Plug Valve Design Features
This post is initiated to give some basic information regarding plug valve design features, it uses, standards, types, advantages, disadvantages etc.
Plug Valve
Plug values are possibly the oldest type of valve in use. Discoveries in ancient ruins indicate that a form of plug valve was used in early Roman water systems. The concept is simple: A rotary cylindrical or tapered plug with an opening through it is inserted into an open body to block or allow the flow of a fluid. When the plug is rotated, it permits flow through the opening and the fluid conduit; a quarter turn in either direction completely n opens or closes the valve quickly. Operator does not have to rely on the valves feel to judge the closure.
They are also called cocks and are generally used for the same full flow service as gate valves where quick shutoff is required. They are sued in steam water oil gas and chemical liquid service. They are not normally used for regulation of flow.
The plug valve like other rotary valves requires minimum installation space, is simple to operate, exhibits fast response, and adds relatively little internal disturbance to flow. Therefore, pressure drop across the valve is low. Plug valves are mainly used for on/off functions, but can also be used for throttling. It contains a cylindrical or tapered plug with a hole that lines up with the direction of flow. Plug valves are not normally used for throttling, but several new designs have been used in some throttling applications. An important characteristic of the plug valve is its adaptability to multiport construction.
When designed in a three-, four-, or five-way configuration, the multiport plug valve can be used to replace as many as four conventional shutoff valves. Plug valves are used throughout the industry in applications where shutoff at low pressures must be ensured and valve cavities are undesirable. There are many types and styles of plug valves used throughout the process industry: lubricated, non-lubricated, rotor, and eccentric plug.
In its basic form it has only three parts, the body, plug and cover. Of these the plug is the only moving part. The porthole on the plug may be round, oblong or diamond shaped and may permit either full or restricted flow through the valve. Most plug valves are lever operated. They are supplied with flanged or threaded ends since heat dissipation during welding may result in distortions in the seat.
Major valve patterns are, regular, venturi, short, round port and multi port. The regular port has a port opening of 70 to 100% of the internal area. The short pattern valve has the overall dimensions of a 150# valve.
Lubricated Plug Valve
Plug valves are available in either a lubricated or non lubricated design. The lubricated valve provides tight shutoff of hard-to-handle fluids. The plug is designed with grooves, which retain a lubricant to seal and lubricate the valve and prevent sticking and provide a hydraulic jacking force to lift the plug within the body, thereby reducing force required for rotary operation. It also assists in sealing the surfaces and valve stem. The plug can be cylindrical or conical in shape.
The straight passage through the port prevents accumulation of sediment or scale, and when rotated 90o, the valve plug wipes away foreign matter. The opening of the port varies between 40 and 100 percent. A full-round port opening design increases the overall plug valve body size.
Lubricated plug valves must be resealed after only a few operations-in some cases after only one cycle-and are therefore considered high-maintenance valves to be used for infrequent operation. Lubricants are inserted using a lubricating gun or stick lubricants are used.
They do not lend themselves to automation because of the uncertainty of the necessary operating torque in remote areas where maintenance is not regularly performed and because there are usually no provisions for mounting hardware.
They are usually used in lines where a little contamination is not a problem. It is best to leave the manufacturer to decide on the lubricant after informing him of the fluid which is passing through the valve. Usual lubricants are molybdenum disulfide and fluorinated plastics.
Non lubricated plug valve
These may range from very small to very large sizes. Of the non lubricated plug valves there are two basic designs, the sleeved and the fully lined. The use of cylindrical plugs is preferred since they do not seize like the tapered plug. They are used in services where contamination is a problem like in the food industry.
Non-lubricated Teflon-sleeved plug valve
To eliminate the need to lubricate plug valves manufacturers of Teflon-sleeved valves take advantage of Teflon as a low-friction sealing material. Manually operated or actuated on-off TFE-sleeved plug valves are widely used in industry as block valves. Some plug valve manufacturers offer special purpose valves such as severe service valves, nuclear classes 2 and 3 valves, jacketed valves, fire-tested valves, chlorine valves, control valves, sampling valves, leak detector valves, and valves for gravity and vacuum systems.
A sleeved plug valve is one in which the plug is surrounded by a conically shaped sleeve of Teflon (see Fig. above), The sleeve is first fitted into the body and acts as a seat for closure once the plug is installed. The plug is continually under pressure from the bonnet assembly to maintain sleeve and plug contact. This means that even in very low pressure applications the operating torque of a plug valve remains high. Plug valves are top-entry valves which means that the closure element (plug) and seats may be accessed through the top of the valve without removing the valve from the line. Unlike ball valves, plug valves are generally not available with a wide variety of seat materials, primarily utilizing Teflon and a few derivatives.
Some advantages of the sleeved plug valve are tight shutoff at low pressure, sleeve and seat protection at the port opening to eliminate wear, and, unlike the ball valve, there are no pockets or cavities around the plug where material may become trapped and cause the valve to seize,
Until recently the plug valve has been considered a manual valve for on-off applications. Some valve manufacturers have recently introduced characterized plugs with specially shaped port openings to provide flow control characteristics in modulating service.
