The opening and closing parts of the stop valve (Globe Valve) are plug shaped valve discs. The sealing surface is flat or conical. The valve disc moves in a straight line along the centerline of the fluid. The movement forms of the valve stem include lifting rod type (the valve stem rises and falls, and the hand wheel does not rise and fall) and lifting rotary rod type (the hand wheel and the valve stem rotate together to rise and fall, and the nut is set on the valve body). The stop valve is only applicable to full opening and full closing, and is not allowed to be adjusted or throttled.
The stop valve is a forced sealing valve, so when the valve is closed, pressure must be applied to the valve disc to force the sealing surface not to leak. When the medium enters valve 6 from below the valve clack, the resistance to be overcome by the operating force is the friction between the valve stem and the packing and the thrust generated by the pressure of the medium. The force to close the valve is greater than the force to open the valve, so the diameter of the valve stem should be larger, otherwise the valve stem will bend. In recent years, since the appearance of the self sealing valve, the medium flow direction of the stop valve has changed from the upper part of the valve disc to the valve chamber. At this time, under the effect of the medium pressure, the force to close the valve is small, while the force to open the valve is large, and the diameter of the valve stem can be reduced accordingly. At the same time, under the action of medium, this type of valve is also tight. It has been stipulated in the "Three Chemical Supplies" of valves in China that the flow direction of stop valves shall be from top to bottom.
When the stop valve is opened, when the opening height of the valve clack is 25%~30% of the nominal diameter, the flow has reached a greater value, indicating that the valve has reached the full open position. Therefore, the full open position of the stop valve shall be determined by the stroke of the valve disc.
The stop valve has the following advantages:
1. Simple structure, easy to manufacture and maintain.
2. The working stroke is small and the opening and closing time is short.
3. Good sealing performance, low friction between sealing surfaces, long service life.
The disadvantages of the stop valve are as follows:
1. The fluid resistance is large, and the force required for opening and closing is large.
2. It is not applicable to medium with particles, high viscosity and easy coking.
3. Poor regulation performance.
Bernard globe valves are divided into external thread type and internal thread type according to the position of stem thread. According to the flow direction of the medium, there are straight through type, direct current type and angle type. Globe valves are divided into packing seal globe valves and bellows seal globe valves.
Structure of stop valve:
The structural forms of the stop valve body include straight through type, direct flow type and right angle type. Straight through type is a more common structure, but its fluid resistance is greater. Direct flow fluid has less resistance and is mostly used for fluid containing solid particles or with high viscosity. The right angle valve body is mostly forged, which is applicable to the stop valve with smaller diameter and higher pressure.
The installation and maintenance of stop valve shall pay attention to the following matters:
1. The hand wheel and handle operated stop valve can be installed at any position of the pipeline.
2. Hand wheel, handle and hoist mechanism are not allowed to be used for lifting.
3. The flow direction of the medium shall be consistent with the arrow direction shown on the valve body.
Main standards of stop valve:
1. GB12233-89 Universal Valves - Iron Globe Valves and Lift Check Valves
2. GB12235-89 Flanged Steel Globe Valves and Lift Check Valves for General Purpose Valves
3. JB/T53174-94 Quality Grading of Globe Valves
4. JB/T53165-92 High Pressure Balanced Stop Valve
5. GB/T587-93 Marine Blue Bronze Stop Valves
6. GB/T590-93 Marine Flanged Cast Iron Stop Valves
7. GB8464-87 Internal Screw Connection Gate Valve, Globe Valve, Ball Valve, Check Valve, Basic Size, Iron Globe Valve
8. GB8465.2-87 Basic Dimensions of Internal Threaded Gate, Globe, Ball and Check Valves Iron Globe Valves
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