Structure, Working Principle, and Maintenance of Rotary Plug Valves

Plug valve is a quarter-turn valve that uses a cylindrical or conical plug to control linear flow through the valve body. The plug has a straight-through opening. As a full-bore structure, the opening area is equal to the valve's inlet and outlet area. The valve plug can be designed with various flow areas, shapes, and functional flow ports. Commonly used opening designs allow for small pressure drops at maximum flow area. The shape of the plug can also give it throttling characteristics. Under throttling conditions, the plug is positioned in the middle, and the fluid undergoes two pressure drops at the plug. Firstly, the airflow hits the conical valve seat surface in front of the plug and changes direction through the guiding effect of the cone surface before entering the plug. This design avoids the sharp angle of the plug bore hole. Since the valve plug can be designed with multiple flow areas, shapes, and functions, plug valves can be used in throttling conditions.

Equipment structure and application of plug valves

• Structure: Plug valves are mainly composed of valve body, plug, and packing.

• Main application: Plug valves are usually used as blowout valves and venting valves in pipeline systems.

Throttling principle of plug valves

Under throttling conditions, the plug is positioned in the middle, and the fluid undergoes two pressure drops at the plug. When the plug rotates away from the full open position, the inlet flow area is reduced and a pressure drop is generated. The fluid then moves within the full open area of the plug, where pressure is restored. Another throttling occurs at the outlet.

For the conical valve seat design of the plug valves, when the gas flows into the plug under throttling conditions, the airflow hits the conical valve seat surface in front of the plug and changes direction through the guiding effect of the cone surface before entering the plug, instead of hitting the sharp angle of the plug bore hole directly. Since the valve plug can be designed with multiple flow areas, shapes, and functions for throttling conditions, the plug valves can be used as throttling valves.

Operation of plug valves

• The valve must be manually operated by one person, and the use of force rods and multiple operators is prohibited.

• Check the valve's open and closed signs before operating the valve.

• Fully open the valve: Rotate the handwheel (or handle) counterclockwise until the valve position indicator is parallel to the pipeline. After the handwheel is turned to the appropriate position, turn it back 1/4 turn.

• Fully close the valve: Rotate the handwheel (or handle) clockwise until the valve position indicator is perpendicular to the pipeline. After the handwheel is turned to the appropriate position, turn it back 1/4 turn.

• The use of brute force when operating the valve is strictly prohibited.

• Plug valves can be used for short-term throttling, but they should not be used for long-term throttling conditions.

Inspection, maintenance, and care of plug valves

• Check the plug valves every month and perform quarterly maintenance every year, including in March, June, September, and December.

• Check the flexibility of the plug valves operation. The plug valves should be operated by one person without force rods.

• Check whether there is any leakage at all sealing points of the plug valve body and flange. The plug valve should be kept leak-free.

• Check whether there is corrosion in all exposed parts of the plug valve.

• Check whether there is water accumulation or lack of lubricant in the actuator of the plug valve.

• Perform maintenance on the actuator of the plug valves. If the inspection finds that the lubricating oil does not cover all contact surfaces of the actuator, remove all old lubricating oil and apply enough new lubricating oil to all contact surfaces.

• Clean the equipment and touch up any paint on the surface of the equipment.

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