CORROSION IN APPARATUS.--All natural water is a solution of oxygen and may be regarded also as a weak solution of the hypothetical carbonic acid. It therefore causes iron to rust more or less quickly; and since no paint is absolutely waterproof, especially if it has been applied to a surface already coated locally with spots of rust, iron and steel cannot be perfectly protected by its aid. More particularly at a few inches above and below the normal level of the water in a holder, therefore, the metal soon begins to exhibit symptoms of corrosion which may eventually proceed until the iron is eaten away or becomes porous. One method of prolonging the life of such apparatus is to give it fresh coats of paint periodically; but unless the old layers are removed where they have cracked or blistered, and the rust underneath is entirely scraped off (which is practically impossible), the new paint films will not last very long. Another more elegant process for preserving any metal like iron which is constantly exposed to the attack of a corrosive liquid, and which is readily applicable to acetylene holders and their tanks, depends on the principle of galvanic action. When two metals in good electrical contact are immersed in some liquid that is capable of attacking both, only that metal will be attacked which is the more electro-positive, or which (the same thing in other words) is the more readily attacked by the liquid, evolving the more heat during its dissolution. As long as this action is proceeding, as long, that is, as some of the more electro- positive material is present, the less electro-positive material will not suffer. All that has to be done, therefore, to protect the walls of an acetylene-holder tank and the sides of its bell is to hang in the seal, supported by a copper wire fastened to the tank walls by a trustworthy electrical joint (soldering or riveting it), a plate or rod of some more electro-positive metal, renewing that plate or rod before it is entirely eaten away. [Footnote: Contact between the bell and the rod may be established by means of a flexible metallic wire; or a separate rod might be used for the bell itself.] If the iron is bare or coated with lead (paint may be overlooked), the plate may be zinc; if the iron is galvanised, i.e., coated with zinc, the plate may be aluminium or an alloy of aluminium and zinc. The joint between the copper wire and the zinc or aluminium plate should naturally be above the water-level. The foregoing remarks should be read in conjunction with what was said in Chapter II., about the undesirability of employing a soft solder containing lead in the construction of an acetylene generator. Here it is proposed intentionally to set up a galvanic couple to prevent corrosion; there, with the same object in view, the avoidances of galvanic action is counselled. The reason for this difference is self-evident; here a foreign metal is brought into electrical contact with the apparatus in order that the latter may be made electro-negative; but when a joint is soldered with lead, the metal of the generator is unintentionally made electro-positive. Here the plant is protected by the preferential corrosion of a cheap and renewable rod; in the former case the plant is encouraged to rust by the unnecessary presence of an improperly selected metal.
OTHER ITEMS IN GENERATING PLANT.--It has been explained in Chapter II. that the reaction between calcium carbide and water is very tumultuous in character, and that it occurs with great rapidity. Clearly, therefore, the gas comes away from the generator in rushes, passing into the next item of the plant at great speed for a time, and then ceasing altogether. The methods necessarily adopted for purifying the crude gas are treated of in Chapter V.; but it is manifest now that no purifying material can prove efficient unless the acetylene passes through it at a uniform rate, and at one which is as slow as other conditions permit. For this reason the proper position of the holder in an acetylene installation is before the purifier, and immediately after the condenser or washer which adjoins the generator. By this method of design the holder is filled up irregularly, the gas passing into it sometimes at full speed, sometimes at an imperceptible rate; but if the holder is well balanced and guided this is a matter of no consequence. Out of the holder, on the other hand, the gas issues at a rate which is dependent upon the number and capacity of the burners in operation at any moment; and in ordinary conditions this rate is so much more uniform during the whole of an evening than the rate at which the gas is evolved from the carbide, that a purifier placed after the holder is given a far better opportunity of extracting the impurities from the acetylene than it would have were it situated before the holder, as is invariably the case on coal-gas works.
