The Forth Bridge/Air Compressors

by Wilhelm Westhofen

Air Compressors.

There was nothing in these different from any ordinary type of compressors. They consisted of a pair of horizontal engines coupled, and directly acting upon a pair of double-acting air-compressing cylinders. There were two pairs of 16+¹⁄₂-in. cylinders with 2-ft. stroke, and three pairs of 12-in. cylinders with 2-ft. stroke, all working at 60 lb. steam pressure. They were run up to any speed required to produce the necessary maximum pressure, which in the case of Inchgarvie on one occasion amounted to 37 lb. per square inch in the air-chamber of the south-west caisson. The air-compressing cylinders were water-jacketted, with a continuous current of cold water passing through. For the service of the rock drills inside the working chamber, a separate engine was used, which forced air down by a separate pipe lead at 70 lb. per square inch—the rock drills receiving, of course, the benefit of the difference between that pressure and the prevailing one in the air-chamber only, as they had to discharge into the general pressure of the working chamber.

On the top of the men's air-lock a whistle was set which worked by air pressure and which could be regulated from below; by means of this a constant communication was kept up, and more or less air pressure put on in accordance with requirements.

The pipes supplying the air were laid from the compressors straight to the caisson side, but the joint to the pipes inside the caissons had to be made by a long piece of flexible tube, the end of which was attached to a check valve placed upon the air-lock for the admission of the men. The air was forced down this shaft into the working chamber, thus preventing the ascent of any foul air in these parts.

A supply of water for flushing purposes was also laid on, being taken from an overhead tank set about 40 ft. above the level of the staging. Of great importance both for the safety and convenience of the men, and for the progress of the work, were the air-locks. Those by which access was got to, and exit from, the air-chamber, consisted of an inner portion, 3 ft. 6 in. in diameter, in continuation of the ascending shaft, and a circular space all round it about 21 in. wide and 6 ft. high. (See Figs. 38 and 39.) The outer chamber was divided by two partitions into two equal spaces. Each of these spaces was provided with a square door giving communication on the one side to the air-shaft, and on the other side to the outer atmosphere. All doors opened to the inside. To gain admission to the air-chamber it was necessary to enter one of the outer spaces, to close the door (which was pressed against an india-rubber joint), and by turning a small cock to gradually admit compressed air. As soon as the pressure on both sides of the door leading into the air-shaft was equal, this latter fell open and the air-chamber could be reached by an iron ladder fixed in the shaft. To return from the air-chamber to the outside was the reverse of this, the pressure from the outer chamber being exhausted by opening a small cock communicating with the atmosphere.

AIRLOCKS ON CAISSONS FOR ADMITTING AND REMOVING MATERIAL.

The cocks admitting pressure from within, or allowing the same to escape, were made of small bore, to prevent any one from making this change too quickly, as this would not be unaccompanied by risk of injury to the person exposed to it. One man was seriously injured by the india-rubber joint of one door suddenly giving way and exhausting the pressure in an instant of time, instead of taking from one and half to two minutes. Fortunately, the subject was a strong and healthy person, and well accustomed to high-pressure work, otherwise the result might have been fatal. As it was, there was considerable bleeding from the nose, the mouth, and the ears, and some pains in the limbs. The lock for admitting or removing material was different in construction. (See Figs. 40, 41, 42, 43, 44, 45.) The principal portion of it was also a part of the 3 ft. 6 in. air-shaft closed at top and bottom by a sliding door. A recess was left at each end for these doors to enter into when drawn open. and there was a further recess which contained the winding drum for lifting and lowering the buckets. The sliding doors were worked by horizontal hydraulic rams, but could, in the absence of water pressure, be moved by rack and pinion and handwheel. By an interlocking arrangement of simple construction it was impossible to open both doors at once, a contingency which would have been fatal to the men working in the air-chamber below. The winding drum was driven by a worm and wormwheel outside the lock, and actuated by a pair of ordinary reversible engines, the main shaft which carried the drum being provided with air-tight glands at both ends where passing the sides of the air-lock. A chain passed round the drum and over a snatchblock suspended to the under side of the upper door and sliding out and in with it, thus bringing the point of suspension immediately into the centre of the shaft.

AIR-LOCK AND HOISTING GEAR FOR CAISSONS.

The mode of working these looks was as follows: A 3-ton steam crane with fixed jib (see Figs. 36 and 51), but provided with quick-moving slewing gear, was set up near the lock so as to centre the air-shaft with its heaving chain; the top door of the lock was then drawn back and the skip or bucket, about 3 ft. in diameter and 4 ft. high, holding about 1 cubic yard, was lowered down into the lock and the shackle of the inside winding chain attached to it. The upper door was now closed, and a slight turn of the engines tightened the chain and lifted the bucket just clear of the lower door. A cock, or valve, which communicated with the compressed air in the working chamber, was now opened and pressure thus admitted to the lock. The sliding doors were provided with india-rubber joints, and small thumbs or tappets, actuated by a lever, were used to force the door hard up against the india-rubber, in addition to which the gradually increasing air pressure helped to close the joint. When the upper door was thus secured, a turn of the interlocking wheels set the lower door free; this was now drawn back, and the bucket was lowered right down to the bottom of the working chamber, and there filled with the spoil of the excavation, or else the chain was taken off and attached to one already filled. A signal was then given, the bucket was drawn up into the empty lock, the lower door closed, the compressed air let out of the lock, the upper door drawn back, the winding chain taken off, and crane chain attached, the bucket drawn up and discharged on a shoot over the side of the caisson, and the same process repeated as before described. In the drawing shown (Fig. 41) it will be noticed that the bucket in its ascent in the lock touched a lever, and gave indication of its whereabouts by a steam whistle; but, as an additional precaution, small wheel gear was placed in connection with the winding drum-shaft, and, by means of a dial and pointer, indicated the position of the bucket at all points of its course.

The lever K acted upon the long rod L, which was so arranged that one of its ends was always within a hole drilled in the frames of the sliding doors, and that thereby either one door or the other was made absolutely immovable.

The two valves admitting water to the rams working the sliding doors are seen at D D close together; each carried a handwheel, out of which a segment is cut, and into this the rim of the other wheel fitted. In the position here shown the upper valve is closed, and its handwheel cannot be turned unless the handwheel of the lower is turned round to bring the cut-out segment exactly opposite. In that position, however, the lower valve would be closed, and it is, therefore, easy to perceive that, so long as the handwheels remained attached to the valves, it was impossible to open both simultaneously.

Since no workmen passed through these locks, it was not necessary to admit or discharge the air pressure by degrees; the inlet and outlet cocks were, therefore, large, and acted expeditiously to reduce the loss of time.