If you are an engineering, you my like this article.You will learn:
Choosing the Right Valve
°Basic Gate Valve Design
°Basic Globe & Angle Valve Design
°Gate Valve Seating Designs
°Seating Materials
°Globe Valve Seating Designs
°Disc-Stem Connections
°Variations in Stem Operations
°Bonnet & Bonnet Joint Characteristics
°Basic Check Valve Designs
°Basic Ball Valve Designs
°Basic Butterfly Valve Designs
°The Common Materials of Which Valves Are Made
°How to Read Service Rating Marks
Let's start
What's So Important About Choosing the Right Valve?
Valves have long been more than just a simple device for turning on and shutting off flow. Valve design has kept in step with industrial progress - the development of piping techniques, and the ever-growing list of fluids for processing, power, and finished product.
Progress in valve design puts at the piping engineer's elbow a great variety of valve types, each with some special qualification for service. From these he may choose the right one to provide dependable and economical performance in each particular need.
It's a case of carefully matching up the valve's service characteristics with the service requirements. It's a matter of knowing every detail of the job to be done - working pressure, temperature, fluid, volume of flow, corrosive elements, valve operating cycle, etc. Other equally vital considerations are the original valve cost, installation cost, and, of course, the cost of maintenance.
Crane Co., as the world's leading valve manufacturer and supplier, helps customers with the problems of valve selection every day. With this vast background of experience, Crane presents here as a helpful refresher for specifiers and buyers, the more important elements involved in choosing the right valve for the right job.
Principal Valve Types
Gate Valve
Commonly used in industrial piping, this type of valve, as a rule, should be used as a stop valve…to turn on and shut off the flow, as opposed to regulating flow. It gets its name from the gate-like disc which operates at a right angle to the path of flow.
Globe and Angle Valves
The flow through globe valves follow 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. The valve is named after its globular body.
Angle valves, similar in principle and a companion line to the globe, are designed to permit a 90 degree turn in piping and are less resistant to flow.
Check Valve
Sometimes referred to as the non-return valve, the check valve stops backflow in the piping. Unlike the gate and globe valves, this simplest of types operates automatically.
Ball Valve
Unique in design, this valve controls the flow of a wide variety of fluids. It can be opened or closed in a quarter-turn of the operating handle. The name "ball" is derived from the ball-shaped disc located within the body. A hole through the center of this disc provides the straight-through flow which is characteristic of ball valves. Light and durable, these are the valves that are playing increasingly important roles in our nation's missile projects, as well as in industry and commercial buildings.
Butterfly Valve
Here's a valve that is extremely durable, efficient and reliable. The butterfly valve derives its name from the wing-like action of the disc which operates at right angles to the flow. Its chief advantage is a seating surface which is not critical. The reason for this being the disc impinges against a resilient liner to provide bubble tightness with low operating torque.
Basic Gate Valve Design
…how it should be used
Gate valves are by far the most widely used in industrial piping. That's because most valves are needed as stop valves - to fully shut off or fully turn on flow - the only job for which gate valves are recommended.
Gate valves are inherently suited for wide-open service. Flow moves in a straight line, and practically without resistance when disc is fully raised.
Seating is perpendicular or at right angle to the line of flow - meets it head on. That's one reason why gate valves are impractical for throttling service and for too frequent operation.
For instance, a 6-inch gate valve holding fluid at 300 psi, puts a load of over 4 tons on one side of the disc, if there is only atmospheric pressure on the other. While seated tight, there's no wear or undue strain on disc or seats. But each time the valve is "cracked open," there's a threat of wire drawing and erosion of seating surfaces by the high-velocity flow.
Repeated movement of disc near point of closure under high-velocity flow, may create a drag on seating surfaces and cause galling or scoring on downstream side. A slightly opened disc may cause turbulent flow with vibration and chattering of disc.
A gate valve usually requires more turns - more work - to open it fully. Also, unlike many globe valves, the volume of flow through the valve is not in direct relation to number of turns of handwheel.
Since most gate valves used have wedge disc with matching tapered seats, refacing or repairing of the seating surfaces is not a simple operation.
CONCLUSION: Gate valves, while not designed for throttling or too frequent operation are generally ideal for services requiring full flow or no flow.
Gate valves are not designed for throttling
In a slightly opened position high-velocity flow will cause wire drawing and erosion of seating surfaces in gate valves.
Repeated movement of disc near point of closure under high-pressure flow may gall or score seating surfaces on downstream side.
Slightly opened disc in turbulent flow may cause troublesome vibration and chattering.
Basic Globe and Angle Valve Design
…highly essential to piping
Unlike the perpendicular seating in gate valves, globe valve seating is parallel to the line of flow. All contact between seat and disc ends when flow begins. These are advantages for more efficient throttling of flow, with minimum wire drawing and seat erosion.
