PUMPING STATIONS

Pumping station and pneumatic ejectors will normally be required to remove waste from areas which cannot be served hydraulically by gravity sewers. In certain situations, however, a gravity sewer system can be used, but only at the expense of deep trench excavation. Both wastewater pumping and gravity flow sewers may be technically feasible and capable of meeting service requirements, however, they may not be equivalent in economic terms.

When it is not readily apparent which solution would be more economical, the decision to use one or the other should be based on life cycle cost analysis. Initial capital and construction costs for pumps, ejectors, structures, force main, plus operation and

maintenance costs should be compared with cost of deep trench excavation or other special construction methods required for a gravity system. Generally, a gravity sewer system will be justified until its cost exceeds the cost of a pumped system by 10 percent.

Practice 670 210 1160 Publication Date 20Sep95 Page 4 of 10 FLUOR DANIEL

SANITARY SEWER SYSTEMS

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

Pumping Equipment

Pumping equipment used in sanitary sewer systems may be classified into two general types;

centrifugal pumps and pneumatic ejectors. The latter are used only in the smaller installations where centrifugal pumps, if used, would be too large for the application.

Centrifugal pumps fall into the following three general classifications:

Axial - flow or propeller pumps Mixed Flow or angle - flow pumps

Radial - flow pumps (commonly referred to as centrifugal pumps)

The classification into which a pump falls usually can be determined by its specific (Ns) at the point of maximum efficiency.

The specific speed of an impeller may be defined as the speed in rpm (revolution per minute) at which a geometrically similar impeller would run if it were of such size as to deliver 1 gpm against 1 foot of head.

The formula for specific speed is as follows:

Ns= RPM GPM H^3/4 where H is in feet.

Pump Construction

Most pump casings are made of cast iron. Although for special applications where gritty or corrosive liquids are involved, other materials sometimes are specified.

Pneumatic ejectors are usually used for lifting sewage from basement of buildings and small lift stations where their advantage outweigh their low efficiency, which is limited to about 15 percent. Their advantages are the following:

Sewage is completely enclosed an consequently no sewer gases can escape except through the vent.

Operation is fully automatic and the ejector goes into service only when needed.

The relatively few moving parts in contact with sewage require little attention or lubrication.

Ejectors are not easily clogged.

The following is an empirical formula for the approximate capacity of air required to operate an ejector:

V=Q(H+34) 250 where

V = volume of free air required in CFM

H = total head in feet

Q = rate of sewage discharge in GPM

Practice 670 210 1160 Publication Date 20Sep95 Page 5 of 10 FLUOR DANIEL

SANITARY SEWER SYSTEMS

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

Datum

All readings for suction lift, suction head, discharge head, and net positive suction head are taken with reference to the datum which in the case of horizontal shaft, is the elevation of the pump center line and in the case of vertical shaft pumps is the elevation of the entrance eye of the suction impeller.

Suction Lift (Hs)

Suction lift exists where the total suction head is below atmospheric pressure. Total suction lift, as determined on test, is the reading of a liquid manometer at the suction nozzle of the pump converted to feet of liquid and referenced to datum, minus the velocity head at the point of gage attachment.

Suction Head (Hs)

Suction head exists, when the total suction head is above atmospheric pressure, as

determined on test, it is the reading of the gage at the suction of the pump converted to head, in feet, at the point of gage attachment.

Total Discharge Head (Hd)

Total discharge head is the reading of a pressure gage at the discharge of the pump, converted to feet of liquid and referred to datum, plus the velocity head at the point of gage attachment.

Total Head (H)

Total Head (H) is sometimes referred to as total dynamic head or TDH. Total head is the measure of the energy increase per pound of the liquid imparted to it by the pump and is therefore the algebraic difference between the total discharge head and the total suction head.

Total head as determined on test where suction lift, and, where positive suction head exists, the total head is the total discharge head minus the total suction head.

NPSH (Net Positive Suction Head)

The NPSH is the total suction head, in feet of liquid absolute, determined at the suction nozzle and referred to datum, less the vapor pressure of the liquid in feet absolute.

SIPHONS

The siphon in sewerage practice almost invariably refers to an inverted siphon or depressed sewer which would stand full even with no flow. Its purpose is to carry the flow under an obstruction such as stream or depressed highway and to regain as much elevation as possible after the obstruction has been passed.

Practice 670 210 1160 Publication Date 20Sep95 Page 6 of 10 FLUOR DANIEL

SANITARY SEWER SYSTEMS

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

Single And Multiple Barrel Siphons

It is common practice, at least on large sewers, to construct multiple barrel siphons. The objective is to provide adequate self-cleaning velocities under widely varying flow

conditions. The primary barrel is designed so that a velocity of 2 to 3 feet per second will be reached at least once each day, even during the early years of operation. Additional pipe regulated by lateral overflow weirs assist progressively in carrying flows of greater magnitude, that is maximum dry weather flow to maximum storm flow.

Profile

Two considerations which govern the profile of a siphon are provision for hydraulic losses and ease of cleaning. The friction loss through the barrel will be determined by the design velocity. For calculating the head loss it is sound conservative Hazen-Williams C of 100 (Manning n from 0.014 for small sizes to 0.018 for the largest). Siphons may need cleaning more often than gravity sewers. For easy cleaning, siphons should not have any sharp bends either vertical or horizontal; only smooth curves of adequate radius should be used.

HYDRAULIC