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PIPELINE PUMPS
Centrifugal pumps are used for pumping petroleum products like HSD, SKO, MS,Naphtha, ATF, LPG etc. in cross country pipelines. Pipeline pumps are designed as perAPI 610.
Pump efficiency is of primary importance because of the power required to pump the liquid. Generally pipeline systems install pumps in series because the differential head is primarily made up of energy loss due to friction and shutdown of one of the units would result in only a partial loss in the thruput capacity. For pipelines where the differential head is mostly static, pumps are installed in parallel. Series arrangement in static system would be unsuitable because the differential head required could not be obtained unless all of the units were operating.
Double suction single stage, double stage or multistage pump design is chosen depending
on the system characteristics and initial & ultimate capacities to be pumped. The single
and two stage double suction pumps are inherently balanced axially. The multistage units
utilize opposed impeller design to obtain axial balance. Electric motors are normally used as drivers. Pumps are also driven using gas turbines.
CENTRIFUGAL PUMPS – MAJOR COMPONENTS
Centrifugal pumps consist of set of rotating vanes, enclosed within a housing or casing, used to impart energy to a fluid through centrifugal force. The pump has two main parts: a rotating element which includes an impeller and a shaft, and a stationary element made up of a casing (volute or solid), stuffing box, and bearings.
The general components of a centrifugal pump, both stationary and rotary, are shown in the figures below.
Resized to 78% (was 904 x 562) - Click image to enlarge
STATIONARY COMPONENTS
CASING
Casings are generally of two types: volute and circular. The impellers are fitted inside the casings. Volute casings build a higher head; circular casings are used for low head and high capacity.
A volute is a curved funnel increasing in area to the discharge port. As the area of the cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid. Volute casing helps to balance the hydraulic pressure on the shaft of the pump. However, running volute-style pumps at a lower capacity than the recommended capacity can put lateral stress on the shaft of the pump, increasing wearand tear on the seals and bearings, and on the shaft itself. Double-volute casings are used when the radial thrusts become significant at reduced capacities.
A circular casing has stationary diffusion vanes surrounding the impeller periphery that convert velocity energy to pressure energy. Conventionally, the diffusers are applied to multi-stage pumps. The casings can be designed either as solid casings or split casings. Solid casing implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. A split casing implies two or more parts are fastened together.
When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing. Casing Wear rings act as the seal between the casing and the impeller.
SUCTION AND DISCHARGE NOZZLE
The suction and discharge nozzles are part of the casings itself. They commonly have the following configurations.
1. End suction/Top discharge - The suction nozzle is located at the end of, and concentric to, the shaft while the discharge nozzle is located at the top of the case perpendicular to the shaft. This pump is always of an overhung type and typically has lower NPSHr because the liquid feeds directly into the impeller eye.
2. Top suction Top discharge nozzle -The suction and discharge nozzles are located at the top of the case perpendicular to the shaft. This pump can either be an overhung type or between-bearing type but is always a radially split case pump.
3. Side suction / Side discharge nozzles - The suction and discharge nozzles arelocated at the sides of the case perpendicular to the shaft. This pump can have either an axially or radially split case type.
SEAL CHAMBER AND STUFFING BOX
Seal chamber and Stuffing box both refer to a chamber, either integral with or separate from the pump case housing that forms the region between the shaft and casing where sealing media are installed. When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. Both the seal chamber
and the stuffing box have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump pressure casing. When the pressure at the bottom of the chamber is below atmospheric, it prevents air leakage into the pump. When the pressure is above atmospheric, the chambers prevent liquid leakage out of the pump. The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for proper temperature control.
Parts of a simple Seal Chamber
Gland: The gland is a very important part of the seal chamber or the stuffing box. It gives the packings or the mechanical seal the desired fit on the shaft sleeve. It can be easily adjusted in axial direction. The gland comprises of the seal flush, quench, cooling, drain, and vent connection ports as per the standard codes like API 682.
Throat Bushing: The bottom or inside end of the chamber is provided with a stationary device called throat bushing that forms a restrictive close clearance around the sleeve (or shaft) between the seal and the impeller.
Internal circulating device or pumping ring refers to device located in the seal chamber to circulate seal chamber fluid through a cooler or barrier/buffer fluid reservoir.
Mechanical Seal: Mechanical seals are designed to prevent leakage between a rotatingshaft and its housing under conditions of extreme pressure, shaft speed and temperature.
