A plug valve is an on-off or a shut-off valve. A plug valve is a quarter turn valve and is used where quick and frequent operation is essential.
Conventional plug valves have relatively poor throttling characteristics. Special trims are required when used for throttling. Plug valve can have the following port patterns :
- Regular Pattern
- Venturi Pattern
- Short Pattern
Regular pattern valves have full area seat ports. Venturi pattern valves have reduced area seat ports. Operating torques are lower for venturi pattern plug valves.
Plug valves are of following types
- Lubricated plug
- Non lubricated plug
Lubricated Plug valves
In a lubricated plug valve lubricant is injected under pressure between the plug face and body seat to reduce friction and provide port sealing. The lubricant must be compatible with process fluid and its operating conditions. Manufacturers generally recommend lubricant suitable for the process fluid. 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.
Non-Lubricated Plug valves
Non-lubricated plug valve incorporates mechanical design features to reduce friction between the plug face and body seat during operation.
There are three types of nonlubricated plug valves :
- Lift-type plug valve
- Elastomer sleeved plug valve
- Fully lined plug valve
Lift-type plug valves have a means of mechanically lifting the tapered plug slightly to disengage it from the seating surface to permit easy rotation. The mechanical lifting can be achieved by an external lever. Some designs have an inherent feature which uses line pressure to ensure that the line pressure can not cause taper locking of the plug / plug movement into the taper. i.e. These valves are of 'Pressure Balance' Design. Some designs use spring balance to prevent locking of plug in the taper. Spring balance valves are generally not used for class 300 and above ratings.
Elastomer sleeved and lined valves require regular maintenance and have temperature limitations. Sleeved and lined plug valves are particularly used for caustic, chlorine and acid services. The type of liner to be used depends on the process fluid and operating conditions.
Teflon-sleeved valves take advantage of Teflon as a low-friction sealing material. The plug is surrounded by a conically shaped sleeve of Teflon. 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. Teflon sleeved valves are frequently used in molten sulphur applications.
Fully lined plug valves use relatively inexpensive ductile iron or cast iron body. The body and plug are fully lined with Teflon. This gives the valve the ability to resist corrosive fluids using iron as the body material. When selecting a fully lined valve, it is important to consider some specific service conditions to ensure valve performance. For example not all manufacturers recommend their valves for vacuum service. Some designs use melt processable thermoplastics that are molded into dovetail recesses in the body to lock the liner into place. Such designs are suitable for vacuum. Some manufacturers use dissimilar lining materials (PFA and TFE) on the sliding surfaces between the plug and liner to minimize the valve operating torque.
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.
Industry Codes and Standards
- API 6D - Specification for Pipeline valves
- API 599 - Metal Plug Valves - Flanged, Threaded and Welding Ends
- API 598 - Valve Inspection and Testing
- API 6FA - Fire Test for Valves
- BS 5188 - Specification for cast iron plug valves
- BS 5353 - Specification for steel plug valves
- BS 6755 Pt. 1 - Specification for production pressure testing requirements.
- BS 6755 Pt. 2 - Specification for fire type-testing requirements.
- MSS SP 61 - Pressure testing of steel valves
- MSS SP 25 - Standard marking system for valves, fittings, flanges & unions
- MSS SP 45 - Bypass and drain connections
Operators for Plug Valves
Most operators use the following guidelines to select the type of operators for Plug valves. When the effort to operate the lever exceeds 350 N geared operators are provided.
Note that plug valves require larger operating torques than a similar sized ball valve and hence the operating levers are usually much larger. Care should be taken during layout to allow room for the operation of the handle.
| Operator | Size | Rating |
|---|---|---|
| Handwheel | 6-inch and below | Class 150 and 300 |
| Handwheel | 4-inch and below | Class 600 |
| Handwheel | 3-inch and below | Class 900 |
| Handwheel | 2-inch and below | Class 1500 and 2500 |
| Gear Operator | 8-inch and above | Class 150 and 300 |
| Gear Operator | 6-inch and above | Class 600 |
| Gear Operator | 4-inch and above | Class 900 |
| Gear Operator | 3-inch and above | Class 1500 and 2500 |
