Agenda - 11-23-1987 - Laserfiche WebLink

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4 , 128 �2 d SYSTEMS DESIGN CONSIDERATIONS ✓ where as gravity lines must be Once the design engineer has made a decision to design a pressure aligned in a continuous down- • sewer system, he must then decide ward slope. Deep cuts for gravity on design parameters that would lines become quite expensive, effect the operation of the system, especially where rock and high i The most important considerations water tables are present. C. Narrow and Shallow Trenches - would be design flow, friction loss. • pump type, scouring velocity and Pressure sewer service lines and air venting. force mains can be easily installed inr narrow trenches just below the • frost line. In the case of the Friction losses are determined by Sunbelt states, the pipe has to be • the Hazin-Williams method with laid deep enough for mechanical appropriate C factor. A C factor of protection. 4 150 is used most frequently in STEP D. Minimum Equipment & Labor • pressure sewer designs. A scouring , Pressure sewers can be installed velocity of 2 to 5 f/sec. is normally • with a trencher and backhoe. The II recommended for the service lines force mains usually are installed and force mains. However, since a in the right-of-ways along the road. 4 high percentage of solids and grease In comparison, gravity sewer in- is left in the septic tank, some en- stallations require large power gineers use a minimum scouring • shovels and it may be necessary to install the mains in the streets. velocity of 1 f/sec. Air venting is Labor and equipment require- important to prevent binding at the ments to handle large diameter high points of the pressure mains. gravity sewer pipe in comparison . There is a limit to how many homes to small PVC pipe can be substan- tially higher. can be paralleled together in one E. Road and Bridge Crossings - 4 system depending on pump head Small diameter pressure sewer capabilities and the TDH require- service lines and pressure mains • ments of the system. About 500 can be bored under roads with homes would be the upper limit with minimum difficulty (assuming no relatively flat topography. rock) since precise slope and align- ment are not critical. Small force ADVANTAGES OF EFFLUENT mains can be installed across � bridges without the use of pump PRESSURE SEWERS • stations. If freezing is of concern. VERSUS GRAVITY the small pipe can be insulated A.Small Diameter Plastic Pipe-One F. Reduction or Elimination of Pump 4 of the advantages of pressure Stations and Manholes - Another sewers over gravity systems is the advantage of pressure sewers is ability to use small diameter the ability to eliminate manholes plastic pipe for the service lines and pumping stations within limit- and force mains. In pressure ations of the grinder pumps head 4 systems,the service line is normal- capabilities. Manholes are not ly 1-14" I.D. and 1-1/2" to 6" I.D. required with pressure sewers. 4 • for force mains. This is in.com- _ However, a valve box is recom- parison to gravity service lines of . mended about every 1000 feet or 4" and 6"and gravity mains up to ' major change in direction. If inter- 4 14" and 16" for a comparable mediate pump stations are re- number of house connections. quired in a pressure sewer system, 4 Consequently, there is a major they are usually much smaller cost advantage in the piping col- pumps and wetwells due to lower lection system for pressure sewers flow requirements. 4 versus gravity. G.Infiltration & Inflow - One of the B. Pipe Alignment - Based on the major problems with gravity sewers 4 pumping capability of the pump, is the problem of heavy infiltration the service lines and force mains . due to broken pipe and leaky can run up and downhill following joints. In the flooding seasons this 4 the contour (within shutoff head can cause problems at the treat- limitations) of the topography 4 __ _ _ _ __ 4 ., CRAIG B.MORGAN AND ASSOCIATES,P.A.• de a 4