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江南大學(xué)太湖學(xué)院
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CENTRIFUGAL PUMPS IN THE CHEMICAL INDUSTRY
Abstract : A centrifugal pump converts the input power to kinetic energy in the liquid by accelerating the liquid by a revolving device - an impeller. The most common type is the volute pump. Fluid enters the pump through the eye of the impeller which rotates at high speed. The fluid is accelerated radially outward from the pump chasing. A vacuum is created at the impellers eye that continuously draws more fluid into the pump . This article stresses on a series of centrifugal pumps，F(xiàn)rom a brief introduction to the principles.
Keywords : centrifugal pump ,Introduction ,Working principle , Cavitation ， Mechanism of Cavitation ，Solution and Remedies
1. Introduction
Pump ,device used to raise ,transfer ,or compress liquids and gases .Four general classes of pumps for liquids are described below .In all of them ,steps are taken to prevent cavitation (the formation of a vacuum) ,which would reduce the flow and damage the structure of the pump .Pumps used for gases and vapors are usually known as compressors .The study of fluids in motion is called fluid dynamics.
Water pump ,device for moving water from one location to another ,using tubes or other machinery .Water pumps operate under pressures ranging from a fraction of a pound to more than 10,000 pounds per square inch .Everyday examples of water pumps range from small electric pumps that circulate and aerate water in aquariums and fountains to sump pumps that remove water from beneath the foundations of homes .
One type of modern pumps used to move water is the centrifugal pump .Early version of the centrifugal pump ,the screw pump ,consists of a corkscrew-shaped mechanism in a pipe that ,when rotated ,pulls water upward .Screw pumps are often used in waste-water treatment plants because they can move large amounts of water without becoming clogged with debris .In the ancient Middle East the need for irrigation of farmland was a strong inducement to develop a water pump .Early pumps in this region were simple devices for lifting buckets of water from a source to a container or a trench .Greek mathematician and inventor Archimedes is thought to have devised the first screw pump in the third century BC .Later Greek inventor Ctesibius develop the first lift pump .During the late 17th and early 18th Centuries AD ,British engineer Thomas Savery ,French physicist Denis Papin ,And British blacksmith and inventor Thomas Newcomen contributed to the development of a water pump that used steam to power the pump’ piston .The steam-powered water pump’s first wide use was in pumping water out of mines .Modern-day examples of centrifugal pumps are those used at the Grand Coulee Dam on the Columbia River .This pump system has the potential to irrigate over one million acres of land .
Also known as rotary pumps ,centrifugal pumps have a rotating impeller ,also known as a blade ,that is immersed in the liquid .Liquid enters the pump near the axis of the impeller ,and the rotating impeller sweeps the liquid out toward the ends of the impeller blades at high pressure .The impeller also gives the liquid a relatively high velocity that can be converted into pressure in a stationary part of the pump ,known as the diffuser .In high-pressure pumps ,a number of impeller may be used in series ,and the diffusers following each impeller may contain guide vanes to gradually reduce the liquid velocity .For lower-pressure pumps ,the diffuser is generally a spiral passage ,known as a volute ,with its cross-sectional area increasing gradually to reduce the velocity efficiently .The impeller must be primed before it can begin operation ,that is ,the impeller must be surrounded by liquid when the pump is started .This can be done by placing a check valve in the suction line ,which holds the liquid in the pump when the impeller is not rotating .If this valve leaks ,the pump may need to be primed by the introduction of liquid from an outside source such as the discharge reservoir .A centrifugal pump generally has a valve in the discharge line to control the flow and pressure .For low flows and high pressures ,the action of the impeller is largely radial .For higher flows and lower discharge pressure ,the direction of the flow within the pump is more nearly parallel to the axis of the shaft ,and the pump is said to have an axial flow .The impeller in this case acts as a propeller .The transition from one set of floe conditions to the other is gradual ,and for intermediate condition , the device is called a mixed-flow pump .
