The principle of the return trap has been described in "Boiler Accessories" but its practical form and application will be taken up here. The type shown in Fig. 62 has all of its working parts outside of the trap. It consists of a cast iron bowl pivoted at G and H. There is an opening through G connecting with the inside of the bowl. The pipe K connects through C with an interior pipe opening near the top (see Fig. 63.) The pipe D connects with a receiver into which all of the returns are brought. A is a check valve allowing water to pass through in the direction shown by the arrow. E is a pipe connecting with the boiler below the water line. B is a check opening toward the boiler and K a pipe connected with the steam main or drum. The action of the trap is as follows. As the bowl fills with water from the receiver it overbalances the weighted lever and drops to the bottom of the ring. This opens the valve C and admits steam at boiler pressure to the top of the trap. Being at a higher level the water flows by gravity into the boiler, through the pipe E. Water and steam are kept from passing out through D by the check A.

Return Traps 1000141

Fig. 60.

Return Traps 1000142

Fig. 61.

When the trap has emptied itself the weight of the ball raises it to the original position, which movement closes the valve C and opens the small vent F. The pressure in the bowl being relieved, water flows in from the receiver through D until the trap is filled, when the process is repeated. In order to work satisfactorily the trap should be placed at least 3 feet above the water level in the boiler and the pressure in the returns must always be sufficient to raise the water from the receiver to the trap against atmospheric pressure which is theoretically about 1 pound for every 2 feet in height. In practice there will be more or less friction to overcome, and suitable adjustments must be made for each particular case. Fig. 64 shows another form acting upon the same principle except in this case the steam valve is operated by a bucket or float inside of the trap. The pipe connections are practically the same as with the trap just described.

Return Traps 1000143

Fig. 62.

Return Traps 1000144

Fig. 63.

Return traps are more commonly used in smaller plants where it is desired to avoid the expense and care of a pump.