The directly proportionate relation of size of seat opening to number of turns of handwheel, a distinctive feature of plug-type globe valves, permits close flow regulation. An operator can gauge the rate of flow by the number of turns of the wheel.
Shorter disc travel - with fewer turns required to operate globe valves - saves considerable time and work - also wear on valve parts.
Whatever wear occurs as the result of frequent or severe operation presents less of a maintenance problem than in gate valves. Seat and disc in most globe valves can be repaired without removing the valve from the pipe line.
CONCLUSION: Globe valves, while not recommended where resistance to flow and pressure drop would be objectionable, are generally ideal for throttling, and preferable for frequent operation.
Basic Angle Valve Design
The angle valve effectively utilizes globe valve seating principle while providing for a 90 degree turn in piping. It is less resisting to flow than the globe valve it displaces. Requires fewer joints; saves makeup time and labor.
Gate Valve Seating Designs…
Solid Wedge Disc
The most widely used disc in gate valves - the solid wedge-shaped disc - with matching tapered body seating surfaces. Favored for its strong, simple design and single part.
Can be installed in any position without danger of jamming due to misalignment of parts.
Ideal for steam service, and well suited for water, air, oil, gas, and many other fluids.
Most practical for turbulent flow because there's nothing inside to vibrate and chatter.
Refacing of the tapered disc surfaces isn't easy, but there's little need for it is valve is used fully opened or fully closed.
Might be subject to some sticking when subjected to extreme temperature changes where body contracts more than disc. For such conditions, Crane flexible wedge disc is recommended.
Double Disc
This parallel-faced double disc makes closure by descending between matching seats in valve body. As the valve is being closed, a lower spreader (or in some cases, a disc wedge) strikes a stop in the bottom of the body. Further closure brings the upper spreader into contact with the lower spreader so that the discs are forced outward against the seats.
First opening movement releases discs, and continued operation raises them clear of seat openings.
Widely used on water service, in waterworks and sewage disposal plants; also on oil and gas, in cross-country pipe lines.
Generally unsuited for steam. Rapid expansion and high velocity of steam flow tend to vibrate loose internal parts in disc assembly, hastening wear.
Exposure of closed valve to rise in external temperature may cause dangerous increase in internal pressure, if non-compressible liquid is trapped between discs.
Because discs and body seats are perpendicular and parallel, repairing or refacing to compensate for wear is easier than on a tapered wedge disc.
Should be installed with stem above horizontal for best results. Many spreader mechanisms are subject to jamming when installed with stem below horizontal line.
Their Service Characteristics
Flexible Wedge Disc
Developed especially to overcome sticking in high-temperature service with extreme temperature changes. The shape of the flexible disc can be likened to two wheels on a very short axle. The "axle" or spud at the center of the disc is amply strong to carry the two halves of the disc together at all times…and yet, it permits a degree of action between them. It is this "flexibility" that makes the disc tight on both faces over a wide range of pressures…prevents sticking during temperature changes, and assures minimum operating torque.
Although each disc face can move independently of the other…up to two full degrees…the construction is one-piece. There are no loose parts to cause harmful vibration.
Split Wedge Disc
A 2-piece, wedge disc that seats between matching tapered seats in body.
Spreader device is simple, and integral with disc halves.
When closing, last turn of handwheel forces discs against the seats. When opening, the first turn releases the discs from the seats.
Seating Materials…
Key to valve performance
The seat and disc constitute the "heart" of a valve; do most of its work. The material from which these parts are made, therefore, becomes important. The tougher the service, the more severe the demand on seating. Valve manufacturers recognize this fact by providing a wider choice of seating materials as valves go up the pressure-temperature scale or are offered for more rigorous service.
For relatively low pressure and temperatures and for ordinary fluids, seating materials are not a particularly difficult problem. Bronze and iron valves usually have bronze or bronze-faced seating surfaces, or iron valves may be all iron. Nonmetallic "composition" discs are available for tight seating on hard-to-hold fluids such as air or gasoline.
As pressures and temperatures increase or as the service becomes severe, careful consideration must be given to many factors, no one which can be overemphasized to the detriment of others. Long, trouble-free life requires the proper combination of hardness, wear-resistance, resistance to corrosion, erosion, galling, seizing, and temperature. Nor does it follow that a satisfactory combination in one instance will serve equally well in all others. Type of valve is a limiting factor, too.
Selection of seating materials for corrosive fluids, regardless of pressure-temperature, is almost endless. Included are many types of alloys, as well as linings or coatings of many kinds.
Valve Catalog your best guide
Safest policy in specifying seating materials is in close adherence to valve manufacturer's recommendations, usually found in catalogs, otherwise supplied on request.
Globe Valve Seating Designs..
