The water supply is vital to the life of the furnace plant and must be continuous, since the failure of the water supply on the tuyeres for two seconds would almost certainly cause them to be burnt, and only a few seconds more would be necessary to burn the coolers and cooling plates as well. For this reason no amount of care expended to secure a reliable and constant water supply to the furnace is too great.

It is obvious that even with duplicate units in the pumping plant there must be an interruption of a few minutes from time to time in the supply, and it is therefore universal in good modern practice to have a stand pipe with a capacity sufficient to take care of the furnace water supply for a limited period. What this period should be depends upon the judgment or, one might almost say, the individual caprice of the designer of the plant. I have seen some plants at which the stand pipe looked hardly bigger than the smoke pipe of a domestic stove, while at many others it is of the size that would do credit to a large city.

It is probable that the stand pipe should be sufficient to maintain a full supply of cooling water on all the furnaces for at least one hour. Then in case of a serious failure of the pumping plant the furnaces would be shut down and the water supply slacked to a very small percentage of its normal flow and in this way they would be saved from injury for several hours. In case of necessity the tuyeres and even the coolers can be pulled if it is obvious that the water supply cannot be restored before the stand pipe supply gives out.

There are three types of stand pipe in common use: first, the straight cylindrical pipe like a stack, second, the elevated tank (generally in modern practice having a spherical or parabolic bottom) supported on a steel tower, and third, a flat tank of relatively small depth placed on top of some building for which it acts as the roof.

The first type is the poorest for the reason that only the upper third of it is useful for storage since when the water falls below that level the head is insufficient to maintain a proper supply. Or if the tank is built so high that a greater proportion is available then the water must be pumped much higher than is necessary to obtain the desired amount of storage, with consequent waste of power and high cost of equipment.

The second type is that which is commonly used as the stand pipe for cities if no hills are available, and for large plants. The tank is elevated to such a level that its whole contents are available for a water supply, and the excess of the maximum over the minimum height of the water level in it is small, so that not much excess pumping is required.

The third type is not so common now as it was a few years ago, but has much to recommend it, embodying as it does a very large storage with a very small fluctuation in height, and being supported on the walls of the building whose roof it forms, its expense is not excessive, although of course very heavy girders across the building are required to sustain its weight.

At some fortunate plants which receive their water supply by gravity little or no tank is used because of the slight probability of interruption to the regular water supply, there being no machinery to fail.