Mechanical seals can be single acting or double acting. Single(acting) mechanical seals have one sealing gap. The lubrication film required by the sliding seal faces is provided by the medium to be sealed. The lubrication film required by the seal faces in double (acting) mechanical seals is provided by a higher pressure buffer medium (sealant liquid) that is compatible with the pumped product. The sealant liquid is at a higher pressure so that any leakage across the seal faces will be the sealant liquid into the pumped product. This buffer serves to separate the product and the atmosphere.
Bearing housing: The bearing housing encloses the bearings mounted on the shaft. The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. The bearing house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating cooling water.
ROTATING COMPONENTS
IMPELLER
The impeller is the main rotating part that provides the centrifugal acceleration to the fluid.
Impellers are classified in many ways.
Based on major direction of flow in reference to the axis of rotation
Radial flow
Axial flow
Mixed flow
Based on suction type
Single-suction: Liquid inlet on one side.
Double-suction: Liquid inlet to the impeller symmetrically from both sides.
Based on mechanical construction
Closed: Shrouds or sidewall enclosing the vanes.
Open: No shrouds or wall to enclose the vanes.
Semi-open or vortex type.
Closed impellers require wear rings and these wear rings present another maintenance problem. Open and semi-open impellers are less likely to clog, but need manual adjustment to the volute or back plate to get the proper impeller setting and prevent internal re-circulation.
Vortex pump impellers are great for solids and "stringy" materials but they are up to 50% less efficient than conventional designs. The number of impellers determines the number of stages of the pump. A single stage pump has one impeller only and is best for low head service. A two-stage pump has two impellers in series for medium head service. A multistage pump has three or more impellers in series for high head service.
SHAFT
The basic purpose of a centrifugal pump shaft is to transmit the torques encountered when starting and during operation while supporting the impeller and other rotating parts. It must do this job with a deflection less than the minimum clearance between the
rotating and stationary parts.
Shaft Sleeve: Pump shafts are usually protected from erosion, corrosion, and wear at the seal chambers, leakage joints, internal bearings, and in the waterways by renewable sleeves. Unless otherwise specified, a shaft sleeve of wear, corrosion, and erosionresistan material shall be provided to protect the shaft. The sleeve shall be sealed at one end. The shaft sleeve assembly shall extend beyond the outer face of the seal gland plate.
Coupling: Couplings compensate for axial growth of the shaft and transmit torque to the impeller.
AUXILIARY COMPONENTS
Auxiliary components generally include the following piping systems for the following services:
Seal flushing , cooling , quenching systems
Seal drains and vents
Bearing lubrication, cooling systems
Seal chamber or stuffing box cooling, heating systems
Pump pedestal cooling systems
Auxiliary piping systems include tubing, piping, isolating valves, control valves, relief valves, temperature gauges and thermocouples, pressure gauges, sight flow indicators, orifices, seal flush coolers, dual seal barrier/buffer fluid reservoirs, and all related ventsand drains.
All auxiliary components shall comply with the requirements as per standard codes like API 610 (refinery services), API 682 (shaft sealing systems) etc.
Resized to 78% (was 904 x 627) - Click image to enlarge
Resized to 72% (was 975 x 617) - Click image to enlarge
WORKING MECHANISM OF A CENTRIFUGAL PUMP
Centrifugal pumps operate using kinetic energy to move fluid utilizing an impeller and a circular pump casing. The impeller produces liquid velocity and the casing forces the liquid to discharge from the pump converting velocity to pressure. This is accomplished by offsetting the impeller in the casing, and by maintaining a close clearance between the impeller and the casing at the cutwater. The fluid enters the pump near the center of the impeller and is moved to its outside diameter by the rotating motion of the impeller.
The faster the impeller revolves or the bigger the impeller is, the higher will the velocity of the liquid. The vanes on the impeller progressively widen from the center of the impeller that reduces speed and increases pressure. This allows centrifugal pumps to produce continuous flows at high pressure. By forcing the fluid through without cupping it, centrifugal pumps can achieve a very high flow rate.
Centrifugal pumps generate flow by using one of three actions: radial flow, mixed flow, and axial flow. These classifications do not rate the performance quality of the pump,they are merely groupings based upon the pump’s action.
Radial flow pumps are centrifugal pumps in which the pressure is developed wholly by centrifugal force. In mixed flow pumps, the pressure is developed partly by centrifugalforce and partly by the lift of the vanes of the impeller on the liquid. Axial flow centrifugal pumps develop pressure by the propelling or lifting action of the vanes of the impeller on the liquid.
