How to choose a valve and valve end connection type
A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
Valves are usually made of metal and they have several different parts. The outer part is called the seat and it often has a solid metal outer casing and a soft inner rubber or plastic seal so the valve makes a closure that’s absolutely tight. The inner part of the valve, which opens and closes, is called the body and fits into the seat when the valve is closed. There’s also some form of mechanism for opening and closing the valve—either a manual lever or wheel (as in a faucet or a stop cock) or an automated mechanism (as in a car engine or steam engine).
The simplest, and very ancient, valve is simply a freely hinged flap which drops to obstruct fluid (gas or liquid) flow in one direction, but is pushed open by flow in the opposite direction. This is called a check valve, as it prevents or “checks” the flow in one direction. Modern control valves may regulate pressure orflow downstream and operate on sophisticated automation systems.
Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on/off and pressure control to dish and clothes washers and taps in the home. Even aerosols have a tiny valve built in. Valves are also used in the military and transport sectors.
There are a large variety of valves and valve configurations to suit all services and conditions; different uses (on/off, control), different fluids (liquid, gas etc; combustible, toxic, corrosive etc) different materials and different pressure and temperature conditions. Valves are for starting or stopping flow, regulating or throttling flow, preventing back flow or relieving and regulating pressure in fluid or gaseous handling applications. Common valve types include: Ball,Butterfly,Check, Diaphragm, Gate, Globe, Knife Gate, Parallel Slide, Pinch, Piston, Plug, Sluice, etc.
The following types of valves are used in a variety of applications, these descriptions may provide a basic guideline in the selection of valves.
Because of their excellent operating characteristics, ball valves are used for the broadest spectrum of isolation applications and are available in a wide range of sizes and materials and are available in full fl ow and full through conduit. Advantages – quick acting, straight through fl ow in either direction, low pressure drop, bubble tight shut off & operating torque, easily actuated. Disadvantages – temperature limitiations on seating material, long “relative” face to face dimension.
The butterfly valve derives its name from the wing-like action of the disc which operates at right angles to the fl ow. It’s main advantage is a seating surface which is not critical. It is designed for fl ow isolation. The disc impinges against a resilient liner to provide bubble tightness with low operating torque. Compact and with a simple construction, butterfly valves facilitate easy pipe arrangement. Due to disc, they have slightly reduced fl ow characteristics. Advantages – quick acting, good regulating characteristics, compact & light, low pressure isolation. Butterfly valves are simple, reliable and range in size from 40mm to 1000mm and beyond. They can be controlled by a notched lever, handwheel as well as by pneumatic or electric actuator. A shaft turns a disc 90º within a pipe. The disc angle within the pipe can provide a restriction varying from drip-tight through to almost full fl ow (except very small sizes).
Sometimes referred to as a non-return valve, the check valve prevents back-flow in the piping by constantly keeping fluid flowing in one direction. Check valves operate automatically. Some piston/disc check valves are spring loaded for fast operation, (minimum cracking pressure should be specified). Vertical downwards flow requires a spring loaded check valve.
Swing Check Valves: – Swing & Wafer checks cease reverse flow with a flap that swings onto a seat. Use swing checks only for forward flow that is horizontal or vertical upward.
Piston Check Valves: – Piston & Spring checks cease reverse flow with a spring loaded plunger.
Ball Check Valves:– Ball checks have a ball that slides into a hole as forward flow slows. Consider a ball check for semi-solids such as pulp or effluent.
Titlting Disc Check Valves: – Tilting Disc check valves are similar to Swing check valves but in most installations, slamming is minimised upon reversal of fl ow so noise and vibration are reduced.
Globe/Stop Valves: – The flow path through globe valves follows a changing course, thereby causing increased resistance to flow and considerable pressure drop. Because of the seating arrangements, globe valves are the most suitable for throttling flow, however avoid extremely close throttling when the repeatable pressure drop exceeds 20%. Close throttling creates excess noise, vibration and possible damage to valves and piping. The valve is named after is globular body. Compared to gate valves, globe valves are designed to open and close more quickly. Their flow characteristics can be changed by re-configuring their discs. Advantages – best shut off (not drip tight above 50NB) & regulating. Disadvantages – high pressure drop (head loss), unidirectional.
