Selection and Structural Features of Plug Valves

Selection and Structural Features of Plug Valves


Plug valves, are valves with closing members or plug-shaped cylinders that rotate 90 degrees to connect or separate the passages on the plug from those on the valve body, thereby opening or closing the valve.
 
The shape of the plug can be cylindrical or conical. In cylindrical plug valves, the passages are generally rectangular, while in conical plug valves, the passages are trapezoidal. These shapes make the structure of plug valves compact. They are most suitable for use as on-off valves and for diverting flow, and depending on the nature of the application and the erosion resistance of the sealing surfaces, they can sometimes be used for throttling.
 
Testing Methods for Plug Valves:
 
1. During the strength test of plug valves, the medium is introduced from one end, and the remaining passages are sealed. The plug is rotated to various fully open working positions for testing. The valve body should not leak to be considered qualified.

2. During the sealing test, the cavity of through-type plug valves should be kept at the same pressure as the passage. The plug is rotated to the closed position, and checks are conducted from the other end. Then the plug is rotated 180°, and the test is repeated. For three-way or four-way plug valves, the cavity should be kept at the same pressure as one end of the passage, and the plug is rotated to the closed position, with pressure introduced from the right-angle end and checks conducted from the other end.

3. Before testing, a thin layer of non-acidic lubricating oil is allowed to be applied to the sealing surface. Leakage and enlarged water droplets within the specified time are considered qualified. The testing time for plug valves can be shorter, generally ranging from 1 to 3 minutes as per the nominal diameter requirements.

4. Plug valves for gas use should undergo an air tightness test at 1.25 times the working pressure.

Selection and Structural Features of Plug Valves:
 
Plug valves offer low resistance to fluid flow, quick and convenient operation, and are mainly used in pipelines for cutting off, diverting, and changing the direction of flow. They are adaptable to multi-channel structures, allowing one valve to have two, three, or even four different flow paths, simplifying pipeline system design and reducing the number of valves and connecting fittings required in equipment.
 
Plug valves were among the earliest valve components adopted by people. Due to their simple structure, rapid opening and closing (requiring only a quarter turn of the plug), ease of operation, and low fluid resistance, they are still widely used today. They are primarily used in low-pressure, small-diameter applications and where medium temperatures are not high.
 
Plug valves should be selected based on their structural characteristics and the functions they can achieve:
 
1. For applications involving medium distribution and changing the direction of flow, with working temperatures not exceeding 300°C, nominal pressure PN ≤ 1.6 MPa, and nominal diameters not exceeding 300 mm, multi-channel plug valves are recommended.

2. In food processing plants, pharmaceutical factories, and other equipment and pipelines handling milk, fruit juice, beer, etc., austenitic stainless steel plug valves with fixed conical plugs are recommended.

3. For branch lines, refining, and cleaning equipment in oil and natural gas fields, with nominal pressure levels not exceeding Class 300 and nominal diameters not exceeding 300 mm, oil-sealed conical plug valves are recommended.

4. In oil and natural gas fields, with nominal pressure levels not exceeding Class 7500, nominal diameters not exceeding 900 mm, and working temperatures not exceeding 340°C, oil-sealed conical plug valves are recommended.

5. In large-scale chemical industries with corrosive media in pipelines and equipment, and where rapid opening or closing is required, polytetrafluoroethylene sleeve-sealed conical plug valves can be selected for nitric acid-based media; and CrI8Ni 2MO2Ti stainless steel-lined polytetrafluoroethylene sleeve-sealed conical plug valves can be selected for acetic acid-based media.

6. In gas, natural gas, and HVAC system pipelines and equipment, with nominal diameters not exceeding 200 mm, packed conical plug valves are recommended.