CENTRIFUGAL PUMP DESIGN
CAPACITY OR PUMP FLOW RATE
Capacity of a pump means the flow rate with which liquid is moved or pushed by the pump to the desired point in the process. The flow rate Q is defined as the external volume flow per unit of time in m3/s (l/s and m3/h are also commonly used). The capacity depends on a number of factors like:
Process liquid characteristics i.e. density, viscosity
Size of the pump and its inlet and outlet sections
Impeller size
Impeller rotational speed RPM
Size and shape of cavities between the vanes
Pump suction and discharge temperature and pressure conditions.
As liquids are essentially incompressible, the capacity is directly related with the velocity of flow in the suction pipe.
PUMP HEAD
The head H of a pump is the useful kinetic energy transmitted by the pump to the liquid, expressed in metres. In other words, head is a measurement of the height of a liquid column that the pump could create from the kinetic energy imparted by the pump to the liquid. It is independent of the density of the product, i.e. a centrifugal pump will generate the same head H for all fluids irrespective of the gravity . The gravity determines the pressure within the pump and forms part of the pump power input P.
PUMP SIZE SELECTION
The data needed for selecting the pump size is the capacity Q and the head H at the required duty point. The pump size and speed can be determined from the family of performance curves. The other parameters of the pump selected, such as efficiency ,input power P and NPSH, can be established from the appropriate individual performance curve.
CALCULATING THE POWER CONSUMPTION
Pump Power Input
The pump power input P of a centrifugal pump is the mechanical energy at the pump coupling or pump shaft absorbed from the drive. It is determined using the following equation:
The pump power input P in kW can also be directly read with sufficient accuracy off the curves.
PUMP CURVE
Pump's operating characteristics are shown by its “Characteristics Performance Curve” by plotting its capacity i.e. flow rate against its developed head. The pump performance curve also shows its efficiency (Best Efficiency Point – BEP), required input power (inBHP), NPSHreq, speed (in RPM), and other information such as pump size and type, impeller size, etc. This curve is plotted for a constant speed (rpm) and a given impeller diameter (or series of diameters). It is generated by tests performed by the pump manufacturer. Pump curves are based on the specific gravity and kinematic viscosity of water, unless stated otherwise.
The duty conditions determine which curve is more favourable, a flat or a steep curve. With a steep curve the capacity changes less than with a flat curve under the same differential head conditions.
The steep curve thus possesses better control characteristics.
A typical performance curve is shown below:
The plot starts at zero flow. The head at this point corresponds to the shut-off head point of the pump. The curve then decreases to a point where the flow is maximum and the head minimum. This point is called the run-out point. The pump curve is relatively flat and the head decreases gradually as the flow increases.
The pump's range of operation is from the shut-off head point to the run-out point. The pump cannot operate beyond the run-out point.
PIPING CHARACTERISTIC (SYSTEM CHARACTERISTIC)
The piping system head curve is the change in flow with respect to head of the system. The system head H is plotted against the capacity Q to get the curve. It represents the relationship between flow and hydraulic losses in a system in a graphic form. Hydraulic losses in piping systems are composed of pipe friction losses, valves, elbows and other fittings, entrance and exit losses and losses from changes in pipe size by enlargement or reduction in diameter. Since friction losses vary as a square of the flow rate, the system curve is parabolic in shape. By plotting the system head curve and pump curve together, it can be determined where the pump will operate on its curve.
OPERATING POINT
An operating point is defined as the point of intersection between the pump curve and the system characteristics curve. The operating point of a pump can be changed only by altering the speed or diameter of the impeller or by modifying the piping characteristic
SUCTION CHARACTERISTICS
Net Positive Suction Head (NPSH)
Pumps can pump only liquids, not vapors. The satisfactory operation of a pump requires that vaporization of the liquid being pumped does not occur at any condition of operation. This is so desired because when a liquid vaporizes its volume increases very much. For example, 1 ft3 of water at room temperature becomes 1700 ft3 of vapor at the same temperature. Hence, if we are to pump a fluid effectively, it must be kept always in the liquid form.
Rise in temperature and fall in pressure induces vaporization.
The vaporization begins when the vapor pressure of the liquid at the operating temperature equals the external system pressure, which, in an open system is always equal to atmospheric pressure. Any decrease in external pressure or rise in operating temperature can induce vaporization and the pump stops pumping. Thus, the pump always needs to have a sufficient amount of suction head present to prevent this vaporization at the lowest pressure point in the pump.
NPSH is a measure to prevent liquid vaporization. Net Positive Suction Head (NPSH) is the energy in the liquid in the centre line of the pump to prevent liquid vaporization. NPSH is referred to as either required or available NPSH.