Stop Check Valves (SDNR): – Stop check valves are essentially the same as Globe valves, except there is no mechanical connection between the stem and the disc. They provide a combination Stop valve and a Piston check valve in one valve. However, they are not designed for throttling. They are used in steam boiler outlet piping when two or more boilers are connected to a common header. Valves must be installed with pressure under the disc, and when the stem is raised, only boiler pressure can raise the disc whenever boiler pressure exceeds header pressure. They prevent backflow from the header to boiler.
Wedge Gate Valves: – Commonly used in industrial piping for stop or isolating – to turn on and shut off the flow as opposed to regulating flow. Gate valves are named from the gate-like disc which operates at a right angle to the path of the flow. Gate valves are general service valves that can be made in a broad spectrum of sizes using a variety of different materials. Wedge gate valves are metal seated but are also available with resilient seat insert if drip tight shut off is required. They can meet the demands of a wide range of pressure and temperature conditions and is available in full port. Advantages – low pressure drop, straight through flow either direction. Disadvantages – slow acting, bulky. Not drip tight shut off (over 150NB). Do not partially open as this will cause damage to seat/disc.
Knife Gate Valves: – Useful for many applications in larger sized pipework (50mm up). Unlike traditional gate valves, they are able to throttle (at lower pressures) depending on line media and degree of opening. Metal seated knife gate valves are not leak tight shut off. Some knife gate valves have a resilient seat in order to ensure they close drip-tight. Available in v-port, o-port and lined they are ideally suited for the control of effluent, slurries, waste products, semi solids, pulp, bulk powders. Most knife gate valves are designed for single flow direction.
Parallel Slide Gate Valves: – Popular in steam applications as the energised disc design handles thermal expansion without sticking like wedge gate valves. Another advantage is lower torque then wedge gate valves especially in venturi (Ferranti) reduced bore configuration. Parallel slide valves consist of two parallel gates that are energised against the seat at all times by springs or a wedging spreader bar between the two gates. No mechanical stress is exerted between the discs, and the valve is not subjected to dangerous strains in opening and closing. This design of valve maintains fluid tightness without the aid of wedging action. These valves are used for saturated and super heated steam.
Pipeline Slab Gate Valves: – Available in parallel solid slab and expanding 2 piece wedging slab. Both styles protect the seat area from the flow in all operating positions. These valves have a full through conduit configuration with a hole in the slab. Slab style gate valves have seats that are spring energised. The expanding slab features two opposed sliding v-shape segments that maintain pressure against the seats. These valves are for API6D pipeline applications but are also used for API6A wellhead valves. All these valves are made in metal to metal and soft seat configuration.
Plug valves are valves with cylindrical or conically-tapered “plugs” which can be rotated inside the valve body to control flow through the valve. The plugs in plug valves have one or more hollow passageways going sideways through the plug, so that fluid can flow through the plug when the valve is open. Plug valves are simple and often economical.
Characteristics of Valves by Types
Common Metal Types used in Valve Manufacture
The following is a general review of common valve materials used in general industrial, commercial and process valve construction.
Aluminum – A non-ferrous metal, very lightweight, approximately one-third the weight of steel. Aluminum exhibits excellent atmospheric corrosion resistance, but can be very reactive with other metals. In valves, aluminum is mainly used as for exterior components such as a hand wheels or identification tags.
Copper – Among the most important properties of wrought copper materials is their thermal and electrical conductivity, corrosion resistance, wear resistance, and ductility. Wrought copper performs well in high temperature applications and is easily joined by soldering or brazing. Wrought copper is generally only used for fittings.
Bronze – One of the first alloys developed in the Bronze Age is generally accepted as the industry standard for pressure rated bronze valves and fittings. Bronze has a higher strength than pure copper, is easily cast, has improved machinability, and is very easily joined by soldering or brazing. Bronze is very resistant to pitting corrosion, with general resistance to a wide range of chemicals.
