Fig. 156. Crown Vent Stack and Waste Stack Standing Close Together, Giving Loop Effect in Pipe Ventilation..
Fig. 157. Method of Securing Loop.
Near Vent Stack.
To avoid separate stacks for scattered fixtures, what is termed the continuous system of soil pipe is frequently employed when practicable. This means offsetting the main so as to be able to include all the fixtures of a toilet-room without making long branch wastes. If vent lines are also offset in this manner, some provision for water-washing the offset should be made, as the products of corrosion or other foreign matter might otherwise fall into and choke the bend at the foot of the upper vertical part. Especially is this true when plain wrought pipe is used. Lavatory wastes are generally used to wash vent lines in such cases.
Some city ordinances permit the continuous system practically without vents, merely requiring the fixture connections to be not over 3 feet in length, and requiring either vents or non-siphoning traps where the stack cannot be brought within reach of the 3-foot limit placed on branch connections.
A plan of offsetting, some modification of which may be used in any kind of system, is shown in Fig. 159, which makes plain the work of offsetting soil waste and vent lines without incurring the risk of having trouble with the vent pipe sooner or later. It provides for
Fig. 158. Method of Ventilating.
Pipes where Neither Waste throwing the corrosion of the vent line, both above and below the offset, into the soil line, where it will be washed into the sewer by the water discharged from the closets and other fixtures. By simply offsetting the vent line, the corrosion from the pipe above the offset will fall into the bend, drift out into the horizontal part slightly, and finally choke up the horizontal vent altogether. As shown by the engraving, commencing with the main soil line at the first fixture, a branch line is made, and the branch then becomes the main soil line, leaving the vertical part for the vent. Next comes the offset, and after that another branch line for soil fixtures, again leaving the vertical pipe for the vent, so that whatever falls down the vent, either above or below the offset, lands in the soil pipe and is carried away with the water. With this arrangement, the only possible chance for the vent to clog with corrosion is in the horizontal part of the vent offset. What corrosion takes place in a piece of horizontal pipe, is not sufficient to warrant consideration in itself. There is no other corrosion to be taken care of, except that which forms in the few feet of vertical pipe between A and B, which will not be enough to restrict materially the area of the pipe. It is best to make the piece of pipe between A and B as short as possible.
Stack nor Crown Vent Stack are Near the Fixture.
With the continuous system, several offsets, simple or more or les3 complex, as shown in Fig. 159, may be necessary in the same stack, according to location of fixtures and the scheme of venting and trapping. Fig. 153 shows a group of fixtures piped diametrically opposite to the continuous stack idea. The main stack does not deviate in favor of odd fixtures. Regular open wall-traps are used. The crown vents are assembled into one stack, and carried up independently or into the stack above the highest fixture. As before stated, the plan shown in Fig. 153 is faulty in that it favors evaporation of the trap seals by putting the extra duty of a line-vent current on the siphon or crown-vent branch.
Fig. 159. Method of Offsetting Soil Waste and Vent Lines..
Anti-siphon traps often simplify ventilation problems, especially in awkward situations where it would be very difficult to vent a fixture properly with pipe. Fig. 160 illustrates an example of this kind, in which non-siphoning traps are used on bath and lavatory without any form of crown or branch line vents. In good practice, bath traps are placed convenient to reach, having screw-top hand-hole with cover in full view at the floor-level.
Soil Stacks. The size to make a soil stack is largely a matter of opinion. There are examples of 10-inch stacks serving 40 closets with the usual complement of lavatories and urinals. There are also instances where as many as 75 closets and numerous other fixtures all discharge into a 5-inch stack which has never given any indication of being too small. Although common usage requires a 4-inch soil stack, there seems little advantage in adhering to this dimension in small and simple installations. When the plumbing was designed for the city of Pullman, 111., more than twenty years ago, 3-inch soil stacks were used for small dwellings, and in some cases they were placed in a party wall, so as to afford service for two adjoining houses. The plumbing regulations of Washington, D. C, have allowed for some years past the construction of 3-inch soil stacks for dwellings having only a single bathroom, and the practice has been justified by favorable results. When it is considered that the outlet of a closet is rarely more than 2 1/2 inches in diameter, it appears that a size smaller than 4 inches is often allowable.
Fig. 160. Anti-Siphoning Traps Dispensing with Necessity for.
The size does not increase with the number of fixtures. Very few of a hundred closets in a building would ordinarily be flushed simultaneously. A 5-inch stack would answer well for 100 closets in a tall building where the toilet-rooms are superimposed, as shown in Fig. 161, which outlines the soil, waste, and vent pipes of several groups of fixtures, rain-water leaders, etc. If the same number of closets were at one elevation, and the fall only moderate, common sense would dictate a 6,8, or 10-inch line, with 4-inch fixture branches. The velocity with which the water will flow away should be a prime factor, but sizes in soil and waste pipes are far more a matter of empiricism than in supply work. A soil pipe not too large is self-scouring in a sense. This point is erroneously argued in favor of small waste pipes. If a soil pipe too small for the duty should be installed, ordinary usage would develop the fact quickly. But in a Waste outlet, where grease is likely to accompany the water, a pipe large enough to carry the waste easily when the pipe is new, may become choked after a considerable period of time, and merely because it is of the size so-called "self-scouring." A house line which may be much too large for the waste will be likely to choke from floating matter adhering to the sides above the water line until overhanging ridges are formed that break down in the channel. Being too heavy for the water to push along, this matter acts as a dam, and complete stoppage soon results. This is why large sewers are built with elliptical bottom section. Having variable flows to take care of, the depth of water produced by ordinary usage cleanses the conduit, and keeps it in much better condition than if round conduits of the same capacity were employed.