NPSH Required: NPSH required is a function of the pump design and is determined based on actual pump test by the vendor. As the liquid passes from the pump suction to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquidstrikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH required is the positive head absolute required at the pump suction to overcome these pressure drops in the pump and maintain the majority of the liquid above its vapor pressure. Centrifugal pumps will only operate satisfactorily if there is no build up of vapour within the pump. Therefore the pressure head at the NPSH datum point must exceed the vapour pressure head of the product.
The manufacturer usually tests the pump with water at different capacities, created by throttling the suction side. When the first signs of vaporization induced cavitation occur, the suction pressure is noted. This pressure is converted into the head. This head number is published on the pump curve and is referred as the "net positive suction head required (NPSHr) or sometimes in short as the NPSH.
Thus the Net Positive Suction Head (NPSH) is the total head at the suction flange of the pump less the vapor pressure
converted to fluid column height of he liquid.
NPSHr is a function of pump design.
The NPSH is always positive since it is expressed in terms of absolute fluid column height. The NPSH required varies with speed and capacity within any particular pump. The NPSH required increase as the capacity is increasing because the velocity of the liquid is increasing, and as anytime the velocity of a liquid goes up, the pressure or head comes down. The NPSH is independent of the fluid density.
NPSH available: NPSHa is a function of system in which the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction, to be sure that the pump selected does not cavitate.
It is calculated based on system or process conditions. NPSH available is defined as:
NPSHa = Pressure head + Static head - Vapor pressure head of product – Friction head loss in the piping, valves and fittings.
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Significance of NPSHr and NPSHa
The NPSH available must always be greater than the NPSH required for the pump to operate properly. It is normal practice to have at least 2 to 3 feet of extra NPSH available at the suction flange to avoid any problems at the duty point.
PUMPING STATION
The location and spacing of pump stations depend on the hydraulic design of the pipeline, the topographic features of the pipeline route, the type and properties of the design fuel,the operating characteristics of the pumping units selected, and the friction head losses for the selected size of pipe. Maintenance must also be considered when selecting the location and spacing of the stations and type of equipment to be used.
Location. The location of pump stations depends mainly on pipeline design.
A pump station in the wrong location cannot pump at the required flow rate or will pump the required rate but at a reduced pressure. Because of rough terrain, a pump station may have to be located either downstream or upstream from the best design location to a better operating site along the pipeline.
Layout. Layout plans for each pump station are based on the pumping requirements. These plans give the location of tanks, pumps, and manifolds.
At the main pumping station (pumping station associated with refinery tanks or storage tanks at mother terminal), fuel storage tanks should be located toprovide gravity flow of fuel to the pumps while not allowing heavy vapors to accumulate in operating areas. Pumps and other installation equipment must be readily accessible to maintenance and operating personnel.
The pumping station should be located as close as possible to the base terminal from which it receives fuel. The initial station and the line connecting it to the base terminal must have enoug pressure to maintain the required design flow rate under all conditions.
Provision must be made for administrative, food and other housekeeping facilities. The pump stations should be sheltered from the weather.
· Pump Units. Pump units are classified as booster and mainline. Pump units used are determined from flow requirements, pipeline characteristics,and pump unit performance curves.
Booster pumps are installed to supply the required suction pressure betweentank farm installations and mainline pump stations.
Mainline pipeline pumps are installed along the pipeline at the pump stations to maintain the pressure and flow of products within the line.
Centrifugal Pumps in Parallel
Centrifugal pumps are used in parallel when required head can be generated from the designed pump but flow rate requirement is more than the pump’s discharge capability and cannot be met with a single pump.
When the system characteristic curve is considered with the curve for pumps in parallel, the operating point at the intersection of the two curves represents a higher volumetric flow rate than for a single pump and a greater system head loss. A greater system head loss occurs with the increased fluid velocity resulting from the increased volumetric flow rate. Because of the greater system head, the volumetric flow rate is actually less than twice the flow rate achieved by using a single pump.
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Operating Point for Two Parallel Centrifugal Pumps
Centrifugal Pumps in Series
Centrifugal pumps are used in series to overcome a larger system head loss than one pump can compensate for individually i.e. when the required flow rate can be generated from the designed pump but head requirement is more than the pump’s capability and cannot be met with a single pump.
Two identical centrifugal pumps operating at the same speed with the same volumetric flow rate contribute the same pump head. Since the inlet to the second pump is the outlet of the first pump, the head produced by both pumps is the sum of the individual heads. The volumetric flow rate from the inlet of the first pump to the outlet of the second remains the same.
