ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 47-2012 VALVES.pdf

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1、47.1CHAPTER 47VALVESFundamentals. 47.1Manual Valves 47.3Automatic Valves 47.4Balancing Valves 47.9Multiple-Purpose Valves 47.10Safety Devices 47.11Self-Contained Temperature Control Valves 47.11Pressure-Reducing Valves 47.12Check Valves. 47.12Stop-Check Valves 47.13Backflow Prevention Devices. 47.13

2、Steam Traps 47.13FUNDAMENTALSALVES are the manual or automatic fluid-controlling elementsVin a piping system. They are constructed to withstand a spe-cific range of temperature, pressure, corrosion, and mechanicalstress. The designer selects and specifies the proper valve for theapplication to give

3、the best service for the economic requirements.Valves have some of the following primary functions: Starting, stopping, and directing flowRegulating, controlling, or throttling flowPreventing backflow Relieving or regulating pressureThe following service conditions should be considered beforespecify

4、ing or selecting a valve:1. Type of liquid, vapor, or gasIs it a true fluid or does it contain solids?Does it remain a liquid throughout its flow or does it vaporize?Is it corrosive or erosive?2. Pressure and temperatureWill these vary in the system?Should worst case (maximum or minimum values) be c

5、onsid-ered in selecting correct valve materials?3. Flow considerationsIs pressure drop critical?Should valve design be chosen for maximum wear?Is the valve to be used for simple shutoff or for throttling flow?Is the valve needed to prevent backflow?Is the valve to be used for directing (mixing or di

6、verting) flow?4. Frequency of operation Will the valve be operated frequently?Will valve normally be open with infrequent operation?Will operation be manual or automatic?Nomenclature for basic valve components may vary from man-ufacturer to manufacturer and according to the application. Figure 1show

7、s representative names for various valve parts.Body RatingsThe rating of valves defines the pressure-temperature relation-ship within which the valve may be operated. The valve manu-facturer is responsible for determining the valve rating. ASMEStandard B16.34 should be consulted, and a valve pressur

8、e classshould be identified. Inlet pressure ratings are generally expressedin terms of the ANSI/ASME class ratings and range from ANSIClass 150 through 2500, depending on the style, size, and materialsof construction, including seat materials. Automatic control valvesare usually either Class 125 or

9、Class 250. Tables in the standard andin various books show pressure ratings at various operating temper-atures (ASME Standard B16.34; Lyons 1982; Ulanski 1991).MaterialsASME Standard B16.34 addresses requirements for valves madefrom forgings, castings, plate, bar stock and shapes, and tubularproduct

10、s. This standard identifies acceptable materials from whichvalves can be constructed. In selecting proper valve materials, thevalve body-bonnet material should be selected first and then thevalve plug and seat trim.Other factors that govern the basic materials selection includePressure-temperature r

11、atings Corrosion-resistance requirements Thermal shock Piping stress Fire hazard Types of materials typically available includeCarbon steelDuctile ironCast ironStainless steelsBrassBronzePolyvinyl chloride (PVC) plasticBodies. Body materials for small valves are usually brass,bronze, or forged steel

12、 and for larger valves, cast iron, cast ductileThe preparation of this chapter is assigned to TC 6.1, Hydronic and SteamEquipment and Systems.Fig. 1 Valve Components(Courtesy Anvil Intl.)47.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)iron, or cast steel as required for the pressure and serv

13、ice. Listingsof typical materials are given in Lyons (1982) and Ulanski (1991).Seats. Valve seats can be machined integrally of the body mate-rial, press-fitted, or threaded (removable). Seats of different materi-als can be selected to suit difficult application requirements. Thevalve seat and the v

14、alve plug or disk are sometimes referred to as thevalve trim and are usually constructed of the same material selectedto meet the service requirements. The trim, however, is usually of adifferent material than the valve body. Replaceable compositiondisks are used in conjunction with the plug in some

15、 designs in orderto provide adequate close-off.Maximum permissible leakage ratings for control valve seats aredefined in Fluid Controls Institute (FCI) Standard 70-2.Stems. Valve stem material should be selected to meet serviceconditions. Stainless steel is commonly used for most HVAC appli-cations,

16、 and bronze is commonly used in ball valve construction.Stem Packings and Gaskets. Valve stem packings undergo con-stant wear because of the movement of the valve stem, and both thepackings and body gaskets are exposed to pressure and pressurevariations of the control fluid. Manufacturers can supply

