The trap and its vent are so closely allied that it is best to consider them under the same heading.
The trap is a vessel containing a body of water, the duty of which is to obstruct and prevent the entrance of sewer air and gases into the house. All plumbing ordinances recognize the necessity of a trap under each fixture, and upon the application of proper principles in its construction, installation, and venting, a large part of the successful operation of the modern system of plumbing depends.
Plate XI. Connections For S-Traps - Venting
Connections. Plate // for S Traps
A trap to be entirely satisfactory and sanitary should possess a good seal, be self-scouring, non-siphonable, have the least possible opportunity for the collection of filth, have no partitions within itself, and depend upon no mechanical contrivance to make a seal.
To secure all these features in the same trap is a difficult matter, but the claim is made for several traps now on the market that they meet these requirements, and the non-siphonable requirement having been solved, they require no venting.
If an absolutely non-siphonable trap could be produced, there would be no need of the venting system, and the cost of the average plumbing system would thereby be reduced approximately one-third.
It is true that several traps have been introduced which have withstood severe siphonage tests remarkably well. A very important question arises, however, as to what results these traps will show after they have been in service for a time, become fouled and in other ways reached the trap's normal condition. Some few plumbing ordinances now allow the use of these so-called non-siphonable traps without the use of the trap vent. The vast majority of ordinances, however, still adhere to the venting of the trap as a safeguard against siphonage, and it would seem at the present time a wise stand to take.
Before considering the special subject of S-traps, it will be well to consider some of the general features of the trap question.
By the seal of the trap is meant the depth of water between the outlet of the trap and the dip, that is, the depth of water which prevents the entrance of gases from the sewer.
A safe depth of seal is 2 in.
A much greater depth of seal might be secured for many traps, but the argument against it is that it presents a larger body of stagnant waste than is necessary. A small seal is dangerous, as it may more easily be destroyed by evaporation. Evaporation is a great menace to the trap seals of fixtures which do not have their seals renewed in the everyday use of the fixture; and the conveyance onto the trap seal of air through the trap vent increases the evil.
Internal partitions are dangerous, for sewer gas may pass into the house through defects that may exist in the partition above the water line.
Formerly traps with mechanical seals were much in use, but are now generally prohibited. The mechanical device employed was usually a heavy ball or float, which gave opportunity for the collection of grease and other filth about itself, resulting in the stoppage of the trap.
The trap seal may be destroyed by back pressure, capillary attraction, momentum, evaporation, and siphonage.
The trap seal may be forced by back pressure, that is, the pressure of gases generated in the sewer.
This evil has been practically eliminated by carrying the vertical stacks through the roof, but was a serious matter in the old-style system, in which each stack ended at the highest fixture.
The action of capillary attraction takes place in the trap when threads, pieces of cloth, etc., happen to dip into the seal and extend over into the outlet. By this means, a drop at a time, the seal may be, and often is, broken. There is no remedy that can be applied to this evil, for its existence is never known. A trap may lose its seal by momentum, that is, in flowing out of the trap, the rush of the waste is so strong that it may carry a part of the seal with it.
This is the tendency in some traps working on the centrifugal principle. In these traps the waste inlet and outlet are on a tangent, resulting in a whirling motion which is so strong as to endanger the seal. These traps have great scouring qualities, which is an excellent feature.
Occasionally traps on top floors may lose a part of their seal by its being blown out by gusts of wind passing over the top of the stack.
Siphonage, however, is the worst evil which the trap has to contend with. For the purpose of the consideration of the action of siphonage it is considered that the trap in Fig. A, Plate II, is without a vent.
In that case, if a vacuum or partial vacuum were formed by any means in the lower part of the trap outlet, the atmospheric pressure exerted on the house side of the trap seal would force the contents out of the trap into the waste pipe. In other words, the contents would be sucked out of the trap. If conditions are such that a vacuum is produced as above, the only way in which siphonage of the trap can be prevented is by bringing a supply of air into the trap at or near its crown.