pumps onboard
Principle of centrifugal pump
Head.
The ideal head is based on the flow deviation of an ideal liquid, and does not include the effect of flow losses in the impeller due to friction and turbulence, and it includes none of the hydraulic flow losses that occur in the pump's casing. These losses are represented by an additional correction factor called the hydraulic efficiency.
The ideal head is based on the flow deviation of an ideal liquid, and does not include the effect of flow losses in the impeller due to friction and turbulence, and it includes none of the hydraulic flow losses that occur in the pump's casing. These losses are represented by an additional correction factor called the hydraulic efficiency.
Power requirement.
The actual horsepower required to drive the pump exceeds the theoretical, because of additional losses not accounted for in the hydraulic efficiency. The sources of the additional losses include the following:
a. Mechanical losses, which include losses due to friction in the pump's bearings, seals, and packing. The power necessary to overcome mechanical losses increases with decreasing specific speed and capacity.
b. Disk friction, which includes the loss due to the friction between the liquid in the pump's casing and the outer surfaces of its rotating impeller. The power necessary to overcome this loss increases with the impeller diameter and operating speed.
c. Internal leakage across wearing rings, inter-stage bushings, and internal balancing devices, which results in the re-circulation within the pump's casing of a fraction of the liquid being discharged from the impeller.
The actual horsepower required to drive the pump exceeds the theoretical, because of additional losses not accounted for in the hydraulic efficiency. The sources of the additional losses include the following:
a. Mechanical losses, which include losses due to friction in the pump's bearings, seals, and packing. The power necessary to overcome mechanical losses increases with decreasing specific speed and capacity.
b. Disk friction, which includes the loss due to the friction between the liquid in the pump's casing and the outer surfaces of its rotating impeller. The power necessary to overcome this loss increases with the impeller diameter and operating speed.
c. Internal leakage across wearing rings, inter-stage bushings, and internal balancing devices, which results in the re-circulation within the pump's casing of a fraction of the liquid being discharged from the impeller.
Net Positive Suction Head (NPSH)
This is a measure of the pump suction conditions. The manufacturer gives the curve plotting the NPSH vs. Capacity.
This is a measure of the pump suction conditions. The manufacturer gives the curve plotting the NPSH vs. Capacity.
Construction of centrifugal pump
Casings
Centrifugal pump casings are made in two or more sections, for ease of maintenance.
They are usually split along a vertical plan that passes through the shaft axis, referred to as axially split, or along a plane that is perpendicular to the shaft, referred to as axially split, or along a plane that is perpendicular to the shaft, referred to as horizontal or radial split pump. Marine pump are usually mounted vertically, i.e. the pump lies below the driving motor, since this requires less horizontal or deck space, than with a horizontal mounting.
The Casing performs three important functions:
a. It guides the liquid being pumped from the inlet to the eye of the impeller. In some cases guide vanes are provided to straighten the flow of liquid entering the impeller, and to break up vortices formed by the entry of the liquid, that cause re-circulation of liquid back out of the Impeller's eye.
b. It collects discharged liquid from the periphery of the impeller, and converts a portion of the Kinetic energy (velocity .head) to Potential energy (pressure head). This is pressure recovery or diffusion.
c. It guides the liquid discharged from the first stage to the inlet of the next stage (in a multistage pump), or to the outlet of the pump.
Types of Casings : Regardless of the type, the Casing is a pressure containing boundary. Its thickness must be sufficient to withstand the design pressure. In addition, a pump's Casing structure must be suitably strengthened to withstand not only hydrostatic pressures but also stresses that result from vessel's motion. External ribs may be added to the casing to increase its strength.
Volute type Casings: The volute or scroll, has a gradually increasing radius and cross-sectional area, that surrounds the periphery of the Impeller. The increase in volume thus produced converts the kinetic energy into pressure head. Volutes are used with many single-stage radial and mixed-flow centrifugal pumps.
The volute is some time designed so that the increase in its cross-sectional area is proportional to the angular advancement from the tongue or cut-water to the throat.
With this configuration the pumped fluid is assumed to have a constant average velocity in all sections of the volute during operation at design capacity. As an alternative to maintaining a constant velocity, a volute may be designed so that the fluid passing through it maintains a constant angular momentum, at the design capacity.
The radial clearance between the outside diameter of the impeller and the volute's tongue should be small, to limit the amount of leakage; however, if the gap is too small excessive pressure pulsations, turbulence, and noise can result. This gap or cut-water clearance is frequently kept so as to optimise pump performance, while at the same time allowing a range of impeller diameters to be used with the same casing . To reduce hydraulic shock and separation losses, the angle used for the volute's tongue often matches the absolute fluid flow angle at the design capacity.
In lieu of a spiral volute, some centrifugal pumps are furnished with a circular or concentric collector that has a constant radius and cross-sectional area around the periphery of the impeller. Except when operating at shut-off (with a closed discharge valve and no through-flow), the velocity in this type of a collector increases, as the flow channel progresses angularly from the tongue to the throat.
