The great thing to guard against is leakage. If the pipes were simply buried in the ground, it would be almost impossible to trace leakage, or even to know of its existence. The income of the company might be wasting away, and the loss never suspected until the quarterly returns from the meters were obtained from the inspectors. Only then would it be discovered that there must be a great leak (or it might be several leaks) somewhere. But how would it be possible to trace them among 20 or 30 miles of buried pipes? We cannot break up the public streets. The very existence of the concern depends upon (1) the daily checking of the meter returns, and comparison with the output from the air compressors, so as to ascertain the amount of leakage; (2) facility for tracing the locality of a leak; and (3) easy access to the mains with the minimum of disturbance to the streets. It will be readily understood, from the drawings, how this is effected. First, the pipes are laid in concrete troughs, near the surface of the road, with removable concrete covers strong enough to stand any overhead traffic. At intervals there are junctions for service connections, with street boxes and covers serving as inspection chambers.
These chambers are also provided over the ball-valves, which serve as stop-valves in case of necessity, and are so arranged that in case of a serious breach in the portion of main between any two of them, the rush of air to the breach will blow them up to the corresponding seats and block off the broken portion of main. The air space around the pipe in the concrete trough will convey for a long distance the whistling noise of a leak; and the inspectors, by listening at the inspection openings, will thus be enabled to rapidly trace their way almost to the exact spot where there is an escape. They have then only to remove the top surface of road metal and the concrete cover in order to expose the pipe and get at the breach. Leaks would mostly be found at joints; and, by measuring from the nearest street opening, the inspectors would know where to break open the road to arrive at the probable locality of the leak. A very slight leak can be heard a long way off by its peculiar whistling sound.
COMPRESSED AIR POWER
The next point is to obtain a daily report of the condition of the mains and the amount of leakage. It would be impracticable to employ an army of meter inspectors to take the records daily from all the meters in the district. We therefore adopt the method of electric signaling shown in the second drawing. In the engineer's office, at the central station, is fixed the dial shown in Fig. 1. Each consumer's meter is fitted with the contact-making apparatus shown in Pig. 4, and in an enlarged form in Figs. 5 and 6, by which a current is sent round the electro-magnet, D (Fig. 1), attracting the armature, and drawing the disk forward sufficiently for the roller at I to pass over the center of one of the pins, and so drop in between that and the next pin, thus completing the motion, and holding the disk steadily opposite the figure. This action takes place on any meter completing a unit of measurement of (say) 1,000 cubic feet, at which point the contact makers touch. But suppose one meter should be moving very slowly, and so retaining contact for some time, while other meters were working rapidly; the armature at D would then be held up to the magnet by the prolonged contact maintained by the slow moving meter, and so prevent the quick working meters from actuating it; and they would therefore pass the contact points without recording.
A meter might also stop dead at the point of contact on shutting off the air, and so hold up the armature; thus preventing others from acting. To obviate this, we apply the disengaging apparatus shown at L (Fig. 4). The contact maker works on the center, m, having an armature on its opposite end. On contact being made, at the same time that the magnet, D, is operated, the one at L is also operated, attracting the armature, and throwing over the end of the contact maker, l¹, on to the non-conducting side of the pin on the disk. Thus the whole movement is rendered practically instantaneous, and the magnet at D is set at liberty for the next operation. A resistance can be interposed at L, if necessary, to regulate the period of the operation. The whole of the meters work the common dial shown in Fig. 1, on which the gross results only are recorded; and this is all we want to know in this way. The action is so rapid, owing to the use of the magnetic disengaging gear, that the chances of two or more meters making contact at the same moment are rendered extremely small. Should such a thing happen, it would not matter, as it is only approximate results that we require in this case; and the error, if any, would add to the apparent amount of leakage, and so be on the right side.
Of course, the record of each consumer's meter would be taken by the inspector at the end of every quarter, in order to make out the bill; and the totals thus obtained would be checked by the gross results indicated by the main dial. In this way, by a comparison of these results, a coefficient would soon be arrived at, by which the daily recorded results could be corrected to an extremely accurate measurement. At the end of the working day, the engineer has merely to take down from the dial in his office the total record of air measured to the consumers, also the output of air from the compressors, which he ascertains by means of a continuous counter on the engines, and the difference between the two will represent the loss. If the loss is trifling, he will pass it over; if serious, he will send out his inspectors to trace it. Thus there could be no long continued leakage, misuse, or robbery of the air, without the company becoming aware of the fact, and so being enabled to take measures to stop or prevent it. The foregoing are absolutely essential adjuncts to any scheme of public motive power supply by compressed air, without which we should be working in the dark, and could never be sure whether the company were losing or making money.
With them, we know where we are and what we are doing.
Referring to the estimates given in Table I., I may explain that the item of repairs and renewals covers 10 per cent. on boilers and gas producers, 5 per cent. on engines, 5 per cent. on buildings, and 5 per cent. on mains. Considering that the estimates include ample fitting shops, with the best and most suitable tools, and that the wages list includes a staff of men whose chief work would be to attend to repairs, etc., I think the above allowances ample. Each item also includes 5 per cent. for contingencies.
I have commenced by giving all the preceding detail, in order to show the groundwork on which I base the estimate of the cost of compressed air power to consumers, in terms of indicated horse power per annum, as given in Table II. I may say that, in estimating the engine power and coal consumption, I have not, as in the original report, made purely theoretical calculations, but have taken diagrams from engines in actual use (although of somewhat smaller size than those intended to be employed), and have worked out the results therefrom. It will, I hope, be seen that, with all the safeguards we have provided, we may fairly reckon upon having for sale the stated quantity of air produced by means of the plant, as estimated, and at the specified annual cost; and that therefore the statement of cost per indicated horse power per annum may be fairly relied upon. Thus the cost of compressed air to the consumer, based upon an average charge of 5d. per 1,000 cubic feet, will vary from £6 14s. per indicated horse power per annum to £18 13s. 3d., according to circumstances and mode of application.
A compressed air motor is an exceedingly simple machine--much simpler than an ordinary steam engine. But the air may also be used in an ordinary steam engine; and in this case it can be much simplified in many details. Very little packing is needed, as there is no nuisance from gland leakage; the friction is therefore very slight. Pistons and glands are packed with soapstone, or other self-lubricating packing; and no oil is required except for bearings, etc. The company will undertake the periodical inspection and overhauling of engines supplied with their power, all which is included in the estimates. The total cost to consumers, with air at an average of 5d. per 1,000 cubic feet, may therefore be fairly taken as follows:
Min. Max. Cost of air used £6 14 0½ £18 13 3 Oil. waste, packing, etc. 1 0 0 1 0 0 Interest, depreciation, etc., 12½ per cent. on £10, the cost of engine per indicated horse power 1 5 0 1 5 0 -------- --------- £8 19 0½ £20 18 3
The maximum case would apply only to direct acting engines, such as Tangye pumps, air power hammers, etc., where the air is full on till the end of the stroke, and where there is no expansion. The minimum given is at the average rate of 5d. per 1,000 cubic feet; but as there will be rates below this, according to a sliding scale, we may fairly take it that the lowest charge will fall considerably below £6 per indicated horse power per annum.--Journal of Gas Lighting.