Materials
Plug valve bodies and closure elements can be supplied in most sizes in the following materials or combinations of materials:
| Alloy 20 (CN7M) | Carbon steel (WCB) |
| CD4MCu | Ductile iron (A395) |
| Hastelloy B (N-12M) | Hastelloy C (CW-12M) |
| Monel (M-35) | Nickel (CZ1OO) |
| 316 stainless steel (CF8M) | 316L stainless steel (CF3M) |
| 304 stainless steel (CF8) | 304L stainless steel (0F3) |
The TFE-sleeved tapered plug valve with the unique chemical-resistant properties of TFE and high-alloy body and bonnet construction provides the process industry with a corrosion-resistant valve, capable of tight shutoff, requiring minimal maintenance. A completely fluorocarbon lined plug valve is also available to allow the use of a lower-alloy body, bonnet, and trim materials while still providing excellent corrosion resistance.
Fully lined plug valve
The second style in which non lubricated plug valves are available is fully lined (see Fig. 3.3). Having seen how manufacturers of plug valves, as well as many other valve manufacturers, are taking advantage of the properties of Teflon, it makes sense that it also represents an opportunity for them to use it as an alternative to high-alloy body materials. In fact, that is exactly the concept of the fully lined plug valve. A relatively inexpensive ductile iron body and plug are fully lined and coated with Teflon. This gives the valve the ability to resist all the aggressive properties of a fluid using iron as the body material. Of course, if the external environment is equally aggressive, then an alloy or appropriately coated valve body should be used.
Some manufacturers of fully lined plug valves use dissimilar lining materials (PFA and TFE) on the sliding surfaces between the plug and liner to minimize the valve operating torque. This design utilizes the low coefficient of friction of fluorocarbon plastics to reduce the cost of an automated valve package when lower sizing torques can be used.
When selecting a fully lined valve, it is important to consider some specific service conditions to ensure valve performance. Vacuum is an especially severe service for fully lined valves. Not all manufacturers recommend their valves for vacuum service, but some may use melt processable thermoplastics that may be molded into dovetail recesses in the body to lock the liner into place (see Fig. above).
If a lining is thin and is subjected to continuous abrasion, it may be destroyed, thereby exposing the less corrosion resistant metallic body to the aggressive line fluid. Some valve manufacturers use an injection molding process to vary and control lining thicknesses to suit the mechanical requirements of the various valve components.
Multiport or Rotor plug valves
These are a variation of both the lubricated or non-lubricated plug valve. In the three way form a single L shaped passage connects any of the two ports with a T shaped opening. The four-way variety has two cutouts on opposite died that connect any two adjacent ports of the four ports or may otherwise connect two out of four available piping systems.
One variation on the traditional sleeved plug valve is the rotor valve (see Fig. 3.4). Rotor valves are available in 1/4 through 8 in (6 to 200 mm) sizes in bronze or 316 stainless steel with a wide variety of end connections. Like the sleeved plug valve the rotor plug uses Teflon as the sealing material in a cavity-free chamber, but instead of rotating the plug within the sleeve the sleeve rotates with the plug and effectively seats against the polished internal body surface. Valves of this type are often used in sanitary applications or where an internal cavity may effect operation. Rotor plug valves are offered with many different end connections’ and flow patterns in the multiport design (see Fig. 3.5). Among the standard multiport pattern selections are right angle L, three-port T, and combinations of the L and T. On a specialty basis they are available in 6- to 24-way porting patterns
Eccentric plug valves
Eccentric plug valves are used extensively in the water and waste industry because they offer straight-through flow and high capacity combined with tight shutoff. The design uses a resilient, coated plug segment that rotates 90o from open to closed into a raised eccentric metal body seat (see Fig. 3.6). Corrosion-resistant welded nickel, epoxy, or plastic-coated seats are used to resist corrosion. Eccentric plug valves are available with a variety of resilient plug facings to provide tight shutoff without the use of sealing lubricants, even with grit or other solid particles in the flow. As the plug closes, it moves toward the seat without scraping the seat or body walls (see Fig. 3.7). This eliminates plug binding and wear, They are available in sizes 1/2 to 72 in (12 to 1800 mm) and are used for the shutoff and throttling of raw sewage, grit slurries, air, water, gases, and all types of sludge.
To meet the requirements of a broad range of applications, eccentric plug valves are available with a wide selection of body materials such as Ni-resist, ductile iron, carbon steel, and stainless steel. They are available in screwed, flanged, mechanical joint, bell, or grooved end styles. Manual and powered actuators are available for shutoff and throttling control, The actuator may be a lever, handwheel, chainwheel, pneumatic or electric motor.
As the eccentric plug rotates 90o from open to closed, it moves into a raised eccentric seat.
In the open position, the segmented plug is out of the flow path. Flow is straight through, flow capacity is high.
As the plug closes, it moves toward the seat without scraping the seat or body walls so there is no binding or wear. Flow is still straight through making the throttling characteristic of this valve ideal for gasses, liquids, and slurries.
In the closed position, the plug makes contact with the seat. When furnished with a resilient facing, the plug is pressed firmly into the seat for dead-tight shutoff. Eccentric plug and seat design assure lasting shutoff because the plug continues to be pressed against the seat until firm contact is made.
