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Avoiding Cavitation

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  • Increasing the available static pressure at the pump location (positioning the pump at a lower level, e.g. from a roof plant room into the basement to benefit from the increased static head from the higher water column. The pump performance remains unchanged).
  • Lowering the fluid temperature (reducing the vapour pressure pD).
  • Changing the pump characteristics (reducing the driven speed and/or different pump design with a lesser Pump NPSHrequired).
 

Pump

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In a narrower sense, in contrast to blowers and compressors for gases and compressible media, pumps for incompressible media are referred to as liquid pumps. This means that the volume of the fluid remains approximately constant as the pressure increases. In practice, this also includes liquid mixtures with a low solid or gas content.Depending on the direction of the energy flow, pumps belong to the working machines. 

Number of Pole Pairs

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nsync = synchronous speed f = frequency of the 3-phase power supply p = number of pole pairs of the 3-phase motorKindly note that the above noted relation results in revs per second if the frequency unit is Hz.
Pole pair numbers123456
Synchronous speed (at 50 Hz) n [rpm]300015001000750600500
Synchronous speed (at 60 Hz) n [rpm]360018001200900720600
The rated full-load speed of three phase asynchronous motors is, due to slip, a few percent below the synchronous speed.

Non-Newtonian Fluids

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This is thus indicated that there is no line coherence between deformation and shear tensors and that the viscosity rate is time-related.Non-Newtonian flow characteristics can be observed with the following fluids:
  • gaseous soot in oil varnish
  • cereal corn suspensions in water
  • waste water sludgesewage
  • toothpaste
  • mortar
  • soap solutions
Examples: Thixotropic characteristics can be observed on numerous kinds of varnishes and lacquers, that means the viscosity is time-related. On stirring with constant angular velocity a rather great resistance is being experienced at first, whilst after a time the viscosity becomes noticeable less, tending towards its lower limits. Plastic and pseudoplastic characteristics can be observed on many industrial sludge’s e.g. suspensions of lime (mortar) and chalk (toothpaste). Below the shear rate (flow limit)they behave like solid bodies and above as fluids.Colloidal fluids, such as caoutchouc or solution of soap in water, behave like thixotropic fluids but will not recover their original viscosity when agitation is stopped. The viscosity of such fluids decreases as agitation or shear rate is increased. The reverse (dilettante) characteristic scan be observed on some high-concentrated suspensions., their viscosity increases with increasing agitation or shear velocity rate.Both the pump curves and the classical methods for calculation of the pipe friction loss assume a constant viscosity and are not valid for non-Newtonian fluids. Depending on the flow behaviour of the pumped medium, they can at best be used as an approximation. For many non-Newtonian media, there are therefore special calculation or approximation methods for sizing and selection.

Rated Pressure

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It is stated without a unit quantity. Its numerical value states the permissible working gauge pressure in bar at a temperature of 20°C (68°F) to which a component or specified material may be subjected. In the field of pump technology, it is common to use pressure ratings according to DIN 2401 for the range from PN 1.6 to PN 200.Preferential designation for pressure ratings are 1.6; 2.5; 4; 6; 10; 16; 25; 40….

Flow velocity

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v = Q / Av – Average flow velocity Q – Flow rate A – Area flowed throughFor a circular cross section, the following results are obtained withA = p / 4 – D2v = 4 – Q / (p – D2)D – Diameter of the circular area flowed through (internal pipe diameter)c or v are usually used as symbols for the average flow velocity. Often v is used for the local velocity, while c is used as a symbol for the mean velocity. The DIN 24260 provides the symbol v for the average speed.The average flow velocity is an important parameter when selecting the optimum pipe diameter for a new pipeline.

Required Pump Shaft Power

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The shaft power requirement or the power input of the pump are, as the hydraulic performance, also shown in a graph.
  • It demonstrates the dependence of power input on the flow rate.
  • Max. shaft power requirements are reached at max. flow for many pump types.
The drive motor is to be sized to suit that point of the pump.Small pumps (e.g. heating circulators) are typically equipped with motors that allow operation over the entire characteristic curve. This reduces the number of types, and as a result, easier stocking of spare parts is guaranteed.For larger pumps, several motor options are usually offered so that the appropriate drive can be selected according to the operating conditions. If the specified duty point of a pump is located on the left hand portion of the duty curve with a corresponding lower power input it is feasible to select a smaller size motor. In such case however exists the hazard of overloading that size motor if the actual duty point allows a higher flow rate than that calculated (a more flat system curve).As in practice there is always the danger of the duty point having shifted it must be recommended to select the motor to drive a pump with power reserve of approx. 5 to 20% above that theoretically required.To determine the operating costs of a pump it must be principally distinguished between the required shaft power P2 [kW] of the pump and the electrical power input of the motor P1 [kW].The latter is the basis for determining the operating costs. It can, In case that only the shaft power requirements P2 are given, be determined by dividing that value by the motor efficiency.The electric power input P1 is stated where pump and drive motor are an integrated unit such as submersible pumps. Here it is even customary to state both P1 and P2 values.The required shaft power P2 is generally given in the case of aggregates, where pump and motor are coupled or rigidly connected to non-submersible pumps. This is necessary to allow the use of most distinctly different types of motors – beginning from IEC-standards up to the special design motors – with their varying electrical criteria such as power inputs and efficiencies.