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PIPELINE PUMPS
Centrifugal pumps are used for pumping petroleum products like HSD, SKO, MS,Naphtha, ATF, LPG etc. in cross country pipelines. Pipeline pumps are designed as perAPI 610.
Pump efficiency is of primary importance because of the power required to pump the liquid. Generally pipeline systems install pumps in series because the differential head is primarily made up of energy loss due to friction and shutdown of one of the units would result in only a partial loss in the thruput capacity. For pipelines where the differential head is mostly static, pumps are installed in parallel. Series arrangement in static system would be unsuitable because the differential head required could not be obtained unless all of the units were operating.
Double suction single stage, double stage or multistage pump design is chosen depending
on the system characteristics and initial & ultimate capacities to be pumped. The single
and two stage double suction pumps are inherently balanced axially. The multistage units
utilize opposed impeller design to obtain axial balance. Electric motors are normally used as drivers. Pumps are also driven using gas turbines.
CENTRIFUGAL PUMPS – MAJOR COMPONENTS
Centrifugal pumps consist of set of rotating vanes, enclosed within a housing or casing, used to impart energy to a fluid through centrifugal force. The pump has two main parts: a rotating element which includes an impeller and a shaft, and a stationary element made up of a casing (volute or solid), stuffing box, and bearings.
The general components of a centrifugal pump, both stationary and rotary, are shown in the figures below.
Resized to 78% (was 904 x 562) - Click image to enlarge
STATIONARY COMPONENTS
CASING
Casings are generally of two types: volute and circular. The impellers are fitted inside the casings. Volute casings build a higher head; circular casings are used for low head and high capacity.
A volute is a curved funnel increasing in area to the discharge port. As the area of the cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid. Volute casing helps to balance the hydraulic pressure on the shaft of the pump. However, running volute-style pumps at a lower capacity than the recommended capacity can put lateral stress on the shaft of the pump, increasing wearand tear on the seals and bearings, and on the shaft itself. Double-volute casings are used when the radial thrusts become significant at reduced capacities.
A circular casing has stationary diffusion vanes surrounding the impeller periphery that convert velocity energy to pressure energy. Conventionally, the diffusers are applied to multi-stage pumps. The casings can be designed either as solid casings or split casings. Solid casing implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. A split casing implies two or more parts are fastened together.
When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing. Casing Wear rings act as the seal between the casing and the impeller.
SUCTION AND DISCHARGE NOZZLE
The suction and discharge nozzles are part of the casings itself. They commonly have the following configurations.
1. End suction/Top discharge - The suction nozzle is located at the end of, and concentric to, the shaft while the discharge nozzle is located at the top of the case perpendicular to the shaft. This pump is always of an overhung type and typically has lower NPSHr because the liquid feeds directly into the impeller eye.
2. Top suction Top discharge nozzle -The suction and discharge nozzles are located at the top of the case perpendicular to the shaft. This pump can either be an overhung type or between-bearing type but is always a radially split case pump.
3. Side suction / Side discharge nozzles - The suction and discharge nozzles arelocated at the sides of the case perpendicular to the shaft. This pump can have either an axially or radially split case type.
SEAL CHAMBER AND STUFFING BOX
Seal chamber and Stuffing box both refer to a chamber, either integral with or separate from the pump case housing that forms the region between the shaft and casing where sealing media are installed. When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. Both the seal chamber
and the stuffing box have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump pressure casing. When the pressure at the bottom of the chamber is below atmospheric, it prevents air leakage into the pump. When the pressure is above atmospheric, the chambers prevent liquid leakage out of the pump. The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for proper temperature control.
Parts of a simple Seal Chamber
Gland: The gland is a very important part of the seal chamber or the stuffing box. It gives the packings or the mechanical seal the desired fit on the shaft sleeve. It can be easily adjusted in axial direction. The gland comprises of the seal flush, quench, cooling, drain, and vent connection ports as per the standard codes like API 682.
Throat Bushing: The bottom or inside end of the chamber is provided with a stationary device called throat bushing that forms a restrictive close clearance around the sleeve (or shaft) between the seal and the impeller.
Internal circulating device or pumping ring refers to device located in the seal chamber to circulate seal chamber fluid through a cooler or barrier/buffer fluid reservoir.
Mechanical Seal: Mechanical seals are designed to prevent leakage between a rotatingshaft and its housing under conditions of extreme pressure, shaft speed and temperature.
