Modern Machine-Shop Practice Part 260

8th. When an engine has no auxiliary or starting, but an _impulse_ valve that merely lets a puff of steam into the receiver, this impulse valve may leak, and if the escape or relief valve on the receiver is too much loaded, it may gag the H. P. piston by giving it high pressure steam on both sides, and this may throw the valve off its seat. Similarly, if the engine has an auxiliary or starting valve, and it leaks, high pressure steam may be admitted to both sides of the L. P. piston, thus gagging it and causing its slide valve to throw back and away from its seat.

9th. The cylinders may be choked with water, and the drain c.o.c.ks choked up.

10th. The crank shaft bearings may be screwed up too tightly.

11th. The air or the circulating pump may be choked with water, either the air pump overflow valve or the circulating discharge valve being secured down.[64]

[64] The air pump overflow valve should never be permanently fastened down. More engines have been broken down from this than from almost any other neglectful cause, because, from great leaks in the condenser tubes and engines standing for a length of time, a larger quant.i.ty of water may require to be got rid of during the first few strokes of the pump than can pa.s.s through the small air or vapor pipe, which is usually fitted from the hot well either into the bilge or else overboard. Unless the valve in this overflow pipe is heavy enough of itself (which is very rarely the case), it should be loaded by a spring or weight, so that when the puff of the air pump causes it to lift, and the vessel is rolling, sea water may not pa.s.s into the hot well. To avoid this, some engineers erroneously fasten this valve down. An experienced engineer states that in his experience five engines have been broken down from this cause alone.

12th. From the engines being allowed to stand a long time in one position, and the glands being too tightly packed. An engine should be turned a little daily when not in use.

13th. From the piston rings being set out too tight to the cylinder bore.

14th. From the throttle or stop valve being shut, as from its spindle being broken.

15th. From the eccentric sheave, or wheel, having shifted on the shaft, some eccentrics having a key that is not sunk in the sheave, which is done so that the eccentric may shift rather than break if it should seize in its strap.

16th. From the H. P. piston leaking badly, or its ring being broken, which will permit the cylinder to fill with steam and the slide valve to unseat.

17th. If the engine has been overhauled, the forward eccentric may have been connected to the wrong end of the link, thus giving an improper motion to the slide valve.

18th. The expansion may be set to cut off too early in the stroke.

19th. From the air pump rod, or from the circulating pump rod being broken, or from the valves being broken.

20th. From the cylinder casing or the receiver being cracked so as to admit steam to both sides of the piston at the same time.

A defective vacuum, or loss of vacuum, may occur from the following causes:

1st. From the glands of the low pressure cylinder leaking.

2d. From the pet c.o.c.k of the air pump being left open.

3d. From the joints of the connections about the condenser leaking.[65]

[65] To discover a leak about a condenser, pa.s.s an exposed light, as a candle, about the joints, etc., and where there is a leak the flame will be drawn in towards the condenser.

4th. From the condenser being cracked, and therefore leaky.

5th. From the injection c.o.c.k or valve being closed.

6th. From the condenser tubes being foul for lack of being cleaned. From the L. P. cylinder escape valves or cylinder c.o.c.ks being leaky, and therefore letting in air.

7th. From the slide valve and piston of the L. P. cylinder leaking.

8th. From the air pump valve being leaky or broken. From the circulating pump being defective, as from having leaky valves.

9th. From the Kingston injection valve not being properly opened, or from its outside orifice being choked.

10th. The bilge injection may be so connected with the air pump or condenser as to impair the vacuum when its valve is accidentally stuck and its stop c.o.c.k is left open.[66]

[66] It is obvious that a defective vacuum may or may not prevent an engine from starting, according to the degree of defectiveness.

The princ.i.p.al causes of heating are:

1st. The bearing caps being screwed down too tight.

2d. The bearings being left uncovered, thus allowing the brick dust used for cleaning the machinery, the dirt from coaling the ship, or the sand used for cleaning the decks, to get into the bearing.

3d. The oil grooves in the bra.s.ses being worn out or too shallow, or the bra.s.ses not being cleared at the sides.

4th. Improper fitting of the distance pieces or fit strips between the bra.s.ses.

5th. Bad oil or too light an oil.

