Gas-Engines and Producer-Gas Plants Part 4

[Ill.u.s.tration: FIG. 45.--Rubber bags on gas-pipes.]

Care should be taken to provide the gas-pipe with a drain-c.o.c.k, at a point near the engine. By means of this c.o.c.k, any air in the pipe can be allowed to escape before starting; otherwise the engine can be set in motion only with difficulty. If the engine be provided with an incandescent tube, the gas-supply pipe of the igniter should be fitted with a small rubber pouch or bag, in order to obviate fluctuations in the burner flame, caused by variations in the pressure (Fig. 44). As a general rule, the supply-pipe should be connected with the main pipe on the forward side of the bags and gas-governors. The main pipe and all other piping near the engine should extend underground, so that free access to the motor from all sides can be obtained, without possibility of injury.

=Anti-pulsators, Bags, Pressure-Regulators.=--The most commonly employed means of preventing fluctuation of nearby lights, due to the sharp strokes of the engine, consists in providing the gas-supply pipe with rubber bags (Fig. 45), which form reservoirs for the gas and, by reason of their elasticity, counteract the effect produced by the suction of the engine. Nevertheless, in order to insure a supply of gas at a constant pressure, which is necessary for the perfect operation of the engine, there are generally used, in addition to the bags, devices called gas-governors, or anti-pulsators (Fig. 46).

Although these devices are constructed in different ways, the underlying principle is the same in all. They comprise a metallic casing, containing a flexible diaphragm of rubber or of some fabric impermeable to gas. Suction of the engine creates a vacuum in the casing. The diaphragm bends, thereby actuating a valve, which cuts off the gas supply. During the three following periods (compression, explosion, and exhaust) the gas, by reason of its pressure on the diaphragm, opens the valve and fills the casing, ready for the next suction stroke.

[Ill.u.s.tration: FIG. 46.--An anti-pulsator.]

Other devices, which are never sold with the engine, but are rendered necessary by reason of the conditions imposed by the gas supply are sold under the name "pressure-regulators" (Fig. 47). They consist of a bell, floating in a reservoir containing water and glycerine (or mercury), and likewise actuate a valve which partially controls the flow of gas. This valve being balanced, its mechanical action is the more certain. Such devices are very effective in maintaining the steadiness of lights. On the other hand, they are often an obstacle to the operation of the engine because they reduce the flow and pressure of the gas too much. In order to obviate this difficulty, a pressure-regulator should be chosen with discrimination, and of sufficiently large size to insure the maintenance of an adequate supply of gas to the engine. Frequent examinations should be made to ascertain if the bell of the regulator is immersed in the liquid. In the case of anti-pulsators, care should be taken that they are not spattered with oil, which has a disastrous effect on rubber. Anti-pulsators are generally mounted about 4 inches from a wall, in order that the diaphragm may be actuated by hand, if need be.

[Ill.u.s.tration: FIG. 47.--A pressure-regulator.]

=Precautions.=--In order not to strain the rubber of the bags or of the anti-pulsators, it is advisable to place a stop-c.o.c.k in advance of these devices so that they can not be filled while the motor is at rest.

The capacity of the rubber bags that can be bought in the market being limited, it is necessary to place one, two, or three extra bags in series (Figs. 48 and 49), for large pipes; but it should be borne in mind that the total section of the branch pipes should be at least equal to that of the main pipe. It is also advisable to extend the tube completely through the bag as shown in Figs. 48 and 49.

[Ill.u.s.tration: FIGS. 48-49.--Arrangement of rubber bags.]

If there be two branch pipes the minimum diameter which meets this requirement is ascertained as follows: Draw to any scale a semicircle having a diameter equal or proportional to that of the main pipe (Fig.

50). The sides of the isosceles triangle inscribed within this semicircle give the minimum diameter of each of the branch pipes.

Sometimes engines are provided with a c.o.c.k having an arrangement by means of which the gas feed is permanently regulated, according to the quality and pressure of the gas and according to the load at which the engine is to run. This renders it possible to open the c.o.c.k always to the same point (Fig. 51).

[Ill.u.s.tration: FIG. 50.]

[Ill.u.s.tration: FIG. 51.]

=Air Suction.=--In a special chapter the precautions to be taken to counteract the influence of the suction of the engine in causing vibration will be treated. The manner in which the suction of air is effected necessarily has as marked an influence on the operation of the engine as the supply of gas, since air and gas const.i.tute the explosive mixture.