For many reasons, such as capacity for isolation when being recharged or repaired, it is highly desirable that each item in an acetylene plant shall be separated, or capable of separation, from its neighbours; and this observation applies with great force to the holder and the decomposing vessel of the generator. In all large plants each vessel should be fitted with a stopcock at its inlet and, if necessary, one at its outlet, being provided also with a by-pass so that it can be thrown out of action without interfering with the rest of the installation. In the best practice the more important vessels, such as the purifiers, will be in duplicate, so that unpurified gas need not be passed into the service while a solitary purifier is being charged afresh. In smaller plants, where less skilled labour will probably be bestowed on the apparatus, and where hand-worked cocks are likely to be neglected or misused, some more, automatic arrangement for isolating each item is desirable. There are two automatic devices which may be employed for the purposes in view, the non-return valve and the water-seal. The non-return valve is simply a mushroom or ball valve without handle, lifted off its seat by gas passing from underneath whenever the pressure of the gas exceeds the weight of the valve, but falling back on to its seat and closing the pipe when the pressure decreases or when pressure above is greater than that below. The apparatus works perfectly with a clean gas or liquid which is not corrosive; but having regard to the possible presence of tarry products, lime dust, or sludge, condensed water loaded with soluble impurities, &c., in the acetylene, a non-return valve is not the best device to adopt, for both it and the hand-worked cock or screw- down valve are liable to stick and give trouble. The best arrangement in all respects, especially between the generator and the holder, is a water-seal. A water-seal in made by leading the mouth of a pipe delivering gas under the level of water in a suitable receptacle, so that the issuing gas has to bubble through the liquid. Gas cannot pass backwards through the pipe until it has first driven so much liquid before it that the level in the seal has fallen below the pipe's mouth; and if the end of the pipe is vertical more pressure than can possibly be produced in the apparatus is necessary to effect this. Omitting the side tube b, one variety of water-seal is shown at D in Fig. 7 on page 103. The water being at the level l, gas enters at a and bubbles through it, escaping from the apparatus at c. It cannot return from c to a without driving the water out of the vessel till its level falls from f to g; and since the area of the vessel is much greater than that of the pipe, so great a fall in the vessel would involve a far greater rise in a. It is clear that such a device, besides acting as a non-return valve, also fulfils two other useful functions: it serves to collect and retain all the liquid matter that may be condensed in the pipe a from the spot at which it was originally level or was given a fall to the seal, as well as that condensing in c as far as the spot where c dips again; and it equally acts as a washer to the gas, especially if the orifice g of the gas-inlet pipe is not left with a plain mouth as represented in the figure, but terminates in a large number of small holes, the pipe being then preferably prolonged horizontally, with minute holes in it so as to distribute the gas throughout the entire vessel. Such an apparatus requires very little attention. It may with advantage be provided with the automatic arrangement for setting the water-level shown at d and e. d is a tunnel tube extending almost to the bottom of the vessel, and e is a curved run-off pipe of the form shown. The lower part of the upper curve in e is above the level f, being higher than f by a distance equal to that of the gas pressure in the pipes; and therefore when water is poured into the funnel it fills the vessel till the internal level reaches f, when the surplus overflows of itself. The operation thus not only adjusts the quantity of water present to the desired level so that a cannot become unsealed, but it also renews the liquid when it has become foul and nearly saturated with dissolved and condensed impurities from the acetylene. It would be a desirable refinement to give the bottom of the vessel a slope to the mouth of e, or to some other spot where a large-bore draw-off cock could be fitted for the purpose of extracting any sludge of lime, &c., that may collect. By having such a water-seal, or one simpler in construction, between the generator and the holder, the former may be safely opened at any time for repairs, inspection, or the insertion of a fresh charge of carbide while the holder is full of gas, and the delivery of acetylene to the burners at a specified pressure will not be interrupted. If a cock worked by hand were employed for the separation of the holder from the generator, and the attendant were to forget to close it, part or all of the acetylene in the holder would escape from the generator when it was opened or disconnected.
Especially when a combined washer and non-return valve follows immediately after a generator belonging to the shoot type, and the mouth of the shoot is open to the air in the plant-house, it is highly desirable that the washer shall be fitted with some arrangement of an automatic kind for preventing the water level rising much above its proper position. The liquid in a closed washer tends to rise as the apparatus remains in use, water vapour being condensed within it and liquid water, or froth of lime, being mechanically carried forward by the stream of acetylene coming from the decomposing chamber. In course of time, therefore, the vertical depth to which the gas-inlet pipe in the washer is sealed by the liquid increases; and it may well be that eventually the depth in question, plus the pressure thrown by the holder bell, may become greater than the pressure which can be set up inside the generator without danger of gas slipping under the lower edge of the shoot. Should this state of things arise, the acetylene can no longer force its way through the washer into the holder bell, but will escape from the mouth of the shoot; filling the apparatus-house with gas, and offering every opportunity for an explosion if the attendant disobeys orders and takes a naked light with him to inspect the plant.
