Positive-displacement pumps can operate with lower suction pressures or high suction lifts because they can create stronger vacuums. Suction lift will be greater if the pressure in a closed tank is greater than atmospheric pressure. Submergence is often confused with either suction static head or static lift. For vertical pumps, submergence relates the liquid level to the setting of the pump.
For horizontal pumps, submergence relates to the height of liquid level necessary in the source vessel or tank to prevent the formation of vortexing and the resulting flashing of vapors in the pump suction. As the pressure on a liquid is decreased, there is a tendency for the bubbles of vapor to be liberated. The vapor pressure of a liquid is the pressure at which the first bubble of vapor appears at a given temperature.
For other fluids, refer to standard references e. The amount and type of suspended solids entrained in the liquid can affect the characteristics and behavior of that liquid. Increased concentrations of solids increase the specific gravity, viscosity, and abrasiveness of a liquid. The type and concentration of suspended solids can affect the style of pump selected and the materials of construction. Suspended solids also affect the selection of impeller design in centrifugal pumps, which in turn affects the wear rate, efficiency, and power consumption. Small amounts of dissolved gases have little effect on flow rate or other pumping requirements.
If large amounts of gas enter the liquid through piping leaks or as a result of vortexing in vessels, the specific gravity of the liquid will decrease. Viscosity offers resistance to flow because of friction within the fluid.
Viscosity levels have a significant impact on pump type selection, efficiency, head capacity, and warm-up. The viscosity of all liquids varies with temperature. For viscosities of liquids, refer to standard industry references e. The corrosive nature of the fluid being pumped has a bearing on pump type selection, materials of construction, and corrosion allowance.
Special mechanical seals and flushing arrangements may be required. Subscripts 1 and 2 refer to locations along a pipe. An examination of each of the terms in Eq. Velocity Head. Velocity head is the potential energy that has been converted to kinetic energy. Velocity head can be expressed as. The velocity head increases the amount of work required of a pump. The velocity head is included in the total dynamic head on the centrifugal-pump curves. The energy contained in the liquid as a result of its elevation relative to a datum is called the elevation head and is expressed as Z in Eq.
Head losses are potential energy that has been lost because of frictional resistance of the piping system pipe, valves, fittings, and entrance and exit losses.
Unlike velocity head, friction head cannot be ignored in system calculations. Head loss values vary as the square of the flow rate. Head losses can be a significant portion of the total head. Control losses occur on the discharge side of a centrifugal pump that has been equipped with a backpressure valve to control flow rate.
As the liquid flows through the control valve, energy is lost. For pump applications, control losses are treated separately from head losses, even though they are included in the hf term in Eq. Acceleration head is used to describe the losses associated with the pulsating flow of reciprocating pumps. Theoretically, acceleration head should be included in the hf term of Eq. It is also equal to the difference in pressure-gauge readings converted to feet across an existing operating pump discounting velocity head.
Suction head is defined as the sum of the suction-vessel operating gauge pressure converted to feet , the vertical distance between the suction-vessel liquid level and the pump reference point, less head losses in the suction piping [discounting change in velocity,. Discharge head is defined as the sum of the discharge-vessel operating gauge pressure converted to feet , the liquid level in the discharge vessel above the pump reference point, pressure drop because of friction in the discharge piping, and control losses discounting velocity head.
It can be expressed as.
NPSH is defined as the total suction head in feet of liquid absolute at the pump centerline or impeller eye less the vapor pressure in feet of the liquid being pumped. It is determined either by test or calculation by the pump manufacturer for the specific pump under consideration. NPSHR is a function of liquid geometry and the smoothness of the surface areas.
Nomenclature - Hydrodynamics of Pumps - Christopher E. Brennen n. Coordinate measured normal to a surface. N. Specific speed. N(R. N.) Cavitation nuclei. PDF | The subject of this monograph is the fluid dynamics of liquid turbomachines , particularly pumps. Rather than attempt a general treatise on.
NPSHR is determined on the basis of handling cold water. Field experience coupled with laboratory testing have confirmed that centrifugal pumps handling gas-free hydrocarbon liquids and water at elevated temperatures will operate satisfactorily, with harmless cavitation and less NPSHR than would be required for cold water. If this is not the case, cavitation or flashing may occur in the pump suction. Cavitation occurs when small vapor bubbles appear in the liquid because of a drop in pressure and then collapse rapidly with explosive force when the pressure is increased in the pump.
Cavitation results in decreased efficiency, capacity, and head and can cause serious erosion of pump parts. Flashing causes the pump suction cavity to be filled with vapors and, as a result, the pump becomes vapor locked. This usually results in the pump freezing up, which is called pump seizure.
NPSHA is not a function of the pump itself but of the piping system for the pump. It can be calculated from. NPSHA decreases with increases in liquid temperature and pipe friction losses. Because pipe friction losses vary as the square of the flow, NPSHA also varies as the square of the flow.
Unless subcooled, a pure-component hydrocarbon liquid is typically in equilibrium with the vapors in a pressure vessel. Thus, increases in the vessel operating pressures are almost fully offset by a corresponding increase in the vapor pressure. When this occurs,. The Hydraulic Inst. When an existing pumping system exhibits insufficient NPSH margin, it is too late to use these solutions without going through an expensive change.
Most of these problems can be traced to suction flow restrictions orifice plates, plugged strainers, partially closed valves, etc.
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Positive displacement pumps. Jump to: navigation , search. Category : 4. Sections About. Categories Uncategorized. Posted on August 27, by admin Leave a comment. Low pressure and vacuum:Hopper off-loading and Trickle valves.
First approximation design methods:Universal conveying characteristics method. Material property influences:Dicalcium phosphate. Material property influences:Dense phase sliding bed flow. Material property influences:Conveying modes and Dilute phase non-suspension flow. Conveying capability:The influence of materials and Low pressure conveying — Part I.
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