Silicone Bronze – Has the ductility of copper but much more strength. Silicon bronze has equal or greater corrosion resistance to that of copper. Commonly used as a stem material in pressure-rated valves, silicon bronze has greater resistance to stress corrosion cracking than common brasses.
Aluminum Bronze – The most widely accepted disc material used in butterfly valves, aluminum bronze is heat treatable and has the strength of steel. Formation of an aluminum oxide layer on exposed surfaces makes this metal very corrosion resistant. Not recommended for high pH wet systems.
Brass – Generally good corrosion resistance. Susceptible to de-zincification in specific applications; excellent machinability. Primary uses for wrought brass are for ball valve stems and balls, and iron valve stems. A forging grade of brass is used in commercial ball valve bodies and end pieces.
Grey Iron – An alloy of iron, carbon and silicon; easily cast; good pressure tightness in the as-cast condition. Grey iron has excellent dampening properties and is easily machined. It is the standard material for bodies and bonnets of Class 125 iron body valves. Grey iron has corrosion resistance that is improved over steel in certain environments.
Ductile Iron – Has composition similar to gray iron. Special treatment modifies metallurgical structure, which yields higher mechanical properties; some grades are heat treated to improve ductility. Ductile iron has the strength properties of steel using similar casting techniques to that of grey iron and is used for class 250 (as well as class 125 in larger sizes).
Carbon Steel – Very good mechanical properties; good resistance to stress corrosion and sulfides. Carbon steel has high and low temperature strength, is very tough and has excellent fatigue strength. Mainly used in gate, globe, and check valves for applications up to 454ºC, and in one-, two-, and three-piece ball valves. Can be forged or cast, with forgings being superior especially for larges sizes in very high classes.
3% Nickel Iron – Improved corrosion resistance over gray and ductile iron. Higher temperature as well as corrosion resistance and mechanical properties. Very resistant to oxidising atmospheres.
Nickel-Plated Ductile Iron – Nickel coatings have received wide acceptance for use in chemical processing. These coatings have very high tensile strength, 50 to 225 ksi. To some extent, the hardness of a material is indicative of its resistance to abrasion and wear characteristics. Nickel plating is widely specified as a disc coating for butterfly valves. For industrial and petroleum ball valves, superior electroless nickel plating (ENP) is used in carbon steel valve components and is in fact superior to stainless steel in hardness but with similar corrosion properties.
400 Series Stainless Steel – An alloy of iron, carbon, and chromium. This stainless is normally magnetic due to its martensitic structure and iron-content. 400 series stainless steel is resistant to high temperature oxidation and has improved physical and mechanical properties over carbon steel. Most 400 series stainless steels are heat-treatable. The most common applications in valves are, for stem material in butterfly valves, and trim components such as seat, backseat bushings, discs, wedges etc. in cast steel gate, globe and check valves.
316 Stainless Steel – An alloy of iron, carbon, nickel, and chromium. A non-magnetic stainless steel with more ductility than 400 series SS. Austenitic in structure, 316 stainless steel has very good corrosion resistance to a wide range of environments, is not susceptible to stress corrosion cracking (however it is not suitable for higher levels of H2S typically found in wellhead applications) and is not affected by heat treatment. Very commonly used in valve body and/or trim material.
17-4 PH Stainless Steel – Is a martensitic precipitation/age hardened stainless steel offering high strength and hardness. 17.4 PH withstands corrosive attack better than any of the 400 series stainless steels and in most conditions its corrosion resistance closely approaches that of 300 series stainless steel. 17.4 PH is primarily used as a stem material for butterfly and ball valves as well as any valve application requiring a superior strength stem.
Alloy 20Cb-3 – This alloy has higher amounts of nickel and chromium than 300 series stainless steel and with the addition of columbium, this alloy retards stress corrosion cracking and has improved resistance to sulfuric acid. Alloy 20 is widely used in all phases of chemical processing.
Monel – Is a nickel-copper alloy used primarily as interior trim on all types of valves. One of the most specified materials for corrosion resistance to sea and salt water. Monel is also very resistant to strong caustic solutions.
