2009年1月6日星期二
Two Basic Requirements for Trouble-Free Operation of Centrifugal Pumps
Centrifugal pumps are the ultimate in simplicity. In general there are two basicrequirements that have to be met at all the times for a trouble free operation and longerservice life of centrifugal pumps.The first requirement is that no cavitation of the pump occurs throughout thebroad operating range and the second requirement is that a certain minimum continuousflow is always maintained during operation.A clear understanding of the concept of cavitation, its symptoms, its causes, andits consequences is very much essential in effective analyses and troubleshooting of thecavitation problem.Just like there are many forms of cavitation, each demanding a unique solution,there are a number of unfavorable conditions which may occur separately orsimultaneously when the pump is operated at reduced flows. Some include:o Cases of heavy leakages from the casing, seal, and stuffing boxo Deflection and shearing of shaftso Seizure of pump internalso Close tolerances erosiono Separation cavitationo Product quality degradationo Excessive hydraulic thrusto Premature bearing failuresEach condition may dictate a different minimum flow low requirement. The finaldecision on recommended minimum flow is taken after careful “techno-economical”analysis by both the pump user and the manufacturer.The consequences of prolonged conditions of cavitation and low flow operationcan be disastrous for both the Centrifugal Pumps and the process. Such failures in hydrocarbonservices have often caused damaging fires resulting in loss of machine, production, andworst of all, human life.Thus, such situations must be avoided at all cost whether involving modificationsin the Centrifugal Pumps and its piping or altering the operating conditions. Proper selection andsizing of pump and its associated piping can not only eliminate the chances of cavitationand low flow operation but also significantly decrease their harmful effects.
2009年1月4日星期日
Select an Energy-Efficient Centrifugal Pump
Overview
Centrifugal pumps handle high flow rates, provide smooth, nonpulsating delivery, and regulate the flow rate over a wide range without damaging the pump. Centrifugal pumps have few moving parts, and the wear caused by normal operation is minimal. They are also compact and easily disassembled for maintenance. The design of an efficient pumping system depends on relationships between fluid flow rate, piping layout, control methodology, and pump selection. Before a Centrifugal pump is selected, its application must be clearly understood.
Centrifugal pump Performance
Centrifugal pump are generally divided into three classes: radial flow, mixed flow, and axial flow. Since they are designed around their impellers, differences in impeller design allow manufacturers to produce pumps that can perform efficiently under conditions that vary from low flow rate with high head to high flow rate with low head. The amount of fluid a Centrifugal pump moves depends on the differential pressure or head it supplies. The flow rate increases as the head decreases. Manufacturers generally provide a chart that indicates the zone or range of heads and flow rates that a particular pump model can provide.
Before you select a pump model, examine its performance curve, which is indicated by its head-flow rate or operating curve. The curve shows the pump’s capacity (in gallons per minute [gpm]) plotted against total developed head (in feet). It also shows efficiency (percentage), required power input (in brake-horsepower [bhp]), and suction head requirements (net positive suction head requirement in feet) over a range of flow rates.
Pump curves also indicate pump size and type, operating speed (in revolutions per minute), and impeller size (in inches). It also shows the pump’s best efficiency point (BEP). The pump operates most cost effectively when the operating point is close to the BEP.
Pumps can generally be ordered with a variety of impeller sizes. Each impeller has a separate performance curve (see Figure 1). To minimize pumping system energy consumption, select a pump so the system curve intersects the pump curve within 20% of its BEP, and select a midrange impeller that can be trimmed or replaced to meet higher or lower flow rate requirements. Select a pump with high efficiency contours over your range of expected operating points. A few points of efficiency improvement can save significant energy over the life of the pump.
2008年12月28日星期日
Pumping system characteristics
Resistance of the system: head
Pressure is needed to pump the liquid through the system at a certain rate. This pressure hasto be high enough to overcome the resistance of the system, which is also called “head”. Thetotal head is the sum of static head and friction head:
Static head
Static head is the difference in height between the source and destination of the pumpedliquid (see Figure 2a). Static head is independent of flow (see Figure 2b). The static head at acertain pressure depends on the weight of the liquid and can be calculated with this equation:
Head (in feet) = Pressure (psi) X 2.31/Specific gravity
Static head consists of:
Static suction head (hS): resulting from lifting the liquid relative to the pump center line.The hS is positive if the liquid level is above pump centerline, and negative if the liquidlevel is below pump centerline (also called “suction lift)
Static discharge head (hd): the vertical distance between the pump centerline and thesurface of the liquid in the destination tank.
b) Friction head (hf)
This is the loss needed to overcome that is caused by the resistance to flow in the pipe andfittings. It is dependent on size, condition and type of pipe, number and type of pipe fittings,flow rate, and nature of the liquid. The friction head is proportional to the square of the flowrate as shown in figure 3. A closed loop circulating system only exhibits friction head (i.e. notstatic head).
