外文翻譯--化工工業(yè)離心泵

1、<p>　　CENTRIFUGAL PUMPS IN THE CHEMICAL INDUSTRY</p><p>　　Abstract : A centrifugal pump converts the input power to kinetic energy in the liquid by accelerating the liquid by a revolving device - an im

2、peller. 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

3、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</p><p>　　Keywords : centrifugal pump ,Introduction ,Working principl

4、e , Cavitation ， Mechanism of Cavitation ，Solution and Remedies</p><p>　　1. Introduction</p><p>　　Pump ,device used to raise ,transfer ,or compress liquids and gases .Four general classes of p

5、umps 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 a

6、re usually known as compressors .The study of fluids in motion is called fluid dynamics.</p><p>　　Water pump ,device for moving water from one location to another ,using tubes or other machinery .Water pumps

7、 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 fo

8、untains to sump pumps that remove water from beneath the foundations of homes . </p><p>　　One type of modern pumps used to move water is the centrifugal pump .Early version of the centrifugal pump ,the screw

9、 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 c

10、logged 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</p><p>　　Also known as rotary pumps ,centri

11、fugal 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 im

12、peller 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

13、of impeller may be used in</p><p>　　2.The Centrifugal Pump </p><p>　　The centrifugal pump is by far the most widely used type in the chemical and petroleum industries .It will pump liquids with

14、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

15、 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 directl</p><p>　　In this type of pump ,the fluid is fed to

16、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 deliv

17、ery sides arises from the conversion of kinetic energy into pressure energy .</p><p>　　The impeller consists of a series of curved vanes so shaped that the flow within the pump is as smooth as possible .The

18、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

19、 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 wi</p><p>　　The liquid enters the casing of the p

20、ump，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 tangentia

21、l 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．</p><p>　　This

22、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

23、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 moti

24、on of the liquid relative to the pump casing--and the r</p><p><b>　　c.</b></p><p>　　is the velocity of the liquid relative to the vane and is the tangential velocity of the tip of t

25、he 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

26、 of the impeller blades．The pump will therefore operate at maximum efficiency only over a narrow range of conditions．</p><p>　　Virtual head of a centrifugal pump</p><p>　　The maximum pressure is

27、 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 f

28、or 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)．</p><p><

29、b>　　d</b></p><p>　　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。</p><p>　　If the fluid is trav

30、eling with a velocity u and at an angle θ to the tangential direction．The angular momentum of this mass of fluid</p><p>　　= dM (urcosθ)</p><p>　　The torque acting on the fluid dτ is equal to the

31、 rate of change of angular momentum with time，as it goes through the pump</p><p>　　Dτ = dM α/αt(urcosθ)=2πrbρdrα/αt(urcosθ) (2.1)</p><p>　　The volumetric rate of flow of liqui

32、d through the pump：</p><p>　　Q=2πrbα/αt (2.2)</p><p>　　Dr =Q ρ d(urcosθ) (2.3)</p><p>　　The total torque act

33、ing 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

34、 at the discharge．</p><p>　　Thus，τ=Q ρ(cos- cos)</p><p>　　The advantages and disadvantages of the centrifugal pump</p><p>　　The main advantages are：</p><p>　　(1) It is

35、 simple in construction and can，therefore， be made in a wide range of materials</p><p>　　(2)There is a complete absence of valves．</p><p>　　(3)It operates at high speed(up to 100 Hz)and，the

36、refore，can be coupled directly to</p><p>　　an electric motor. In general，the higher the speed the smaller the pump and motor for a give n duty．</p><p>　　(4)It gives a steady delivery．</p>

37、<p>　　(5)Maintenance costs are lower than for any other type of pump．</p><p>　　(6)No damage is done to the pump if the delivery line becomes blocked，provided it is not run in this condition for a prol

38、onged period．</p><p>　　(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．</p><p>　　(8)L