2.The Centrifugal Pump
The centrifugal pump is by far the most widely used type in the chemical and petroleum industries .It will pump liquids with very wide ranging properties and suspensions with a high solids content including ,for example ,cement slurries ,and may be constructed from a very wide rang of corrosion resistant materials .The whole pump casing may be constructed from plastic such as polypropylene or it may be fitted with a corrosion-resistant lining .Because it operates at high speed ,it may be directly coupled to an electric motor and it will give a high flow rate for its size .
In this type of pump ,the fluid is fed to the centre of a rotating impeller and is thrown outward by centrifugal action .As a result of the high speed of rotation the liquid acquires a high kinetic energy and the pressure difference between the suction and delivery sides arises from the conversion of kinetic energy into pressure energy .
The impeller consists of a series of curved vanes so shaped that the flow within the pump is as smooth as possible .The greater the number of vanes on the impeller ,the greater is the control over the direction of the liquid and hence the smaller are the losses due to turbulence and circulation between the vanes .In the open impeller ,the vanes are fixed to a central hub ,whereas in the closed type the vanes are held between two supporting plates and leakage across the impeller is reduced .As will be seen later ,the angle of the tips of the blades very largely determines the operating characteristics of the pump .
The liquid enters the casing of the pump，normally in an axial direction，and is picked up by the vanes of the impeller．In the simple type of centrifugal pump，the liquid discharges into a volute，a chamber of gradually increasing cross—section with a tangential outlet．A volute type of pump is shown in Fig.(a)．In the turbine pump[-Fig．(b)]the liquid flows from the moving vanes of the impeller through a series of fixed vanes forming a diffusion ring．
This gives a more gradual change in direction to the fluid and more efficient conversion of kinetic energy into pressure energy than is obtained with the volute type．The angle of the leading edge of the fixed vanes should be such that the fluid is received without shock．The liquids flows along the surface of the impeller vane with a certain velocity whilst the tip of the vane is moving relative to the casing of the pump．The direction of motion of the liquid relative to the pump casing--and the required angle of the fixed vanes—is found by compounding these two velocities．In Fig．c，
c.
is the velocity of the liquid relative to the vane and is the tangential velocity of the tip of the vane；compounding these two velocities gives the resultant velocity of the liquid．It is apparent，therefore，that the required vane angle in the diffuser is dependent on the throughput，the speed of rotation，and the angle of the impeller blades．The pump will therefore operate at maximum efficiency only over a narrow range of conditions．
Virtual head of a centrifugal pump
The maximum pressure is developed when the whole of the excess kinetic energy of the fluid is converted into pressure energy. As indicated below．the head is proportional to the square of the radius and to the speed，and is of the order of 60m for a single—stage centrifugal pump；for higher pressures，multistage pumps must be used．Consider the liquid which is rotating at a distance of between r and r+dr from the centre of the pump(Fig．d)．
d
The mass of this element of fluid dm is given by 2πrdrdρ，where ρ is the density of the fluid and 6 is the width of the element of fluid。
If the fluid is traveling with a velocity u and at an angle θ to the tangential direction．The angular momentum of this mass of fluid
= dM (urcosθ)
The torque acting on the fluid dτ is equal to the rate of change of angular momentum with time，as it goes through the pump
Dτ = dM α/αt(urcosθ)=2πrbρdrα/αt(urcosθ) (2.1)
The volumetric rate of flow of liquid through the pump：
Q=2πrbα/αt (2.2)
Dr =Q ρ d(urcosθ) (2.3)
The total torque acting on the liquid in the pump is therefore obtained integrating dτ between the limits denoted by suffix 1 and suffix 2，where suffix 1 refers to the conditions at the inlet to the pump and suffix 2 refers to the condition at the discharge．
Thus，τ=Q ρ(cos- cos)
The advantages and disadvantages of the centrifugal pump
The main advantages are：
(1) It is simple in construction and can，therefore， be made in a wide range of materials
(2)There is a complete absence of valves．
(3)It operates at high speed(up to 100 Hz)and，therefore，can be coupled directly to
an electric motor. In general，the higher the speed the smaller the pump and motor for a give n duty．
(4)It gives a steady delivery．
(5)Maintenance costs are lower than for any other type of pump．
(6)No damage is done to the pump if the delivery line becomes blocked，provided it is not run in this condition for a prolonged period．
(7)It is much smaller than other pumps of equal capacity．It can，therefore，be made into a sealed unit with the driving motor and immersed in the suction tank．
(8)Liquids containing high proportions of suspended solids are readily handled．
The main disadvantages are：
(1)The single—stage pump will not develop a high pressure．Multistage pumps will develop greater heads bat they are very much more expensive and cannot readily be made in corrosion—resistant material because of their greater complexity．It is generally better to use very high speeds in order to reduce the number of stages required．
(2)It operates at a high efficiency over only a limited range of conditions; this applies especially to turbine pumps．
(3)It is not usually self-priming.