Mechanical seals can be single acting or double acting. Single(acting) mechanical seals have one sealing gap. The lubrication film required by the sliding seal faces is provided by the medium to be sealed. The lubrication film required by the seal faces in double (acting) mechanical seals is provided by a higher pressure buffer medium (sealant liquid) that is compatible with the pumped product. The sealant liquid is at a higher pressure so that any leakage across the seal faces will be the sealant liquid into the pumped product. This buffer serves to separate the product and the atmosphere.
Bearing housing: The bearing housing encloses the bearings mounted on the shaft. The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. The bearing house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating cooling water.
ROTATING COMPONENTS
IMPELLER
The impeller is the main rotating part that provides the centrifugal acceleration to the fluid.
Impellers are classified in many ways.
Based on major direction of flow in reference to the axis of rotation
Radial flow
Axial flow
Mixed flow
Based on suction type
Single-suction: Liquid inlet on one side.
Double-suction: Liquid inlet to the impeller symmetrically from both sides.
Based on mechanical construction
Closed: Shrouds or sidewall enclosing the vanes.
Open: No shrouds or wall to enclose the vanes.
Semi-open or vortex type.
Closed impellers require wear rings and these wear rings present another maintenance problem. Open and semi-open impellers are less likely to clog, but need manual adjustment to the volute or back plate to get the proper impeller setting and prevent internal re-circulation.
Vortex pump impellers are great for solids and "stringy" materials but they are up to 50% less efficient than conventional designs. The number of impellers determines the number of stages of the pump. A single stage pump has one impeller only and is best for low head service. A two-stage pump has two impellers in series for medium head service. A multistage pump has three or more impellers in series for high head service.
SHAFT
The basic purpose of a centrifugal pump shaft is to transmit the torques encountered when starting and during operation while supporting the impeller and other rotating parts. It must do this job with a deflection less than the minimum clearance between the
rotating and stationary parts.
Shaft Sleeve: Pump shafts are usually protected from erosion, corrosion, and wear at the seal chambers, leakage joints, internal bearings, and in the waterways by renewable sleeves. Unless otherwise specified, a shaft sleeve of wear, corrosion, and erosionresistan material shall be provided to protect the shaft. The sleeve shall be sealed at one end. The shaft sleeve assembly shall extend beyond the outer face of the seal gland plate.
Coupling: Couplings compensate for axial growth of the shaft and transmit torque to the impeller.
AUXILIARY COMPONENTS
Auxiliary components generally include the following piping systems for the following services:
Seal flushing , cooling , quenching systems
Seal drains and vents
Bearing lubrication, cooling systems
Seal chamber or stuffing box cooling, heating systems
Pump pedestal cooling systems
Auxiliary piping systems include tubing, piping, isolating valves, control valves, relief valves, temperature gauges and thermocouples, pressure gauges, sight flow indicators, orifices, seal flush coolers, dual seal barrier/buffer fluid reservoirs, and all related ventsand drains.
All auxiliary components shall comply with the requirements as per standard codes like API 610 (refinery services), API 682 (shaft sealing systems) etc.
Resized to 78% (was 904 x 627) - Click image to enlarge
Resized to 72% (was 975 x 617) - Click image to enlarge
WORKING MECHANISM OF A CENTRIFUGAL PUMP
Centrifugal pumps operate using kinetic energy to move fluid utilizing an impeller and a circular pump casing. The impeller produces liquid velocity and the casing forces the liquid to discharge from the pump converting velocity to pressure. This is accomplished by offsetting the impeller in the casing, and by maintaining a close clearance between the impeller and the casing at the cutwater. The fluid enters the pump near the center of the impeller and is moved to its outside diameter by the rotating motion of the impeller.
The faster the impeller revolves or the bigger the impeller is, the higher will the velocity of the liquid. The vanes on the impeller progressively widen from the center of the impeller that reduces speed and increases pressure. This allows centrifugal pumps to produce continuous flows at high pressure. By forcing the fluid through without cupping it, centrifugal pumps can achieve a very high flow rate.
Centrifugal pumps generate flow by using one of three actions: radial flow, mixed flow, and axial flow. These classifications do not rate the performance quality of the pump,they are merely groupings based upon the pump’s action.
Radial flow pumps are centrifugal pumps in which the pressure is developed wholly by centrifugal force. In mixed flow pumps, the pressure is developed partly by centrifugalforce and partly by the lift of the vanes of the impeller on the liquid. Axial flow centrifugal pumps develop pressure by the propelling or lifting action of the vanes of the impeller on the liquid.
