Pumps and Pumping Stations!10

By Eleanor Freeman,2014-06-08 13:57
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Pumps and Pumping Stations!10 Pumps and Pumping Stations!10

Pumps and Pumping Stations

    Pumps and pumping machinery serve the following purposes in water systems: (1)lifting water from its sourcesurface or ground??either immediately to the

    community through highlift installationsor by low lift to purification works

    (2)boosting water from lowservice to highservice areasto separate fire

    suppliesand to the upper floors of manystoried buildingsand(3)transporting

    water through treatment worksbackwashing filtersdraining component

    settling tanksand other treatment unitswithdrawing deposited solids and

    supplying water especially pressure water to operating equipment.

    Today most water and wastewater pumping is done by either centrifugal pumps or propeller pumpsHow the water is directed through the impeller determines the type of pump. There is (1) radial flow in openor closedimpeller pumps

    with volute or turbine casingsand single or double suction through the eye of the impeller(2)axial flow in propeller pumps, and (3)diagonal flow in mixed

    flowopenimpeller pumps. Propeller pumps are not centrifugal pumps. Both can be referred to as rotodynamic pumps.

Openimpeller pumps are less efficient than closedimpeller pumps, but they

    can pass relatively large debris without being clogged. Accordingly, they are useful in pumping wastewaters and sludges. Singlestage pumps have but one

    impeller, and multistage pumps have two or more, each feeding into the next higher stage. Multistage turbine well pumps may have their motors submerged, or they may be driven by a shaft from the prime mover situated on the floor of the pumping station.

    In addition to centrifugal and propeller pumps, water and wastewater systems may include (1) displacement pumps, ranging in size from handoperated

    pitcher pumps to the huge pumping engines of the last century built as steam-driven units; (2)rotary pumps equipped with two or more rotors ; (3)hydraulic rams utilizing the impulse of large masses of lowpressure water

    to drive much smaller masses of water one half to one sixth of the driving

    water through the delivery pipe to higher elevations in synchronism with the

    pressure waves and sequences induced by water hammer; (4)jet pumps or jet ejectorsused in wells and dewatering operationsintroducing a highspeed jet

    of air or water through a nozzle into a constricted section of pipe; (5)air lifts in which air bubblesreleased from upwarddirected air pipe, lift water from a well or sump through an eductor pipe; and (6)displacement ejectors housed in a pressure vessel in which waterespecially wastewateraccumulates and from

    which it is displaced through an eductor pipe when a floatoperated valve is

    tripped by the rising water and admits compressed air to the vessel.

    Pumping units are chosen in accordance with system heads and pump

    characteristics. The system head is the sum of the static and dynamic heads against the pump. As such, it varies with required flows and with changes in storage and suction levels. When a distribution system lies between pump and distribution reservoir, the system head responds also to fluctuations in demand. Pump characteristics depend on pump size, speed, and design. For a given speed N in revolution per minute, they are determined by the relationships between the rate of discharge, Q, usually in gallons per minute, and the head H in feet, the efficiency E in percent, and the power input P in horsepower. For purposes of comparison, pumps of given geometrical design are characterized also by their specific speed Ns, the hypothetical speed of a homologous (geometrically similar) pump with an impeller diameter D such that it will discharge 1 gpm against as 1-ft head. Because discharge varies as the product of area and velocity, and velocity varies as H1/2, Q varies as D2H1/2. But velocity varies also as πDN/60. Hence H1/2 varies as DN, or N varies as


    Generally speaking, pump efficiencies increase with pump size and capacity. Below specific speeds of 1000 units, efficiencies drop off rapidly. Radial-flow pumps perform well between specific speeds of 1000 and 3500 units; mixed-flow pumps in the range of 3500 to 7500 units; and axial-flow pumps after that up to 12,000 units. For a given N, high-capacity, low-head pumps have the highest specific speeds. For double-suction pumps, the specific speed is computed for half the capacity. For multistage pumps, the head is distributed between the stages. This keeps the specific speed high and with it, also, the efficiency.

    Specific speed is an important criterion, too, of safety against cavitation, a phenomenon accompanied by vibration, noise, and rapid destruction of pump impellers. Cavitation occurs when enough potential energy is converted to kinetic energy to reduce the absolute pressure at the impeller surface below the vapor pressure of water at the ambient temperature. Water then vaporizes and forms pockets of vapor that collapse suddenly as they are swept into regions of high pressure. Cavitation occurs when inlet pressures are too low or pump capacity or speed of rotation is increased without a compensating rise in inlet pressure. Lowering a pump in relation to its water source, therefore, reduce cavitation.

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