FISHER直通阀基础.ppt

1、Introduction to Control Valve,Process control loop,Representation of a typical process control Final control element (FCE) typically control valve,Role of control valve,Control of pressure at the chamber,Symbol,ACTUATOR,POSITIONER,VALVE,ISA Definition of control valve,Instrument Society of America (

2、ISA) defines the control valve as a power operated device that modulates the fluid flow rate in a process control system,Control valve classification,Sliding stem control valve,Major components in SS control valve,Major components in SSV,Valve body assembly Bonnet Pressure Retaining Components Bolti

3、ng Seat ring Plug Stem Cage Trim Materials Packing Seal ring Gasket and shim Actuator assembly,Major components in SS control valve,Valve Body,Main fluid boundary and pressure containing component Valve body Includes Provision for securing internal parts End connections that allows installation in t

4、he pipeline Provision for attaching the bonnet,Bonnet,Also a major pressure containing component and fluid boundary Bolted or threaded into valve body Bonnet locates and guides the valve stem and it includes a bore for packing (packing box) Also includes yoke boss or some other means of mounting an

5、actuator,Trim,What is trim? Trim refers to all internal, wetted components It includes the valve plug, cage, seat ring Factors to be considered for Trim selection Mechanical suitability of material (Strength and wear) Fluid compatibility with the material (general corrosion resistance/erosion/cracki

6、ng) Thermal expansion due to temperature Galling effect,Trim Components,Plug Provides throttling control and shutoff in globe valves Seating surface on the valve plug must be capable of withstanding the seat loads required for shutoff In post-guided or port-guided constructions, the plug material mu

7、st provide good resistance to galling in conjunction with the bushing or seat ring material respectively,Trim Components,Seat ring Seat rings in globe valves work with the valve plug to arrest leakage. They must be able to withstand the seat loads required for shutoff as well as against erosion In p

8、ort-guided designs, the seat ring material must provide good galling and sliding wear resistance in conjunction with the plug material,Trim Components,Cage Provides plug guidance and flow characterization The cage transfers a portion of the bonnet bolt loading to the seat ring to hold it in place an

9、d maintain gasket loading At high temperature and pressure there will be galling effect between cage and plug Cages must provide good sliding wear properties in combination with the plug material Cage must withstand circumferential loading caused by the pressure differential across the cage wall,Tri

10、m Components,Bushing It serves as a guide surface for the top of the plug or the stem directly above the plug The bushing must be resistant to galling in conjunction with the plug or stem material Stem The valve stem must be resistant to both general corrosion and pitting so that leakage and/or dama

11、ge to the packing will not occur The stem must be strong enough to sustain the actuator loads without buckling or yielding,Gasket,Provides sealing between mating surfaces Bonnet gasket : Between Body and bonnet Cage gasket : Between cage and bonnet Seat ring gasket : Between seat ring and body,Packi

12、ng,To prevent leakage through valve stem Packing is compressed to form a tight seal between the packing box wall and the valve plug stem Different type of standard packing Spring loaded V-ring PTFE packing Double PTFE packing Graphite packing,Basic specification of control valve,Body/Bonnet material

13、 Body Bonnet rating End connection Flow capacity Flow characteristics Shutoff class Trim designation Bonnet style Packing,Body/Bonnet material carbon steel body,Most commonly used carbon steel bodies are WCC and LCC WCC Weldable carbon steel cast Can be used between -29 to 427 deg C Above 427 deg C

14、graphitization can occur All carbon steel bodies supplied by Fisher are killed carbon steel,Body/Bonnet material carbon steel body,LCC Low temperature carbon steel cast Same chemical composition as WCC Quenched and tempered in order to meet impact requirement Can be used between -46 to 343 deg C Cha

15、rpy impact tested and certified,Body/Bonnet material Alloy steels,Addition of Chromium and molybdenum enhances the resistance to graphitization and tempering at elevated temperature WC6 : -29 to 593 deg C WC9 : -29 to 593 deg C,Body/Bonnet material Stainless steel,For increased high temperature capa

16、bility and for increased pressure retaining capability, Alloys with still more chromium and molybdenum are specified To avoid sensitization (formation of carbides) carbon content is kept low in stainless steel CF8M, CF3M, CG8M, CF8C Where to find: ,Body/ bonnet rating,The first consideration in valv

17、e selection is to select valve body/bonnet rating. Rating - The maximum pressure withstanding capability of body at the given temperature Body bonnet rating is expressed in terms of ANSI (American National Standards Institute) class rating. The common ratings are class 150, 300, 600, 900, 1500 and 2