CENTRIFUGAL PUMP DESIGN
CAPACITY OR PUMP FLOW RATE
Capacity of a pump means the flow rate with which liquid is moved or pushed by the pump to the desired point in the process. The flow rate Q is defined as the external volume flow per unit of time in m3/s (l/s and m3/h are also commonly used). The capacity depends on a number of factors like:
Process liquid characteristics i.e. density, viscosity
Size of the pump and its inlet and outlet sections
Impeller size
Impeller rotational speed RPM
Size and shape of cavities between the vanes
Pump suction and discharge temperature and pressure conditions.
As liquids are essentially incompressible, the capacity is directly related with the velocity of flow in the suction pipe.
PUMP HEAD
The head H of a pump is the useful kinetic energy transmitted by the pump to the liquid, expressed in metres. In other words, head is a measurement of the height of a liquid column that the pump could create from the kinetic energy imparted by the pump to the liquid. It is independent of the density of the product, i.e. a centrifugal pump will generate the same head H for all fluids irrespective of the gravity . The gravity determines the pressure within the pump and forms part of the pump power input P.
PUMP SIZE SELECTION
The data needed for selecting the pump size is the capacity Q and the head H at the required duty point. The pump size and speed can be determined from the family of performance curves. The other parameters of the pump selected, such as efficiency ,input power P and NPSH, can be established from the appropriate individual performance curve.
CALCULATING THE POWER CONSUMPTION
Pump Power Input
The pump power input P of a centrifugal pump is the mechanical energy at the pump coupling or pump shaft absorbed from the drive. It is determined using the following equation:
The pump power input P in kW can also be directly read with sufficient accuracy off the curves.
PUMP CURVE
Pump's operating characteristics are shown by its “Characteristics Performance Curve” by plotting its capacity i.e. flow rate against its developed head. The pump performance curve also shows its efficiency (Best Efficiency Point – BEP), required input power (inBHP), NPSHreq, speed (in RPM), and other information such as pump size and type, impeller size, etc. This curve is plotted for a constant speed (rpm) and a given impeller diameter (or series of diameters). It is generated by tests performed by the pump manufacturer. Pump curves are based on the specific gravity and kinematic viscosity of water, unless stated otherwise.
The duty conditions determine which curve is more favourable, a flat or a steep curve. With a steep curve the capacity changes less than with a flat curve under the same differential head conditions.
The steep curve thus possesses better control characteristics.
A typical performance curve is shown below:
The plot starts at zero flow. The head at this point corresponds to the shut-off head point of the pump. The curve then decreases to a point where the flow is maximum and the head minimum. This point is called the run-out point. The pump curve is relatively flat and the head decreases gradually as the flow increases.
The pump's range of operation is from the shut-off head point to the run-out point. The pump cannot operate beyond the run-out point.
PIPING CHARACTERISTIC (SYSTEM CHARACTERISTIC)
The piping system head curve is the change in flow with respect to head of the system. The system head H is plotted against the capacity Q to get the curve. It represents the relationship between flow and hydraulic losses in a system in a graphic form. Hydraulic losses in piping systems are composed of pipe friction losses, valves, elbows and other fittings, entrance and exit losses and losses from changes in pipe size by enlargement or reduction in diameter. Since friction losses vary as a square of the flow rate, the system curve is parabolic in shape. By plotting the system head curve and pump curve together, it can be determined where the pump will operate on its curve.
OPERATING POINT
An operating point is defined as the point of intersection between the pump curve and the system characteristics curve. The operating point of a pump can be changed only by altering the speed or diameter of the impeller or by modifying the piping characteristic
SUCTION CHARACTERISTICS
Net Positive Suction Head (NPSH)
Pumps can pump only liquids, not vapors. The satisfactory operation of a pump requires that vaporization of the liquid being pumped does not occur at any condition of operation. This is so desired because when a liquid vaporizes its volume increases very much. For example, 1 ft3 of water at room temperature becomes 1700 ft3 of vapor at the same temperature. Hence, if we are to pump a fluid effectively, it must be kept always in the liquid form.
Rise in temperature and fall in pressure induces vaporization.
The vaporization begins when the vapor pressure of the liquid at the operating temperature equals the external system pressure, which, in an open system is always equal to atmospheric pressure. Any decrease in external pressure or rise in operating temperature can induce vaporization and the pump stops pumping. Thus, the pump always needs to have a sufficient amount of suction head present to prevent this vaporization at the lowest pressure point in the pump.
NPSH is a measure to prevent liquid vaporization. Net Positive Suction Head (NPSH) is the energy in the liquid in the centre line of the pump to prevent liquid vaporization. NPSH is referred to as either required or available NPSH.
