Modern Machine-Shop Practice Part 252

Then 859.375/(37.6 996) = 22.94 pounds water per I. H. P. per hour, the rate that would be due to using an entire cylinder full of steam at 25 pounds pressure every stroke. But as the period of consumption is represented by B A (_b_ _a_ being the stroke), the following correction is required:

(22.94 3.03)/3.45" = 20.15; 3.03 inches being the portion B A, and 3.45 inches being the whole length _b_, _a_. This correction allows for the effects of clearance as well as compression, since, if more clearance had existed, the pressure at A would not have been reached till later in the stroke, and the consumption line B A would have been longer.

[Ill.u.s.tration: Fig. 3375.]

But such a rate can never be realized in practice. Under the best attainable conditions, such as about the load indicated on the diagram, or more on a large engine with steam tight valves and piston, and well protected cylinder and pipes, the unindicated loss will seldom be less than 10 per cent., and it will be increased by departure from any of the above conditions to almost any extent. It will increase at an accelerating ratio as the load is diminished, so that such calculations applied to light load diagrams would be deceptive and misleading; in fact, they have but little practical value, except when made for comparison with tests of actual consumption for the purpose of determining the amount of loss under certain given conditions.

DEFECTIVE DIAGRAMS.

In seeking the causes that may produce a defective diagram, the following points should be remembered:

The indicator must be kept in perfect order, thoroughly clean and well lubricated, so that its parts will move freely. It should always be cleaned throughout after using.

The motion of the indicator drum should be an exact copy, on a reduced scale, of that of the piston at every point in the stroke.

The steam pipes from the cylinder to the indicator, if any are used, must be large enough to give a free and full pressure of steam, and care must be taken that the water of condensation does not obstruct them or enter the indicator.

The cord should be as strong as possible, or if long, fine wire should be subst.i.tuted.

The pencil should be held to the card with just sufficient force to make a fine line with a sharp pencil.

The diagram should be as long as the atmospheric line, any difference in this respect showing unequal tension of the cord, probably from unequal pressure of the pencil to the paper or card.

A fall in the steam line could arise from too small a steam pipe, and this could be tested by a diagram taken from the steam chest. It could also occur from too small a steam port or an obstructed steam pa.s.sage as well as from a leaky piston.

An expansion curve that is higher than it should be may arise from a leaky valve, letting in steam after the cut off had occurred, or if at the later point of expansion curve, it may be caused by the steam being wet or containing water, which evaporates as the temperature falls from the expansion.

An expansion curve that is lower than it should be may be caused by a leaky piston, by a valve that leaks on the exhaust side but not on the steam side, or if the exhaust valve is separate from the steam valve, it may leak while the steam valve is tight.

It may also be caused by the cylinder being unduly cooled, as from water acc.u.mulating in a steam jacket.

There are many defects in the adjustment of the valve gear, or of improper proportion in the parts, that may be clearly shown by a diagram, while there are defects which might exist and that would not be shown on the diagram.

It is possible, for example, that a steam valve and the engine piston may both leak to the same amount, and as a result the expansion curve may appear correct and not show the leak.

Insufficient valve lead would be shown by the piston moving a certain portion of its stroke before the steam line attained its greatest height in Fig. 3376, in which from A upwards, the admission line, instead of rising vertically, is at an angle to the right, showing that the piston had moved a certain portion of its stroke before full pressure of steam was admitted.

That too small a steam port or steam pipe did not cause this defect may be known from the following reasoning:

The port opened when the pencil was at A, which shows that the valve had lead. At this time the piston was near the dead centre and moving slower than it was when the pressure reached its highest point on the diagram, and since the steam line is fairly parallel with the atmospheric line, it shows that the port was large enough to maintain the pressure when the piston was travelling fast, and therefore ample when the piston was moving slow.

The remedy in this case is to set the eccentric back.

With less compression the point A would be lower.

Excessive lead is shown in Fig. 3377 by the loop at A, where the compression curve extends up to the steam line, and the lead carries the admission line above it, because of the piston moving against the incoming steam.

To mark in the theoretical compression curve, the vacuum line and the clearance line must be drawn in as in the figure, and ordinates must be drawn.

According to the diagram, in Fig. 3377, the compression is clearly defined to have begun at C, and at that time the s.p.a.ce filled by steam is represented by the distance from C to the clearance line. The pressure above vacuum (or total pressure) of the steam in the cylinder when the compression began is represented by the length or height of the dotted line 1.

Now suppose the piston to have moved from the point C, where compression began, to line 2 (which is midway between line 1 and the clearance line), and as the compressed steam occupies one-half the s.p.a.ce it did when the piston was at C, therefore the steam pressure will be doubled, and line 2 may be drawn making it twice as high as line 1.

[Ill.u.s.tration: Fig. 3376.]

[Ill.u.s.tration: Fig. 3377.]

Line 2 is now the starting point for getting the next ordinate, and line 3, must be marked midway between line 2 and the clearance line, and twice as high as line 2, because at line 3 the steam will occupy half the s.p.a.ce it did at line 2. Line 4 is obviously midway between line 3 and the clearance line.

Through the tops of these lines we may draw the theoretical compression curve, which is shown dotted in.

