Water in Mines. — Practically all coal mines are more or less troubled with water, and in a very large number of eases it has to be raised from the mine by mechanical appliances.

Part of the rain which falls on the surface sinks into the ground and percolates through the different strata. Some rocks are easily penetrated by water, others are not. The result of this is that water often lodges in certain beds such as sandstone and limestone, and when these beds are penetrated by a shaft they give off much water. If water bearing beds lie some considerable distance above the coal seams that are to be worked, the water may be confined in them by putting a watertight lining, called tubbing, in the shafts. If on the other hand the water-bearing beds are near the coal, or if the coal-seam is near the surface, then the breaking of the strata caused by the working of the coal allows the water to come down into the workings. in such cases tubbing the shaft has no effect, and the water must be removed from the workings by an adit level or by pumps.

Acid Water. — Water in its passage through the rocks often takes up a considerable amount of mineral matter in solution, and mine waters which contain sulphates, chlorides or free acids, usually cause trouble with the pumps. This trouble is due to the corrosive action of the water on the working parts of the pumps. These parts have frequently to be made of gun-metal instead of iron to enable them to withstand corrosion.

Adits. — These are mines driven from a point on the surface whose level is lower than that of the workings. Since the majority of our coalfields lie in hollows in the older rocks, and the seams are much lower than the surface, their drainage by means of an adit is rarely possible.

Winding Water. — In a few cases where the amount of water coming into the workings of the mine is small, it may be collected in a sump or lodgment at the bottom of the shaft and raised in tanks fitted below the cages. This method is seldom satisfactory, and in most cases where water has to be dealt with it is done by means of pumps.

Pumping. — Two systems of draining mines by pumps are in general use. In the one the pump is placed in the mine and is operated by a steam-engine on the surface, the power being transmitted through a line of moving rods. In the other, both motor and pumps are placed in the mine. The motive power may be transmitted to the pump by steam, compressed air, hydraulic power, or electricity. Of these methods of transmitting power electricity is the most economical and convenient, especially where long distances are involved. The pumps commonly used in mines are Bucket Pumps, Plunger Pumps, and Turbine Pumps.

In the bucket pump (Fig. 84) we have a hollow piston or bucket with a valve on its top which opens upwards. This bucket works up and down in a smooth bore pipe, called a working barrel, at the bottom of which a second valve is placed. This valve also opens upwards, and is known as the clack or receiving valve. A pipe, in the upper end of which this valve is placed, passes from the working barrel into the water, and is known as the suction or snore pipe. The upward movement of the bucket creates a partial vacuum in the space below, into which the water rises until ultimately it passes through the receiving valve and into the working barrel. On the down stroke of the bucket the receiving valve closes and prevents the water from going back, and it passes up through the bucket. Thus the water is lifted and discharged at every up stroke.

A column of water 33.87 feet high balances the atmospheric pressure of 14.7 lbs. per square inch, so that a pump of this kind must be placed within this distance of the surface of the water with which it has to deal. Owing to the impossibility of obtaining a perfect vacuum, and because of clearance spaces, the pump should not be more than from 18 to 22 feet above the level of the water to be lifted.

Plunger or Force Pump. — This pump differs from the bucket type by having a solid piston or plunger. The water is drawn up the suction pipe and through the receiving valve, while the plunger is on the up stroke just as in the bucket pump. On the down stroke the water is forced through a delivery valve and into the discharge pipe. A sectional diagram of a plunger pump is shown in Fig. 85.

Both bucket and plunger pumps when installed in the shaft are usually worked by rods driven by an engine on the surface. They provide very efficient means of dealing with the water but are cumbersome and costly to install.

Three Throw Plunger Pumps. — This type of force pump (Fig. 86) is frequently used for pumping water from dip workings, where the pump must be placed some distance away from the shaft. In such cases the power must be transmitted over a considerable distance, and for this reason electricity is almost invariably chosen. To suit electrical power where the speed of the motor is high, the pumps are made with a short stroke and run at a fairly high speed. The motor is generally connected to the pumps by spur gearing, and three separate pumps with their plungers set at an angle of 120 degrees to each other are driven from the same shaft. The object of this triple arrangement is to equalise the load on the motor. To reduce the risk of breaking the gear, the motor pinion is often made of raw hide and geared into a steel wheel, having machine-cut teeth. In many cases rotary pumps of the turbine type are now used for dip pumping.

Turbine Pumps. — Great strides have recently been made in the design of rotary pumps, and it is now possible to get pumps of this class that will raise water to a height of 1000 feet or more. These pumps are made on the turbine principle, a number of circular impellers being mounted on a single shaft, each one occupying a separate chamber. The water enters along the axis, is caught by the revolving impellers, and discharged radially. It passes from one chamber to another until a velocity — or rather its pressure equivalent — sufficient to overcome the head to be dealt with has been acquired. A pump of this type is shown in Fig. 87, while a section of a four-stage pump is shown in Fig. 88. The speed of rotation of the impellers is high, usually from 900 to 1500 revolutions per minute, and the pump is coupled direct to an electric motor, no intermediate gear being necessary. Turbine pumps of the type shown are now being extensively employed for mine pumping, and, owing to their low first cost, high efficiency, freedom from trouble, and the small space they occupy, are rapidly replacing all other types where circumstances favour their adoption. Under certain circumstances turbine pumps are not the best, as, for instance, in pumping water from the face of a dook where the work is being advanced. In such a case the water cannot be allowed to collect, and as a turbine pump cannot deal with air and water it would require constant priming. Another case is that where a very small quantity of water has to be raised against a high head. Here the pump is quite capable of doing the work, but it does not do it efficiently. In cases such as the above a ram pump is more suitable than a turbine.

Pipe Columns. — With all classes of pumps the water is carried from the pump to the point of delivery in a line of pipes. These pipes are often made of cast-iron and are usually in 9-feet lengths with flanges at each end. A joint is placed between each length, and the pipes are fastened by bolts which pass through the flanges. In some cases lengths of steel tubing are used for the same purpose. In such cases the lengths are often joined by screwing the ends into a thimble. Where pipes have to be carried up a shaft they must be carefully supported. This is done by fitting cross buntons in the shaft and fastening the pipes to them by means of collars. This method of fixing a pipe column is shown in Fig. 89.

Siphons. — Sometimes in working a coal-seam it may dip so as to form a depression in which water may collect. In such cases if the lowest point of the depression is above the level of the shaft bottom and no more than 25 feet or so below the level of the highest intervening part, the water may be removed by means of a siphon. The siphon is a line of pipes, carried over an obstruction, with valves fitted at each end. The vertical height on the suction side must be 25 feet or less, and in all cases less than the vertical height on the discharge side. The pipes are filled with water, a tap being fitted at the highest point to allow the contained air to escape. Once the pipes are full and the air tap closed, the valves are opened and water begins to flow from the higher to the lower level. The amount of water that will pass in a given time depends on the area of the pipe, the length of the pipe, and the difference in head or pressure between the two ends. Like the pump, the action of the siphon depends on atmospheric pressure. A diagram showing the general conditions involved in using a siphon is shown in Fig. 90.