A modified or semi-concentric Casing, is a variation of the concentric design, in which the radius and cross-sectional area of the collector remain constant over only a portion of the casing's circumference. Volute-type multistage pumps often have axially split casings, with the flow passage between successive stages referred to as crossovers, being either integrally cast or weld into casing.
Liquid collected in each inter stage volute is frequently decelerated in the corresponding crossover before being directed to the eye of the impeller in the pump's next stage. The pumped liquid is also usually decelerated in the casing's discharge nozzle, which forms the transition from the throat of the last stage volute to the pump's outlet port.
A multi-vaned diffuser consists of a number of diverging vanes, mounted in a ring that surrounds the periphery of the impeller. In diffuser type multistage pumps, the flow channels from the discharge of one stage to the suction of the next are generally formed by a series of vanes located on the back side of each diffuser or included in the diaphragms, also referred to as stage pieces, that separate adjacent stages.
With this configuration, the pump's rotor, together with its stationary diffusers and stage pieces, can often be inserted into a radially split Casing as an assembled cartridge. Because of the casing's cylindrical shape, these units are frequently referred to as "barrel" pumps.
Many axial-flow pumps are also fitted with a diffuser that is located downstream from the discharge of the propeller.
The "axial diffuser" is used to convert the tangential velocity component of the absolute flow leaving the propeller into pressure head. It is also used to straighten this flow, so that the pumped fluid is discharged from the Casing in an axial direction.
Vane-less diffuser Casings
When a wide range of operation is anticipated, a radial-flow centrifugal pump is sometimes fitted with a vane-less diffuser. The pressure recovery with this type of a Casing occurs in an annular passage that surrounds the impeller.
Neglecting friction, fluid passing through this channel, which can have a constant width or can flare outwards slightly, is assumed to follow free-vortex flow. For a vane-less diffuser with parallel walls, the reduction in fluid velocity will, therefore, be proportional to the increase in the radius of the diffuser's flow channel. A volute-type collector is often attached to the outlet of a vane-less diffuser.
To maintain a proper clearance between the impeller and the casing, so as to reduce leakage to a minimum, wear rings are fitted. As the leakage increases, due to wear on these rings, the clearances need to be checked at regular intervals. If found excessive, the wear rings are renewed during over-haul, along with the bearings.
In case of single-suction impellers, a stationary wear-ring is fitted behind the rear shroud so as to take up the thrust.
Sometimes rotating wear rings are fitted on the outer hubs of the impeller. To reduce leakage, serrations or grooves are present on the inside surface. These also reduce damage in case the impeller should be forced towards the casing by some transient external force. They also lessen damage caused by trapped foreign bodies, which may be carried through with the liquid. In multi-stage pumps, replaceable bushes are used to limit leakage and maintain proper clearances.
The pumps thrust bearing absorbs loads that are applied in the axial direction. The pump's shaft should be free to expand/contract in response to changes in axial load or temperature, for which reason it should be constrained axially at only one location.
A pump shaft that is rigidly coupled to the shaft of its driver, therefore, is generally not fitted with a separate thrust bearing but is, instead, supported axially by the thrust bearing in the driver. The remaining pump bearings, which are typically configured so that they absorb only radial loads, are frequently referred to as line bearings.
In a single-stage pump with an impeller centred on rotor, a bearing is installed at each end of the shaft. The type of bearing used depends for a specific application depends on the magnitude and orientation of applied loads, the temperatures, speed and the method of lubrication. Single row deep grove ball bearings are used in most centrifugal pumps. For increased loads, a double row deep grooved ball bearing is used. Bearings are usually lubricated with grease, and this must be of the proper type, depending on the temperature to which it will be subjected.
Grease is, easier to apply then oil, and can easily keep contaminants away from the running surface. If the heat generated is negligible, oil is not required as a coolant. The grease is pressure fed from a nipple, and should be renewed at the recommended intervals. To provide additional protection for the bearing, a 'slinger' is mounted on the shaft, external to the housing, which prevents dirt and moisture from travelling along the shaft into the bearing.
A pump operating below its first critical speed is a 'stiff shaft' pump, while one operating above its first critical speed a 'flexible shaft' pump.
This is difficult since the casing is stationary and the shaft is rotating at a high speed. The provision of multiple packing rings inside a gland has traditionally been used, since this does the job with the least of friction, is easy to replace and cheap.
The portion of the shaft in the way of gland packing is protected by a sleeve, which is hardened or has a wear resistant coating. To limit the temperature rise due to friction, a certain amount of 'controlled leakage' is allowed which can be easily adjusted by tightening/loosing the gland of the pump
If the pressure of the liquid at the base of the stuffing box is below atmospheric pressure, air can be drawn into the pump. In addition to resulting in a loss of lubrication to the packing, air entering the casing through the stuffing box can have a detrimental effect on pump performance.
Whenever it is possible for the base of the stuffing box to be under a vacuum, high pressure liquid is often injected into the stuffing box through a lantern ring or seal cage that is sandwiched between two of the intermediate rings of packing.
Pumps furnished for applications involving contaminated fluids may also be fitted with a lantern ring so that clean liquid can be injected into the stuffing box to flush contaminents away from the packing.
