More Information
| NPSHA | Net positivesuction head available is the absolute pressure of the liquid at the inlet of the pump. Back |
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NPSHA is a function of the elevation, the temperature, and the pressure of the liquid, and is expressed in units of absolute pressure (psia). Any variation in these three liquid characteristics will change the NPSHA. Accurate determination is critical for any centrifugal pump application. The net positive suction head reuired (NPSHR) remains unchanged for a given head. flow, rotational speed, and impeller diameter, but changes with wear and liquids. |
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| Specific
Gravity |
Ratio of the density of one substance to that of a reference substance at a specified tempeature. Back |
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Water at 4ºC is used as the reference for solids and liquids. Air is used as the reference for gases. The specific gravity of a liquid affects the input energy requirements (or brake horsepower) of centrifugal pumps. It also affects the energy required to more the liquid, and therefore must be used in determining the pumps horsepower. |
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| Vapour Pressure | Pressure at which a pure liquid can exist in equilibrium with its vapour at a specifid temperature. Back |
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Fluids at temperatures greater than their specified (critical) temperature will exist as a vapour with no distinction between gas and liquid phases. This is a single phase liquid. At less than the critical temperature, the two fluid phases can coexist; the denser fluid phase exists as a liquid and the less dense phase as a vapour. At the critical temperature, the liquid phase is stable at pressures exceeding the vapour pressure and the gas phase is stable at pressures less than the vapour pressure. For a fluid to be liquid its surface pressure must be equal to or greater than the vapour pressure at the prevailing temperature. Higher pressures are needed to maintain volatile materials in their liquid states. The surface pressure of a liquid must be greater than its vapour pressure to maintain satisfactory operation of a centrifugal pump. |
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| Viscosity | Resistance of a fluid to sheer motion - its internal friction. The viscosity of a fluid varies with temperature.Back |
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The molecules of a liquid have an attraction for each other, and they resist movement. This resistance to flow is expressed as its viscosity. Dynamic viscosity can be defined as the ratio of shearing stress to the rate of deformation. As viscosity varies with temperature it is always expressed at a specific temperature. A change in viscosity alters liquid handling characteristics of the pumping system; more or less energy may be required to perform the same amount of work. In a centrifugal pump the increase in viscosity reduces the pressure energy (head) produced while increasing the rate of energy input. |
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