化工原理英文課件：Chapter2-1Pipe, Fittings, and Valves

1、 Chapter2 Transportation of Fluids In order to make a fluid flow from one point to another in a pipe, it is necessary to have a driving force. Sometimes this force is supplied by gravity. Usually, the driving force is supplied by a mechanical device such as a pump or blower. Mechanical devices incre

2、ase the mechanical energy of the fluid. This energy may be used to increase the velocity , the pressure, or the elevation of the fluid, Generally, the word pump designates a machine or device for moving an incompressible liquid. Fans, blowers, and compressors are devices for moving gas (usually air)

3、. 2.1Pipe, Fittings, and Valves lThe piping system including pipe, fittings and vales 2.1.1 Pipe and tubing lThere is no clear-cut distinction between the terms pipe and tubing. Generally speaking, (1) pipe is heavy-walled and relatively large in diameter and comes in moderate lengths of 6 or 12 m;

4、tubing is thin-walled and often comes in coils several hundred feet long. (2)Metallic pipe can be threaded; tubing usually cannot. (3)Pipe walls are usually slightly rough; tubing has very smooth walls. (4)Lengths of pipe are joined by screwed, flanged, or welded fittings; pieces of tubing are conne

5、cted by compression fittings, flare fittings, or soldered fittings. size lStandard nominal diameters: (1) Pipe For large pipe, more than 12 in. in diameter, the nominal diameters are the actual outside diameters; for small pipe the nominal diameter does not correspond to any actual dimension. The no

6、minal value is close to the actual inside diameter for 3- to 12-in. pipe, but for very small pipe this is not true. lRegardless of wall thickness, the outside diameter of all pipe of a given nominal size is the same, to ensure interchangeability of fittings. lThus the designation “2-in. nickel IPS p

7、ipe” means nickel pipe having the same outside diameter as standard 2-in. steel pipe. (2) Tubing The size of tubing is indicated by the outside diameter. The normal value is the actual outer diameter, to within very close tolerances. Selection of pipe sizes lThe pipe size selected for a particular i

8、nstallation depends mainly on the cost of the pipe and fittings and the cost of energy needed for pumping the fluid. lThe cost of the pipe and the annual capital charges increase with about the 1.5 power of the pipe diameter, lwhile the power cost for turbulent flow varies with the -4.8 power of the

9、 diameter. lFor turbulent flow of liquids in steel pipes larger than 1 in. (25 mm) in diameter, the optimum velocity is 36. 0 1 . 0 745.10 m uopt where uoptoptimum velocity, m/s mmass flow rate, kg/s fluid density, kg/m3 lFor water and similar fluids uopt is 0.9 to 1.8 m/s; lfor air or steam at low

10、to moderate pressures uopt is 6 to 24 m/s. lFor flow in heat exchanger tubes, the optimum design velocity is often higher than that given by Eq. (2.1-1) because of improved heat transfer at high fluid velocities. lWhen flow is by gravity from overhead tanks or when a viscous liquid is being pumped,

11、low velocities are favored, in the range of 0.06 to 0.24 m/s. Joints and fittings lThe methods used to join pieces of pipe depend in part on the properties of the material but primarily on the thickness of the wall. lThick-walled tubular products are usually connected by screwed fittings, by flanges

12、, or by welding. lPieces of thin-walled tubing are joined by soldering or by compression or flare fittings. lPipe made of brittle materials such as glass or carbon or cast iron is joined by flanges or bell-and-spigot joints. lFor joining pieces of large steel pipe in process piping, especially for h

13、igh-pressure service, welding has become the standard method. lWelding makes stronger joints than screwed fittings do, and since it does not weaken the pipe wall, lighter pipe can be used for a given pressure. lProperly made welded joints are leakproof, whereas other types of joints are not. lAlmost

14、 the only disadvantage of a welded joint is that it cannot be opened without destroying it. lEnvironmental protection legislation considers flanged and screwed joints to be sources of emission of volatile materials. Allowances for expansion lAlmost all pipes are subjected to expand and contract due

15、to varying temperatures lProvision is made in all high-temperature lines for taking up expansion, so that the fittings and valves are not put under strain. This is done by bends or loops in the pipe, by packed expansion joints, by bellows or packless joints, and sometimes by flexible metal hose. lTh

16、is section deals with the transportation of liquids through channels. l Liquid are sometimes moved by gravity from elevated tanks, or from a “blowcase”, but by far the most common devices for the purpose are pumps. Pumps increase the mechanical energy of the liquid. The two major classes are positiv

17、e-displacement pumps and centrifugal pumps. Positive-displacement units apply pressure directly to the liquid by the reciprocating piston, or by rotating members. Centrifugal pumps generate high rotational velocities, then, convert the resulting kinetic energy of the liquid to pressure energy. A typ

18、ical pump application is shown diagrammatically in figure. The pump is installed in a pipeline to provide the energy needed to draw liquid from a reservoir and discharge at the exit of the pipeline. z2 z2 above the level of the liquid. At pump itself, the liquid enters the suction connection at stat

