Welded stop valve Structure and principle of
Globe valve The opening and closing parts of (stop valve, globe valve) are plug-shaped valve discs. The sealing surface is flat or conical, and the valve disc moves in a straight line along the center line of the fluid. The movement forms of the valve stem include the lifting rod type (the valve rod rises and falls, and the hand wheel does not rise and fall), and there are also the lifting and rotating rod type (the hand wheel and the valve rod rotate and lift together, and the nut is set on the valve body). The globe valve is only suitable for full opening and full closing, and adjustment and throttling are not allowed.
The globe valve is a forced sealing valve, so when the valve is closed, pressure must be applied to the disc to force the sealing surface not to leak. When the medium enters valve 6 from the lower part of the valve disc, the resistance to be overcome by the operating force is the friction force 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 rod bending fault will occur. In recent years, since the appearance of self sealing valve, the medium flow direction of the stop valve changes from the top of the valve disc into the valve chamber. Under the action of medium pressure, the force to close the valve is small, but the force to open the valve is large, so 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 more tight. In China, the "top-down" valve is used.
When the valve reaches the nominal valve opening height of 30%, it means that the valve has been opened at a high flow rate of 25%. Therefore, the full open position of the stop valve should be determined by the stroke of the disc.
Classification of globe valves
1. According to the channel direction of the stop valve, it can be divided into:
1) Straight through globe valve
2) Once through globe valve : in the direct flow or Y-type globe valve, the flow passage of the valve body and the main channel form an oblique line, so the damage degree of the flow state is smaller than that of the conventional stop valve, so the pressure loss through the valve is correspondingly small.
3) Angle globe valve In Angle Globe valve, the direction of fluid only needs to be changed once, so that the pressure drop through the valve is smaller than that of conventional globe valve.
4) Plunger type globe valve: this type of globe valve is a variant of conventional stop valve. In this valve, the disc and seat are usually designed based on the principle of plunger. The valve disc is polished into a plunger and connected with the valve stem. The sealing is realized by two elastic sealing rings which are set on the plunger. The two elastic sealing rings are separated by a ring and the sealing ring around the plunger is pressed firmly by the load applied on the valve cover by the bonnet nut. The elastic seal ring can be replaced and can be made of various materials. The valve is mainly used for "opening" or "closing". However, it is equipped with a special form of plunger or special sleeve ring, which can also be used to adjust the flow rate.
2. According to the position of thread on the valve stem, it can be divided into:
1) Globe valves with threaded stem on the outside of the valve body. The advantages of this structure are that the valve stem is not eroded by the medium and is easy to be lubricated.
2) Lower threaded stem globe valve: the thread of the stop valve stem is in the valve body. In this structure, the thread of the valve rod is in direct contact with the medium and is easy to be eroded and unable to be lubricated. This structure is used in small diameter and low temperature places.
The globe valve has the following advantages:
1. It is simple in structure and convenient in manufacture and maintenance.
2. The working stroke is small and the opening and closing time is short.
3. Good sealing performance, small friction force between sealing surfaces and 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 suitable for medium with particles, high viscosity and easy coking.
3. Poor regulation performance.
The types of 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. According to the sealing form, the stop valve has packing seal and bellows seal.
The structure of the stop valve is as follows:
The structure of globe valve body has straight through type, direct current type and right angle type. Straight through type is a common structure, but its fluid resistance is large. The resistance of direct flow fluid is small, and it is mostly used for the fluid with solid particles or high viscosity. The right angle valve body is mostly forged, which is suitable for the globe valve with smaller diameter and higher pressure.
The installation and maintenance of the stop valve should pay attention to the following matters:
1. The stop valve operated by hand wheel and handle can be installed in any position of pipeline.
2. Hand wheel, handle and large moving mechanism are not allowed to be used for lifting.
3. The flow direction of the medium should be consistent with the arrow direction shown in the valve body.