It is indispensable that every acetylene apparatus shall be fitted with a safety-valve, or more correctly speaking a vent-pipe. The generator must have a vent-pipe in case the gas-main leading to the holder should become blocked at any time, and the acetylene which continues to be evolved in all water-to-carbide apparatus, even after the supply of water has been cut off be unable to pass away. Theoretically a non-automatic apparatus does not require a vent-pipe in its generator because all the gas enters the holder immediately, and is, or should be, unable to return through the intermediate water seal; practically such a safeguard is absolutely necessary for the reason given. The holder must have a safety-valve in case the cutting-off mechanism of the generator fails to act, and more gas passes into it than it can store. Manifestly the pressure of the gas in a water-sealed holder or in any generator fitted with a water-sealed lid cannot rise above that corresponding with the depth of water in the seal; for immediately the pressure, measured in inches of water, equals the depth of the sealing liquid, the seal will be blown out, and the gas will escape. Such an occurrence, however, as the blowing of a seal must never be possible in any item of an acetylene plant, more especially in those items that are under cover, for the danger that the issuing gas might be fired or might produce suffocation would be extremely great. Typical simple forms of vent-pipe suitable for acetylene apparatus are shown in Fig. 7. In each case the pipe marked "vent" is the so-called safety-valve; it is open at its base for the entry of gas, and open at its top for the escape of the acetylene into the atmosphere, such top being in all instances carried through the roof of the generator-house into the open air, and to a spot distant from any windows of that house or of the residence, where it can prove neither dangerous nor a nuisance by reason of its odour. At A is represented the vent-pipe of a displacement vessel, which may either be part of a displacement holder or of a generator working on the displacement principle. The vent-pipe is rigidly fixed to the apparatus. If gas is generated within the closed portion of the holder or passes through it, and if the pressure so set up remains less than that which is needed to move the water from the level l to the levels l' and l", the mouth of the pipe is under water, and acetylene cannot enter it; but immediately such an amount of gas is collected, or such pressure is produced that the interior level sinks below l", which is that of the mouth of the pipe, it becomes unsealed, and the surplus gas freely escapes. There are two minor points in connexion with this form of vent-pipe often overlooked. At the moment when the water arrives at l" in the closed half of the apparatus, its level in the interior of the vent-pipe stands at l', identical with that in the open hall of the apparatus (for the mouth of the vent-pipe and the water in the open hall of the apparatus are alike exposed to the pressure of the atmosphere only). When the water, then, descends just below l" there is an amount of water inside the pipe equal in height to the distance between l' and l"; and before the acetylene can escape, it must either force this water as a compact mass out of the upper mouth of the vent-pipe (which it is clearly not in a position to do), drive it out of the upper mouth a little at a time, or bubble through it till the water is gradually able to run downwards out of the pipe as its lower opening is more fully unsealed. In practice the acetylene partly bubbles through this water and partly drives it out of the mouth of the pipe; on some occasions temporarily yielding irregular pressures at the burners which cause them to jump, and always producing a gurgling noise in the vent- pipe which in calculated to alarm the attendant. If the pipe is too small in diameter, and especially if its lower orifice is cut off perfectly horizontal and constricted slightly, the water may refuse to escape from the bottom altogether, and the pipe will fail to perform its allotted task. It is better therefore to employ a wide tube, and to cut off its mouth obliquely, or to give its lower extremity the shape of an inverted funnel. At the half of the central divided drawing marked B (Fig. 7) is shown a precisely similar vent-pipe affixed to the bell of a rising holder, which behaves in an identical fashion when by the rising of the bell its lower end is lifted out of the water in the tank. The features described above as attendant, upon the act of unsealing of the displacement-holder vent-pipe occur here also, but to a less degree; for the water remaining in the pipe at the moment of unsealing is only that which corresponds with the vertical distance between l' and l", and in a rising holder this is only a height always equal to the pressure given by the bell. Nevertheless this form of vent-pipe produces a gurgling noise, and would be better for a trumpet-shaped mouth. A special feature of the pipe in B is that unless it is placed symmetrically about the centre of the bell its weight tends to throw the bell out of the vertical, and it may have to be supported at its upper part; conversely, if the pipe is arranged concentrically in the bell, it may be employed as part of the guiding arrangement of the bell itself. Manifestly, as the pipe must be long enough to extend through the roof of the generator-house, its weight materially increases the weight of the bell, and consequently the gas pressure in the service; this fact is not objectionable provided due allowance is made for it. So tall a vent-pipe, however, seriously raises the centre of gravity of the bell and may make it top-heavy. To work well the centre of gravity of a holder bell should be as low as possible, any necessary weighting being provided symmetrically about its circumference and close to its bottom edge. The whole length of an ascending vent-pipe need not be carried by the rising bell, because the lower portion, which must be supported by the bell, can be arranged to slide inside a wider length of pipe which is fixed to the roof of the generator-house at the point where it passes into the open air.