39、iquids containing high proportions of suspended solids are readily handled．</p><p>　　The main disadvantages are：</p><p>　　(1)The single—stage pump will not develop a high pressure．Multistage pum

40、ps 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 t

41、o reduce the number of stages required． </p><p>　　(2)It operates at a high efficiency over only a limited range of conditions; this applies especially to turbine pumps． </p><p>　　(3)It is no

42、t usually self-priming.</p><p>　　(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．</p><p>　　

43、(5)Very viscous liquids cannot he handled efficiently．</p><p>　　3. Cavitation in centrifugal pump</p><p>　?。?）The term ‘cavitation’ comes from the Latin word cavus, which means a hollow space

44、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. </p><p>　　In any discussion on centrifugal

45、 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

46、 discussion.</p><p>　　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 th

47、rough the pump. </p><p>　　Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles or Gas bubbles. </p><p>　　1.Vapor bubbles are formed due to the vaporisation of a pro

48、cess liquid that is being pumped. The cavitation condition induced by formation and collapse of vapor bubbles is commonly referred to as Vaporous Cavitation. </p><p>　　2.Gas bubbles are formed due to the pre

49、sence 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 Gaseo

50、us Cavitation. </p><p>　　（2）Important Definitions: To enable a clear understanding of mechanism of cavitation, definitions of following important terms are explored.</p><p>　　·   

51、      Static pressure,</p><p>　　·         Dynamic pressure,</p><p>　　·         Total pres

52、sure,</p><p>　　·         Static pressure head,</p><p>　　·         Velocity head, </p><p>　　·

53、60;       Vapour pressure.</p><p>　　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 descri

54、bes 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 d

55、uct and outside the duct, disregarding any airflow inside the duct. In energy terms, the static pressure is a measure of the potential ener</p><p>　　Dynamic pressure：A moving fluid stream exerts a pressure h

56、igher than the static pressure due to the kinetic energy (½ 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 th

57、e 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.</p><p>　　Total pressure：The sum

58、 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.</p><p>　　Velocity head：Vapor pre

59、ssure 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 an

60、d 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.</p><p>　?。?） Cavitation Damage：Cavitation can

61、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 cavitati

62、on. 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。<

63、/p><p>　　1）Cavitation Enhanced Chemical Erosion</p><p>　　Pumps operating under cavitation conditions become more vulnerable to corrosion and chemical attack. Metals commonly develop an oxide layer

64、 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 (cavit

65、ation & oxidation) then work together to rapidly remove metal from the pump casing and impeller. Stainless steels are not invulnerable to t</p><p>　　2）Materials Selection</p><p>　　There is

66、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

67、in longer life and customer value in their ability to withstand cavitation energies, so that attention to pump construction materials is valuable and productive.</p><p>　　Where cavitation is not a problem or

68、 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

69、even though many of those pumps experience some cavitation.</p><p>　?。?）Mechanism of Cavitation：The phenomenon of cavitation is a stepwise process as shown in Figure (below).</p><p>　　Step One,

70、Formation of bubbles inside the liquid being pumped.</p><p>　　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

71、 a pumping operation?</p><p>　　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 pr

72、essure at the operating temperature, or the temperature of the liquid rises,</p><p>　　raising the vapor pressure such that it becomes equal to or greater than the operating pressure at the liquid surface. Fo

73、r 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 oper

74、ating 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.</p><p>　　Just like in a closed container, vaporizat

75、ion 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.</p><p>　　Step Two, Growth of bubbles Unless

76、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 v

77、ane 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

78、been estimated to be in t</p><p>　　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 pr

79、essure 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 approx

80、imately the same point on each impeller vane. Bubbles collapse non-symmetrically such that the surrounding l</p><p>　　After the bubble collapses, a shock wave emanates outward from the point of collapse. Th

81、is 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

82、 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 </p><p&g

83、t;　　(5)Solution and Remedies：For vaporization problems (cavitation)</p><p>　　(1.To cure vaporization problems you must either increase the suction head, lower the fluid temperature, or decrease the N.P.S.H.