(4)If a non-return valve is not incorporated in the delivery or suction line, the liquid will run back into the suction tank as soon as the pump stops．
(5)Very viscous liquids cannot he handled efficiently．
3. Cavitation in centrifugal pump
（1）The term ‘cavitation’ comes from the Latin word cavus, which means a hollow space or a cavity. Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure.
In any discussion on centrifugal pumps various terms like vapor pockets, gas pockets, holes, bubbles, etc. are used in place of the term cavities. These are one and the same thing and need not be confused. The term bubble shall be used hereafter in the discussion.
In the context of centrifugal pumps, the term cavitation implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump.
Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles or Gas bubbles.
1.Vapor bubbles are formed due to the vaporisation of a process liquid that is being pumped. The cavitation condition induced by formation and collapse of vapor bubbles is commonly referred to as Vaporous Cavitation.
2.Gas bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped (generally air but may be any gas in the system). The cavitation condition induced by the formation and collapse of gas bubbles is commonly referred to as Gaseous Cavitation.
（2）Important Definitions: To enable a clear understanding of mechanism of cavitation, definitions of following important terms are explored.
· ??????? Static pressure,
· ??????? Dynamic pressure,
· ??????? Total pressure,
· ??????? Static pressure head,
· ??????? Velocity head,
· ??????? Vapour pressure.
Static pressure ：The static pressure in a fluid stream is the normal force per unit area on a solid boundary moving with the fluid. It describes the difference between the pressure inside and outside a system, disregarding any motion in the system. For instance, when referring to an air duct, static pressure is the difference between the pressure inside the duct and outside the duct, disregarding any airflow inside the duct. In energy terms, the static pressure is a measure of the potential energy of the fluid.
Dynamic pressure：A moving fluid stream exerts a pressure higher than the static pressure due to the kinetic energy (? mv2) of the fluid. This additional pressure is defined as the dynamic pressure. The dynamic pressure can be measured by converting the kinetic energy of the fluid stream into the potential energy. In other words, it is pressure that would exist in a fluid stream that has been decelerated from its velocity ‘v’ to ‘zero’ velocity.
Total pressure：The sum of static pressure and dynamic pressure is defined as the total pressure. It is a measure of total energy of the moving fluid stream. i.e. both potential and kinetic energy.
Velocity head：Vapor pressure is the pressure required to keep a liquid in a liquid state. If the pressure applied to the surface of the liquid is not enough to keep the molecules pretty close together, the molecules will be free to separate and roam around as a gas or vapor. The vapor pressure is dependent upon the temperature of the liquid. Higher the temperature, higher will be the vapor pressure.
（3） Cavitation Damage：Cavitation can destroy pumps and valves, and cavitation causes a loss of efficiency in pumps immediately, and also a continuously increasing loss of efficiency as the equipment degrades due to erosion of the pump components by cavitation. Therefore It is important to understand the phenomena sufficiently to predict and therefore reduce cavitation and damage from cavitation, and also to diagnose and find practical solutions to cavitation problems。
1）Cavitation Enhanced Chemical Erosion
Phenomenon of Cavitation
Pumps operating under cavitation conditions become more vulnerable to corrosion and chemical attack. Metals commonly develop an oxide layer or passivated layer which protects the metal from further corrosion. Cavitation can remove this oxide or passive layer on a continuous basis and expose unprotected metal to further oxidation. The two processes (cavitation & oxidation) then work together to rapidly remove metal from the pump casing and impeller. Stainless steels are not invulnerable to this process.