6th. If the bra.s.ses are too slack and thump or pound, the back of the bra.s.s may be stretched by pening, causing the sides of the bra.s.s to close in upon and bind the crank journal or crank pin, and this will cause heating.

For other information concerning the engine see as follows:

Page.

Angularity of connecting rod 375 The slide valve 376 Double ported and griddle valves 377 Balanced valves 377 Piston valves 378 Separate cut off valves 378 Reversing gears 383 Finding the working results of a slide valve 376 Condensing engines 442, 444 Calculations on the mechanical powers 405 The unit of power 407 Calculating horse power 407 Calculations of safety valves 409 Heat, water, and steam 410 The expansion of steam 411 The conversion of heat into work 411 The indicator 413 Indicator diagrams 414, 421 The barometer 415 Calculating the horse power from indicator diagrams 419 Finding the steam of water consumption from an indicator 421

Figs. 3405 and 3406 represent a triple expansion marine engine, the construction being as follows:

The high pressure cylinder has a piston valve and the intermediate and low pressure cylinders flat valves. Each cylinder has a link motion, and all three link motions are shifted from the same shaft, which is moved by a steam reversing gear. At _a_, Fig. 3405, are the eccentrics for the link B, for the high pressure cylinder; _b'_, _b'_ are those for link B', for the intermediate cylinder; and _c'_ _c'_ are those for the link C', for the low pressure cylinder. From each link are rods E, Fig. 3406, connected to arms on the shaft F _f_, to an arm on which is connected the rod G, from the worm wheel H, whose actuating worm I is on a crank shaft operated by the small steam cylinder J. The slide spindles D work in guides, and their cross heads C span the edges of the links, gibs being provided to take up the wear.

The gear for turning the engine when there is no steam in the main boilers is constructed as follows:

On the shaft of the wheel _m_, Fig. 3405, is a worm _n_ operating a worm wheel _p_, on whose shaft is a worm which operates the large worm wheel shown on the main crank shaft.

Figs. 3407 and 3408 represent the compound engines of the steamship _Poplar_, concerning which _The Engineer_ (from which the engravings are taken) says:

"Both the cylinders of these engines are fitted with piston valves, placed at the back of the cylinders and worked by the single eccentric valve gear, which has been so largely adopted and so successfully carried out by this firm in triple expansion as well as compound engines. It will be noticed that whilst this valve gear permits of the cylinders being close together, it allows of the crank shaft being made in two similar pieces, and affords exceptionally long main and crank pin bearings, of the former of which there are only three, instead of the usual four. In the case of the _Poplar_ the cylinders are 29 in. and 55 in. in diameter and 33 in. stroke, and the crank pins are 11 in. long, whilst the centre main bearing, which does duty for both the engines, is 23-3/4 in. in length, each of the outer bearings being 18 in. in length, the diameter of the crank shaft being 9-1/2 in. Another very interesting feature about these compact little engines is the design of the front framework. Instead of the ordinary upright columns in front of each engine there is an arrangement which gives exceptional stiffness to the whole structure whilst affording the fullest possible accessibility to the main working parts, and which has the appearance of an arch, from the shoulders of which there are branches worked up to receive the feet of the cylinders, thus accommodating the close centres and providing for the support of the reversing wheel without in the least obstructing the gear below. The condenser is divided horizontally through the centre on a plan strongly advocated by the builders, the whole of the base of the engines being cast in one piece and made level on the under side, so as to enable it to receive support from, and be bolted to, the engine seating immediately beneath the crank shaft, as well as round the margin."

[Ill.u.s.tration: Fig. 3405.]

[Ill.u.s.tration: Fig. 3406.]

[Ill.u.s.tration: Fig. 3407.]

[Ill.u.s.tration: Fig. 3408.]

CHAPTER XLIII.--MARINE BOILERS.

Boilers for marine engines are, in England, made of special qualities of plate, the best being termed Yorkshire, and a nearly equal grade, Staffordshire. The plates for the sh.e.l.l, the furnace bottoms and the gusset stays are made of Staffordshire, while the tube plates, furnace tops, and such parts as require to be f.l.a.n.g.ed and are subject to more intense heat, are made of Yorkshire plate, which has more ductility.