Stellite – Cobalt base alloy, one of the best all-purpose hard facing alloys. Very resistant to heat, abrasion, corrosion, impact, galling, oxidation, thermal shock and erosion. Stellite takes a high polish and is used in steel valve seat rings. Normally applied with transfer plasma-arc; Stellite hardness is not affected by heat treatment.
Hastelloy C – A high nickel-chromium molybdenum alloy, which has outstanding resistance to a wide variety of chemical process environments including strong oxidisers such as wet chlorine, chlorine gas, and ferric chloride. Hastelloy C is also resistant to nitric, hydrochloric, and sulfuric acids at moderate temperatures.
The main parts of the most usual type of valve are the body and the bonnet. These two parts form the casing that holds the fluid going through the valve.
Valve Body : this is the pressure-containing component and main part of a valve that contains the internal parts or trim, and can have many different forms. Sometimes also referred to as valve casing or shell. Metal valve bodies can be cast or forged and are available in a wide range of materials. Since the body is in direct contact with the fluid, selecting the right body material for the application is very important.
Valve Bonnet : The valve bonnet acts as a cover on the valve body, both can be screwed, bolted or welded together. It contains the stem , valve packing and at the top it supports the actuator. To access the valve internals for maintenance, the bonnet has to be removed. When the bonnet is welded, there will be zero leakage (no fugitive emissions), but the easy removal of the bonnet as with the screwed or bolted version is of course not possible. Between the body and bolted bonnet is a gasket to get a tight seal. Not all valves have a bonnet (like ball valves), because the construction design is different.
Valve Stem : The valve stem provides the necessary movement to the disc, plug or the ball for opening or closing the valve, and is responsible for the proper positioning of the disk. It is connected to the valve handwheel, actuator, or the lever at one end and on the other side to the valve disc. In gate or globe valves, linear motion of the disc is needed to open or close the valve, while in plug, ball and Butterfly valves, the disc is rotated to open or close the valve.
Stems are usually forged, and connected to the disk by threaded or other techniques. To prevent leakage, in the area of the seal, a fine surface finish of the stem is necessary.
There are five types of valves stems:
RISING STEM WITH OUTSIDE SCREW AND YOKE
The exterior of the stem is threaded, while the portion of the stem in the valve is smooth. The stem threads are isolated from the flow medium by the stem packing. Two different styles of these designs are available; one with the handwheel attached to the stem, so they can rise together, and the other with a threaded sleeve that causes the stem to rise through the handwheel. This type of valve is indicated by “O. S. & Y.” is a common design for NPS 2 and larger valves.
RISING STEM WITH INSIDE SCREW
The threaded part of the stem is inside the valve body, and the stem packing along the smooth section that is exposed to the atmosphere outside. In this case, the stem threads are in contact with the flow medium. When rotated, the stem and the handwheel to rise together to open the valve.
NON RISING STEM WITH INSIDE SCREW
The threaded part of the stem is inside the valve and does not rise. The valve disc travels along the stem, like a nut if the stem is rotated. Stem threads are exposed to the flow medium, and as such, are subjected to the impact. That is why this model is used when space is limited to allow linear movement, and the flow medium does not cause erosion, corrosion or abrasion of the stem material.
This valve stem does not rotate or turn. It slides in and out the valve to open or close the valve. This design is used in hand-operated lever rapid opening valves. It is also used in control valves are operated by hydraulic or pneumatic cylinders.
This is a commonly used model in ball, plug, and Butterfly valves. A quarter-turn motion of the stem open or close the valve.
In the main Menu “Valves” you will find some links to detailed (large) images of Rising and NON Rising Stem valves.
Valve Trim : The internal parts of a valve that are in direct contact with the fluid are collectively referred to as valve trim. It typically includes disc, seats and stem and the term is mainly used with linear motion valves like gate and globe (control) valves. The seats can be from metal (integral to the body) , called hard seats, or of softer materials like PTFE, FKM (Viton), EPDM and are called soft seats. The hard seats usually have a small amount of leakage, while soft seats give 100% tight shutoff, but are limited in temperature and can be damaged more easily.
