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1 A sanitary sewer (a subsurface open channel), runs 8,000' along an island. Lift stations are 2000' apart. The pipe slopes S = 0.005 between them,...
1 A sanitary sewer (a subsurface open channel), runs 8,000’ along an island. Lift stations are 2000’ apart. The pipe slopes S = 0.005 between them, so the invert drops 10’ between stations. At each if the first three stations, a dry-pit pump (HO p 8-4) pushes wastewater10’ up to the start of the next 2000’ gravity flow section. The 4th pump at S l ’ feeds a 5,000’ force main (pressure pipe) to a treatment plant on the mainland. The static lift Dz is 40’from the wet well of this 4th pump to the plant intake. The sewer collects 30 gpd/ft. Thus, the average daily flow pushed from this last pump station is Qavg= 240,000 gpd.
a What is the best type of pump to use at the three lift stations where Dz = 10’, hl ~ 5’, TDH ~ 15’?
b Wastewater accumulates in wetwells until it triggers the pump. Actual pumping averages 6 hrs/day, leaving capacity for heavier flows. Efficiency = 0.8. Find BHP’s of each of the 3 lift pumps.
c Find force main diameter for vavg = 8 fps, 6 hours pumping. Pipe “f” = 0.02. Find line friction. Pipe bend, exit and valve local losses SKl . Find Ep or TDH (total dynamic head) for the 4th pump
d For 4th pump Qdes and TDH, find appropriate model from either p 7-11 Fig 9.25 (select impeller dia) or HO p. 7-12 Fig 5.10 (which pump, what RPM?). Recall P = Tw (T is proportional to radius).
e Rather than dry pit, positive intake pressure (Fig 5-6 p 7-5), a platform pump with a suction line (upper left p 7-5) is desired. Cavitation occurs if intake NPSH (P 7-13) = po/g - pvar /g - zs – hl = 32’ - zs – hl is lower than the required NPSH shown on the pump curve. If local, entrance and line losses total hl = 6’, use the plot for chosen one of Fig 9.25 or Fig 5.10 to find maximum zs height for this Qdes