GENERAL
In industrial piping, the control of flow is very important. Mechanical devices used for flow control are called valves. The principal functions of valves are:
· Starting and Stopping Flow
· Regulating or Throttling Flow
· Preventing Back Flow
· Regulating Pressure
· Relieving Pressure
Steel valves are classified by nominal pipe size (NPS) and by pressure-temperature service ratings. There are two service ratings; a Primary Rating and a Cold Working Pressure Rating:
· The Primary Rating is a pressure rating established by standards and accepted practice at an elevated temperature.
· The Cold Working Pressure Rating is the rating at ambient temperature (minus 20°F to 100°F). This rating is referred to as the CWP (Cold Working Pressure) Rating. This rating is of most interest when determining hydrostatic testing limits.
The Primary Rating is expressed in terms of steam. Steam ratings are used as a basis for determining the suitability of a material for a given application. The Cold Working Pressure Rating is usually designated by the mark WOG, which stands for Cold Water, Oil or Gas, non-shock.
Cast and forged steel valves bear a mark such as 150, 300, 600, etc.
These figures denote the maximum pressure in pounds per square inch (psi) at a certain temperature (usually
800 °F) for which an item is suited. A certain 600-pound valve may be suited for 600-pound pressure at temperatures up to 850 °F. But if the temperature exceeds that point, say up to 1000 °F, the valve is not recommended for pressures over 170 pounds.
As shown in the table above, all ratings are the maximum allowable non-shock pressure (psig) at the tabulated temperature (°F). The pressure-temperature tables should be consulted to select the pressure class of product required to meet the conditions of the intended service.
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VALVE CONSTRUCTION
Valves are manufactured in standard sizes ranging from 1/2 inch through 36 inch nominal pipe size. Valves may be as simple as a small plug valve (cock) having only four parts, or as complex as a motor-operated control valve having hundreds of parts. The valves most frequently encountered will have several parts that are functionally comparable and bear the same or similar names.
Body
The valve body connects to the system piping, houses the internal valve parts, and provides a pressure boundary as well as the passage for fluid flow. Bodies are made from a great variety of metals and alloys. Thus, a body material can be selected on the basis of compatibility with system piping and suitability for service conditions.
Most valves for ordinary uses have bodies made from appropriate grades of iron, steel or bronze castings. Cast bodies are prone to have material flaws that may elude detection by all surface type nondestructive examinations and their detection is only possible through use of volumetric methods such as ultrasonic or radiographic examination. Forgings contain fewer defects and are therefore generally used for the bodies of valves in critical service piping. Some very large valve bodies are made from two or more forgings that are machined and welded together, then run through a final machining process to true up interfacing surfaces. For critical services, a radiographic examination is then performed.
Trim Materials
The removable internal metal parts that contact the line fluid are collectively known as the valve trim. This includes parts such as the seat ring, disc or plug, glands, spacers, guides, bushings, and internal springs. Parts not considered trim include the body, bonnet, packing, yoke, and similar items. Valve trim parts are frequently made of materials that are compatible with but not the same as the body material. Likewise, trim parts of different materials are often used within the same valve assembly. Discs or plugs, and seat rings are pressure-retaining parts and need to conform to specification requirements.
Disc
Most valves have a disc or plug that stops flow through the valve when pressed against a stationary seat, or seats, in the body (closed) position. In the open position, the disc is moved away from the seat to allow fluid flow. The plug or ball performs the same functions in plug valves and ball valves. Many different styles of discs are used. Some bear little resemblance to the flat, circular shape for which they were originally named, but the name is almost universally
recognized for this valve part.
Seat
Valves may be provided with integral seats or replaceable seat rings, depending on the valve size. Small valves generally have screwed-in or welded-in seat rings, while larger valves have seating surfaces made up of a hardened treatment of the base body metal. Fluid flow through the valve is shut off by the seal formed between the disc and the seat.
The valve leakage rate is a function of the effectiveness of this seal.
VALVE CATEGORIES
The main valve categories, based on the different body styles include:
· Gate Valves
· Globe Valves
· Check Valves
· Plug Valves
· Ball Valves
· Butterfly Valves
· Safety Relief Valves
Besides these main categories, various other types of valves such as 3-Way, Swing, Stop Check, Diaphragm, and Pinch valves are available for special purposes. Control Valves, which may use basic design features of any of these main valve categories but which serve special operating functions, are described in later in this manual. Valves can be obtained with ends flanged, threaded for screwed connections, recessed for socket welding, or beveled for butt welding.
Gate Valves
Gate Valves have a significant feature of having less flow obstruction and lower turbulence within the valve creating only a small pressure drop across the valve. The main variations of gate valve designs are by the type of disc or
wedge. These include the solid wedge disc, double disc, flexible wedge disc and split wedge disc and vented disc. Solid wedges are of one piece construction, solid web type. The seating surfaces are precision machined to a mirror finish to provide full seating contact between the wedge and seats.
The double disc makes closure by descending between two parallel or tapered seats in the valve body.
