Brake Horse Power
The power required to drive the pump is that required to overcome all the losses and supply the energy added to the fluid. These losses include the friction of flow trough the impeller and turbulent losses, the disk friction or energy required just to rotate the impeller in the fluid, the leakage of fluid from the periphery back to the eye of the impeller, and the mechanical friction losses in the bearing, stuffing boxes, and wearing rings.
The liquid horsepower is the energy absorbed in the fluid leaving the pump. The brake horsepower is the energy requirement of the pump per unit of time.
The power required to drive the pump is that required to overcome all the losses and supply the energy added to the fluid. These losses include the friction of flow trough the impeller and turbulent losses, the disk friction or energy required just to rotate the impeller in the fluid, the leakage of fluid from the periphery back to the eye of the impeller, and the mechanical friction losses in the bearing, stuffing boxes, and wearing rings.
The liquid horsepower is the energy absorbed in the fluid leaving the pump. The brake horsepower is the energy requirement of the pump per unit of time.
Efficiency
The efficiency of a centrifugal machine is the ratio of the fluid horsepower to brake horsepower.
Cavitation
When a centrifugal pump is operating at high rates, the high velocities occurring at certain points in the eye of the impeller or at the van tips cause local pressures to fall below the vapor pressure of the liquid. Vaporization occurs at this points, forming bubbles which collapse violently upon moving along to a region of higher pressure or lower velocity. This momentary vaporization and destructive collapse of the bubbles is called cavitation and is to be avoided if maximum capacity is to be obtained and damage to the pump prevented. The shock of bubble collapse causes severe pitting of the impeller and creates considerable noise and vibration. Cavitation may be reduced or eliminated by reducing the pumping rate or by slight alterations in impeller design to give better streamlining. Cavitation usually does not occur at low flow rates on any given pump.
Specific Speed
For single-stage side-suction impellers, or one stage of a multistage pump, the specific speed Ns, is a convenient concept.
The efficiency of a centrifugal machine is the ratio of the fluid horsepower to brake horsepower.
Cavitation
When a centrifugal pump is operating at high rates, the high velocities occurring at certain points in the eye of the impeller or at the van tips cause local pressures to fall below the vapor pressure of the liquid. Vaporization occurs at this points, forming bubbles which collapse violently upon moving along to a region of higher pressure or lower velocity. This momentary vaporization and destructive collapse of the bubbles is called cavitation and is to be avoided if maximum capacity is to be obtained and damage to the pump prevented. The shock of bubble collapse causes severe pitting of the impeller and creates considerable noise and vibration. Cavitation may be reduced or eliminated by reducing the pumping rate or by slight alterations in impeller design to give better streamlining. Cavitation usually does not occur at low flow rates on any given pump.
Specific Speed
For single-stage side-suction impellers, or one stage of a multistage pump, the specific speed Ns, is a convenient concept.
Ns = specific speed.
N = revolution per second.
Q = volume of fluid per second.
-w = “total developed head”. This value is gotten from Barnoulli equation.
The specific speed is dimensionless if consistent units are used.
The characteristic curves of pump represent performance from zero flow to maximum flow, and the specific speed would vary from zero to infinity, respectively. For classifying impellers a single value must be selected. The point of maximum efficiency is usually selected for calculation of the specific speed. The usual range is from 0.03 to 0.87 when so calculated and expressed as the dimensionless ration given above. The lower values apply to radial-flow centrifugal pumps and the higher specific speeds to axial-flow propeller pumps.
Unfortunately, current practice omits gc and expresses Q in gallons per minute, N in revolutions per minute, and w in foot-pounds force per pound-mass. In these units specific speeds vary from 500 to 15000 which may be converted to the dimensionless ratio by dividing by 17200.
Deep well pump
The centrifugal pump is capable of reduction in size to an extent which permits the construction of a multistage unit which will fit into well casings as small as 4 in. in diameter. Deep well pump assembly can be lowered down to the water level. The motor may be submerged with the pump or kept at the surface level operating trough a long drive shaft extending down to the pump.
A two-stage deep well pump is supported by the discharge piping. Some pumps use no protective tubing around the shaft and depend upon flowing liquid to lubricate rubber shaft bearings mounted at intervals. In the pump illustrated the oil lubricant is sealed from the water by the labyrinth packing.
Sump Pump
These are small single-stage vertical pumps used to drain shallow pits or sumps. They are of the same general construction as vertical process pumps but are not designed for severe operating conditions.
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