When it is impractical to provide clean high pressure liquid to the stuffing box, grease is sometimes injected through the lantern-ring connection.
The primary sealing surfaces in a mechanical seal are the polished faces of one stationary ring and one rotating ring, that are separated by a thin film of fluid. One of these mating faces is frequently a hard material, such as aluminium oxide or tungsten carbide, while the second is a softer material which has self lubrication, such as carbon.
Two secondary seals (o-rings) prevent leakage from occurring between the rotating ring and the shaft, and between the stationary ring and the gland. In many seals, the rotating ring is flexibly mounted on the pump's shaft with springs or with either a metallic or an elastomeric bellow.
This arrangement enables the face of the rotating seal ring to remain parallel to that of the stationary mating ring, with limited radial or axial shaft movement, and permits the axial position of the rotating ring to be self-adjusting to account for minor wear of the two mating surfaces. Sometimes the stationary ring is flexibly mounted. Mechanical seals are cooled and lubrited by a portion of the pumped liquid that is re-circulated through either an internal port or an external line connecting the discharge side of the casing to the seal area.
Pumped liquid may contain abrasive particles, which are often passes through a cyclone-type abrasive separator, prior to being injected into the seal area. The mechanical seals used in many critical applications have a gland that has a built-in auxiliary, stuffing box. In the event of a mechanical seal failure, two or more rings of packing can be installed in this stuffing box to enable pump operation to continue, until the mechanical seal can be replaced.
However, because the auxiliary packing would receive no lubrication when the mechanical seal is functioning properly, it is important that this packing be installed only after a mechanical seal failure has occurred.
The actual replacement of a leaking mechanical seal can be time consuming if a partial disassembly of the pump is required, to permit both the seal being replaced and the new seal to pass over the end of the shaft. One way to eliminate the need for this is to use a split mechanical seal. By splitting the mechanical seal's sealing elements into halves, they can be replaced without removing other parts of the pump.
Centrifugal pump casings are made in two or more sections, for ease of maintenance.
They are usually split along a vertical plan that passes through the shaft axis, referred to as axially split, or along a plane that is perpendicular to the shaft, referred to as axially split, or along a plane that is perpendicular to the shaft, referred to as horizontal or radial split pump. Marine pump are usually mounted vertically, i.e. the pump lies below the driving motor, since this requires less horizontal or deck space, than with a horizontal mounting.
The Casing performs three important functions:
a. It guides the liquid being pumped from the inlet to the eye of the impeller. In some cases guide vanes are provided to straighten the flow of liquid entering the impeller, and to break up vortices formed by the entry of the liquid, that cause re-circulation of liquid back out of the Impeller's eye.
b. It collects discharged liquid from the periphery of the impeller, and converts a portion of the Kinetic energy (velocity .head) to Potential energy (pressure head). This is pressure recovery or diffusion.
c. It guides the liquid discharged from the first stage to the inlet of the next stage (in a multistage pump), or to the outlet of the pump.
Types of Casings : Regardless of the type, the Casing is a pressure containing boundary. Its thickness must be sufficient to withstand the design pressure. In addition, a pump's Casing structure must be suitably strengthened to withstand not only hydrostatic pressures but also stresses that result from vessel's motion. External ribs may be added to the casing to increase its strength.
Volute type Casings: The volute or scroll, has a gradually increasing radius and cross-sectional area, that surrounds the periphery of the Impeller. The increase in volume thus produced converts the kinetic energy into pressure head. Volutes are used with many single-stage radial and mixed-flow centrifugal pumps.
The volute is some time designed so that the increase in its cross-sectional area is proportional to the angular advancement from the tongue or cut-water to the throat.
With this configuration the pumped fluid is assumed to have a constant average velocity in all sections of the volute during operation at design capacity. As an alternative to maintaining a constant velocity, a volute may be designed so that the fluid passing through it maintains a constant angular momentum, at the design capacity.
The radial clearance between the outside diameter of the impeller and the volute's tongue should be small, to limit the amount of leakage; however, if the gap is too small excessive pressure pulsations, turbulence, and noise can result. This gap or cut-water clearance is frequently kept so as to optimise pump performance, while at the same time allowing a range of impeller diameters to be used with the same casing . To reduce hydraulic shock and separation losses, the angle used for the volute's tongue often matches the absolute fluid flow angle at the design capacity.
In lieu of a spiral volute, some centrifugal pumps are furnished with a circular or concentric collector that has a constant radius and cross-sectional area around the periphery of the impeller. Except when operating at shut-off (with a closed discharge valve and no through-flow), the velocity in this type of a collector increases, as the flow channel progresses angularly from the tongue to the throat.
A modified or semi-concentric Casing, is a variation of the concentric design, in which the radius and cross-sectional area of the collector remain constant over only a portion of the casing's circumference. Volute-type multistage pumps often have axially split casings, with the flow passage between successive stages referred to as crossovers, being either integrally cast or weld into casing.