Pressure is needed to pump the liquid through the system at a certain rate. This pressure hasto be high enough to overcome the resistance of the system, which is also called “head”. Thetotal head is the sum of static head and friction head:
Static head
Static head is the difference in height between the source and destination of the pumpedliquid (see Figure 2a). Static head is independent of flow (see Figure 2b). The static head at acertain pressure depends on the weight of the liquid and can be calculated with this equation:
Head (in feet) = Pressure (psi) X 2.31/Specific gravity
Static head consists of:
Static suction head (hS): resulting from lifting the liquid relative to the pump center line.The hS is positive if the liquid level is above pump centerline, and negative if the liquidlevel is below pump centerline (also called “suction lift)
Static discharge head (hd): the vertical distance between the pump centerline and thesurface of the liquid in the destination tank.
b) Friction head (hf)
This is the loss needed to overcome that is caused by the resistance to flow in the pipe andfittings. It is dependent on size, condition and type of pipe, number and type of pipe fittings,flow rate, and nature of the liquid. The friction head is proportional to the square of the flowrate as shown in figure 3. A closed loop circulating system only exhibits friction head (i.e. notstatic head).
2008年12月23日星期二
What are pumps and pumping systems?
Pumping systems account for nearly 20% of the world’s electrical energy demand and rangefrom 25-50% of the energy usage in certain industrial plant operations (US DOE, 2004).
Pumps have two main purposes:
Transfer of liquid from one place to another place (e.g. water from an undergroundaquifer into a water storage tank)
Circulate liquid around a system (e.g. cooling water or lubricants through machines andequipment)
The main components of a pumpingsystem are:
Pumps (different types of pumps areexplained in section 2)
Prime movers: electric motors,diesel engines or air system
Piping, used to carry the fluid
Valves, used to control the flow inthe system
Other fittings, controls andinstrumentation
End-use equipment, which havedifferent requirements (e.g. pressure,flow) and therefore determine thepumping system components andconfiguration. Examples include heat exchangers, tanks and hydraulic machines.
The pump and the prime mover are typically the most energy inefficient components.
Pumps have two main purposes:
Transfer of liquid from one place to another place (e.g. water from an undergroundaquifer into a water storage tank)
Circulate liquid around a system (e.g. cooling water or lubricants through machines andequipment)
The main components of a pumpingsystem are:
Pumps (different types of pumps areexplained in section 2)
Prime movers: electric motors,diesel engines or air system
Piping, used to carry the fluid
Valves, used to control the flow inthe system
Other fittings, controls andinstrumentation
End-use equipment, which havedifferent requirements (e.g. pressure,flow) and therefore determine thepumping system components andconfiguration. Examples include heat exchangers, tanks and hydraulic machines.
The pump and the prime mover are typically the most energy inefficient components.
2008年12月21日星期日
Components of a centrifugal pump
The main components of a centrifugal pump are shown in Figure 9 and described below:
Rotating components: an impeller coupled to a shaft
Stationary components: casing, casing cover, and bearings.
Figure 9. Main Components of a Centrifugal Pump (Sahdev)
a) ImpellerAn impeller is a circular metallic disc with a built-in passage for the flow of fluid. Impellersare generally made of bronze, polycarbonate, cast iron or stainless steel, but other materialsare also used. As the performance of the pump depends on the type of impeller, it isimportant to select a suitable design and to maintain the impeller in good condition.
The number of impellers determines the number of stages of the pump. A single stage pumphas one impeller and is best suited for low head (= pressure) service. A two-stage pump hastwo impellers in series for medium head service. A multi-stage pump has three or moreimpellers in series for high head service.
