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).
2008年12月28日星期日
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
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