
riveted through the frame. It is important to secure this frame very firmly
to the rest of the ship, with which view it is advisable to form a great
palm, like the palm of a vice, on its inner superior corner, which,
projecting into the ship, may be secured by breast-hook plates to the
sides, whereby the strain which the screw causes will be distributed over
the stern, instead of being concentrated on the rivets of the frame.
479. _Q._--Are there several lengths of screw shaft?
_A._--There are.
480. _Q._--How then are these secured to one another?
_A._--The best mode of securing the several lengths of shaft together is by
forging the shafts with flanges at the ends, which are connected together
by bolts, say six strong bolts in each, accurately fitted to the holes.
[Illustration: Fig 44. End of the Screw Shaft of Correo, showing the mode
of receiving the Thrust. A, discs; B, tightening wedge.]
481. _Q._--How is the thrust of the shaft usually received?
_A._--In some cases it is received on a number of metal discs set in a box
containing oil; and should one of these discs stick fast from friction, the
others will be free to revolve. This arrangement, which is represented in
fig. 44, is used pretty extensively and answers the purpose perfectly. It
is of course necessary that the box in which the discs A are set, shall be
strong enough to withstand the thrust which the screw occasions. Another
arrangement still more generally used, is that represented in figs. 55 and
56, p. 331. It is a good practice to make the thrust plummer block with a
very long sole in the direction of the shaft, so as to obviate any risk of
canting or springing forward when the strain is applied, as such a
circumstance, if occurring even to a slight extent, would be very likely to
cause the bearing to heat.
482. _Q._--Are there not arrangements existing in some vessels for enabling
the screw to be lifted out of the water while the vessel is at sea?
_A._--There are; but such arrangements are not usual in merchant vessels.
In one form of apparatus the screw is set on a short shaft in the middle of
a sliding frame, which can be raised or lowered in grooves like a window
and the screw shaft within the ship can be protruded or withdrawn by
appropriate mechanism, so as to engage or leave free this short shaft as
may be required. When the screw has to be lifted, the screw shaft is drawn
into the vessel, leaving the short shaft free to be raised up by the
sliding frame, and the frame is raised by long screws turned round by a
winch purchase on deck. A chain or rope, however, is better for the purpose
of raising this frame, than long screws; but the frame should in such case
be provided with pall catches like those of a windlass, which, if the rope
should break, will prevent the screw from falling.
DETAILS OF THE PADDLES AND PADDLE SHAFT.
483. _Q._--What are the most important details of the construction of
paddle wheels?
_A._--The structure of the feathering wheel will be hereafter described in
connection with an account of the oscillating engine; and it will be
expedient now to restrict any account of the details to the common radial
paddle, as applied to ocean steamers. The best plan of making the paddle
centres is with square eyes, and each centre should be secured in its place
by means of eight thick keys. The shaft should be burred up against the
head of these keys with a chisel, so as to prevent the keys from coming
back of their own accord. If the keys are wanted to be driven back, this
burr must be cut off, and if made thick, and of the right taper, they may
then be started without difficulty. The shaft must of course be forged with
square projections on it, so as to be suitable for the application of
centres with square eyes. Messrs. Maudslay & Co. bore out their paddle
centres, and turn a seat for them on the shaft, afterward fixing them on
the shaft with a single key. This plan is objectionable for the two
reasons, that it is insecure when new, and when old is irremovable. The
general practice among the London engineers is to fix the paddle arms at
the centre to a plate by means of bolts, a projection being placed upon the
plates on each side of the arm, to prevent lateral motion; but this method
is inferior in durability to that adopted in the Clyde, in which each arm
is fitted into a socket by means of a cutter--a small hole being left
opposite to the end of each arm, whereby the arm may be forced back by a
drift.
484. _Q._--How are the arms attached to the outside rings?
_A._--Some engineers join the paddle arms to the outer ring by means of
bolts; but unless very carefully fitted, those bolts after a time become
slack sideways, and a constant working of the parts of the wheel goes on in
consequence. Sometimes the part of the other ring opposite the arm is
formed into a mortise, and the arms are wedged tight in these holes by
wedges driven in on each side; but the plan is an expensive one, and not
satisfactory, as the wedges work loose even though riveted over at the
point. The best mode of making a secure attachment of the arms to the ring,
consists in making the arms with long T heads, and riveting the cross piece
to the outer ring with a number of rivets, not of the largest size, which
would weaken the outer ring too much. The best way of securing the inner
rings to the arms is by means of lugs welded on the arms, and to which the
rings are riveted.
485. _Q._--What are the scantlings of the paddle floats?
