A blow-off tank and connections are shown in Fig. 46. Water enters the condensing tank from the boiler through the pipe a. When released from pressure, some of the water instantly flashes into steam and escapes to the atmosphere through the vapor pipe, b. Hot water entering the tank causes cold water from the bottom of the tank to overflow through the pipe c, to the house sewer outside of the main drain trap. An equalizing pipe, d, admits air to the overflow pipe and thus prevents the water being siphoned out of the tank.

Size Of Blow-Off Tanks

A blow-off tank should be large enough to hold one gauge of water from the steam boiler. In blowing off a steam boiler, one gauge of water is the most that should be blown off at one time, and if the tank is large enough to hold that quantity it will be sufficiently large for all purposes. The size of tank required can be found by multiplying the length of the steam boiler in feet by the diameter in feet and multiplying the product by one-third (4 inches being considered the depth of one gauge of water). This product will be the capacity in cubic feet of the tank required.

Example

What capacity blow-off tank will be required for a steam boiler 18 feet long and 5 feet in diameter?

Solution

18 X 5 X 1/3 = 30 cubic feet, and 30 X 7.5 = 225 gallons capacity.

Stock sizes of blow-off tanks can be found in the following table:

Table X - Dimensions And Capacities Of Tanks

When ordering blow-off tanks the order should be accompanied by a sketch showing the location and size of the several outlets.

When several boilers are connected in battery one blow-off tank will suffice for all, provided sufficient time is allowed between blowing off the several boilers for the water in the tank to cool, or providing provision is made for replacing the hot water with cool water.

Size Of Tank Outlets

Blow-off outlets to steam boilers are seldom over two inches in diameter; therefore the inlet to blow-off tanks need not be over 2 inches, iron-pipe size. The outlet, however, should be 2 1/2 or 3 inches in diameter, so the water will enter the sewer at a slow velocity. The vapor pipe should be 2 inches in diameter, and if it extends over 100 feet should be 2 1/2 inches in diameter. Drips from high-pressure plants do not require a condensing tank but may connect to an atmospheric steam trap discharging into the house sewer outside of the main drain trap.

Blow-offs from low-pressure boilers need not pass through either a condensing tank or a steam trap, but may discharge freely into the house sewer outside of the main drain trap.