Gaskets : To get a seal between two static valve parts (like two body parts bolted together), a gasket is installed. This can be in a fibrous material, graphite, PTFE or a metallic gasket like a spiral wound gasket.
Ports : The ports of a valve are the valve ends that are connected to the piping. The ports can have different forms (see valve end connections).
Most valves have two connections or “ports”, and are therefore referred to as “two-way” valves (1 inlet and 1 outlet); when the valves has multiple ports, for instance to mix of divert a flow, they are referred to as “3-way” of “4-way” valves, depending on the number of ports. Typically a three-way ball valve comes with a T- or L-shaped drilling of the ball. The L-bore allows to guide the flow from one specific port to another and block off a third. The T-port can do the same, but is more commonly used to mix (2 inlets, 1 outlet port) or divert (1 inlet, 2 outlet ports) flow.
In instrumentation, 5 ports or more are not uncommon.
Hand-operated valves are usually equipped with a handwheel attached to the valve’s stem or Yoke nut which is rotated clockwise or counter clockwise to close or open a valve. Globe and gate valves are opened and closed in this way.
Hand-operated, quarter turn valves, such as Ball, Plug or Butterfly, has a lever for actuate the valve.
There are applications where it is not possible or desirable, to actuate the valve manually by handwheel or lever. These applications include:
- Large valves that must be operated against high hydrostatic pressure
- Valves they must be operated from a remote location
- When the time for opening, closing, throttle or manually controlling the valve is longer, than required by system-design criteria
These valves are usually equipped with an actuator.
An actuator in the broadest definition is a device that produces linear and rotary motion of a source of power under the action of a source of control.
Basic actuators are used to fully open or fully close a valve. Actuators for controlling or regulating valves are given a positioning signal to move to any intermediate position. There a many different types of actuators, but the following are some of the commonly used valve actuators:
- Gear Actuators
- Electric Motor Actuators
- Pneumatic Actuators
- Hydraulic Actuators
- Solenoid Actuators
For more information about Actuators see main Menu “Valves” –Valve Actuators–
The following are some of the commonly used valve classifications, based on mechanical motion:
- Linear Motion Valves. The valves in which the closure member, as in gate, globe, diaphragm, pinch, and lift Check Valves, moves in a straight line to allow, stop, or throttle the flow.
- Rotary Motion Valves. When the valve-closure member travels along an angular or circular path, as in butterfly, ball, plug, eccentric- and Swing Check Valves, the valves are called rotary motion valves.
- Quarter Turn Valves. Some rotary motion valves require approximately a quarter turn, 0 through 90°, motion of the stem to go to fully open from a fully closed position or vice versa.
CLASSIFICATION OF VALVES BASED ON MOTION
|VALVE TYPES||LINEAR MOTION||ROTARY MOTION||QUARTER TURN|
|VALVE TYPES||LINEAR MOTION||ROTARY MOTION||QUARTER TURN|
Pressure-temperature ratings of valves are designated by class numbers. ASME B16.34, Valves-Flanged, Threaded, and Welding End is one of the most widely used valve standards. It defines three types of classes: standard, special, and limited. ASME B16.34 covers Class 150, 300, 400, 600, 900, 1500, 2500, and 4500 valves.
Valve end connections
Valves can have different types of ends to mate with the piping in the installation.
1. Screwed Valve End Connections. – Male threads of various forms may be used for special purposes, but as a rule screwed end valves have female pipe threads, wither tapered for assembly to taper threaded pipe, or parallel for assembly to taper or parallel threaded pipe. In taper-to-taper and in taper-to-parallel connections, the pressure-tight joint is made on the threads. In parallel-to-parallel connections, the pressure tight joint is made by compressing a grummet or gasket against the end face of a valve. Screwed ends, usually confined to pipe sizes of 150mm and smaller, are widely used for bronze valves and to a lesser extent in iron and steel valves.
2. Flanged Valve End Connections. – Flanged-end valves are easy to install or remove from a pipeline, being bolted to the mating pipe flanges. To ensure a tight seal, a gasket is usually fitted between the machined facing of the flanges. The type of gasket, which can be non-metallic, metallic or a combination of both, depends upon service conditions and upon the type of flange facing employed. Bronze and iron valves are normally supplied with plain (flat) facings, and steel valves with plain (flat), raised or male facings, although female, tongue and groove, or ring-joint types are available. Flanged end valves are made in sizes from 15mm upwards.
