Plug Valve Working Principle:
Plug valve is the oldest member of valve family and used to start or stop fluid flow. It is a rotational motion valve. Body, plug (tapered or cylindrical) and cover are the basic parts of plug valve. The plug is a solid disc with a bored passage at a right angle to the longitudinal axis of the plug. It turned through 900 to allow fluid passage or to present a blank fuse to prevent flow.
In an open position, the bored passage in the plug lines up with inlet and outlet ports of the valve body. When the plug turned 900 from the open position, solid part of the plug blocks the ports and stops fluid flow. Because of their design, these offer quick action, simple operation, minimum installation space, and less pressure drop.
The two-port plug is the simplest and common type of plug valve with two positions, open to allow flow and close to stop the flow. Three-way plug valves are also available. In this type, fluid flow from one port could be directed to either second or third port and even connect all three ports together. Three-way plug valve also shifts flow between three ports.
Plug valve port designs:
Plug valve ports are available in disc, rectangular, round and diamond shapes. Disc port plug is either round or cylindrical with the taper. A rectangular port plug is commonly used that represents at least 70% of the equivalent pipe cross-sectional area. In round port plug, it has round opening through the plug. If the round port plug size is equal or greater than pipe inner diameter, it refers as a full port. On the other hand, if the port opening is smaller than the pipe inner diameter, the port refers as a standard port.
Valves having standard round ports used only where flow restriction is not essential. Diamond port plug designed for throttling purpose. Diamond port plug valves are venturi restricted flow type.
Plug Valve types:
These valves classified as lubricating and non-lubricating.
1. Lubricating plug valve:
These valves use a lubricant which provided under pressure between the body seat and plug face. Lubricant provides less friction between the metal parts of the valve and increases valve life. Multiple grooves around the body and plug port opening supplied with grease prior to actuation. This lubricant (grease) seals the gap between body and plug and also seals plug motion. Grease injected for lubrication through fitting at the top of the valve stem through the check valve. The lubricant must be compatible with temperature and working fluid nature. Lubricated plug valves can be large as 24 inches and have capabilities to handle pressure up to 6000 psig.
2. Non-lubricated plug valve:
Non-lubricating plug valve uses a tapered or cone-shape that acts as a wedge and presses the sleeve alongside the body cavity. Sleeve reduces friction between the body and plug mechanism. Lift type and elastomer sleeve are the two basic types of non-lubricating plug valve. Lift type non-lubricating plug provides with a mechanically lifting arrangement to disengage tapered plug from the seating surface. Mechanical lifting accomplished with external lever or cam.
Common non-lubricating plug valves have elastomer sleeve. Elastomer sleeve is a TFE sleeve which surrounds the plug completely. It is engaged and locked in place through a metal body. This design results in a primary seal maintained between the sleeve and the plug at all times irrespective of the valve position. TFE sleeve has less coefficient of friction and also self-lubricated. These valves used where maintenance to be kept at a minimum level.
Plug valve applications:
Plug valves use for low pressure, low temperature. While some high pressure, high-temperature designs are also available. These valves permit easy lining of the body with corrosive chemical materials such as polytetrafluoroethylene (PTFE), sulfur and hydrogen fluoride. These are ideal for on off, moderate throttling and diversion applications. They are applied in liquid and gas, vacuum, food processing, non-abrasive slurry and pharmaceutical services. Abrasive and sticky fluids can also handle with special design.