GEAR PUMP HYDRAULICS

GEAR PUMP

GEAR PUMP CONSISTS OF TWO SPUR GEARS CLOSELY FITTED IN HOUSING. ONE GEAR WHEEL IS CALLED DRIVE GEAR GETS THE POWER FROM AN ELECTRIC MOTOR OR DIESEL ENGINE AND DRIVES THE OTHER GEAR WHEEL. END PLATES ARE PROVIDED TO COVER THE SIDE OF THE HOUSING AND TO ACCOMMODATE THE GEAR WHEEL SHAFT BEARINGS. IN THE HIGH PRESSURE GEAR PUMP WEAR PLATES ARE USED IN BETWEEN THE END PLATE AND GEAR WHEELS. BEARING ARE USED TO SUPPORT THE GEAR WHEEL SHAFT.THE CLEARANCE BETWEEN THE MOVING PARTS AND STATIONARY PLATE IS VARY FROM 0.002” TO 0.005”. WHEN THE GEAR WHEEL ROTATES, THE OIL IS CARRIED AROUND THE PERIPHERY OF THE REVOLVING GEARS FROM THE SUCTION SIDE TO DISCHARGE SIDE.THE TEETHS ARE MESHING BETWEEN THE TWO GEAR AND THUS PREVENT RETURN OF OIL FROM THE DELIVERY TO SUCTION

GEAR PUMPS ARE BASICALLY HYDRAULICALLY UNBALANCED. THE PRESSURE THAT DEVELOPED AT THE DISCHARGE PORT HAS A TENDENCY TO CROWED THE GEAR WHEELS OVER AGAINST THE INTAKE SIDE OF THE HOUSING

            A SUCCESSFUL ATTEMPT HAS BEEN MADE TO BALANCE THE GEARS HYDRAULICALLY BALANCED PRESSURE IS CONDUCTED BY PASSAGES FROM THE PRESSURE PORT SUITABLE POCKETS DIAMETRICALLY OPPOSITE.

            GEAR PUMP DOES NOT EMPLOY GASKET,BUT BODY JOINTS ARE GROUND TO AN OIL TIGHT FIT AND ASSEMBLED WITH ‘O’ RING PACKING SERVICE HAVE BEEN BUILT IN CAPACITIES RANGING ACTION FROM A FRACTION OF A LITER TO 300 LITER AND ABOVE THE SPEED OF THE PUMP VARIES FROM 900 TO 3500 RPM AND IT CAN DEVOLOP200.KG/CM².PRESSURE.THE VOLUMETRIC EFFICIENCY OF A GEAR PUMP VARIES FROM 85% TO 90%. THE VOLUMETRIC LOSSES IN THE PUMP ARE CAUSED BY INTERNAL LEAK THROUGH THE MATTING SURFACE,LOSSES DUE TO THE JET ACTION AT THE LINE OF CONTACT OF THE TEETH AND LOSSES IN INLET PRESSURE DUE TO CENTRIFUGAL ACTION OF FLUID.

MATERIALS

• EXTERNALS (HEAD, CASING, BRACKET) - IRON, DUCTILE IRON, STEEL, STAINLESS STEEL, HIGH ALLOYS, CAST STEEL
• INTERNALS (SHAFTS) - STEEL, STAINLESS STEEL, HIGH ALLOYS,
• INTERNALS (GEARS) - STEEL, STAINLESS STEEL,HEAT TREATED OR NITRIDED
• BUSHING - CARBON, BRONZE, SILICON CARBIDE, NEEDLE BEARINGS
• SHAFT SEAL - PACKING, LIP SEAL, COMPONENT MECHANICAL SEAL,

 ADVANTAGES

1.ABILITY TO HANDLE A WIDE RANGE OF VISCOSITIES OF FLUID

2.LESS SENSITIVITY TO CAVITATION (THAN CENTRIFUGAL STYLE PUMPS)

3.RELATIVELY SIMPLE TO MAINTAIN AND REBUILD.

4.AS THE PUMPING ACTION IS QUITE FINITE, THE OUTPUT IS VERY CONTROLLABLE. DOUBLING THE SIZE OR SPEED, IN THEORY SIMPLY DOUBLES THE OUTPUT

 DISADVANTAGES

1.THE FLUID MUST BE FREE OF ABRASIVES.