Liquid collected in each inter stage volute is frequently decelerated in the corresponding crossover before being directed to the eye of the impeller in the pump's next stage. The pumped liquid is also usually decelerated in the casing's discharge nozzle, which forms the transition from the throat of the last stage volute to the pump's outlet port.
Diffuser type Casings
Multi-vaned diffusers. In multistage pumps, where pressure recovery must be accomplished in the limited space between adjacent stages, multi-vaned diffusers are often used. This type of a pressure-recovery device, is also used in some single stage radial flow pumps due to its typically high peak efficiency.A multi-vaned diffuser consists of a number of diverging vanes, mounted in a ring that surrounds the periphery of the impeller. In diffuser type multistage pumps, the flow channels from the discharge of one stage to the suction of the next are generally formed by a series of vanes located on the back side of each diffuser or included in the diaphragms, also referred to as stage pieces, that separate adjacent stages.
With this configuration, the pump's rotor, together with its stationary diffusers and stage pieces, can often be inserted into a radially split Casing as an assembled cartridge. Because of the casing's cylindrical shape, these units are frequently referred to as "barrel" pumps.
Many axial-flow pumps are also fitted with a diffuser that is located downstream from the discharge of the propeller.
The "axial diffuser" is used to convert the tangential velocity component of the absolute flow leaving the propeller into pressure head. It is also used to straighten this flow, so that the pumped fluid is discharged from the Casing in an axial direction.
Vane-less diffuser Casings
When a wide range of operation is anticipated, a radial-flow centrifugal pump is sometimes fitted with a vane-less diffuser. The pressure recovery with this type of a Casing occurs in an annular passage that surrounds the impeller.
Neglecting friction, fluid passing through this channel, which can have a constant width or can flare outwards slightly, is assumed to follow free-vortex flow. For a vane-less diffuser with parallel walls, the reduction in fluid velocity will, therefore, be proportional to the increase in the radius of the diffuser's flow channel. A volute-type collector is often attached to the outlet of a vane-less diffuser.
Impellers and Wear rings
The strength of the impeller's vanes and shrouds must be sufficient to withstand hydraulic forces. Impellers can be of various types single entry or double entry. To maintain a proper clearance between the impeller and the casing, so as to reduce leakage to a minimum, wear rings are fitted. As the leakage increases, due to wear on these rings, the clearances need to be checked at regular intervals. If found excessive, the wear rings are renewed during over-haul, along with the bearings.
In case of single-suction impellers, a stationary wear-ring is fitted behind the rear shroud so as to take up the thrust.
Sometimes rotating wear rings are fitted on the outer hubs of the impeller. To reduce leakage, serrations or grooves are present on the inside surface. These also reduce damage in case the impeller should be forced towards the casing by some transient external force. They also lessen damage caused by trapped foreign bodies, which may be carried through with the liquid. In multi-stage pumps, replaceable bushes are used to limit leakage and maintain proper clearances.
Bearings
The net axial and radial loads applied to the pump's rotating assembly are transmitted to the bearing that support its shaft.The pumps thrust bearing absorbs loads that are applied in the axial direction. The pump's shaft should be free to expand/contract in response to changes in axial load or temperature, for which reason it should be constrained axially at only one location.
A pump shaft that is rigidly coupled to the shaft of its driver, therefore, is generally not fitted with a separate thrust bearing but is, instead, supported axially by the thrust bearing in the driver. The remaining pump bearings, which are typically configured so that they absorb only radial loads, are frequently referred to as line bearings.
In a single-stage pump with an impeller centred on rotor, a bearing is installed at each end of the shaft. The type of bearing used depends for a specific application depends on the magnitude and orientation of applied loads, the temperatures, speed and the method of lubrication. Single row deep grove ball bearings are used in most centrifugal pumps. For increased loads, a double row deep grooved ball bearing is used. Bearings are usually lubricated with grease, and this must be of the proper type, depending on the temperature to which it will be subjected.
Grease is, easier to apply then oil, and can easily keep contaminants away from the running surface. If the heat generated is negligible, oil is not required as a coolant. The grease is pressure fed from a nipple, and should be renewed at the recommended intervals. To provide additional protection for the bearing, a 'slinger' is mounted on the shaft, external to the housing, which prevents dirt and moisture from travelling along the shaft into the bearing.
Shaft design
The pump shaft should be strong enough to withstand the loads, and stiff enough to avoid excessive deflection, which could cause the rotating components to touch. The natural frequency should also not coincide with the forced frequency, resulting in resonance and vibration problems. pump's critical speed be varied at the design stage by adjusting the span between the bearings, the diameter of the shaft, the choice of material for varying the modulus of elasticity and the weight of the rotating assembly. A pump operating below its first critical speed is a 'stiff shaft' pump, while one operating above its first critical speed a 'flexible shaft' pump.
Gland Packings
The opening provided for the shaft to emerge from the casing must be sealed to prevent the leakage of the liquid being pumped. This is difficult since the casing is stationary and the shaft is rotating at a high speed. The provision of multiple packing rings inside a gland has traditionally been used, since this does the job with the least of friction, is easy to replace and cheap.
