A Pumps

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  • Topic: Pump, Fluid dynamics, Viscosity
  • Pages : 7 (2101 words )
  • Download(s) : 157
  • Published : March 14, 2013
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A pump :is a device that moves fluids (liquid or gases), or sometimes slurries, by mechanical action. Pumps can be classified into three major groups according to the method they use to move the fluid: direct lift, displacement, and gravity pumps. Pumps operate by some mechanism (typically reciprocating or rotary), and consume energy to perform mechanical work by moving the fluid. Pumps operate via many energy sources, including manual operation, electricity, engines, or wind power

Type of pumps:

1- positive displacement pump
A positive displacement pump makes a fluid move by trapping a fixed amount and forcing (displacing) that trapped volume into the discharge pipe .Some positive displacement pumps use an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant through each cycle of operation Positive displacement pump behavior and safety

Positive displacement pumps theoretically can produce the same flow at a given speed (RPM) no matter what the discharge pressure. Thus, positive displacement pumps are constant flow machines. However, a slight increase in internal leakage as the pressure increases prevents a truly constant flow rate. A positive displacement pump must not operate against a closed valve on the discharge side of the pump, because it has no shutoff head like centrifugal pumps. A positive displacement pump operating against a closed discharge valve continues to produce flow and the pressure in the discharge line increases until the line bursts, the pump is severely damaged, or both. A relief or safety valve on the discharge side of the positive displacement pump is therefore necessary. The relief valve can be internal or external. The pump manufacturer normally has the option to supply internal relief or safety valves. The internal valve is usually only used as a safety precaution. An external relief valve in the discharge line, with a return line back to the suction line or supply tank provides increased safety.

Positive displacement types
a- External Gear pump
External gear pumps are a popular pumping principle and are often used as lubrication pumps in machine tools, in fluid power transfer units, and as oil pumps in engines. External gear pumps can come in single or double (two sets of gears) pump configurations with spur (shown), helical, and herringbone gears.  Helical and herringbone gears typically offer a smoother flow than spur gears, although all gear types are relatively smooth.  Large-capacity external gear pumps typically use helical or herringbone gears. Small external gear pumps usually operate at 1750 or 3450 rpm and larger models operate at speeds up to 640 rpm. External gear pumps have close tolerances and shaft support on both sides of the gears.  This allows them to run to pressures beyond 3,000 PSI / 200 BAR, making them well suited for use in hydraulics.  With four bearings in the liquid and tight tolerances, they are not well suited to handling abrasive or extreme high temperature applications. Advanteg:

* High speed
* High pressure
* No overhung bearing loads
* Relatively quiet operation
* Design accommodates wide variety of materials
Disadvantages
* Four bushings in liquid area
* No solids allowed
* Fixed End Clearances
b-Reciprocating Pumps
In a reciprocating pump, a volume of liquid is drawn into the cylinder through the suction valve on the intake stroke and is discharged under positive pressure through the outlet valves on the discharge stroke. The discharge from a reciprocating pump is pulsating and changes changes only when the speed of the pump is changed. This is because the intake is always a constant volume. Often an air chamber is connected on the discharge side of the pump to provide a more even flow by evening out the pressure...
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