This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Where the blow-off pipe connects with a sewer some means must be provided for cooling the water or the expansion and contraction caused by the hot water flowing through the drain pipes will start the joints and cause leaks. For this reason it is customary to pass the water through a blow-off tank. A form of wrought iron tank is shown in Fig. 68. It consists of a receiver supported on cast-iron cradles. The tank ordinarily stands nearly full of cold water.
The pipe from the boiler enters above the water line, and the sewer connection leads from near the bottom as shown. A vapor pipe is carried from the top of the tank above the roof of the building. When water from the boiler is blown into the tank cold water from the bottom flows into the sewer and the steam is carried off through the vapor pipe. The equalizing pipe is to prevent any siphon action which might draw the water out of the tank after a flow was once started. As only a part of the water is blown out of a boiler at one time the blow-off tank can be of a comparatively small size. A tank 24" X 48" should be large enough for boilers up to 48 inches in diameter and one 36" X 72" should care for a boiler 72 inches in diameter. If smaller quantities of water are blown off at a time smaller tanks can be used. The sizes given above are sufficient for batteries of 2 or more boilers, as one boiler can be blown off and the water allowed to cool before a second one is blown off. Cast iron tanks are often used in place of wrought iron and these may be sunken in the ground if desired.
Cast Iron Seamless Tubular Steam Heater.