Resistance to the suction of air should be carefully avoided, for which reason the length of the pipe should be reduced to a minimum, and its cross-section kept at least equal to that of the air inlet of the engine. Since the quality of street-gas varies with each city, the proper proportions of gas and air are not constant. In order that these proportions may be regulated, it is a matter of some importance to fit some suitable device on the pipe. Good engines are provided with a plug or flap valve. Generally the air-pipe terminates either in the hollowed portion of the frame, or in an independent pot, or air chest. The first arrangement is not to be recommended for engines over 20 to 25 horse-power. Accidents may result, such as the breaking of the frame by reason of back firing, of which more will be said later. If an independent chest be employed, its closeness to the ground renders it possible for dust easily to pa.s.s through the air-holes in the walls at the moment of suction, and even to enter the cylinder, where its presence is particularly harmful, leading, as it does, to the rapid wear of the rubbing surfaces. This evil can be largely remedied by filling the air-chest with cocoa fiber or even wood fiber, provided the latter does not become packed down so as to prevent the air from pa.s.sing freely. Such fibers act as air-filters. Regular cleaning or renewal of the fiber protects the cylinder from wear. In a general way, care should be taken, before fitting both the gas and air pipes, to tap the pipes, elbows, and joints lightly with a hammer on the outside in order to loosen whatever rust or sand may cling to the interior; otherwise this foreign matter may enter the cylinder and cause perturbations in the operation of the engine. Under all circ.u.mstances, care should be taken not to place the end of the air-pipe under the floor or in an enclosed s.p.a.ce, because leakage may occur, due to the bad seating of the air-valve, thereby producing a mixture which may explode if the flame leaps back, as we shall see in the discussion of suction by pipes terminating in the hollow of the frame. On the other hand, sand or sawdust should not be sprinkled on the floor.

=Exhaust.=--For the exhaust, cast-iron or drawn pipes as short as possible should be used. Not only the power of the engine, but also its economic consumption, can be markedly affected by the employment of long and bent pipes. Resistance to the exhaust of the products of combustion not only causes an injurious counter-pressure, but also prevents the clearing of the cylinder of burnt gases, which contaminate the aspired mixture and rob it of much of its explosiveness. The necessity of evacuating the cylinder as completely as possible is, nevertheless, not always reconcilable with local surroundings. To a certain extent, the objections to long exhaust-pipes are overcome by rigorously avoiding the use of elbows. Gradual curves are preferable. In the case of very long pipes it is advisable to increase their diameter every 16 feet from the exhaust. The exhaust-chest should be placed as near as possible to the engine; it should never be buried; for the joints of the inlet and outlet pipes of the exhaust-chest should be easily accessible, so that they may be renewed when necessary. The author recommends the placing of the exhaust-chest in a masonry pit, which can be closed with a sheet-metal cover. For engines of 20 horse-power and upward, these joints should be entirely of asbestos. Pipes screwed directly into the casting are liable to rust. Exposed as they are to the steam or water of the exhaust, they cannot be detached.

[Ill.u.s.tration: FIG. 52.--Method of mounting pipes.]

The water, which results from the combination of the hydrogen of the gas with the oxygen of the air, is deposited in most cases at the bottom of the exhaust-chest. It is advisable to fit a plug or iron c.o.c.k in the base of the chest. Alkaline or acid water will always corrode a bronze c.o.c.k. In order that the pipes may not also be attacked, they are not disposed horizontally, but are given a slight incline toward the point where the water is drained off. If pipes of some length be employed, they should be able to expand freely without straining the joints, as shown in the accompanying diagram (Fig. 52), in which the exhaust-chest rests on iron rollers which permit a slight displacement.

For the sake of safety, at least that portion of the piping which is near the engine should be located at a proper distance from woodwork and other combustible material. By no means should the exhaust discharge into a sewer or chimney, even though the sewer or chimney be not in use; for the unburnt gases may be trapped, and dangerous explosions may ensue at the moment of discharge.

The joints or threaded sleeves employed in a.s.sembling the exhaust-pipe should be tested for tightness. The combined action of the moisture and heat causes the metal to rust and to deteriorate very rapidly at leaky spots.

When several engines are installed near one another, each should be provided with a special exhaust-pipe; otherwise it may happen, when the engines are all running at once, that the products of combustion discharged by the one may cause a back pressure detrimental to the exhaust of the next.

It is possible to employ a pipe common to all the exhausts if the pipe starts from a point beyond the exhaust-chests, in which case Y-joints and not T-joints are to be used.

The manner of securing the pipes to walls by means of detachable hangers, lined with asbestos, is shown in a general way in the accompanying Fig. 53. The object of this arrangement is to render detachment easy and to prevent the transmission of shocks to the masonry.

The precautions to be taken for m.u.f.fling the noise of the exhaust will be discussed later.

The end of the exhaust-pipe should be slightly curved down in order to prevent the entrance of rain. Exhaust-pipes are subjected to considerable vibration, due to the sudden discharge of the gases. To protect the joints, the pipes should be rigidly fastened in place.

[Ill.u.s.tration: FIG. 53.--Method of securing pipes to walls.]

=Legal Authorization.=--In most countries gas-engines may be installed only in accordance with the provision of general or local laws, which impose certain conditions. These laws vary with different localities, for which reason they are not discussed here.

CHAPTER IV

FOUNDATION AND EXHAUST

The reader will remember from what has already been said that a gas-engine is a motor which, more than any other, is subjected to forces, suddenly and repeatedly exerted, producing violent reactions on the foundation. It follows that the foundation must be made particularly resistant by properly determining its shape and size and by carefully selecting the material of which it is to be built.