18、500. The rating depends on the material, pressure and temperature.,Body/ bonnet rating,End connection,End connection provide the means for installation of control valves in pipelines Different types of end connections are Screwed end connection Welded end connection Raised face flange (RF) Flat face

19、 flange (FF) Separable flange Ring type joint (RTJ) Wafer style,End connection Screwed end,Popular for small control valves because of their economy and ease of installation Not available for bodies above 2 inch in size Aligning the threaded in valve with pipelines needs extreme care and mismatch ma

20、y lead to damage to thread in valve,End connection Welded end,For high pressure and high temperature applications Valve body is directly welded into the pipeline Leak tight at all pressure and temperature Economical Socket weld ends are common for smaller valve sizes and butt weld ends are standard

21、for sizes 2-1/2 inch and larger valve bodies Precise alignment of valve body with pipe line is mandatory for welded end connection,Butt Weld Socket Weld,Butt weld joint,Butt Weld end Connection,End connection Raised Face (RF),Most common end connection Flange has circular, raised faces with concentr

22、ic circular grooves for good gasket sealing and resistance to gasket blowout Gasket only covers the raised face portion of the flange As line bolting is tightened, high stresses are produced at the perimeter of each flange Not available for brittle/Soft material like Cast Iron, Bronze etc.,End conne

23、ction Flat Face (FF),Allows full face contact with gasket clamped between the flanges Full face contact has the advantage of minimizing flange stresses caused by forces of line bolting This type of flanges are common in cast iron bodies which are brittle and would crack if raised face flanges were u

24、sed. Flat face flanges are also common in brass and aluminum bodies These materials are soft and would bend if raised face flanges were used,End connection Separable flange,Flanges are manufactured independently of the valve body Split rings are required to secure the flange to the body and to trans

25、mit the bolting force to the gasket and mating connection If the body material is expensive separable flanges may result in economy since flanges can be made of an economical material like carbon steel,End connection Ring type joint (RTJ),Well suited for extremely high pressure applications but gene

26、rally not used at high temperature Mating grooves in the body and pipe flanges retain a solid metal seal ring As the line bolting is tightened the ring deforms and conforms to the grooves Pressure assisted seal since as the pressure increases the ring pushes against the grooves Soft iron or Monel is

27、 generally used as gasket material,End connection Wafer style,Generally called as Wafer style or flangeless Includes a raised-face contact surface in the body casting Long line bolting sandwiches the body and gasket between the pipe flanges This is more economical body style since it requires less m

28、aterial for construction Ball and butterfly valves are commonly available with this body style,Flow capacity,Flow capacity of the valve is generally specified as Cv in ISA term Definition of Cv Valve manufacturer determines Cv by test and by calculation according to ANSI/FCI standards Sliding stem v

29、alve capacities are published at different percentage of opening Rotary valves capacities are published for different degree of rotation,The number of US gallons of water per minute at 60 deg F that will pass through the valve with 1 psi pressure drop across the valve,Flow capacity & Factors that dr

30、ives Cv,Flow capacity,Determinants of Cv Port size Travel Travel Capacity Flow Geometry,Flow characteristics,The relationship between the flow coefficient and valve travel as the valve travels from 0 to 100 percent open Purpose of characterizing a control valve To match the valve gain with system ga

31、in To provide uniform control loop stability over expected range of operating conditions,Flow characteristics,Common flow characteristics are Quick opening Higher gain at lower valve travel Preferred for on/off service Linear Constant gain through out valve travel Preferred for constant pressure dro

32、p Equal percentage Lower gain at lower travel and higher gain at higher travel,Flow characteristics,Flow characteristics,Quick opening,Linear,Equal percentage,Shutoff class,The ability of a control valve to stop flow is shutoff ANSI/FCI 70-2 came up with different shutoff classes to standardize leak

33、age across control valve To provide a range of shutoff options SSV has different type of plug and seat construction Rotary valves have various seal configuration and materials to offer a range of shutoff,Shutoff class,Importance of control valve shutoff To maintain proper operation of a process If a

34、 control valve leaks excessively in closed position the leakage may affect the downstream processes & end product quality To avoid potential damage to the valve in certain applications If a valve in boiler feed water application leaks, the high velocity streams that leaks across the seat can erode critical seating surfaces,Shutoff class,ANSI/FCI/IEC Leakage Classes,Shutoff cla