ANS)A gear pump uses the meshing of gears to pump fluid by displacement.[1] They are one of the most common types of pumps for hydraulic fluid power applications. 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 (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.
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 viscous fluids.
Many variations exist, including; helical and herringbone gear sets (instead of spur gears), lobe shaped rotors similar to Roots Blowers (commonly used as superchargers), and mechanical designs that allow the stacking of pumps. The most common variations are shown below (the drive gear is shown blue and the idler is shown purple).
Suction and pressure ports need to interface where the gears mesh (shown as dim gray lines in the internal pump images). Some internal gear pumps have an additional, crescent shaped seal (shown above, right).
Generally used in: diesel oil, crude oil, lubes oil & sludge etc.
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 :a motor drives one gear and it in turn drives the other gear. A shaft supports each
gear 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.
Q64) EXPLAIN WORKING OF CENTRIFUGAL PUMP?
A pump is a machine used to raise liquids from a low point to a high point. In a centrifugal pump liquid enters the centre or eye of the impeller and flows radially out between the vanes, its velocity being increased by the impeller rotation. A diffuser or volute is then used to convert most of the kinetic energy in the liquid into pressure.
The arrangement of a centrifugal pump is shown diagrammatically in figure below
Fig: Centrifugal pumping operation
A vertical, single stage, single entry, centrifugal pump for general marine duties is shown in Figure here. The mainframe and casing, together with a motor support bracket, house the pumping element assembly. The pumping element is made up of a top cover, a pump shaft, an impeller, a bearing bush and a sealing arrangement around the shaft. The sealing arrangement may be a packed gland or a mechanical seal and the bearing lubrication system will vary according to the type
of seal. Replaceable wear rings are fitted to the impeller and the casing. The motor support bracket has two large apertures to provide access to the pumping element, and a coupling spacer is fitted between the motor and pump shaft to enable the removal of the pumping element without disturbing the motor.
Fig: Single entry centrifugal pump
A vertical multi-stage single-entry centrifugal pump used for deep-well cargo pumping is shown in Figure below. This can be considered as a series of centrifugal pumps arranged to supply
one another in series and thus progressively increase the discharge pressure. The pump drive is located outside the tank and can be electric, hydraulic or any appropriate means suitable for the location.
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.
Centrifugal pumps, while being suitable for most general marine duties, are not self-priming and require some means of removing air from the suction pipeline and filling it with liquid. Where the liquid to be pumped is at a level higher than the pump, opening an air cock near the pump suction will enable the air to be forced out as the pipeline fills up under the action of gravity. If the pump is below sea water level, and seawater priming is permissible in the system, then opening a seawater injection valve and the air cock on the pump will effect priming.
Alternatively an air-pumping unit can be provided to individual pumps or as a central priming system connected to several pumps. The water ring or liquid ring primer can be arranged as an individual unit mounted on the pump and driven by it, or as a motor driven unit mounted separately and serving several pumps. The primer consists of an elliptical casing in which a vaned rotor revolves. The rotor may be separate from the hub and provide the air inlet and discharge ports as shown in Figure down. Alternatively another design has the rotor and hub as one piece with ports on the cover. The rotor vanes revolve and force a ring of liquid to take up the elliptical shape of the casing. The water ring, being elliptical,
advances and recedes from the central hub, causing a pumping action to occur. The suction piping system is connected to the air inlet ports and the suction line is thus primed by the removal of air. The air removed from the system is discharged to atmosphere. A reservoir of water is provided to replenish the water ring when necessary.
When starting a centrifugal pump the suction valve is opened and the discharge valve left shut: then the motor is started and the priming unit will prime the suction line. Once the pump is primed the delivery valve can be slowly opened and the quantity of liquid can be regulated by opening or closing the delivery valve. When stopping the pump the delivery valve is closed and the motor stopped.
Regular maintenance on the machine will involve attention to lubrication of the shaft bearing and ensuring that the shaft seal or gland is not leaking liquid. Unsatisfactory operation or loss of performance may require minor or major overhauls. Common faults, such as no discharge, may be a result of valves in the system being shut, suction strainers blocked or other faults occurring in the priming system. Air leaks in the suction piping, a choked impeller or too tight a shaft gland can all lead to poor performance.
When dismantling the pump to remove the pumping element any priming pipes or cooling water supply pipes must be disconnected. Modern pumps have a coupling spacer, which can be removed to enable the pumping element to be withdrawn without disturbing the motor: the impeller and shaft can then be readily separated for examination. The shaft- bearing bush together with the casing and impeller wear rings should be examined for wear.
Q65) Why centrifugal pump is not self –priming?
ANS) This is because of its churning effect it is unable to remove air positively, as mass of air is relatively zero.
Q66) what is Hot well & why it is kept heated?
ANS) Hot Well recollects the steam after the work is done and it is condensed. Boiler water tank is known as the hot well because boiler feed pump takes suction from the hot well and gives it to the boiler through feed check valve. It can be called by three different names, they are:
Hot well - because the water collected is hot
Cascade tank - because it collects the water from the condenser
Observation tank - because it is used for observe for any oil or dirt entering the system
If any traces of oil are found in the system, it indicates that there is a crack in the steam heating line inside the fuel oil tanks. A sight glass is placed to observe the traces of oil or dirt present in the system.
If oil is present in the system then it forms a coating in tubes of the boiler, which may lead to lesser heat transfer to the water in the boiler.
Water is kept heated to avoid oxidation of feed water & also to avoid thermal stress of boiler.
Q67) HOW TO MEASURE MAIN BEARING CLEARENCE OF 2-