=The Foundation Materials.=--Well-hardened brick should be used. The top course of bricks should be laid on edge. It is advisable to increase the stability of the foundation by longitudinally elongating it toward the base, as shown in the accompanying diagram (Fig. 54).

As a binding material, only mortar composed of coa.r.s.e sand or river sand and of good cement, should be used. Instead of coa.r.s.e sand, crushed slag, well-screened, may be employed. The mortar should consist of 2/3 slag and 1/3 cement. Oil should not in any way come into contact with the mortar; it may percolate through the cement and alter its resistant qualities.

As in the construction of all foundations, care should be taken to excavate down to good soil and to line the bottom with concrete, in order to form a single ma.s.s of artificial stone. A day or two should be allowed for the masonry to dry out, before filling in around it.

When the engine is installed on the ground floor above a vaulted cellar, the foundation should not rest directly on the vault below or on the joists, but should be built upon the very floor of the cellar, so that it pa.s.ses through the planking of the ground floor without contact.

[Ill.u.s.tration: FIG. 54.--Method of building the foundation.]

When the engine is to be installed on a staging, the method of securing it in place ill.u.s.trated in Fig. 55 should be adopted.

Although a foundation, built in the manner described, will fulfill the usual conditions of an industrial installation, it will be inadequate for special cases in which trepidation is to be expected. Such is the case when engines are to be installed in places where, owing to the absence of factories, it is necessary to avoid all nuisance, such as noise, trepidations, odors, and the like.

[Ill.u.s.tration: FIG. 55.--Elevated foundation.]

=Vibration.=--In order to prevent the transmission of vibration, the foundation should be carefully insulated from all neighboring walls. For this purpose various insulating substances called "anti-vibratory" are to be recommended. Among these may be mentioned horsehair, felt packing, cork, and the like. The efficacy of these substances depends much on the manner in which they are applied. It is always advisable to interpose a layer of one of these substances, from one to four inches thick, between the foundation and the surrounding soil, the thickness varying with the nature of the material used and the effect to be obtained. Between the bed of concrete, mentioned previously, and the foundation-masonry and between the foundation and the engine-frame, a layer of insulating material may well be placed. Preference is to be given to substances not likely to rot or at least not likely to lose their insulating property, when acted upon by heat, moisture or pressure.

Here it may not be amiss to warn against the utilization of cork for the bottom of the foundation; for water may cause the cork to swell and to dislocate the foundation or destroy its level.

The employment of the various substances mentioned does not entail any great expense when the foundations are not large and the engines are light. But the cost becomes considerable when insulating material is to be employed for the foundation of a 30 to 50 horse-power engine and upwards. For an engine of such size the author recommends an arrangement as simple as it is efficient, which consists in placing the foundation of the engine in a veritable masonry basin, the bottom of which is a bed of concrete of suitable thickness. The foundation is so placed that the lateral surfaces are absolutely independent of the supporting-walls of the basin thus formed. Care should be taken to cover the bottom with a layer of dry sand, rammed down well, varying in thickness with each case. This layer of sand const.i.tutes the anti-vibratory material and confines the trepidations of the engine to the foundation.

As a result of this arrangement, it should be observed that, being unsupported laterally, the foundation should be all the more resistant, for which reason the base-area and weight should be increased by 30 to 40 per cent. The expense entailed will be largely offset by saving the cost of special anti-vibratory substances. In places liable to be flooded by water, the basin should be cemented or asphalted.

When the engine is of some size and is intended for the driving of one or more dynamos which may themselves give rise to vibrations, the dynamos are secured directly to the foundation of the engine, which is extended for that purpose, so that both machines are carried solidly on a single base.

The foregoing outline should not lead the proprietor of a plant to dispense with the services of experts, whose long experience has brought home to them the difficulties to be overcome in special cases.

It should here be stated, as a general rule, that the bricks should be thoroughly moistened before they are laid in order that they may grip the mortar.

After having been placed on the foundation and roughly trimmed with respect to the transmission devices, the engine is carefully leveled by means of hardwood wedges driven under the base. This done, the bolts are sealed by very gradually pouring a cement wash into the holes, and allowing it to set. When the holes are completely filled and the bolts securely fastened in place, a shallow rim, or edge of clay, or sand is run around the cast base, so as to form a small box or trough, in which cement is also poured for the purpose of firmly binding the engine frame and foundation together. When, as in the case of electric-light engines, single extra-heavy fly-wheels are employed, provided with bearings held in independent cast supports, the following rule should be observed to prevent the overheating due to unlevelness, which usually occurs at the bus.h.i.+ngs of these bearings: That part of the foundation which is to receive such a support should rest directly on the concrete bed and should be rigidly connected at the bottom with the main foundation. When the foundation is completely blocked up, the fly-wheel bearing with its support is hung to the crank-shaft; and not until this is effected is the masonry at the base of the support completed and rigidly fixed in its proper position.