To find the amount of steam actually saved by the compression, we have to consider the compression curve only, beginning at the point of the diagram where it is considered that the compression actually began, and ending where the compression line joins the admission line, and the horizontal distance between these two points represents the length of the cylinder bore filled by the compression.

To find the average amount to which the steam is compressed, we must draw within this length of the diagram, and within the boundaries of the compression curve, and the line of no pressure ordinates corresponds to those given for finding the average shown pressure of a diagram, as explained with reference to that subject, taking care to have the end ordinates s.p.a.ced half as wide as the intermediate ones, as explained with reference to Fig. 3372.

CHAPTER XLI.--AUTOMATIC CUT OFF ENGINES.

An automatic cut off engine is one in which the valve gear is so acted upon by the governor as to keep the speed of the engine uniform under variations of the load the engine drives, and notwithstanding variations in the boiler pressure. This it accomplishes by varying the point in the piston stroke at which the live steam is cut off. This is economical because it enables the engine to use the steam more expansively than is possible with engines having throttling governors, which govern the engine speed by wire drawing the steam.

There are two princ.i.p.al forms of automatic cut off engines, first, those in which the steam valve spindle or rod is released from the parts that move it to open for admission, while dash pots, weights, or springs close the valve to effect the cut off; and second, those in which the travel of the valve is varied so as to alter the point of cut off.

The first usually employ fly ball governors which actuate cams or stops to trip the valves for the steam cylinders. The second usually employ wheel governors or speed regulators, as they are sometimes termed.

The distinctive features in the action of the first, of which the Corliss engine is the most important, is that as two admission and two exhaust valves are used, therefore the amount of the valve lead, the point of exhaust and amount of the compression remain the same at whatever point in the piston stroke the cut off may occur; whereas in the second, the lead increases, the cut off occurs earlier, and the compression increases in proportion as the cut off occurs earlier in the piston stroke. In this cla.s.s of engine the steam valve travels as quickly when opening the steam port for a short and early period of cut off as it does for a late one, hence the amount of steam port opening is as full, with reference to the piston motion, for an early as it is for a late point of cut off. In other words, there is the same amount of steam port opening for the first, second, third, and fourth inch of piston motion, let the point of cut off occur at whatever point in the piston motion it may. In engines which vary the point of cut off by reducing the travel of the slide valve, this is accomplished by using double ported valves or griddle valves.

[Ill.u.s.tration: Fig. 3378.]

Fig. 3378 represents the arrangement of the valves in a Corliss engine, V and V^{1} being the steam valves and V^{2} and V^{3} the exhaust valves. These valves, it will be seen, extend crossways of the cylinder and are circular. In the figure the valves are shown in the position they would occupy when the piston was at the crank end of the cylinder, as in the figure.

The principles of a Corliss valve gear will be understood from the following, which is derived from a book by the author of this work, and ent.i.tled _Modern Steam Engines_.

In 3379 and 3380 the valve gear (which is the distinctive feature of the engine) is represented with the parts in the position they occupy when the cut off occurs at half stroke, the piston having moved from the head end of the cylinder. In Figs. 3381 and 3382 the parts are shown in position with the crank on the dead centre and the piston at the crank end of the cylinder, valve _v_ having opened its port to the amount of the lead.

Referring to Fig. 3379, motion from the eccentric is imparted by the rod M to the wrist plate Y, to which are connected the rods C, C', for operating the admission valves, whose shafts are seen at S, S', and the rods F, F', for operating the exhaust valves, whose shafts are seen at T, T'.

The mechanism for the steam or admission valves may be divided into three elements: first, that for operating the valve to open the port for admission; second, that for closing the valve to effect the cut off; and third, that which determines the point in the stroke at which the cut off shall occur.

The first consists of the rod M, wrist plate Y, and the rods C and C', which operate the bell cranks _r_ _r_, _r'_ _r'_ which are fast on the valve shafts S, S'. Upon the ends of bell cranks _r_ _r_, _r'_ _r'_, are pivoted latch links _u_, _u'_, which have in them a recess for the latch blocks, of which one is seen at _e_ (the rod R' and its connection with the valve stem being shown broken away to expose _e_ to view). During the admission the latch block abuts against the end _y_ of the recess _w_ and is tripped therefrom by the cam _n'_. The ends of arms _g_ of the latch links abut against the hub of the arms _d_, _d'_ upon which are cams _n_, _n'_, and at _a_, _a'_ are springs for keeping the ends _g_ of latch links _u_, _u'_ against the hubs and cams of _d_, _d'_.

Referring now to the valve mechanism at the head end only, suppose the piston to be at the head end of the cylinder, and latch block _e_ will be seated in the recess provided in _a_ to receive it, and as the bell crank moves, the latch block will be raised by the latch link, which is carried by a crank arm corresponding to that seen at _x_ at the crank end of the cylinder, and as this crank arm is fast upon the valve spindle, the lifting of _e_ will open the valve for admission. As soon, however, as the end _g_ of the latch link meets the cam _n'_, the latch link will be moved so that the end _y_ of its recess will leave contact with the latch block _e_ and the dash pot will cause rod R' to descend instantaneously and close the valve, thus effecting the cut off.

The period of admission, therefore, is determined by the amount of motion the latch link _u'_ is permitted to have before its end _g_ meets the cam _n'_, which trips the latch link, and therefore frees _e_ from the latch link recess.

[Ill.u.s.tration: Fig. 3379.]