If large steam pumps, such as are used for fire pumps and to fill house tanks on tall buildings, were allowed to pump water direct from the city mains, they would cause considerable annoyance while operating by reducing the pressure and thus decreasing the flow of water in other supply systems in the neighborhood. Furthermore, the operation of the pump might cause water ram in the mains that would be annoying to other water consumers and damaging to the water supply system. For these reasons, also to store a supply of water on the premises to provide against shortage should water be temporarily shut off from the street mains, suction tanks should be provided in all large buildings.

Suction tanks usually consist of an open steel tank covered with wooden planking. Sometimes, however, they are enclosed rectangular steel tanks with a manhole and hinged cover, through which access may be had to the interior of the tank.

The supply pipe, a, to suction tanks (Fig. 103) is generally so very large that a ball cock of the full calibre of the pipe would be subjected to too severe a strain, hence large sizes of supply pipes are usually provided with a manifold header so the inlet to the tank can be automatically supplied through several ball cocks, b, as shown in the illustration. Suction pipes from suction tanks to house pumps are usually cross-connected to the street supply, so in case of emergency, as for instance during a fire, water can be pumped direct from the city mains. Suction tanks should have sufficient capacity to store at least one day's supply of water for the entire building; when space permits, it is better to provide capacity for two days' storage. This quantity will tide over any probable period of time that water will be shut off from the street mains.

House tanks are used to store water for the supply of buildings and should be located at least ten feet above the level of the highest fixture to be supplied. There are two kinds of tanks commonly used, wooden tanks and iron tanks. When located outside of buildings on roofs or in other exposed po-sitions, wooden tanks are generally used; when located inside of buildings, iron tanks are generally used. During warm weather moisture condenses on the outside of iron tanks, and if not cared for will drip to the floor and wet both floor and ceiling below. To prevent this a drip pan should be placed under all iron tanks and a drip pipe from the pan extended to some convenient sink or connected to the overflow pipe from the tank.

Lead-lined wooden tanks were formerly extensively used, and in some localities are still, to a limited extent, but owing to the liability of carbonates or sulphates of lead being dissolved from the lining and poisoning the water, lead should not be used for tank linings, particularly in localities where the water is soft.

Fig. 103

Copper-lined wooden tanks are sometimes used. From a chemical standpoint, copper linings are not so objectionable as lead, particularly when the copper is tinned; however, copper linings present so many joints and seams that some of them are liable to leak, and, in some waters, soldered copper joints rapidly disintegrate, owing either to a chemical or galvanic action of the metals.

In extremely tall buildings, fixtures on the lower floors are supplied with water direct from the street mains; the upper floors are supplied with water from the house tank on the roof, and intermediate tanks are installed, so that not more than eight floors of the building are supplied with water from any one tank. In such installations the house supply from the roof tank should be cross-connected to the house supply from all the intermediate tanks and to the house supply for the lower floors, so that in case of necessity the entire building can be supplied with water from the house tank, which can be filled by pumping from the suction tank.

Storage tanks should be provided with overflow pipes of sufficient capacity to safely carry off the greatest quantity of water likely to be discharged by the supply pipe. It is a safe rule to allow for the overflow pipe twice the diameter or four times the sectional area of the supply pipe. Overflow pipes from tanks located on roofs of buildings may discharge onto the roof. Overflow pipes from tanks located inside of buildings should discharge into a properly tapped and water-supplied sink. Under no circumstance should they connect directly to the drainage system.

The size of storage tanks depends upon the number of people to be supplied. They should have sufficient storage capacity for one day's supply, to tide over possible periods of breakdown of pump or boiler. When figuring the capacity of storage tanks, 100 gallons of water per day per capita should be allowed in hotels, hospitals, apartment houses and public institutions.

The general arrangement of pipe connections to a house tank is shown in Fig. 104. The cleanout or emptying pipe is valved and connected to the overflow pipe. The house supply extends a few inches above the bottom of the tank to prevent sediment entering the pipe. Below the valve that controls the house supply is connected a vent pipe to admit air to the house supply and permit it to empty when the valve is shut off. A vapor or relief-pipe from the highest point in the hot water supply system bends over the tank and thus permits the escape of steam. The pump may discharge into the house tank in the manner indicated when the pump is not controlled automatically. When it is, the pump pipe should enter the tank through the bottom and be controlled by a balanced float valve. A drip pan, a, under the tank and extending a few inches on all sides of it, catches the water of condensation and discharges it through the waste pipe, b, into the overflow pipe. When a tank is supplied with water by a pump that is not automatic in operation, a tell-tale pipe should be run from a point in the tank about two inches below the level of the overflow pipe to the engineer's sink. Water flowing through the pipe then notifies the engineer when the tank is full.