Plug Valve 2

1.Design Details
Design detail of plug valve are as follows :
a. There are two basic types of plug valves :
- Lubricated Type
- Nonlubricated Type
b. Plug may be :
- Tapered (More Common) or
- Cylindrical (Less Common)
c. Normally, plugs are tapered downward with bolted access cover to top. Inverted plug valves with bottom access are also available.
d. The term “lubricated” refers to sealant being injected under pressure into grooves within valve body and plug. The sealant helps to prevent internal leakage and also acts as a lubricant to reduce metal-to-metal friction. Nonlubricated valves depend on a resilient liner for sealing. Stem sealing is accomplished in a variety of ways.
e. Plug valve body configurations are :
- Shot Pattern
- Regular Pattern
- Venturi Pattern
- Multiport Patterns

2.Port Configuration

Port openings can be specified in a variety of configurations, such as round, rectangular, oval, trapezoidal, etc., resulting in restriction to flow characteristics that must be examined for specific applications; e.g., regular pattern valves provide a flow area of 40 to 100% of the connecting pipe area. Venturi patterns provide a flow area of 36 to 50% of the connecting pipe area. The area reductions vary with valve type and also with manufacturer. Generally, the greater the opening, the lower pressure drop, and the higher the cost.
Cylindrical plug valves may have a round opening identical to the pipe opening, thus introducing no additional pressure drop. They are not readily available and are seldom used, particularly in large sizes, because of the relatively high cost.

3. Actuators

Geared actuators are normally used for 4 inch and larger nonlubricated plug valves and for 6 inch and larger lubricated plug valves.

4. Limitations

Limitations of plug valves include the following :
a. They are subject to binding and galling.
b. They are not suited for steam service.
c. Lubricated plug valves require periodic lubrication.
d. The lubricant (sealant) may react or contaminate the fluid being carried.
e. Flowing fluid trapped inside the plug port may cause overpressure and failure (cracking) of the plug when the valve is closed.
f. It should be noted when handling water at freezing temperatures that there is a risk that water trapped inside a closed plug will freeze and crack the plug. This can be prevented by drilling a small hole in the plug on the downstream side.

5. Typical Applications

Typical applications of plug valves are in petroleum and natural gas for leakproof isolation, and in slurry service.
It should be noted when handling water at freezing temperatures that there is a risk that water trapped inside a closed plug will freeze and crack the plug. This can be prevented by drilling a small hole in the plug on the downstream side.

6. Eccentric Plug Valves

The eccentric plug valve is a quarter turn nonlubricated type. The valve consist of a body, bonnet, actuating mechanism, and a rotating vane-like plug disc that is eccentrically located off its trunnion shaft and passes flow by positioning the plug out of the flow path and stops flow by positioning the plug against as inlet or outlet port. These valves are manufactured in sizes from ½ inch to 54 inch NPS. Multiported eccentric valves are available in 3 inch to 16 inch pipe sizes.
Eccentric plug valves can be used in on-off operation and in throttling service for waste water, saltwater, corrosive drains, gas or air contaminated with suspended solids, and for liquid slurries. They have excellent flow characteristics and, when the valve is wide open the plug is out of the slurry stream, which reduces wear. In slurry service the flow direction is from under the seat, so that solids do not collect in the valve cavity. The preferred orientation is with the shaft horizontal with the plug in open position.
Special valves provided by bottom ash handling system vendors are often of the eccentric plug type.