_A._--The paddle floats are usually made either of elm or pine; if of the
former, the common thickness for large sea-going vessels is about 2-1/2
inches; if of the latter, 3 inches. The floats should have plates on both
sides, else the paddle arms will be very liable to cut into the wood, and
the iron of the arms will be very rapidly wasted. When the floats have been
fresh put on they must be screwed up several times before they come to a
bearing. If this be not done, the bolts will be sure to get slack at sea,
and all the floats on the weather side may be washed off. The bolts for
holding on the paddle floats are made extra strong, on account of the
corrosion to which they are subject; and the nuts should be made large, and
should be square, so that they may be effectually tightened up, even though
their corners be worn away by corrosion. It is a good plan to give the
thread of the paddle bolts a nick with a chisel, after the nut has been
screwed up, which will prevent the nut from turning back. Paddle floats,
when consisting of more than one board, should be bolted together edgeways,
by means of bolts running through their whole breadth. The floats should
not be notched to allow of their projection beyond the outer ring, as, if
the sides of the notch be in contact with the outer ring, the ring is soon
eaten away in that part, and the projecting part of the float, being
unsupported, is liable to be broken off.
486. _Q._--Do not the wheels jolt sideways when the vessel rolls?
_A._--It is usual to put a steel plate at each end of the paddle shafts
tightened with a key, to prevent end play when the vessel rolls, but the
arrangement is precarious and insufficient. Messrs. Maudslay make their
paddle shaft bearings with very large fillets in the corner, with the view
of diminishing the evil; but it would be preferable to make the bearings of
the crank shafts spheroidal; and, indeed, it would probably be an
improvement if most of the bearings about the engine were to be made in the
same fashion. The loose end of the crank pin should be made not spheroidal,
but consisting of a portion of a sphere; and a brass bush might then be
fitted into the crank eye, that would completely encase the ball of the
pin, and yet permit the outer end of the paddle shaft to fall without
straining the pin, the bush being at the same time susceptible of a slight
end motion. The paddle shaft, where it passes through the vessel's side, is
usually surrounded by a lead stuffing box, which will yield if the end of
the shaft falls; this stuffing box prevents leakage into the ship from the
paddle wheels: but it is expedient, as a further precaution, to have a
small tank on the ship's side immediately beneath the stuffing box, with a
pipe leading down to the bilge to catch and conduct away any water that may
enter around the shaft.
487. _Q._--How is the outer bearing of the paddle wheels supplied with
tallow?
_A._--The bearing at the outer end of the paddle shaft is sometimes
supplied with tallow, forced into a hole in the plummer block cover, as in
the case of water wheels; but for vessels intended to perform long voyages,
it is preferable to have a pipe leading down to the oil cup above the
journal from the top of the paddle box, through which pipe oil may at any
time be supplied.
488. _Q._--Will you explain the method of putting engines into a steam
vessel?
_A._--As an illustration of this operation it may be advisable to take the
case of a side lever engine, and the method of proceeding is as follows:--
First measure across from the inside of paddle bearers to the centre of the
ship, to make sure that the central line, running in a fore and aft
direction on the deck or beams, usually drawn by the carpenter, is really
in the centre. Stretch a line across between the paddle bearers in the
direction of the shaft; to this line, in the centre of the ship where the
fore and aft mark has been made, apply a square with arms six or eight feet
long, and bring a line stretched perpendicularly from the deck to the
keelson, accurately to the edge of the square: the lower point of the line
where it touches the keelson will be immediately beneath the marks made
upon the deck. If this point does not come in the centre of the keelson, it
will be better to shift it a little, so as to bring it to the centre,
altering the mark upon the deck correspondingly, provided either paddle
shaft will admit of this being done--one of the paddle brackets being
packed behind with wood, to give it an additional projection from the side
of the paddle bearer. Continue the line fore and aft upon the keelson as
nearly as can be judged in the centre of the ship; stretch another line
fore and aft through the mark upon the deck, and look it out of winding
with the line upon the keelson. Fix upon any two points equally distant
from the centre, in the line stretched transversely in the direction of the
shaft; and from those points, as centres, and with any convenient radius,
sweep across the fore and aft line to see that the two are at right angles;
and, if not, shift the transverse line a little to make them so. From the
transverse line next let fall a line upon each outside keelson, bringing
the edge of the square to the line, the other edge resting on the keelson.
A point will thus be got on each outside keelson, perpendicularly beneath
the transverse line running in the direction of the shaft, and a line drawn
between those two points will be directly below the shaft. To this line the
line of the shaft marked on the sole plate has to be brought, care being
taken, at the same time, that the right distance is preserved between the
fore and aft line upon the sole plate, and the fore and aft line upon the
central keelson.