A refinement upon this vent-pipe is represented at C, where it is rigidly fastened to the tank of the holder, and has its internal aperture always above the level of the water in the apparatus. Rigidly fixed to the crown of the bell is a tube of wider diameter, h, which is closed at its upper end. h is always full of gas, and its mouth is normally beneath the level of the water in the seal; but when the bell rises to its highest permissible position, the mouth of h comes above the water, and communication is opened between the holder and the outer atmosphere. No water enters the vent-pipe from the holder, and therefore no gurgling or irregular pressure is produced. Another excellent arrangement of a vent-pipe, suggested by Klinger of Gumpoldskirchen, is shown at D, a drawing which has already been partly considered as a washer and water-seal. For the present purpose the main vessel and its various pipes are so dimensioned that the vertical height g to f is always appreciably greater than the gas pressure in the service or in the generator or gasholder to which it is connected. In these circumstances the gas entering at a depresses the water in the pipe below the level f to an extent equal to the pressure at which it enters that pipe--an extent normally less than the distance f to g; and therefore gas never passes into the body of the vessel, but travels away by the side tube b (which in former references to this drawing was supposed to be absent). If, however, the pressure at a exceeds that of the vertical height f to g, gas escapes at g through the water, and is then free to reach the atmosphere by means of the vent c. As before, d serves to charge the apparatus with water, and e to ensure a proper amount being added. Clearly no liquid can enter the vent-pipe in this device. Safety-valves such as are added to steam-boilers and the like, which consist of a weighted lever holding a conical valve down against its seat, are not required in acetylene apparatus, for the simpler hydraulic seals discussed above can always be fitted wherever they may be needed. It should be noticed that these vent-pipes only come into operation in emergencies, when they are required to act promptly. No economy is to be effected by making them small in diameter. For obvious reasons the vent-pipe of a holder should have a diameter equal to that of the gas-inlet tube, and the vent-pipe of a generator be equal in size to the gas-leading tube.
FROTHING IN GENERATORS.--A very annoying trouble which crops up every now and then during the evolution of acetylene consists in the production of large masses of froth within the generator. In the ordinary way, decomposition of carbide is accompanied by a species of effervescence, but the bubbles should break smartly and leave the surface of the liquid reasonably free from foam. Sometimes, however, the bubbles do not break, but a persistent "head" of considerable height is formed. Further production of gas only increases the thickness of the froth until it rises so high that it is carried forward through the gas-main into the next item of the plant. The froth disappears gradually in the pipes, but leaves in them a deposit of lime which sooner or later causes obstructions by accumulating at the angles and dips; while during its presence in the main the steady passage of gas to the holder is interrupted and the burners may even be made to jump. Manifestly the defect is chiefly, if not always, to be noticed in the working of carbide-to-water generators. The phenomenon has been examined by Mauricheau-Beaupré, who finds that frothing is not characteristic of pure carbide and that it cannot be attributed to any of the impurities normally present in commercial carbide. If, however, the carbide contains calcium chloride, frothing is liable to occur. A 0.1 per cent. solution of calcium chloride appears to yield some foam when carbide is decomposed in it, and a 1 per cent. solution to foam in a pronounced manner. In the absence of calcium chloride, the main cause of frothing seems to be the presence in the generator of new paint or tar. If a generator is taken into use before the paint in any part of it which becomes moistened by warm lime-water has had opportunity of drying thoroughly hard, frothing is certain to occur; and even if the carbide has been stored for only a short time in a tin or drum which has been freshly painted, a production of froth will follow when it is decomposed in water. The products of the polymerisation of acetylene also tend to produce frothing, but not to such an extent as the turpentine in paint and the lighter constituents of coal-tar. Carbide stored even temporarily in a newly painted tin froths on decomposition because it has absorbed among its pores some of the volatile matter given off by the paint during the process of desiccation.
THE "DRY" PROCESS OF GENERATION.--A process for generating acetylene, totally different in principle from those hitherto considered, has been introduced in this country. According to the original patents of G. J. Atkins, the process consisted in bringing small or powdered carbide into mechanical contact with some solid material containing water, the water being either mixed with the solid reagent or attached to it as water of crystallisation. Such reagents indeed were claimed as crude starch and the like, the idea being to recover a by-product of pecuniary value. Now the process seems to be known only in that particular form in which granulated carbide is treated with crystallised sodium carbonate, i.e., common washing soda. Assuming the carbide employed to be chemically pure and the reaction between it and the water of crystallisation contained in ordinary soda crystals to proceed quantitatively, the production of acetylene by the dry process should be represented by the following chemical equation:
5CaC_2 + Na_2CO_3.10H_2O = 5C_2H_2 + 5Ca(OH)_2 + Na_2CO_3.