The portion of the shaft in the way of gland packing is protected by a sleeve, which is hardened or has a wear resistant coating. To limit the temperature rise due to friction, a certain amount of 'controlled leakage' is allowed which can be easily adjusted by tightening/loosing the gland of the pump
If the pressure of the liquid at the base of the stuffing box is below atmospheric pressure, air can be drawn into the pump. In addition to resulting in a loss of lubrication to the packing, air entering the casing through the stuffing box can have a detrimental effect on pump performance.
Whenever it is possible for the base of the stuffing box to be under a vacuum, high pressure liquid is often injected into the stuffing box through a lantern ring or seal cage that is sandwiched between two of the intermediate rings of packing.
Pumps furnished for applications involving contaminated fluids may also be fitted with a lantern ring so that clean liquid can be injected into the stuffing box to flush contaminents away from the packing.
When it is impractical to provide clean high pressure liquid to the stuffing box, grease is sometimes injected through the lantern-ring connection.
Mechanical Seals
A mechanical seal is an improved form of sealing. This results in reduced shaft sleeve wear and does away with the continuous leakage that must be tolerated with conventional gland packing.The primary sealing surfaces in a mechanical seal are the polished faces of one stationary ring and one rotating ring, that are separated by a thin film of fluid. One of these mating faces is frequently a hard material, such as aluminium oxide or tungsten carbide, while the second is a softer material which has self lubrication, such as carbon.
Two secondary seals (o-rings) prevent leakage from occurring between the rotating ring and the shaft, and between the stationary ring and the gland. In many seals, the rotating ring is flexibly mounted on the pump's shaft with springs or with either a metallic or an elastomeric bellow.
This arrangement enables the face of the rotating seal ring to remain parallel to that of the stationary mating ring, with limited radial or axial shaft movement, and permits the axial position of the rotating ring to be self-adjusting to account for minor wear of the two mating surfaces. Sometimes the stationary ring is flexibly mounted. Mechanical seals are cooled and lubrited by a portion of the pumped liquid that is re-circulated through either an internal port or an external line connecting the discharge side of the casing to the seal area.
Pumped liquid may contain abrasive particles, which are often passes through a cyclone-type abrasive separator, prior to being injected into the seal area. The mechanical seals used in many critical applications have a gland that has a built-in auxiliary, stuffing box. In the event of a mechanical seal failure, two or more rings of packing can be installed in this stuffing box to enable pump operation to continue, until the mechanical seal can be replaced.
However, because the auxiliary packing would receive no lubrication when the mechanical seal is functioning properly, it is important that this packing be installed only after a mechanical seal failure has occurred.
The actual replacement of a leaking mechanical seal can be time consuming if a partial disassembly of the pump is required, to permit both the seal being replaced and the new seal to pass over the end of the shaft. One way to eliminate the need for this is to use a split mechanical seal. By splitting the mechanical seal's sealing elements into halves, they can be replaced without removing other parts of the pump.
When high-temperature fluids are pumped, gland packing and mechanical seals are often cooled by circulating liquid through a surrounds the pump's sealing area.
Overhauling of centrifugal pump
The pump needs to be checked for damage, wear and corrosion / erosion at the routine over-haul. The clearance may need to be checked, alignment should be checked while the pump is warm, as the expansion due to heat while running would be different from the cold condition. Make markings and remove coupling bolt. Rotate the halves together, checking for angular alignment and rim out-of-true. Clean surfaces of pins and bushes, check for roundness — replace damaged bushes.
The key should be a snug fit, else it will lead to damage of key-way and further problems.
The Impeller should be visually inspected for wear erosion and cavitation damage. The impeller should not be excessively worn at any point, either due to contact or due to corrosion. The vanes at the eye and the discharge are the trouble spots, Diffuser blades which have worn at the tips need to be dressed up, to avoide stress raisers.
The shaft needs to be checked, especially the sleeve in way of the stuffing box, which is excessively tightened.
Slip the packing the correct way onto the shaft. When opening the ring, do it sideways, to prevent distortion; especially with lead filled or metallic types.
Swab the insides of metallic packing with the lubricant supplied along with the packing or a light oil, to keep the packing from scratching the shaft and damaging it.
Stagger the packing to ensure that all the gaps do not come in a line. Install the packing so that the lantern ring lines up with the cooling liquid opening. Remember that this ring moves in as the packing is compressed by the gland. First tighten gland by wrench, then back off till finger-tight, and allow packing to leak slightly till it seals itself, before re-tightening the gland. A slight leak is beneficial for lubrication and cooling, since there is high friction.
Use a split Wooden bushing to install the first turn of packing. Then force inside by tightening the gland.
Remove all of the old packing, taking care not to damage the shaft by scratches.
After removing all the packing, check the straightness of the shaft. A bent shaft will cause leakage problems, besides damaging the bearings.
If the neck bushing clearance is great, use stiffer bottom ring or replace the bush. Revolve the shaft and check with the dial gauge, that the shaft is true. To find the right size for packing, measure the bore of the stuffing box and the diameter of the shaft. Subtract shaft diameter from the inside bore, and divide by 2. This takes the guesswork out and ensures a perfect fit.