Impellers can be classified on the basis of:
Major direction of flow from the rotation axis: radial flow, axial flow, mixed flow
Suction type: single suction and double suction
Shape or mechanical construction:
− Closed impellers have vanes enclosed by shrouds (= covers) on both sides (Figure10). They are generally used for water pumps as the vanes totally enclose the water.This prevents the water from moving from the delivery side to the suction side, whichwould reduce the pump efficiency. In order to separate the discharge chamber fromthe suction chamber, a running joint is necessary between the impeller and pumpcasing. This joint is provided by wearing rings, which are mounted either overextended portion of impeller shroud or inside the cylindrical surface of pump casing.A disadvantage of closed impellers is the higher risk of blockage.
− Open and semi-open impellers (Figure 10) are less likely to clog. But to avoidclogging through internal re-circulation, the volute or back-plate of the pump must bemanually adjusted to get the proper impeller setting.
− Vortex pump impellers are suitable for solid and "stringy" materials but they are up to50% less efficient than conventional designs.
Electrical Energy Equipment: Pumps and Pumping Systems
Figure 10. Closed and Open Impeller Types (Sahdev)
b) Shaft
The shaft transfers the torque from the motor to the impeller during the startup and operationof the pump.
c) Casing
The main function of casing is to enclose the impeller at suction and delivery ends andthereby form a pressure vessel. The pressure at suction end may be as little as one-tenth ofatmospheric pressure and at delivery end may be twenty times the atmospheric pressure in asingle-stage pump. For multi-stage pumps the pressure difference is much higher. The casingis designed to withstand at least twice this pressure to ensure a large enough safety margin.
A second function of casing is to provide a supporting and bearing medium for the shaft andimpeller. Therefore the pump casing should be designed to
Provide easy access to all parts of pump for inspection, maintenance and repair
Make the casing leak-proof by providing stuffing boxes
Connect the suction and delivery pipes directly to the flanges
Be coupled easily to its prime mover (i.e. electric motor) without any power loss.
Electrical Energy Equipment: Pumps and Pumping Systems
There are two types of casings
Volute casing (Figure 11) has impellers that are fitted inside the casings. One of the mainpurposes is to help balance the hydraulic pressure on the shaft of the pump. However,operating pumps with volute casings at a lower capacity than the manufacturer’srecommended capacity, can result in lateral stress on the shaft of the pump. This cancause increased wearing of the seals, bearings, and the shaft itself. Double-volute casingsare used when the radial force becomes significant at reduced capacities.
Circular casing has stationary diffusion vanes surrounding the impeller periphery thatconvert speed into pressure energy. These casings are mostly used for multi-stage pumps.The casings can be designed as:
− Solid casing (Figure 12): the entire casing and the discharge nozzle are contained inone casting or fabricated piece.
− Split casing: two or more parts are joined together. When the casing parts are dividedby horizontal plane, the casing is called horizontally split or axially split casing.
Rotating components: an impeller coupled to a shaft
Stationary components: casing, casing cover, and bearings.
Figure 9. Main Components of a Centrifugal Pump (Sahdev)
a) ImpellerAn impeller is a circular metallic disc with a built-in passage for the flow of fluid. Impellersare generally made of bronze, polycarbonate, cast iron or stainless steel, but other materialsare also used. As the performance of the pump depends on the type of impeller, it isimportant to select a suitable design and to maintain the impeller in good condition.
The number of impellers determines the number of stages of the pump. A single stage pumphas one impeller and is best suited for low head (= pressure) service. A two-stage pump hastwo impellers in series for medium head service. A multi-stage pump has three or moreimpellers in series for high head service.
Impellers can be classified on the basis of:
Major direction of flow from the rotation axis: radial flow, axial flow, mixed flow
Suction type: single suction and double suction
Shape or mechanical construction:
− Closed impellers have vanes enclosed by shrouds (= covers) on both sides (Figure10). They are generally used for water pumps as the vanes totally enclose the water.This prevents the water from moving from the delivery side to the suction side, whichwould reduce the pump efficiency. In order to separate the discharge chamber fromthe suction chamber, a running joint is necessary between the impeller and pumpcasing. This joint is provided by wearing rings, which are mounted either overextended portion of impeller shroud or inside the cylindrical surface of pump casing.A disadvantage of closed impellers is the higher risk of blockage.
− Open and semi-open impellers (Figure 10) are less likely to clog. But to avoidclogging through internal re-circulation, the volute or back-plate of the pump must bemanually adjusted to get the proper impeller setting.
