As already stated, the trap seal is broken more frequently by siphonage than by any other means. In fact, siphonage is the greatest obstacle that confronts the trap, and it is the one thing that prevents the attainment of the perfect trap.

In order to study siphonic action in connection with the trap, it will be necessary first to consider the action of the simple siphon, and for this purpose Fig. 34 may be used. The purpose to which the siphon is applied is the transferring of liquids from higher to lower levels, and in form it consists essentially of a tube so bent that one arm is longer than the other. The short arm dips into the liquid to be moved, and the long arm delivers the liquid at the lower level. In order that the liquid may be carried over by siphonic action, it is necessary that the air be exhausted in the long arm, or in other words, that a vacuum be created at this point.

At the instant that a vacuum is by any means formed in the long arm, atmospheric pressure acting upon the surface of the liquid, no longer having any force counterbalancing it, forces the liquid up the short arm, and over the crown of the siphon into the long, or delivery, arm. Unless broken by other means, siphonic action will continue under these conditions, until the water falls to a point below the bottom of the short arm. When it is considered that atmospheric pressure is approximately 15 lbs. per square inch, it will be seen that with a nearly perfect vacuum, siphonic action is strong and rapid. It is necessary that one arm of the siphon be shorter than the other, in order that the heavier column of liquid in the long arm may exert a pulling force or suction upon the liquid in the short arm. If it were not for this relative difference between the two arms of the siphon, the liquid in each would fall back by virtue of its own weight, and the continuance of siphonic action would be impossible. Therefore, the greater the relative difference in length between the two arms, the stronger will be the action of the siphon, unless, indeed, the long arm be carried to such a length that friction overcomes the advantage gained.

The action of the siphon may be destroyed by admitting air at or near the crown, a principle which, as will be seen in the following chapter, is made use of in connection with the plumbing system, to prevent the siphonage of traps. While presenting what may be considered as the greatest obstacle which the plumbing system has to overcome, siphonage is also one of the greatest aids in procuring a high grade of plumbing construction, inasmuch as upon the principle of the siphon depends the action of many excellent plumbing fixtures, valves, and other devices. By referring to Fig. 35 it may be seen wherein the connections of both S and drum traps present the same conditions to be noted in the typical siphon shown in Fig. 34. The arm A of the S trap constitutes the short arm of the siphon, and B, the long arm, with the relative difference between the lengths of the two arms often very great. A vertical column of water in the trap of the size of the waste pipe, constitutes in the drum trap the short arm of the siphon, and D the long arm. With atmospheric pressure in each case acting in the direction of the arrow E, all that is necessary to produce the siphonage of the contents of each trap is the formation of a vacuum or partial vacuum in the long arm. There is usually abundant opportunity in the plumbing system that is unvented, for the formation of this vacuum. In Fig. 35, for instance, it might be formed by the passage of a heavy volume of waste past the entrance of the branches on the two waste fittings on the soil-pipe line. The effect and various causes of siphonage in the plumbing system may best be studied from such an illustration as Fig. 30, which shows a plumbing system such as commonly installed years ago, with no line of soil pipe venting itself through the roof, and without trap ventilation of any description. In such work, the condition most favorable to siphonage is to be found in the fact that the vertical line of soil pipe terminates at the top fixture instead of continuing through the roof with an open end. Suppose now, that a large body of water enters the vertical line of pipe from several fixtures at the same time. Even though the volume is not sufficient to entirely fill the bore of the pipe, owing to its spiral motion in falling, the effect may be nearly equivalent, in which case the air in the pipe is forced ahead of the falling column of water, and a partial vacuum thereby formed in its rear, the effect of which may be felt to a greater or less degree on every trap in the building. By this means each of the traps on the top floor may be siphoned, even the 4-in. water-closet being susceptible to its action. It may readily be seen that in large buildings, where there is a large amount of plumbing of this old-time type, the conditions would often be more severe than those shown in Fig. 36, and the siphonage of traps therefore even more certain. Again, let a heavy body of waste pass down the vertical line of soil pipe from the three floors above the basement. As this column enters the horizontal line, it is naturally retarded; the waste, as a consequence, backs up and fills the pipe at this point, and in attempting to pass out produces a partial vacuum which is certain to be severely felt by the water-closet trap in the basement. It will be noted in Fig. 36 that the waste from the upper bath and lavatory, and from all three fixtures on the second floor has little pitch and is of considerable length. Such conditions are most favorable to trap siphonage, for the lack of fall allows the waste to set back and fill the pipe, and as in the instance noted above, in attempting to. pass off, the waste partially exhausts the air in the pipe, and the siphonage or partial siphonage of the traps follows. It should be understood that very often the trap is affected by siphonic action only slightly, possibly only a few drops of the seal being lost at a time. In the event that the fixture is not in constant use, however, it becomes merely a matter of time when enough of the seal will be thus gradually lost to make its destruction complete. Therefore it is plain that even though siphonic influence may not be severe in the case of a certain trap, it still presents great danger.

Fig. 34   The Siphon.

Fig. 34 - The Siphon.

Fig. 35.   The Siphonage of Traps.

Fig. 35. - The Siphonage of Traps.

Fig. 36.   Siphonage of Traps in Old Fashioned Unvented Plumbing System.

Fig. 36. - Siphonage of Traps in Old-Fashioned Unvented Plumbing System.

A stoppage in the waste outlet may have a result similar to that mentioned of fixtures having long wastes without proper pitch. Much old work was put in like that of the kitchen sink and laundry tubs, and here again is abundant opportunity for siphonic troubles. A heavy flow of waste through the horizontal line of soil pipe might siphon either sink or laundry-tub trap; a heavy flow from either fixture might siphon the other; and either fixture might siphon its own trap. Thus it may clearly be seen that the old style plumbing system was susceptible in many different ways to trap siphonage, and the fatal results coming from ignorance of the conditions which actually existed to such a great extent in the plumbing system, and ignorance of the proper remedy for such troubles can hardly be estimated, but must certainly be of great extent.

Fig. 37 shows another unventilated plumbing system, similar in many respects to that of Fig. 36, in which the siphonage of each fixture is a matter of possibility. These illustrations are not exaggerated in any way, but show truthfully the manner in which the unvented plumbing system, with its great and serious defects, was formerly installed.

Fig. 37 shows not only the great possibility of the siphonage of the fixture traps, but shows also other minor defects common to the old-time plumbing system, such as the use of tee fittings on the drainage system, the entrance of waste into the heel of the lead bend, etc.

For the purpose of comparison. Fig. 38 is given, showing a complete modern system of plumbing, with its main line of vents and its branch vents. The great advance which has been made in plumbing installation since the days when such work as that shown in Figs. 36 and 37 was considered sanitary, may easily be seen in comparing the illustrations.

Fig. 37. Siphonage in Unventilated Plumbing System.

Fig. 37.-Siphonage in Unventilated Plumbing System.

Fig. 38.   The Vented Plumbing System.

Fig. 38. - The Vented Plumbing System.

The application of the principle of trap venting has, without doubt, been the chief cause of such vastly improved sanitary conditions in the plumbing system. It is the venting of fixture traps which has proved the only practical means of preventing trap siphonage, and this subject in theory and in its practical application will be taken up in a special chapter on the subject of venting.