2）Materials Selection
There is no metal, plastic, or any other material known to man, that can withstand the high levels of energy released by cavitation in the forms of heat and pressure. In practice however, materials can be selected that result in longer life and customer value in their ability to withstand cavitation energies, so that attention to pump construction materials is valuable and productive.
Where cavitation is not a problem or not predicted to be a problem, common materials such as cast iron and bronze are suitable for pump construction. There are millions of cast iron and bronze pumps that work fine for 20 years or more without any problem even though many of those pumps experience some cavitation.
（4）Mechanism of Cavitation：The phenomenon of cavitation is a stepwise process as shown in Figure (below).
Step One, Formation of bubbles inside the liquid being pumped.
The bubbles form inside the liquid when it vaporises i.e. phase change from liquid to vapor. But how does vaporization of the liquid occur during a pumping operation?
Vaporization of any liquid inside a closed container can occur if either pressure on the liquid surface decreases such that it becomes equal to or less than the liquid vapor pressure at the operating temperature, or the temperature of the liquid rises,
Collapse of a Vapor Bubble
raising the vapor pressure such that it becomes equal to or greater than the operating pressure at the liquid surface. For example, if water at room temperature (about 77 °F) is kept in a closed container and the system pressure is reduced to its vapor pressure (about 0.52 psia), the water quickly changes to a vapor. Also, if the operating pressure is to remain constant at about 0.52 psia and the temperature is allowed to rise above 77 °F, then the water quickly changes to a vapor.
Just like in a closed container, vaporization of the liquid can occur in centrifugal pumps when the local static pressure reduces below that of the vapor pressure of the liquid at the pumping temperature.
Step Two, Growth of bubbles Unless there is no change in the operating conditions, new bubbles continue to form and old bubbles grow in size. The bubbles then get carried in the liquid as it flows from the impeller eye to the impeller exit tip along the vane trailing edge. Due to impeller rotating action, the bubbles attain very high velocity and eventually reach the regions of high pressure within the impeller where they start collapsing. The life cycle of a bubble has been estimated to be in the order of 0.003 seconds。
Step Three, Collapse of bubbles，As the vapor bubbles move along the impeller vanes, the pressure around the bubbles begins to increase until a point is reached where the pressure on the outside of the bubble is greater than the pressure inside the bubble. The bubble collapses. The process is not an explosion but rather an implosion (inward bursting). Hundreds of bubbles collapse at approximately the same point on each impeller vane. Bubbles collapse non-symmetrically such that the surrounding liquid rushes to fill the void forming a liquid microjet. The micro jet subsequently ruptures the bubble with such force that a hammering action occurs. Bubble collapse pressures greater than 1 GPa (145x106 psi) have been reported. The highly localized hammering effect can pit the pump impeller. The pitting effect is illustrated schematically in this the figure.
After the bubble collapses, a shock wave emanates outward from the point of collapse. This shock wave is what we actually hear and what we call "cavitation". The implosion of bubbles and emanation of shock waves (red color) . In nutshell, the mechanism of cavitation is all about formation, growth and collapse of bubbles inside the liquid being pumped. But how can the knowledge of mechanism of cavitation can really help in troubleshooting a cavitation problem. The concept of mechanism can help in identifying the type of bubbles and the cause of their formation and collapse.
(5)Solution and Remedies：For vaporization problems (cavitation)
(1.To cure vaporization problems you must either increase the suction head, lower the fluid temperature, or decrease the N.P.S.H. Required. We shall look at each possibility:
1).Increase the suction head: ? Raise the liquid level in the tank
? Raise the tank
? Put the pump in a pit
? Reduce the piping losses. These losses occur for a variety of reasons that include :
1. The system was designed incorrectly. There are too many fittings and/or the piping is too small in diameter.
2. A pipe liner has collapsed.
3. Solids have built up on the inside of the pipe.
4. The suction pipe collapsed when it was run over by a heavy vehicle.
5. A suction strainer is clogged.
6. Be sure the tank vent is open and not obstructed. Vents can freeze in cold weather
7. Something