Agenda - 01-04-1988 - Laserfiche WebLink

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., SYSTEMS DESIGN 035 CONSI DERATIONS i Once the design engineer has made where as gravity lines must be a decision to design a pressure aligned in a continuous down- i sewer system, he must then decide ward slope. Deep cuts for gravity on design parameters that would lines become quite expensive, 9 effect the operation of the system. especially where rock and high The most important considerations water tables are present. would be design flow, friction loss, C. Narrow and Shallow Trenches - ' pump.type, scouring velocity and Pressure sewer service lines and air venting. force mains can be easily installed in 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. 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 recommended for the service lines force mains usually are installed and force mains. However, since a in the right-of-ways along the road. 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 velocity of 1 f/sec. Air venting is to install the mains in the streets. important to prevent binding at the Labor and equipment require- high points of the pressure mains. ments to handle large diameter gravity sewer pipe in comparison There is a limit to how many homes to small PVC pipe can be substar•.- can be paralleled together in one Rally higher. system depending on pump head E. Small diameter Bridge p Crossings - capabilities and the TDH require- Smac diameter pressure sewer ments of the system. About 500 can lines and pressure mains homes would be the upper limit with can be bored under roads with relatively flat topography, minimum difficulty (assuming no rock)since precise slope and align- ment are not critical. Small force ADVANTAGES OF EFFLUENT mains can be installed across PRESSURE SEWERS bridges without the use of pump VERSUS GRAVITY - stations. If freezing is of concern. the small pipe can be insulated A.Small Diameter Plastic Pipe-One within limitations. Of the advantages of pressure F. Reduction or Elimination of Pump of the over ntaglty systems is pressure Stations and Manholes - Another ability wens over use small diameter advantage of pressure sewers is plastic the ability to eliminate manholes pipe for the service lines and pumping stations within limit- and force mains. In pressure ations of the grinder pumps head systems,the service line is normal- capabilities. Manholes are not ly 1.W' l.b. and 1-%" to 6" I.D. required with - for force mains. This is in-corn- However, vre bsure sewers. owever. a valve box is recpm- parisort to gravity service lines of - mended about every 1000 feet or 4 smelt"and gravity mains up to major 74" and 16" for a comparable change in direction. if inter- . - number of house Connections, mediate pump stations are re- um Consequently, there is e a ma or pulsed in a pressure sewer system, cost advantage in the piping col- they are usually much smaller lection system for pressure sewers pompand vents. s due to lower flow r ee versus gravity. quirements. B.Pipe Alignment - Based on the G.Infiltration & Inflow - One of the pumping capability of the pump. major problems with gravity sewers the service lines and force mains is the problem of heavy indltleaky can run up and downhill following � due to broken pipe and leaky �� the contour and downhill shutoff following joints.In the flooding seasons this treat- limitations) of the topography can cause problems at the treat- _ _ _ 4 CRAIG S.MORGAN AND ASSOCI�ATES,P.A.• i