Types of pumps

      A pump is a device used to transport fluids, such as liquid (pump) , gases (compressor) or slurries . A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps.Their names describe the method for moving a fluid.

Types

• Positive displacement pump

MORE     A positive displacement pump moves a fluid by trapping a fixed amount of it then forcing (displacing) that trapped volume into the discharge pipe.

Some positive displacement pumps work using 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 given each cycle of operation. positive displacement pumps are in hydraulic systems to a pressure up to 34500 bar.

Positive displacement pump behavior and safety

    Positive displacement pumps, unlike centrifugal or rotor-dynamic pumps, will produce the same flow at a given speed (RPM) regardless what the discharge pressure. Thus, positive displacement pumps are "constant flow machines".

   A positive displacement pump must not be operated against a closed valve on the discharge side of the pump, because it has no shut-off head like centrifugal pumps. When a positive displacement pump operating against a closed discharge valve it will continue to produce flow and the pressure in the discharge line will increase, until the line bursts or 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 should in general only be used as a safety precaution, an external relief valve installed in the discharge line with a return line back to the suction line or supply tank is recommended.

Positive displacement types

A positive displacement pump can be further classified according to the mechanism used to move the fluid:

• Rotary-type positive displacement: internal gear, screw, shuttle block , flexible vane circumferential piston, helical twisted roots (e.g. the Wendelkolben pump) or liquid vacuum pump. A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum
• Reciprocating-type positive displacement:piston or diaphragm pumps
• Linear-type positive displacemet:  chain pumps.

Rotary positive displacement pumps

      Positive displacement rotary pumps are pumps that move fluid using the principles of rotation. The vacuum created by the rotation of the pump captures and draws in the liquid.

 Advantages: Rotary pumps are very efficient because they naturally remove air from the lines, eliminating the need to bleed the air from the lines manually.

Drawbacks: Positive displacement rotary pumps also have their weaknesses. Because of the nature of the pump, the clearance between the rotating pump and the outer edge must be very close, requiring that the pumps rotate at a slow, steady speed. If rotary pumps are operated at high speeds, the fluids will cause erosion. Rotary pumps that experience such erosion eventually show signs of enlarged clearances, which allow liquid to slip through and reduce the efficiency of the pump
 .

Rotary positive displacement pumps can be grouped into three main types
  

•     Gear pumps - a simple type of rotary pump where the liquid is pushed between to gears.

•     Screw pumps - the shape of the internals of this pump usually two screws turning against each other pump the liquid.

•     Rotary vane pumps - similar to scroll compressors, consisting of a cylindrical rotor encased in a similarly shaped housing. As the rotor

1. Gear pumps

       A gear pump uses the meshing of gears to pump fluid by displacement . They are one of the most common types of pumps. Gear pumps are also widely used in chemical installations to pump fluid with a certain viscosity. There are two main variations; external gear pumps which use two external spur gears, and internal gear pumps which use an external and an internal spur gear. Gear pumps are  positive displacement pumps (or fixed displacement), meaning they pump a constant amount of fluid for each revolution. Some gear pumps are designed to function as either a motor or a pump.

Theory of operation

As the gears rotate they separate on the intake side of the pump, creating a void and suction which is filled by  fluid. The fluid is carried by the gears to the discharge side of the pump, where the meshing of the gears displaces the fluid. The mechanical clearances are small— in the order of 10 μm. The tight clearances, along with the speed of rotation, effectively prevent the fluid from leaking backwards.

The rigid design of the gears and houses allow for very high pressures and the ability to pump highly viscus fluids.

External Gear Pumps

 How External Gear Pumps Work

External gear pumps are similar in pumping action to internal gear pumps in that two gears come into and out of mesh to produce flow.  However, the external gear pump uses two  identical gears rotating against each other -- one gear is driven by a motor and it in turn drives the other gear.  Each gear is supported by a shaft with bearings on both sides of the gear.

1.  As the gears come out of mesh, they create expanding volume on the inlet side of the pump. Liquid flows into the cavity and is trapped by the gear teeth as they rotate.

2.  Liquid travels around the interior of the casing in the pockets between the teeth and the casing -- it does not pass between the gears. 

3.  Finally, the meshing of the gears forces liquid through the outlet port under pressure.

Because the gears are supported on both sides, external gear pumps are quiet-running and are routinely used for high-pressure applications such as hydraulic applications. With no overhung bearing loads, the rotor shaft can't deflect and cause premature wear

Advantages

• 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

Internal gear pumps

How Internal Gear Pumps Work 1. Liquid enters the suction port between the rotor (large exterior gear) and idler (small interior gear) teeth. The arrows indicate the direction of the pump and liquid.
2. Liquid travels through the pump between the teeth of the "gear-within-a-gear" principle. The crescent shape divides the liquid and acts as a seal between the suction and discharge ports.
3. The pump head is now nearly flooded, just prior to forcing the liquid out of the discharge port.  Intermeshing gears of the idler and rotor form locked pockets for the liquid which assures volume control.
4. Rotor and idler teeth mesh completely to form a seal equidistant from the discharge and suction ports. This seal forces the liquid 

Advantages

• Only two moving parts
• Only one stuffing box
• Non-pulsating discharge
• Excellent for high-viscosity liquids
• Constant and even discharge regardless of pressure conditions
• Operates well in either direction
• Can be made to operate with one direction of flow with either rotation
• Low NPSH required
• Single adjustable end clearance
• Easy to maintain
• Flexible design offers application customization

 Disadvantages

• Usually requires moderate speeds
• Medium pressure limitations
• One bearing runs in the product pumped
• Overhung load on shaft bearing