Introduction to Plug Valve

A plug valve is a quarter-turn manual valve that uses a cylindrical or tapered plug to permit or prevent straight through flow through the body. The plug has a straight through full port opening. Plug valve can be used in on-off, basic moderate throttling, and diverting services.

The face-to-face standard for plug valves is normally associated with ANSI Standard B16.10. However, many manufacturers have elected to use face-to-face dimensions provided for gate valves so that plug valves can be used as a replacement for gate valve in existing process services.

Plug Valve Design

The most common plug valve design allows for straight through, two way service, with the closure element in the middle of the body. The closure element, which is a plug and a sleeve, is accessible through top entry access in the body and is sealed by a bonnet cap. A diagrammatic representation of plug valve is shown below.

In the open position, the passage in the plug lines up with the inlet and outlet ports of the valve. When the closure element is rotated 90° from the open position, the solid part of the plug blocks the flow passage and stops the flow of fluid.

For some corrosive chemical services (such a hydrochloric acid, sulfuric acid etc), plug valve bodies are completely lined with PTFE, PVDF, PVDC, polyethylene, polypropylene or similar coatings.

Three way plug valves are also commonplace, with third port typically at a right angle from the inlet. With the three way design, the closure element is used to divert or combine the flow.

• Diverting Flow – Flow enters at the inlet and moves through the plug, which channels the flow to one of the two outlets. When plug is moved 90°, the flow is channel to the other outlet. At the midway position, flow may be equally divided to both outlets.
• Combining Flow – Flow is directed from two inlets to a single outlet.

Below figure shows six such three way flow arrangements.

To allow for the correct quarter turn motion without over or under stroking of plug valve, a stop collar arrangement is used. The stop collar is designed so that it fits at the top of the plug and thus turns with the plug stem.  A fixed key is provided on the bonnet cap at quarter turn path. As the plug stem is moved, the fixed key keeps the stop collar and the plug from moving outside the quarter turn range.

Types of Plug Valves

Plug valves are available in various types as given below:

• Lubricating Plug Valve
• Non Lubricating Plug Valve
• Multi Port Plug Valve
• Eccentric Plug Valve
• Expanding Plug Valve

These will be discussed is detail is separate articles.

Plug Valve Applications

• Plug valves are commonly used in low-pressure-low-temperature services, although some high-pressure-high-temperature designs also exists.
• Plug valves are high capacity valves that can be used for directional flow control, even in moderate vacuum systems.
• Plug valves can efficiently handle gas and liquid fuel.
• Plug valves can safely handle extreme temperature flow, such as boiler feed water, condensate, and other such elements.
• Plug valves can be used to regulate the flow of liquids containing suspended solids, for example, slurries.

Advantages and Disadvantages of Plug Valve

Advantages

• Plug valves have simple design with few parts.
• Plug valves open and close easily (except large dimensions).
• Maintenance and repair of plug valves can be done at the place of operation.
• Plug valves have low flow resistance.
• Plug valves provide reliable leak-proofness.

Disadvantages

• Because of high friction, first movement (opening-closing) requires a large amount of force.
• NPS 4 (DN100) and larger plug valves require a gearbox or an actuator.
• Plug valve transitions are narrowing flows due to conic plug.
• Generally, plug valves costs are much larger than ball valves.