When cutting packing, place on shaft to cut rings: If you cut the ring by stretching out the packing, The ends will be at the wrong angle and sealing will be bad. If used for high temperature service, leave a slight gap between the ring to allow for expansion, else shaft may have excessive drag.
Turn a worn sleeve down to a smooth surface, if wear is negligible or replace with a new sleeve. Renew shaft packing. Ensure that the lantern ring is aligned with the sealing hole. Slight gland leakage is beneficial as it provides lubrication for the packing, and also prevents air ingress. Water flinger ring protects the bearing from water ingress and damage due to corrosion. The ring should be a snug fit on the shaft, else use sealing paste.
For shell type bearings, clearances need to be checked, either with appropriate feeler gauges or lead wire gauge. Adjust for proper clearance by means of shims. Causes of bearing overheating are:
a. Grease / Oil level too low or improper grade.
b. Dirt in bearing or moisture.
c. Bearing too tight.
d. Oil seals fitted too closely on shaft.
e. Misalignment.
The key should be a snug fit, else it will lead to damage of key-way and further problems.
The Impeller should be visually inspected for wear erosion and cavitation damage. The impeller should not be excessively worn at any point, either due to contact or due to corrosion. The vanes at the eye and the discharge are the trouble spots, Diffuser blades which have worn at the tips need to be dressed up, to avoide stress raisers.
The shaft needs to be checked, especially the sleeve in way of the stuffing box, which is excessively tightened.
Slip the packing the correct way onto the shaft. When opening the ring, do it sideways, to prevent distortion; especially with lead filled or metallic types.
Swab the insides of metallic packing with the lubricant supplied along with the packing or a light oil, to keep the packing from scratching the shaft and damaging it.
Stagger the packing to ensure that all the gaps do not come in a line. Install the packing so that the lantern ring lines up with the cooling liquid opening. Remember that this ring moves in as the packing is compressed by the gland. First tighten gland by wrench, then back off till finger-tight, and allow packing to leak slightly till it seals itself, before re-tightening the gland. A slight leak is beneficial for lubrication and cooling, since there is high friction.
Use a split Wooden bushing to install the first turn of packing. Then force inside by tightening the gland.
Remove all of the old packing, taking care not to damage the shaft by scratches.
After removing all the packing, check the straightness of the shaft. A bent shaft will cause leakage problems, besides damaging the bearings.
If the neck bushing clearance is great, use stiffer bottom ring or replace the bush. Revolve the shaft and check with the dial gauge, that the shaft is true. To find the right size for packing, measure the bore of the stuffing box and the diameter of the shaft. Subtract shaft diameter from the inside bore, and divide by 2. This takes the guesswork out and ensures a perfect fit.
When cutting packing, place on shaft to cut rings: If you cut the ring by stretching out the packing, The ends will be at the wrong angle and sealing will be bad. If used for high temperature service, leave a slight gap between the ring to allow for expansion, else shaft may have excessive drag.
Turn a worn sleeve down to a smooth surface, if wear is negligible or replace with a new sleeve. Renew shaft packing. Ensure that the lantern ring is aligned with the sealing hole. Slight gland leakage is beneficial as it provides lubrication for the packing, and also prevents air ingress. Water flinger ring protects the bearing from water ingress and damage due to corrosion. The ring should be a snug fit on the shaft, else use sealing paste.
For shell type bearings, clearances need to be checked, either with appropriate feeler gauges or lead wire gauge. Adjust for proper clearance by means of shims. Causes of bearing overheating are:
a. Grease / Oil level too low or improper grade.
b. Dirt in bearing or moisture.
c. Bearing too tight.
d. Oil seals fitted too closely on shaft.
e. Misalignment.
Dismantle a Centrifugal pump for survey:-
Considering the fire pump is a centrifugal pump. Ensure pump is stopped, shut the suction and discharge valve. Isolate the motor electrically by control room circuit breaker. Put Man at work tag. Electric isolation permit obtained. Local panel breaker put off. Check via purging cock that the line has no pressure.
Place the dismantled part on a clean surface. When separating fit and flange faces, use jack bolts and wooden hammers, and never apply force with chisels or drivers. When removing the rotating element, take care to avoid flaw on sliding faces and machined surfaces. Particularly, never damage the mechanical seal mating faces. When removing rotating parts from the shaft, draw off each one carefully after removing the locking device. Handle the long sized-parts such as shaft carefully so that it may not bend. At overhauling, put suitable match marks as many as possible to avoid mistakes when reassembling.
Dismantling:-
1. Remove the distant piece, fitted between the pump and motor coupling after removing both motor and pump side coupling bolts and discs.2. Remove cooling connections to mechanical seal.
3.Remove casing top cover bolts. Once casing cover bolts are removed, pump assembly is free to remove from place along with shaft, bearing housing, bearing, mechanical seal, impeller and impeller shaft with sleeve.
4. On removing the pump assembly, slacken impeller lock nut and remove the impeller from the shaft Remove shaft key. Care should be taken for not loosing the shaft key.