2.THE DISCHARGE OF GEAR PUMPS MUST BE CONTROLLED VIA THE MOTOR SPEED. THROTTLING 3.THE DISCHARGE IS NOT AN ACCEPTABLE MEANS OF CONTROL.

4.GEAR PUMPS RELY ON PRECISION CLEARANCES AND HAVE SEVERAL ROTATING ELEMENTS. THIS 5.MAKES THEM MORE EXPENSIVE THAN A CENTRIFUGAL PUMP

6. GEAR PUMPS ARE LIMITED BY SIZE. WHEN LARGE BULK FLOW RATES ARE REQUIRED THEY     BECOME PHYSICALLY UNSUITABLE COMPARE TO OTHER PUMP TECHNOLOGY

TYPES OF GEAR PUMPS

 1. EXTERNAL GEAR PUMP

EXTERNAL GEAR PUMP

2. INTERNAL GEAR PUMP

INTERNAL GEAR PUMP

3. GEROTOR PUMP

GEROTOR PUMP

 

4. LOBE PUMP

LOBE PUMP

HYDRAULICS PUMPS

PUMPS

Pump is a device to induce fluid flow from one place to another place under pressure, hydraulics systems pumps are used to develop very high pressure to transfer energy from one place to another place.

Pumps are classified in many ways however they are classified as its way of mechanical working arrangement, fluid working principle and by its delivery

BY MECHANICAL OPERATION 

1. ROTARY PUMPS

2. SEMI ROTARY PUMPS

3. RECIPROCATING PUMPS

BY WORKING PRINCIPLE

1. DYNAMIC PRESSURE PUMP

2. POSITIVE   DISPLACEMENT  PUMP

BY DISCHARGE

1. CONSTANT DISCHARGE PUMP

2. VARIABLE DISCHARGE PUMP

3. VARIABLE AND REVERSIBLE DISCHARGE PUMP

ROTARY PUMPS

                A rotor or an impeller rotates inside housing and induces the fluid flow and pressure centrifugal pump, gear pump, vane pump, screw pump, axial and radial piston pump are rotary pumps suction and discharge valves are not employed in rotary pumps

SEMI ROTARY PUMP

                The rotor or its kind arrangement inside a housing moves only a less than half a circle in both the direction it is a hand operating pump. The flap valve arranged on the rotor is used for suction and delivery.

RECIPROCATING PUMP

                Piston pump and plunger pump are reciprocating pumps either it can be operated by hand or any other power this pumps are provided with suction and discharge valves. The suction valves are opened by suction pressure and the delivery valves are opened by fluid pressure. Small hand operated   reciprocating plunger pumps are used in hydraulic system for hydraulic jack, puller and pressure testing etc.

DYNAMIC PRESSURE PUMP

                In dynamic pressure pump a tangential acceleration is imported to the fluid and due to the movement of the fluid from centre to outer track, vacuum is created at the centre this vacuum is continuously filled with the fluid from the suction side because of atmospheric pressure or any other pressure. The tangential acceleration is imported to the liquid by centrifugal force. Centrifugal pump is a dynamic pressure pump. Mostly this type of pumps are used to transfer the fluid from  one place to another

POSITIVE DISPLACEMENT PUMP

                In positive displacement pump one or more chambers are filled while crossing suction port and the fluid is carried over and discharged at the discharge port by the chambers. This type of pump can develop high pressure. Vane   pump, gear pump, screw pump, axial and radial piston pump are positive displacement pumps

CONSTANT DISCHARGE PUMP

                All the positive displacement pumps are made as constant discharge pump. In this type of pump discharge fluid is always constant at particular rpm. The revolution is increased the discharge of fluid also increased and vice versa. gear pump, vane pump, axial piston pump, radial piston pump, screw  pump etc.