Plug Type Disc
Long taper with corresponding seat, giving a wide area of seating contact, makes the plug-type disc superior to all others for severe throttling service, such as blow-off, soot-blower, boiler feed.
Because of wide seat bearing, most cuts and nicks by dirt, scale, and other foreign matter in flow are seldom big enough to cause leakage.
Plug disc shape, in proper combination of metals for service, is most effective in resisting erosive effects of close throttling.
Construction permits replacement of seat if necessary.
Conventional (ordinary) Disc
A good seating design for many not-too-severe services, but not for close throttling.
Disc has relatively narrow contact with body seat - virtually a line bearing. This narrow metal area, in closely throttled high-velocity flow, is subject to erosion and wire-drawing.
Deposit of particles of foreign matter on seat makes tight closure virtually impossible.
Yet uniform deposit on seat, such as coking action in oil refineries, is more easily broken down by the narrow bearing. It makes a tight metal-to-metal contact easier than a wide seat.
Seat and disc can be conveniently serviced.
Needle Point Disc and Seat
Needle point valves are designed to give fine control of flow in small-diameter piping. Their name is derived from their sharp-pointed conical disc and matching seat. They come in globe and angle patterns, in bronze and steel, and find usage on steam, air, water, oil, gas, light liquid, fuel oil, and similar services.
Stem threads are finer than usual so that considerable movement of stem is required to increase or decrease opening through seat.
Usually, these valves have reduced seat diameter in relation to pipe size.
Their Service Characteristic
Composition Disc
A useful design in bronze and iron valves for adaptability to many services and for quick repairs.
Discs available in compositions suitable for steam, hot water, cold water, oil, air, gas, gasoline, and many other fluids. Disc change is quickly made with slip-on disc holder.
Highly regarded for dependable, tight seating on hard-to-hold fluids such as compressed air. Flat face relatively "soft" disc seats against a raised crown in body.
Small particles of foreign matter are imbedded in disc, preventing seat damage and leakage.
Suited for all moderate pressure services except close regulating and throttling, which can rapidly cut out the disc.
Angle Valve Seating
It is well to note and remember the angle valve when looking for globe valves. If there's a right angle turn in the line near where you need a valve, an angle pattern gives you important advantages.
It's available with the same seating variations as shown here for globe valves: plug-type disc, conventional, and composition disc.
Has considerably reduced turbulence, restriction of flow, and pressure drop because flow makes one less change of direction than in globe valve.
Angle valve cuts down on piping installation time, labor, and materials, also reduces number of joints or potential leaks by serving as a valve and a 90 degree elbow.
Disc-Stem Connections…
important factor in valve selection
In Gate Valves
In a gate valve, the sole function of the stem is to raise and lower the disc. In doing its job, the stem should not be subject to corollary stresses and strains of service conditions on the disc.
Thus, with gate valves, especially those used for higher pressure installations, a relatively loose disc-stem connection is desired.
If the connection were rigid, any side thrust on the disc caused by pressure and flow, would readily be transmitted to the stem, and tend to strain and possibly bend it.
A properly fitted loose connection relieves strain on the stem due to any lateral movement of the disc.
In Globe Valves
The stem in a globe valve not only raises and lowers the disc, but also must help guide it squarely to its seat.
Thus, unlike a gate valve, the globe valve disc-stem connection must be relatively close fitting to prevent any extreme lateral motion of the disc that would cause it to cock and seat improperly.
But, once the disc and seat are joined, the disc must stop turning while seating is completed by the stem. This will avoid metal-to-metal friction between disc and seat that would be destructive to seating surfaces.
The solution to this need is a swivel action in globe valve disc-stem connections, which permits true and tight seating without damage to seating surfaces.
Variations in Stem Operations
Although in many valve applications the type of stem operation makes little or no difference, in other cases it can be important. A simple example of the latter is the need for a self-indicator to show open or closed position, as in the case of rising stem valve, or, conversely, the need for a non-rising stem valve because of lack of head room. This shows how stem operating designs are adapted to service needs.
Rising Stem with Outside Screw
On both valves shown here, whether opened or closed, the stem threads always remain outside the valve body. They are not subjected to corrosion, erosion, sediment, or any elements in the line fluid that might damage stem threads inside the valve body. Being outside, they can be lubricated easily when necessary.
Rising Stem with Inside Screw
This is the simplest and most common stem construction for gate, globe, and angle valves in the smaller sizes. Stem turns and rises on threads inside the valve. Position of handwheel indicates position of disc - opened or closed.
Non-rising Stem with Inside Screw
Generally used on gate valves only, this stem does not rise, but merely turns with handwheel. In turning, the stem threads raise or lower the disc. Since stem only rotates, packing wear is less. Ideal where head room is limited.