5. Remove distance ring.
6. Slacken the holding screw and remove mechanical seal's rotating part
7. Slacken bearing housing bolts fitted on casing cover.
8. Remove casing cover from the shaft.9. Remove shaft sleeve from shaft.
10. Remove bearing housing cover
11. Remove bearing retaining circlip.
12. Remove bearing housing along with bearing.
Factors which will decide replacing the parts.
In addition to that every time upon dismantling a pump few consumable items are required to be replaced such as wahers, seal rings, o-rings etc.
Further, during its inspections as per the PMS and surveys, parts are checked inspected and tested. If they are not found in satisfactory condition, must be replaced.
Cleaning and Inspection of parts:
1. Remove bearing from housing for inspection.
2. Clean impeller, bearing , shaft sleeve , casing and the shaft. Inspect and evaluate each parts for any damages, deformation, wear and tears.
3. Measure the clearance between impeller and wear rings for top as well as bottom near rings. If clearance is more than the maker's
recommendation, renew it.
4. Inspect shaft sleeve for any wear down. If sleeve found worn out , renew the same.
5. Check the condition of bearing. If found not good, renew the same.
6. Check the conditions of mechanical seal's rotary and stationary parts. If found damaged, renew it.
7. Check bearing bush at the casing bottom and confirm any wear down. If worn out beyond acceptable limits, renew It.
2. Clean impeller, bearing , shaft sleeve , casing and the shaft. Inspect and evaluate each parts for any damages, deformation, wear and tears.
3. Measure the clearance between impeller and wear rings for top as well as bottom near rings. If clearance is more than the maker's
recommendation, renew it.
4. Inspect shaft sleeve for any wear down. If sleeve found worn out , renew the same.
5. Check the condition of bearing. If found not good, renew the same.
6. Check the conditions of mechanical seal's rotary and stationary parts. If found damaged, renew it.
7. Check bearing bush at the casing bottom and confirm any wear down. If worn out beyond acceptable limits, renew It.
Assembling the pump:
Assembly
Carry out assembly, by reversing the order of disassembly and paying attention as follows.
a. Remove dust and stain from each part by washing it thoroughly with kerosene. Repair it if flaw is found.
b. Fit the locking device perfectly in each rotating part if necessary.
c. When fitting the parts with match marks, be sure no follow them.
d. Install the mechanical seal carefully and confirm its movement by hand after installation.
e. Insert each packing ring in good order softly one by one from the bottom, staggering each joint by 90° or 180°
f. Check up alignment.
Carry out assembly, by reversing the order of disassembly and paying attention as follows.
a. Remove dust and stain from each part by washing it thoroughly with kerosene. Repair it if flaw is found.
b. Fit the locking device perfectly in each rotating part if necessary.
c. When fitting the parts with match marks, be sure no follow them.
d. Install the mechanical seal carefully and confirm its movement by hand after installation.
e. Insert each packing ring in good order softly one by one from the bottom, staggering each joint by 90° or 180°
f. Check up alignment.
g. Turn the shaft by band to see whether it tums smoothly.
Trouble shooting of centrifugal pump
The major problems faced are loss of suction, failure to build-up pressure, vibration, over-heating and motor over-load. Failure to build-up pressure could be due to several reasons including:
a. Loss of suction / air ingress.
b. Discharge valve stuck.
c. Loss of speed due to motor fault (single phasing).
d. Internal damage to the impeller.
e. Excessive wear ring clearance.
Checking alignment:
Pump alignment is checked by means of a dial gauge.
Checking of alignment should be done whenever the shaft, impeller or any component is replaced and there is excessive whipping of the shaft.
Vibration
Pump vibration is usually because of either bearing damage, misalignment or damage to the, rotating components causing un-balance. There have also been cases of heavily clogged impellers causing excessive vibration. If any bearing is the cause, It is preferable to completely strip down the pump, time permitting and change all bearings, at the same time also checking condition of other components like impeller, shaft and wear rings, since the, cause of early bearing damage is usually due to a fault in these components.
Overload
Over-load of the motor will occur due to excessive resistance to motion, due to imbalance, bent shaft, insufficient working clearances, damaged bearings or clogged impellers.
Drop in Capacity
Drop in capacity is usually due to worn parts (excessive clearance of wear rings), or fault in the prime mover (not developing correct rpm). There have been cases of impellers being boxed-up in the wrong way, leading to sudden drop in performance (check the direction before dismantling, and confirm before re-assembly).
Over heating
At low capacities, the power supplied to the pump is no longer converted into mechanical output, which is seen as a drop in efficiency. The excess energy is seen as an increase in the temperatures, which could lead to reduction in working clearances, over heating and damage to bearings seals.
a. Loss of suction / air ingress.
b. Discharge valve stuck.
c. Loss of speed due to motor fault (single phasing).
d. Internal damage to the impeller.
e. Excessive wear ring clearance.
Checking alignment:
Pump alignment is checked by means of a dial gauge.
Checking of alignment should be done whenever the shaft, impeller or any component is replaced and there is excessive whipping of the shaft.