17、 recom-mendations regarding materials and lubricants for specific fluidtemperatures and pressures.Flow Coefficient and Pressure DropFlow through any device results in some loss of pressure. Someof the factors affecting pressure loss in valves include changes in thecross section and shape of the flow

18、 path, obstructions in the flowpath, and changes in direction of the flow path. For most applica-tions, the pressure drop varies as the square of the flow when oper-ating in the turbulent flow range. For check valves, this relationshipis true only if the flow holds the valve in the full-open positio

19、n.For convenience in selecting valves, particularly control valves,manufacturers express valve capacity as a function of a flow coeffi-cient. This coefficient represents the quantity of fluid that passesthrough a fully open control valve at unit pressure drop. Flow coef-ficient Avis defined based on

20、 volumetric flow in cubic metres persecond at a pressure drop of 1 Pa across a valve (CIBSE Guide B).In North America, a flow coefficient Kv is based on flow in cubicmetres per hour at a pressure drop of 100 kPa (1 bar). Sometimes Kvis evaluated in litres per second at 1 kPa pressure drop. Manufactu

21、r-ers may also furnish valve coefficients at other pressure drops. Flowcoefficients apply only to water. When selecting a valve to controlother fluids, be sure to account for differences in viscosity.Figure 2 shows a typical test arrangement to determine the ratingwith the test valve wide open. Glob

22、e valve HV-1 allows adjustingthe supply gage reading (e.g., to 200 kPa) and HV-2 is then adjusted(e.g., to 200 kPa return gage) to allow a test run at a pressure dropof 100 kPa. A gravity storage tank may be used to minimize supplypressure fluctuations. The bypass valve allows fine adjustment ofthe

23、supply pressure. A series of test runs is made with the weighingtank and a stopwatch to determine the flow rate. Further capacitytest detail may be found in International Society for Measurementand Control (ISA) Standard S75.02.CavitationCavitation occurs when the pressure of a flowing fluid dropsbe

24、low the vapor pressure of that fluid (Figure 3). In this two-stepprocess, the pressure first drops to the critical point, causing cavitiesof vapor to form. These are carried with the flow stream until theyreach an area of higher pressure. The bubbles of vapor then sud-denly collapse or implode. This

25、 reduction in pressure occurs whenthe velocity increases as the fluid passes through a valve. After thefluid passes through the valve, the velocity decreases and the pres-sure increases. In many cases, cavitation manifests itself as noise.However, if the vapor bubbles are in contact with a solid sur

26、facewhen they collapse, the liquid rushing into the voids causes highlocalized pressure that can erode the surface. Premature failure ofthe valve and adjacent piping may occur. The noise and vibrationcaused by cavitation have been described as similar to those ofgravel flowing through the system.Wat

27、er HammerWater hammer is a series of pressure pulsations of varying mag-nitude above and below the normal pressure of water in the pipe.The amplitude and period of the pulsation depend on the velocity ofthe water as well as the size, length, and material of the pipe.Shock loading from these pulsatio

28、ns occurs when any movingliquid is stopped in a short time. In general, it is important to avoidquickly closing valves in an HVAC system to minimize the occur-rence of water hammer.When flow stops, the pressure increase is independent of theworking pressure of the system. For example, if water is fl

29、owing at1.5 m/s and a valve is instantly closed, the pressure increase is thesame whether the normal pressure is 500 kPa or 5000 kPa.Water hammer is often accompanied by a sound resembling apipe being struck by a hammer (hence the name). The intensity ofthe sound is no measure of the magnitude of th

30、e pressure. Tests indi-cate that even if 15% of the shock pressure is removed by absorbersor arresters, adequate relief is not necessarily obtained.Velocity of pressure wave and maximum water hammer pressureformulas may be found in the Hydraulic Handbook (Fairbanks Morse1965).NoiseChapter 22 of the

31、2009 ASHRAE HandbookFundamentalspoints out that limitations are imposed on pipe size to control thelevel of pipe and valve noise, erosion, and water hammer pressure.One recommendation places a velocity limit of 1.2 m/s for pipe500 mm and smaller, and a pressure drop of 400 Pa per metre lengthfor pip

32、ing over 50 mm in diameter. Velocity-dependent noise in pip-ing and piping systems results from any or all of four sources: tur-bulence, cavitation, release of entrained air, and water hammer (seeChapter 48 of the 2011 ASHRAE HandbookHVAC Applications).Some data are available for predicting hydrodyn

33、amic noise gener-ated by control valves. ISA Standard 75.01 compiled prediction cor-relations in an effort to develop control valves for reduced noise levels.Fig. 2 Flow Coefficient Test Arrangement Fig. 3 Valve Cavitation at Sharp CurvesValves 47.3Body StylesValve bodies are available in many confi