84、Required. We shall look at each possibility:</p><p>　　1).Increase the suction head: ?Raise the liquid level in the tank </p><p>　　?Raise the tank </p><p>　　?Put the pump in a pit

85、 </p><p>　　?Reduce the piping losses. These losses occur for a variety of reasons that include : </p><p>　　1.The system was designed incorrectly. There are too many fittings and/or the piping

86、is too small in diameter. </p><p>　　2.A pipe liner has collapsed. </p><p>　　3.Solids have built up on the inside of the pipe. </p><p>　　4.The suction pipe collapsed when it was r

87、un over by a heavy vehicle. </p><p>　　5.A suction strainer is clogged.</p><p>　　6.Be sure the tank vent is open and not obstructed. Vents can freeze in cold weather </p><p>　　7.S

88、omething is stuck in the pipe, It either grew there or was left during the last time the system was opened . Maybe a check valve is broken and the seat is stuck in the pipe. </p><p>　　8.The inside of the pi

89、pe, or a fitting has corroded. </p><p>　　9.A bigger pump has been installed and the existing system has too much loss for the increased capacity. </p><p>　　10.A globe valve was used to replace

90、 a gate valve. </p><p>　　11.A heating jacket has frozen and collapsed the pipe. </p><p>　　12.A gasket is protruding into the piping. </p><p>　　13.The pump speed has increased. &l

91、t;/p><p>　　?Install a booster pump </p><p>　　?Pressurize the tank</p><p>　　2) lower the fluid temperature</p><p>　　?Injecting a small amount of cooler fluid at the suct

92、ion is often practical. </p><p>　　?Insulate the piping from the sun's rays. </p><p>　　?Be careful of discharge recirculation lines, they can heat up the suction fluid.</p><p>

93、　　3) reduce the N.P.S.H. Required</p><p>　　?Use a double suction pump. This can reduce the N.P.S.H.R. by as much as 27% or in some cases it will allow you to raise the pump speed by 41% </p><p>

94、;　　?Use a lower speed pump </p><p>　　?Use a pump with a larger impeller eye opening. </p><p>　　?If possible install an Inducer. These inducers can cut N.P.S.H.R. by almost 50%. </p>&l

95、t;p>　　?Use several smaller pumps. Three half capacity pumps can be cheaper than one large pump plus a spare. This will also conserve energy at lighter loads.</p><p>　　(2. For suction cavitation: &

96、lt;/p><p>　　1. Remove debris from suction line. </p><p>　　2. Move pump closer to source tank/sump </p><p>　　3. Increase suction line diameter. </p><p>　　4. Decrease

97、suction lift requirement </p><p>　　5. Install larger pump running slower which will decrease the Net Positive Suction Head Required by the pump(NPSHR). </p><p>　　6. Increase discharge p

98、ressure. </p><p>　　7. Fully open Suction line valve.</p><p>　　(3. For discharge cavitation: </p><p>　　1. Remove debris from discharge line. </p><p>　　2. Dec

99、rease discharge line length </p><p>　　3. Increase discharge line diameter. </p><p>　　4. Decrease discharge static head requirement. </p><p>　　5. Install larger pump, which will

100、maintain the required flow without discharge cavitating. </p><p>　　6. Fully open discharge line valve.</p><p>　　(4. For Recirculation cavitation:</p><p>　　1.Designing the pump f

101、or lower suction-specific speeds and limiting the range of operation to flow capacities above the point of recirculation. </p><p>　　2. Raising the suction head.</p><p>　　Selected from :</p&g

102、t;<p>　　1.J.M.Coucson ,J.F.Richardson ,Chemical Engineering ,Butterworth-Heinemann Ltd.,1995</p><p>　　2. Delgosha, O. C., Patella, R. F., Reboud, J. L.: Experimental and Numerical Studies in a Centri