Double disc with parallel faces are seated by being spread against the body seats. A disc spreader makes contact with a stop in the bottom of the valve and forces the disc apart.
Flexible wedge discs are of one piece construction but are cut out between the two seats in such a way as to provide a small degree of flexibility. It is this “flexibility” that makes the disc tight on both faces over a wide range of pressures. The split wedge disc is a two piece, wedge disc that seats between matching tapered seats in the body. The spreader device is simple, and integral with the disc halves.
Vented discs generally used in cryogenic services are a flexible wedge with a hole drilled in one side to allow even seating pressure on a downstream wedge for positive shutoff.
See also our Safety Valve Books section
Globe Valves
Globe Valves are commonly constructed with its inlet and outlet in line and with its port opening at right angles to the inlet and outlet. This seating construction increases resistance to the flow and permits close regulation of fluid flow. The globe valve is used principally in throttling service to control the flow to any desired degree. Flow is in the
bottom (under the seat) and out the top.
The main variations of globe valve design are by the type of disc. These include plug type, composition and conventional discs.
The plug disc is cone shaped with the seat ring having a matching cone shaped center. The wide bearing surfaces of the long, tapered plug type disc and matching seating offers high resistance to the cutting effects of dirt, scale, and other foreign matter.
The composition disc unit consists of a metal disc holder, composition disc and retaining nut. The flat face of the
composition disc seats like a cap against the seat opening. The disc is normally circular shaped, approximately
a 3/16 inch thick flat piece of material (compressed fiber or plastic). Closure is effected against a thin lip protruding
from and actually constituting the valve seat. The conventional disc is the oldest kind of globe valve. The basic design feature is a flat surfaced though slightly tapered valve seat that is fitted with a disc of convex configuration that used the taper in the seat for closing. This type of seating has only a narrow line of contact that normally assists an easy pressure tight closure.
Check Valves
Check Valves are entirely automatic in their operation and are activated internally by the flow of fluid or gases which they regulate. check valves permit the flow in only one direction and if the flow stops or tries to reverse its direction, the check valve closes immediately and prevents backflow. As soon as the pressure in the line is re-established, the
check valve opens and the flow is resumed in the same direction as before.
There are three basic designs of check valves:
· Swing check
· Lift check
· Wafer (2 swinging flappers closed by a spring)
Swing Check Valves
In the swing check, the disc is hinged at the top and seats against a machined seat in the tilted bridge wall opening. the disc swings freely in an arc from the fully closed position to one providing unobstructed flow. The valve is kept open by flow, with the size of the opening varying with the volume of the flow.
Lift Check Valves
For lift check valves, the flow is the same as through the globe valve. Consequently, there is turbulence within the valve and some pressure drop occurs. Lift check valves can be divided into three different types.
Horizontal-Lift Check Valve
it has an internal construction similar to the globe valve. The disc, which is seated on a horizontal seat, is equipped with guides above and/or below the seat and is guided in its vertical movement by integral guides in the seat bridge or valve bonnet. The disc is seated by backflow, or by gravity when there is no flow, and is free to rise and fall depending on the pressure under it. These valves are normally only installed in the horizontal position, however, can be installed in a vertical position with upward directed flow.
Vertical-Lift Check Valve
it has the same guiding principle as the horizontal lift check. It is equipped with a free-floating guided disc that rests when inoperative on the seat. These valves are of practical use only when installed in a
vertical piping system with an upward directed flow.
Ball Check Valve
It is similar to the horizontal or vertical lift check valve. Instead of a guide disc, a ball serves as the flow control medium. When operating, the ball is constantly in motion, reducing the effect of wear on any particular area of its sphere.
On some vertical-lift check valve designs, the disc or ball is spring loaded for improved performance in vertical applications.
see our Valve Books section
Plug Valves
they are composed of a tapered or cylindrical plug fitted snugly into a correspondingly shaped seat in the valve body. The plug is provided with an opening in line with the flow opening in the valve body. The porthole or flow opening in the plug may be round, oblong or diamond shaped. The valve is opened by turning the plug so that the opening in the valve body and plug are in line and is closed by turning the plug so that the plug opening is at
right angles to the valve body opening. Small plug valves are usually referred to as plug cocks.
Plug valves are either lubricated or non-lubricated. For non-lubricated valves, the plug may be inserted from the top or bottom of the valve body. The use of cylindrical plugs is often preferred since they are less likely to experience galling or freezing than conical plugs. In some designs, plastic seats are often molded into grooves of the plug to provide better seals, and bottom springs assist in operation.
The lubricated plug valve is designed with grooves in the plug which permits the lubricant to seal and lubricate the valve as well as to function as a hydraulic jacking force to lift the plug within the body, thus permitting easy operation. The lubricant is forced into its various distribution channels by a special lubricant gun that fits a button head fitting on top of the plug. The straightway passage through the port offers no opportunity for sediment or scale to collect. The valve plug, when rotated, wipes foreign matter from the plug.
References:
1. Piping/Mechanical Handbook.
2. Piping Handbook