19、ion a and leaves the discharge connection at station b. Since the only friction is that occurring in the pump itself and is accounted for by mechanical efficiency , hf = 0. Then equation (1.3-25) can be written 22 22 a a ab b b u gZ pu gZ p W 2.2-1 The equation(2.2-1) can be divide by g, gives g u Z

20、 g p g u Z g p H a a ab b b 22 22 (2.2-2) The quantity H is called total head, which each term has the dimension of length. Often the head developed by a pump is expressed in meters of fluid. mWNe2.2-3 Brake power is denoted by Ne as mass flow rate m is transported Using the total mechanical-energy-

21、balance equation (2.2-1) on a pump and piping system, the theoretical mechanical energy W added to the fluid by the pump can be calculated. mgHNe 2.2-5 e N N The power delivered to the fluid is also calculated from the mass flow rate and the head developed by pump. If is the fractional efficiency an

22、d N the shaft work delivered to the pump, Eq. (2.2-5)gives lSince most pumps are driven by electric motors, the efficiency of the motor must be taken into account to determine the total electric power to the motor. lHence, the total electric power input equals the shaft work divided by the electric

23、motor drive efficiency e: e N powerelectrictotal 2.2-6 If the suction pressure is only slightly greater than the vapor pressure, some liquid may be flash to vapor inside pump , a process called cavitation , which greatly reduces the pump capacity and causes severe erosion. If the suction pressure is

24、 actually less than the vapor pressure, cavitation will occur in the suction line, and no liquid can be drawn into the pump. To avoid cavitation, the pressure at the pump inlet must exceed the vapor pressure by a certain value. Called the net positive suction head(NPSH). Hg 1 2 To avoid cavitation,

25、the pressure in station 2 must exceed the vapor pressure by a certain value NPSH fg H g u g pp H 2 2 221 2.2-7 From an energy balance, the least height of the suction station 2 of pump above the liquid level may be found as Therefore, suction lift is fg va H g u H g pp NPSH 2 2 2 f v g H g u NPSH g

26、pp H 2 2 21 For a pump taking suction from a reservoir, like that shown in figure, the available NPSH is customarily calculated as 2.2-9 But this term is usually small and is accounted for in the recommended value for NPSH furnished by pump suppliers. the velocity head at the pump inlet u2/2 could b

27、e subtracted from the result given by equation to give more theoretically correct value of the available NPSH Required value of NPSH is: For small centrifugal pump(100gal/min), 2 to 3m For very large pumps, value up to 15m. gf v HH pp NPSH 1 The NPSH is calculated as f v g HNPSH g pp H 1 The suction

28、 lift is calculated by 2.2-9 For the special situation where the liquid is practically nonvolatile (Pv=0), the friction is negligible (hf=0), and pressure at station a atmospheric, the maximum possible suction lift can be obtained by subtracted the required NPSH from the barometric head. lAny decrea

29、se in external pressure or rise in operating temperature can induce vaporization and the pump stops pumping. lThus, 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. lThe NPSH required varies with speed an

30、d capacity within any particular pump. The NPSH required increase as the capacity is increasing. lPump manufacturers curves normally provide this information. lThe NPSH is independent of the fluid density as are all head terms. lThe manufacturer usually tests the pump with water at different capacit

31、ies, 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 so

32、metimes 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 the liquid. NPSH required is a function of the pump design and is determined based on actual pump test by the vendor

33、. 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 . The NPSH required is the positive head absolute required at the pump suction to overcome these pressure drops in the pump and maintain the majo

34、rity of the liquid above its vapor pressure. The available NPSH for a given pump should be at least 1 m more than that required by the manufacturer. Note: It is to be noted that the net positive suction head required NPSH number shown on the pump curves is for fresh water at 20 and not for the fluid

35、 or combinations of fluids being pumped. lWhen the temperature of fluid to be transported increases, the maximum lift of suction will ( ). If the velocity of suction pipe decreases, the maximum suction lift will ( ). If the flow rate remains constant but the diameter of suction pipe decreases, the m

36、aximum suction lift will ( ) A） increase; B ）decrease; C）keep the same D） be uncertain lWhen the suction pressure is ( ) to the vapor pressure, the cavitation will occur lIf a centrifugal pump operates on air, then, liquid from an initially empty suction line。 lPositive-displacement pumps air bindin

37、g. lBefore a centrifugal pump starts up, close the valve in discharge in other to ( ) the start-up power required lThe difference between the theoretical and actual curves results from: ; ; . lWhen higher developed head is required, the best selection is . A) operation in series B) multistage centri

38、fugal pump C) throttled by valve l ( )One pump can develop more H than that two same pumps which work in series. l The methods of adjusting volume flow rate of centrifugal pump are (1) ; (2) ; (3) ; The commonest way in practice is ( ). A centrifugal pump is to be used to extract water from a condenser in which the vacuum is 640 mm of mercury. At the rated discharge, the net positive suction head must be at least 3m above the cavitation vapor pressure of 710mm mercury vacuum. If losses in the suction pipe accounted for a head of 1.5m. W