3. Socket Weld Valve End Connections. – In this type, the ends of a valve are socketed to receive plain-end pipe. A circumferential weld is made on the outside of the pipe so that ‘icicles’ and weld spatter are unable to enter the pipeline. Socket-weld ends are used only on steel valves, and as a rule they are limited to sizes of 50mm and smaller for higher pressure/temperature applications in pipelines not requiring frequent dismantling.
4. Butt Weld Valve End Connections. – In this case, the ends of the valve are bevelled to match wall thickness and machined bevel at the end of a mating pipe. A circumferential weld is made at the abutted mating bevels. ‘Backing rings’ which are basically sleeves fitting inside the pipe, are sometimes used to align the pipe and valve bores also to prevent ‘icicles’ and weld spatter from entering the pipeline. Butt-weld ends are used only on steel valves, normally in sizes 50mm and upwards, for the higher pressure/temperature applications in pipelines which do not require frequent dismantling.
5. Compression Valve End Connections. – This type of valve end has a socket to receive the pipe and is fitted with a screwed union nut. The joint is made by the compression of a ring or sleeve on to the outside of a plain-end pipe, or by compressing a preformed portion of the pipe end. As a rule compression ends are used with copper tubing and steel tubing up to 65mm diameter and are used for low pressures or where pipes may require frequent dismantling.
6. Capillary Valve End Connections. – These valves are soldered to the mating pipe. The ends of the valve have a socket, machined to close tolerances to receive the plain-end pipe. The joint is made by the flow of solder by capillarity along the annular space between the socket and the outside of the pipe. Capillary ends are commonly used with copper tubing and confined to sizes 65mm and smaller. The high temperature use of capillary end valves is limited due to the comparatively low melting point of the solder.
7. Socket Valve End Connections – The ends of the valve are socketed to receive the plain spigot end of the pipe, the seal being made by the insertion of a yarn ring joint, corked with lead. Other forms of socket ends use a rubber scaling ring. These ends are either in the style of flange and socket adaptors for bolting to flanged end valves, or incorporated in the valves. Socket ends are normally associated with cast iron valves for water services in sizes 50mm and upwards.
8.Spigot Valve End Connections – The type of socket used in the coupling or on the pipe end determines the form of the spigot ends. For cast iron pipes with lead joints the spigot end is provided with a raised band, for screwed and bolted gland and other forms of mechanical joint the spigot end is prepared to suit the joint. For asbestos cement connections, the spigot end is finished plain in the same way as the pipe. These spigot ends are normally associated with cast iron valves for water services in sizes 50mm and upwards.
Reduced bore and full bore
A full bore valve has an orifice diameter similar to the end connections, giving the valve maximum flow capacity, low pressure drop and is used when “pigging” is required.
A reduced bore valve has an orifice diameter which is smaller than the end connection which leads to lower capacity, higher pressure drop, higher flow velocity (causing wear) and pigging is no longer possible. Reduced bore valves have a lower price.
These terms are mostly used in association with ball valves.
Based on their way of operation, valves can be divided roughly in two categories : multi-turn valves (also called linear motion valves) and quarter-turn valves (also called rotational motion valves).
With multi-turn valves, a handwheel is turned a certain number of turns to move the closure element up and down in a linear motion to open or close the valve.
The quarter turn valves are operated by a lever and require a quarter turn (90°) of the stem to operate the valve from fully open to fully closed, as in ball valves, butterfly valves and plug valves. Some (three or four-way) ball valves can be operated to half a turn (180°), but for ease, they are also classified as quarter-turn valves.
There are also valves that are self-operated and perform their function totally independent , like check valves, safety valves, float valves and steam traps.
Valves can be automated for different reasons with electric, pneumatic or hydraulic actuators. The first two are the most common in the industry.
Source: China valves Manufacturer – Yaang pipe fitting Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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