There are two styles of service sinks (slopsinks): the trap-to-wall and the trap-to-floor. Theyare used for disposal of wash water, filling swabbuckets, and washing out swabs. The trap-to-walltype requires a 2-inch or 3-inch waste pipe; thetrap-to-floor, a 3-inch waste pipe. For both types,if copper tubing is used, a one size reduction isallowed.Scullery sinks are large sheet metal sinks usedfor washing large pots and pans and for generalscouring purposes. The large amount of greasethat usually passes through a scullery sink makesa 2-inch waste pipe necessary.Drinking fountains carry only clear waterwastes and a 1 1/4-inch waste pipe is suitable. Anindirect drain (covered later in this chapter) shouldbe used.Sizing Sanitary Collecting SewersThe design and sizing of collecting sewers, thesubtrunks, and the main trunk lines are providedby engineers. However, the UT should understandthe factors that contribute to the design and therequirements that must be met.While the unit system is used to size thebuilding sanitary piping and the building drain,the sewage quantities used in sewer designnormally are computed on a contributingpopulation basis. The population to be used indesign depends upon the type of area that thesewer is to serve. If the area is strictly residential,the design population is based on full occupancyof all quarters served. If the area is industrial,the design population is the greatest numberemployed in the area at any time. There are ex-ceptions to the general rule that sewers must bedesigned on a population basis. Among these ex-ceptions are laundry sewers and industrial-wastesewers. The per capita contribution for sewerdesign varies. Typical values are 100 gallons perperson per day for permanent residents and 30gallons per person in the industrial area per 8-hourperiod.The sizing of the sewer includes the averagerate and the extreme (peak) rate of flow (whichoccurs occasionally). The ratio of the peak rateof flow to the average rate of flow may vary withthe area served, because the larger the area or thegreater the number of persons served, the greaterthe tendency for flow to average out. Typical peakflows might range from 6 for small areas downto 1.5 for larger areas.An allowance for infiltration of subsurfacewater is added to the peak flow to obtain thedesign flow. A typical infiltration allowance is 500gallons per inch of pipe diameter, per mile ofsewer per day.Additional capacity to provide for populationincrease is usually included for areas that are likelyto continue to develop. Provision of approxi-mately 25 percent additional capacity over theinitial requirements is advisable.Each length of pipe from one manhole to thenext is sized to carry the design flow. However,to help prevent clogging and to facilitatemaintenance, a minimum size is usually specifiedwhich may be larger than is necessary to carry thedesign flow at the upper ends of the system.Typical minimum sizes are 6-inch pipe for houseand industrial-waste sewers and 8-inch pipe forall other sewers.It is sometimes the practice to select a pipe sizethat will carry the design flow when the pipe ishalf full, thus allowing for expansion. More often,however, sufficient safety factors in the futurepopulation estimate and the peak flow factor areincluded so the pipe may be designed to carry thedesign flow when flowing full.The formulas or tables used in sizing the pipeare based on experiments and experience. One ofthe factors taken into account is the roughnessof the pipe. Asbestos-cement pipe, for example,is smoother than concrete pipe. Because there isless friction on the inside of the asbestos-cementpipe, it will carry a greater flow than concrete pipeof the same size.Another factor is the slope at which the pipewill be laid. The slope will generally be determinedby the fall available on the natural ground areathrough which the sewer runs. The plans forcollecting sewer systems generally show slope (orgrade) in terms of fall per hundred feet. Slope issometimes expressed as a percent rather than ininches per foot. A 1 percent slope means 1 footof frill in a 100-foot length of pipe, or about 1/8inch per foot. A 0.5 percent slope (6 inches in 100feet is about 1/16 inch per foot.Table 7-6 gives the minimum slope for someof the most commonly used pipe sizes. The slopeshould remain constant in the section betweeneach manhole. Each section between successivemanholes should be analyzed and the slope forthat particular section determined. If the fall isrelatively steep, the velocity of the flow is fasterand a smaller pipe size may be used. If the slopeis relatively flat, the velocity is slower and a largerpipe size may be used. In the larger pipe, the depthof flow may decrease to such extent that thevelocity might be no greater than a smaller pipe7-8

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