Vibration
Pump vibration is usually because of either bearing damage, misalignment or damage to the, rotating components causing un-balance. There have also been cases of heavily clogged impellers causing excessive vibration. If any bearing is the cause, It is preferable to completely strip down the pump, time permitting and change all bearings, at the same time also checking condition of other components like impeller, shaft and wear rings, since the, cause of early bearing damage is usually due to a fault in these components.
Overload
Over-load of the motor will occur due to excessive resistance to motion, due to imbalance, bent shaft, insufficient working clearances, damaged bearings or clogged impellers.
Drop in Capacity
Drop in capacity is usually due to worn parts (excessive clearance of wear rings), or fault in the prime mover (not developing correct rpm). There have been cases of impellers being boxed-up in the wrong way, leading to sudden drop in performance (check the direction before dismantling, and confirm before re-assembly).
Over heating
At low capacities, the power supplied to the pump is no longer converted into mechanical output, which is seen as a drop in efficiency. The excess energy is seen as an increase in the temperatures, which could lead to reduction in working clearances, over heating and damage to bearings seals.
Priming of centrifugal pump.
A centrifugal pump cannot pump a gas; therefore, the differential pressure necessary for flow will not be created if the impeller is having air or vapour. Prior to start-up, the pump's Casing should be filled with liquid and vented of all gases.
In applications where the pump must operate with a suction lift, a vacuum must initially be created to draw the liquid, into the eye of the impeller.
Priming a centrifugal pump :
a. The pump can be connected through vents to a central priming system. This type of a system is fitted with either vacuum pumps or air ejectors, and is generally used to prime multiple pumps on the vessel.
b. The pump can be fitted with an integral or independent vacuum pump.
c. A special centrifugal pump that is designed to be self-priming can be used. This type of a pump is typically fitted with a suction chamber that retains liquid when the unit is not operating.
In applications where the pump must operate with a suction lift, a vacuum must initially be created to draw the liquid, into the eye of the impeller.
Priming a centrifugal pump :
a. The pump can be connected through vents to a central priming system. This type of a system is fitted with either vacuum pumps or air ejectors, and is generally used to prime multiple pumps on the vessel.
b. The pump can be fitted with an integral or independent vacuum pump.
c. A special centrifugal pump that is designed to be self-priming can be used. This type of a pump is typically fitted with a suction chamber that retains liquid when the unit is not operating.
As the velocity of the outer tips of the impeller of a centrifugal pump is relatively low, the suction efforts of the pump, when empty rarely exceeds 12mm water gauge and hence must be primed with water and cannot exhaust the contained air in the manner that a displacement pump can.
Self priming action of a attached vacuum pump:-
Self priming action of a attached vacuum pump:-
The vacuum pump has been designed so that it can be mounted at the side of centrifugal pump to make them self priming. It consists of a vacuum pump, clutch actuating device and separate tank. The vacuum pump operate only while priming the main pump and automatically stops as soon as it enters into normal operation.
When motor is started the spring load by the hydraulic pressure piston pushes through the lever the clutch facing against the coupling and through the frictional force the vacuum pump begins operation.When the vacuum pump starts, the air from the main pump's casing top passes through the auto valve and after mixing with some water from the separate tank, is sucked by the vacuum pump and discharged into the separating tank, in the separate tank air is separated from water and discharged outside and thus priming is affected.
When priming has been complicated and the pressure of the main pump has raisin, the water pressure led from the hydraulic pressure piston, pushes the lever, disconnects the clutch and brings the vacuum pump to a stand still. Also auto valve closed automatically.
It is important to keep separate tank to be filled with water, pour machine oil through a small hole inside the friction clutch and see if it can slide axially when the lever is pulled. Check the vacuum pump can move freely when coupling turned by hand.
Gear pump
.
(a) Sketch and describe a gear type pump indicating the flow of fluid.
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 bya 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
1 Gland / Seal Housing
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6 Rotors (2)
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11 Bushes – Back Cover
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16 Locknut
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2 Shaft Sealing
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7 Key
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12 Back cover
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17 Adjusting Screw
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3 Front cover
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8 Body Gaskets
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13 Relief Valve
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18 Plug
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4 Bushes – Front Cover
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9 Following Shaft
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14 Spring
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5 Driving shaft
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10 Body
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15 Sealing Washer
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(b) state the materials that gear type pump components may be manufactured from.
Externals (head, casing, bracket)-Iron, ductile iron, steel, stainless steel, high alloys, composites (PPS, ETFE)
Internals (shafts)-Steel, stainless steel, high alloys, alumina ceramic
Internals (gears)-Steel, stainless steel, PTFE, composite (PPS)
Bushing-Carbon, bronze, silicon carbide, needle bearings
Shaft Seal-rubber oil seal, Gland Packing, lip seal,mechanical seal.
(c) specify THREE applications that are suitable for the employment of gear type pumps.
(1)The gear pumps are used widely for engine lubrication.
(2) For transfer of lub oil and fuel oil in ships.
(3) For hydraulic power transmission in deck machinery
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