34、gurations depending onthe desired service. Usual functions include stopping flow, allowingfull flow, modulating flow between extremes, and directing flow.The operation of a valve can be automatic or manual.The shape of bodies for automatic and manual valves is dictatedby the intended application. Fo

35、r example, angle valves are com-monly provided for radiator control. The principle of flow is thesame for angle and straight-through valve configurations; the man-ufacturer provides a choice in some cases as a convenience to theinstaller.The type or design of body connections is dictated primarily b

36、ythe proposed conduit or piping material. Depending on materialtype, valves can be attached to piping in one of the following ways:Bolted to the pipe with companion flange.Screwed to the pipe, where the pipe itself has matching threads(male) and the body of the valve has threads machined into it(fem

37、ale).Welded, soldered, or sweated.Flared, compression, and/or various mechanical connections tothe pipe where there are no threads on the pipe or the body.Valves of various plastic materials are fastened to the pipe if thevalve body and the pipe are of compatible plastics.MANUAL VALVESSelectionEach

38、valve style has advantages and disadvantages for the appli-cation. In some cases, the design documents provide inadequateinformation, so that selection is based on economics and local stockavailability by the installer and not on what is really required. Goodsubmittal practice and approval by the de

39、signer are required toprevent substitutions. The questions listed in the section on Funda-mentals must be evaluated carefully.Globe ValvesIn a globe valve, flow is controlled by a circular disk forcedagainst or withdrawn from an annular ring, or seat, that surroundsan opening through which flow occu

40、rs (Figure 4). The direction ofmovement of the disk is parallel to the direction of the flow throughthe valve opening (or seat) and normal to the axis of the pipe inwhich the valve is installed.Globe valves are most frequently used in smaller diameter pipesbut are available in sizes up to 300 mm. Th

41、ey are used for throttlingduty where positive shutoff is required. Globe valves for controllingservice should be selected by class, and whether they are of thestraight-through or angle type, composition disk, union or gasketedbonnet, threaded, and solder or grooved ends. Manually operatedflow contro

42、l valves are also available with fully guided V-port throt-tling plugs or needle point stems for precise adjustment.Gate ValvesA gate valve controls flow by means of a wedge disk fittingagainst machined seating faces (Figure 5). The straight-throughopening of the valve is as large as the full bore o

43、f the pipe, and thegate movement is perpendicular to the flow path.Gate valves are intended to be fully open or completely closed.They are designed to allow or stop flow, and should not be used toregulate or control flow. Various wedges for gate valves are avail-able for specific applications. Valve

44、s in inaccessible locations maybe provided with a chain wheel or with a hammer-blow operator.More detailed information is available from valve manufacturers.Plug ValvesA plug valve is a manual fluid flow control device (Figure 6). Itoperates from fully open to completely shut off within a 90 turn.Th

45、e capacity of the valve depends on the ratio of the area of the ori-fice to the area of the pipe in which the valve is installed.The cutaway view of a plug valve shows a valve with an orificethat is considerably smaller than the full size of the pipe. Lubricatedplug valves are usually furnished in g

46、as applications. A plug valveis selected as an on/off control device because (1) it is relativelyinexpensive; (2) when adjusted, it holds its position; and (3) its posi-tion is clearly visible to the operator. The effectiveness of this valveas a flow control device is reduced if the orifice of the v

47、alve is fullyported (i.e., the same area as the pipe size).Ball ValvesA ball valve contains a precision ball held between two circularseals or seats. Ball valves have various port sizes. A 90 turn of thehandle changes operation from fully open to fully closed. BallFig. 4 Globe Valve(Courtesy Anvil I

48、ntl.)Fig. 5 Two Variations of Gate Valve47.4 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)valves for shutoff service may be fully ported. Ball valves for throt-tling or controlling and/or balancing service should have a reducedport with a plated ball and valve handle memory stop. Ball valvesma

49、y be of one-, two-, or three-piece body design (Figure 7).Butterfly ValvesA butterfly valve typically consists of a cylindrical, flanged-end body with an internal, rotatable disk serving as the fluidflow-regulating device (Figure 8). Butterfly valve bodies may bewafer style, which is clamped between two companion flangeswhose bolts carry the pipeline tensile stress and place the waferbody in compression, or lugged style, with tapped holes in thewafer body, which may serve as a future point of disconnection.The disks axis of rotation is the valve stem; it is perpendicular tothe f