NPSH Required: NPSH required is a function of the pump design and is determined based on actual pump test by the vendor. As the liquid passes from the pump suction to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquidstrikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH required is the positive head absolute required at the pump suction to overcome these pressure drops in the pump and maintain the majority of the liquid above its vapor pressure. Centrifugal pumps will only operate satisfactorily if there is no build up of vapour within the pump. Therefore the pressure head at the NPSH datum point must exceed the vapour pressure head of the product.
The manufacturer usually tests the pump with water at different capacities, created by throttling the suction side. When the first signs of vaporization induced cavitation occur, the suction pressure is noted. This pressure is converted into the head. This head number is published on the pump curve and is referred as the "net positive suction head required (NPSHr) or sometimes in short as the NPSH.
Thus the Net Positive Suction Head (NPSH) is the total head at the suction flange of the pump less the vapor pressure
converted to fluid column height of he liquid.
NPSHr is a function of pump design.
The NPSH is always positive since it is expressed in terms of absolute fluid column height. The NPSH required varies with speed and capacity within any particular pump. The NPSH required increase as the capacity is increasing because the velocity of the liquid is increasing, and as anytime the velocity of a liquid goes up, the pressure or head comes down. The NPSH is independent of the fluid density.
NPSH available: NPSHa is a function of system in which the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction, to be sure that the pump selected does not cavitate.
It is calculated based on system or process conditions. NPSH available is defined as:
NPSHa = Pressure head + Static head - Vapor pressure head of product – Friction head loss in the piping, valves and fittings.
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Significance of NPSHr and NPSHa
The NPSH available must always be greater than the NPSH required for the pump to operate properly. It is normal practice to have at least 2 to 3 feet of extra NPSH available at the suction flange to avoid any problems at the duty point.
PUMPING STATION
The location and spacing of pump stations depend on the hydraulic design of the pipeline, the topographic features of the pipeline route, the type and properties of the design fuel,the operating characteristics of the pumping units selected, and the friction head losses for the selected size of pipe. Maintenance must also be considered when selecting the location and spacing of the stations and type of equipment to be used.
Location. The location of pump stations depends mainly on pipeline design.
A pump station in the wrong location cannot pump at the required flow rate or will pump the required rate but at a reduced pressure. Because of rough terrain, a pump station may have to be located either downstream or upstream from the best design location to a better operating site along the pipeline.
Layout. Layout plans for each pump station are based on the pumping requirements. These plans give the location of tanks, pumps, and manifolds.
At the main pumping station (pumping station associated with refinery tanks or storage tanks at mother terminal), fuel storage tanks should be located toprovide gravity flow of fuel to the pumps while not allowing heavy vapors to accumulate in operating areas. Pumps and other installation equipment must be readily accessible to maintenance and operating personnel.
The pumping station should be located as close as possible to the base terminal from which it receives fuel. The initial station and the line connecting it to the base terminal must have enoug pressure to maintain the required design flow rate under all conditions.
Provision must be made for administrative, food and other housekeeping facilities. The pump stations should be sheltered from the weather.
· Pump Units. Pump units are classified as booster and mainline. Pump units used are determined from flow requirements, pipeline characteristics,and pump unit performance curves.
Booster pumps are installed to supply the required suction pressure betweentank farm installations and mainline pump stations.
Mainline pipeline pumps are installed along the pipeline at the pump stations to maintain the pressure and flow of products within the line.
Centrifugal Pumps in Parallel
Centrifugal pumps are used in parallel when required head can be generated from the designed pump but flow rate requirement is more than the pump’s discharge capability and cannot be met with a single pump.
When the system characteristic curve is considered with the curve for pumps in parallel, the operating point at the intersection of the two curves represents a higher volumetric flow rate than for a single pump and a greater system head loss. A greater system head loss occurs with the increased fluid velocity resulting from the increased volumetric flow rate. Because of the greater system head, the volumetric flow rate is actually less than twice the flow rate achieved by using a single pump.
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Operating Point for Two Parallel Centrifugal Pumps
Centrifugal Pumps in Series
Centrifugal pumps are used in series to overcome a larger system head loss than one pump can compensate for individually i.e. when the required flow rate can be generated from the designed pump but head requirement is more than the pump’s capability and cannot be met with a single pump.
Two identical centrifugal pumps operating at the same speed with the same volumetric flow rate contribute the same pump head. Since the inlet to the second pump is the outlet of the first pump, the head produced by both pumps is the sum of the individual heads. The volumetric flow rate from the inlet of the first pump to the outlet of the second remains the same.
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