Q. Please give me your idea on this subject: A claims that the pipe b in trap in Figure 51 should be as high or even higher than shown. He says that a current of air passing through pipe a will syphon out the trap.

On the other hand, B claims that in trap Figure 52, pipe b should be as low as shown, or even lower. Now, if the water comes down pipe b and goes out pipe a with a rush, it will not have a chance to syphon out. Who is right or wrong?

A. The ability to resist syphon-action is not in this case affected by the location of the pipe b, since this does not affect the amount of water in the trap, the depth of seal being apparently the same in both. It should be borne in mind that to syphon out the trap there must be a partial vacuum in the pipe a - i. e., a part of the air has been withdrawn by the descending column of water, so that the atmospheric pressure is less on that side of the water-seal than at b, in which case the atmospheric pressure at b will force out the water until it supplies the vacuum, or what is lacking at a. To accomplish this the water would be forced out at a until it fell below c, so the air could pass through and make the atmospheric pressure the same on both sides. It is for this purpose that traps are vented, so that the vents may supply the air and not oblige the taking out of the water so as to get the air from the other side of the water-seal. So far as the inlet of b in Figure 51 is concerned, it is too high to be of much service to the fixture it is presumably intended to serve, since a slight lowering of the water-line will open direct communication between a and the fixture at the other end of b. In fact, it is not good practice to use one trap for two fixtures; each should have its own trap wherever practicable.

Figure 51.

Figure 53.