This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
192. The fittings commonly employed in steam piping are objectionable for hot-water heating apparatus, because of the great resistance which they offer to the flow of the water, due to the angles being too abrupt. The enlargements commonly made in pipe fittings are of little consequence when steam flows through them, but they retard the flow of water too much in a hot-water heating system, because the motive force of the current is so small in proportion to the amount of resistance offered. Elbows for hot-water service should be made with a radius equal to five times the internal diameter of the pipe, as shown in Fig. 69. Such elbows are commonly made of wrought-iron-or steel pipe, bent up to shape, and, consequently, are commonly called bends.
When common pipe fittings must be employed, the evil may be mitigated somewhat by carefully reaming out the ends of the pipe. The common screw union should never be used in hot-water piping; the right and left coupling should always be employed instead. In large pipes, only the flanged unions should be used.
193. The requirements of valves used in hot-water heating apparatus are very different from those employed in steam heating or in ordinary plumbing. In the latter cases they must close with sufficient force to be tight against considerable pressure, but in hot-water heating they require only to check or direct a current of water having but a very small propelling force. The stems must be packed with equal care, however, to prevent leakage, and the valve bodies must be equally strong, to resist static pressure and rough usage; but the valve proper, i. e., the part which serves to shut the passage, may be of very light construction.
Globe valves offer so much resistance to the passage of water that they should not be employed in hot-water apparatus at any point; therefore, gate valves should be used exclusively in all the piping of a hot-water system.
To prevent the water in a radiator from freezing when the radiator is not in use, a common practice is to have a small hole (about 1/8 inch in diameter) through the valve so that a small circulation will always be maintained in the radiator when the valve is closed.
Air Vents. Any kind of small valve or petcock will serve as an air vent, for hand regulation; but there are many situations where the venting must be performed automatically. In automatic vents the escape valve must be controlled by a float, so that it will remain closed as long as water is present, and will open only when the water is displaced by air. Air vents for steam heating are frequently constructed with a float which serves to close the vent and prevent the escape of water, but the float in a hot-water air vent operates in a very different manner. The buoyancy of the float, when surrounded by water, is depended upon, primarily, to close the valve; therefore, it should be constructed in such a way that it can never fail to be buoyant.
The changes which occur in the temperature of the water in hot-water heating apparatus cannot be utilized to control the air vents. ______