Sliding Stem is Often Useful
The sliding stem valve is useful where quick opening and closing are wanted. A lever takes the place of the handwheel, and stem threads are eliminated. Available in both gate and globe valves.
Stuffing Box Designs Featured on CRANE Valves
Stuffing box must effect a tight seal around the stem to retain pressure inside piping system. Stem must be tight without binding. Packing is subject to wear and must be periodically compressed and eventually replaced.
- Packing Nut without Gland
Used on low-pressure and small-size valves. With wheel and packing nut removed, this type is easier to repack than ordinary gland type on valves with small diameter stem.
- Packing Nut with Gland
Conventional type packing nut with loose gland. Gland has small lip at top edge so that it can be pried out with screwdriver tip if jammed all the way down.
- Bolted Gland
Deep stuffing box with two-piece ball-type gland and flange with swing-type eye bolts. Construction maintains an even load on the packing and prevents binding on the stem even when the gland bolt nuts are pulled up unevenly.
- Injection Type
Add new packing with the twist of a wrench, even under full rated line pressure, and with the disc in any position! No need to backseat the disc. The specially designed ball check valve eliminates possibility of packing extrusion. When the packing reservoir is empty, simply back out the adjustment screw and insert a new pack stick.
- Lantern Type
Superior construction for larger-size high pressure-temperature valves. Cooling chamber with lantern spacer and three rings of packing below to wipe stem clean before it passes into the sealing rings above. Two-piece ball-type gland and flange with swing-type eye bolts.
Bonnet and Bonnet Joint Characteristics
…of gate, globe, and angle valves
Which is best?
In choosing valves, the service characteristics of the bonnet joint should not be overlooked. Bonnets and bonnet joints must provide a leakproof closure for the body. There are many modifications, but the three most common types are screwed-in bonnet, screwed union ring bonnet, and bolted bonnet.
The simplest and least expensive construction, frequently used on bronze gate, globe, and angle valves, and recommended where frequent dismantling is not needed.
When properly designed with running threads, and carefully assembled, the screwed-in bonnet makes a durable pressure-tight seal, suited for many services. On modified steel valve designs such as the lip-seal valve with a weld around the periphery of the body-bonnet juncture, the screwed-in bonnet withstands even high pressures and temperatures.
Screwed Union Ring Bonnet
A good choice for quick dismantling and reassembly - yet a strong, well-reinforced joint.
Convenient where valves need frequent inspection or cleaning - also for quick renewal or changeover of disc in composition disc valves.
Separate union ring applies direct load on bonnet to hold a pressure-tight joint with body. Turning motion used to tighten ring is spent between shoulders of the ring and bonnet. Hence, the point of seal contact between bonnet and body is less subject to wear from frequent opening of the joint. Contact faces are less likely to be injured in handling. Union ring gives the body added strength and rigidity against internal pressure and distortion. While ideal on smaller-size valves, it is impractical on large sizes.
Bolted Bonnet Joint
A practical and commonly used joint for larger-size valves or for higher pressure applications.
Adaptable to all types of gasketing.
Multiple bolting, with small diameter bolts, permits equalized sealing pressure without the excessive torque needed to make large threaded joints. Only small wrenches are needed.
Has practically no limitation for size. Only the highest pressures and temperatures tax its capacity to permanently hold tight.
Lip-Seal Bonnet Joint
Crane's lip seal design features simplicity. The body and bonnet are screwed together until a firm metal-to-metal contact is made between the smoothly machined, flat surfaces on the shoulder of the bonnet and the top of the body. The shoulder of the bonnet is smaller in diameter than the mating area of the body, thus permitting the use of a fillet form of seal weld around the periphery of the connection. Dismantling is accomplished by grinding off the fillet weld and unscrewing the bonnet. The design makes possible compact, relatively lightweight valves ideal for high pressure-temperature services. Absolute tightness, full seating area, and freedom from bonnet joint maintenance are other advantages.
CRANE Pressure-Seal Bonnet Joint
Newest and most effective bonnet joint, developed by Crane, for sealing the highest pressures and temperatures, especially in steam service.
Tightness of seal does not depend on nuts, bolts, and threads as in conventional bonnet joints. Instead, Crane Pressure-Seal bonnet joint utilizes line fluid pressure to seal the joint. The greater the pressure, the tighter the seal.
The actual joint is inside the valve, and is sealed with a wedge-shaped seal ring. Internal fluid pressure acting on the entire underside area of the bonnet, is concentrated at the smaller contacting area of the wedge-shaped ring to make a pressure-tight metal-to-metal joint.
Available in gate, globe, angle, check and stop-check valves.
Basic Check Valve Design
The Swing Check…companion for gate valves
Swing checks work automatically as shown here. But whether used in a horizontal line or vertical line for upward flow, they will not function properly unless installed with pressure under the disc.
Flow through swing checks is in a