− Vortex pump impellers are suitable for solid and "stringy" materials but they are up to50% less efficient than conventional designs.
Electrical Energy Equipment: Pumps and Pumping Systems
Figure 10. Closed and Open Impeller Types (Sahdev)
b) Shaft
The shaft transfers the torque from the motor to the impeller during the startup and operationof the pump.
c) Casing
The main function of casing is to enclose the impeller at suction and delivery ends andthereby form a pressure vessel. The pressure at suction end may be as little as one-tenth ofatmospheric pressure and at delivery end may be twenty times the atmospheric pressure in asingle-stage pump. For multi-stage pumps the pressure difference is much higher. The casingis designed to withstand at least twice this pressure to ensure a large enough safety margin.
A second function of casing is to provide a supporting and bearing medium for the shaft andimpeller. Therefore the pump casing should be designed to
Provide easy access to all parts of pump for inspection, maintenance and repair
Make the casing leak-proof by providing stuffing boxes
Connect the suction and delivery pipes directly to the flanges
Be coupled easily to its prime mover (i.e. electric motor) without any power loss.
Electrical Energy Equipment: Pumps and Pumping Systems
There are two types of casings
Volute casing (Figure 11) has impellers that are fitted inside the casings. One of the mainpurposes is to help balance the hydraulic pressure on the shaft of the pump. However,operating pumps with volute casings at a lower capacity than the manufacturer’srecommended capacity, can result in lateral stress on the shaft of the pump. This cancause increased wearing of the seals, bearings, and the shaft itself. Double-volute casingsare used when the radial force becomes significant at reduced capacities.
Circular casing has stationary diffusion vanes surrounding the impeller periphery thatconvert speed into pressure energy. These casings are mostly used for multi-stage pumps.The casings can be designed as:
− Solid casing (Figure 12): the entire casing and the discharge nozzle are contained inone casting or fabricated piece.
− Split casing: two or more parts are joined together. When the casing parts are dividedby horizontal plane, the casing is called horizontally split or axially split casing.
2008年12月16日星期二
How a centrifugal pump works
Dynamic pumps are also characterized by their mode of operation: a rotating impellerconverts kinetic energy into pressure or velocity that is needed to pump the fluid.There are two types of dynamic pumps: Centrifugal pumps are the most common pumps used for pumping water in industrialapplications. Typically, more than 75% of the pumps installed in an industry arecentrifugal pumps. For this reason, this pump is further described below.
Special effect pumps are particularly used for specialized conditions at an industrial site.2.2.1 How a centrifugal pump worksA centrifugal pump is one of the simplest pieces of equipment in any process plant. Figure 8shows how this type of pump operates:
Liquid is forced into an impeller either by atmospheric pressure, or in case of a jet pumpby artificial pressure.
The vanes of impeller pass kinetic energy to the liquid, thereby causing the liquid torotate. The liquid leaves the impeller at high velocity.
The impeller is surrounded by a volute casing or in case of a turbine pump a stationarydiffuser ring. The volute or stationary diffuser ring converts the kinetic energy intopressure energy.
Electrical Energy Equipment: Pumps and Pumping Systems
2008年11月25日星期二
Hydraulic Pump Motor
A hydraulic pump motor is a device that is composed of three main parts: a reservoir, a pump, and a hydraulic cylinder. These components work together to convert hydraulic energy into mechanical energy. This energy is then used to power various automobile parts and construction machinery.
Hydraulic cylinders have two chambers and a piston rod. The basic concept behind hydraulic systems is fairly simple: force applied to incompressible liquid triggers linear motion in a piston.
When the hydraulic fluid is pumped into the bottom chamber of a hydraulic cylinder, the piston rod is pushed up, pushing the fluid in the other chamber back into the reservoir. This process pressurizes the chamber and extends the piston to its full length, giving the hydraulic cylinder its ability to push.
Hydraulic cylinders have two chambers and a piston rod. The basic concept behind hydraulic systems is fairly simple: force applied to incompressible liquid triggers linear motion in a piston.
When the hydraulic fluid is pumped into the bottom chamber of a hydraulic cylinder, the piston rod is pushed up, pushing the fluid in the other chamber back into the reservoir. This process pressurizes the chamber and extends the piston to its full length, giving the hydraulic cylinder its ability to push.
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