VARIABLE DISCHARGE PUMP

                In variable discharge pump the fluid discharge can be varied at given speed. Radial and axial piston pump vane pump are variable displacement pumps, a hand wheel or lever provided in this pump to alter the stroke length of the piston the quantity of discharge can be reduced or increased at a given speed by moving the lever or hand wheel 

VARIABLE AND REVERSIBLE DISCHARGE PUMP

                The quantity of discharge and direction of flow can be changed .in this type pump without altering the speed and direction of pump. The axial and radial piston pumps are examples the lever provided the pump control the stroke of the piston when the lever is neutral or center there would be no suction and discharge, when the lever is moved to right off center the pump begins to deliver the fluid through the one port and takes suction from other port, the lever is moved to opposite side off center the pump changes or reverse the direction of fluid

CENTRIFUGAL PUMP

WHAT IS  CENTRIFUGAL  PUMP

            A centrifugal pump which employs centrifugal force to deliver liquid from one place to other is called centrifugal pump, here the kinetic energy of the leaving water from the impeller is converted in to potential energy which is utilize axial flow and radial flows

            Centrifugal pump works on the principle that when a certain mass of liquid is made to rotate along the impeller from the central axis of rotation, it impresses a centrifugal head. It causes the water to move radially outwards at higher velocity and causes the water to rise to a higher level. The motion of water is restricted by casing of pump,  it results into pressure build up. In addition, the change in angular momentum of liquid during its flow results  into increase in pressure head.

Impeller

 The rotating part of a centrifugal pump is called ‘impeller’. It consists of a series of backward curved vanes. The impeller is mounted on a shaft which is connected to the shaft of an electric motor.

Functions of impeller

            The functions of impeller is to increase the liquid velocity by its rotation this increasing the energy. Some energy being partially converted pressure energy by suitable design of the impeller vanes suction of the pump being created by the differences in pressure between the suction and discharge side of the impeller    

            When the pump rotates liquid in the casing is swirled by the rotating impeller the swirling action causes liquid to move towards the outside and away from the center the backward curving vanes and the rotation give the liquid a combined radial and circular motion

Volute casing

Volute casing is the type of casing that converts kinetic energy created by the             impeller into pressure. The impeller pushes water into the volute which converts     that energy into pressure and directs the flow toward the discharge point.

  PURPOSE OF VOLUTE CASING
It is provided to reduce the velocity of water after it leaves the impeller and convert part of kinetic energy to pressure energy

Diffuser

                                                         

Diffuser is the another method used in centrifugal pumps to convert energy into pressure. This is through the use of a diffuser. A diffuser functions similarly to a standard volute in that it contains vanes that begin close to the impeller and then gradually extend away from the impeller periphery
 A diffuser is fitted to high-pressure centrifugal pumps. This is a ring fixed to the casing, around the impeller, in which there are passages formed by vanes. The passages widen out in the direction of liquid flow and act to convert the kinetic energy of the liquid into pressure energy. Hydraulic balance arrangements are also usual. Some of the high-pressure discharge liquid is directed against a drum or piston arrangement to balance the discharge liquid pressure on the impeller and thus maintain it in an equilibrium position.

The centrifugal pumps are not self priming why?

A centrifugal pump cannot pump a gas therefore a differential pressure necessary for flow, the flow will not be created if the impeller is having air or vapour.

          A centrifugal pump is a rotodynamic pump. This means that the head developed (in meters of liquid that is pumped) depends on the velocities determined by diameter of the impeller and the impeller speed (rpm.). As the pressure developed is related to the head  by the equation  head = pr / sp. weight, the pressure available will be proportional to the specific weight of the liquid. This means that the pressure (or pressure difference) created with air will be only around 1/800 times that with water ( density of water = 1000 kg/ m3  and dry air at S.T.P has a density of 1.2 kg/m3). Therefore, if the pump is not primed, the suction pressure created will not be sufficient to lift water.

Why centrifugal pump started with discharge valve shut?

            It is a common practice to start large capacity centrifugal pump with the discharge valve closed. If the characteristic curve for a centrifugal pump are examined it will be seen that when the quantity of water discharged is zero the power required by the pump is zero or very small amount. by starting the pump with discharge valve closed the power demand made by the pump on the pump motor is kept to very minimum after the pump has started and the momentary high motor current demand has stabilized the discharge valve is opened.