General principle and installation of forced-seal valve external thread stop valve

General principle and installation of forced-seal valve external thread stop valve

The opening and closing part of the stop valve is a plug-shaped valve flap, and the sealing surface is a plane or a cone surface, and the valve flap moves linearly along the centerline of the fluid. The valve stem is in the form of a lifting rod (the stem moves up and down, the hand wheel does not lift), and there is also a lifting and rotating rod type (the hand wheel rotates and lifts together with the valve stem, and the nut is arranged on the valve body). Shut-off valves are only suitable for full opening and full closing and do not allow adjustments and throttling.

The shutoff valve is a forced seal type valve, so when the valve is closed, pressure must be applied to the valve flap to force the seal surface not to leak. When the medium enters valve 6 from below the valve flap, the resistance to be overcome by the operating force is the friction force of the stem and packing and the thrust generated by the pressure of the medium. The force of closing the valve is greater than the force of opening the valve, so the valve The diameter of the rod must be large, otherwise a failure of the stem bending may occur. In recent years, after the appearance of the self-sealing valve, the flow direction of the medium of the external thread stop valve was changed from the valve flap into the valve chamber. At this time, under the pressure of the medium, the force of closing the valve is small, and the force of the valve is opened. The diameter of the rod can be reduced accordingly. At the same time, this type of valve is also tighter under the influence of the medium. China's valve "three to give" has stipulated that the flow direction of the globe valve shall be adopted from top to bottom.

When the closing valve is opened, the valve opening height is 25% to 30% of the nominal diameter, and the flow rate has reached a large value, indicating that the valve has reached the fully open position. Therefore, the fully open position of the stop valve should be determined by the stroke of the disc.

Assortment of shutoff valves

1. According to the direction of the cut-off valve, it is divided into:

1) Straight-through stop valve

2) DC type stop valve: In the direct or Y type globe valve, the flow channel of the valve body is in a diagonal line with the main flow channel, so the damage degree of the flow state is smaller than the conventional stop valve, so the pressure loss through the valve is also corresponding. It's small.

3) Angle type stop valve: In the angle type stop valve, the fluid only needs to change the direction once, so that the pressure drop through this valve is smaller than that of the conventional structure.

4) Piston stop valve: This type of globe valve is a variant of a conventional globe valve. In this valve, the flap and seat are usually designed based on the plunger principle. The valve flap is ground to connect the plunger to the valve stem. The seal is achieved by two elastic seals that are placed on the plunger. The two elastomeric seals are separated by a collar and the seal around the plunger is secured by the load applied by the bonnet nut to the valve cap. Elastic seals can be replaced and can be made from a wide variety of materials. This valve is mainly used for "open" or "closed", but it is available with a special plunger or special collar, which can also be used to regulate the flow .

2. According to the position of the thread on the globe valve stem can be divided:

1) Upper threaded stem shut-off valve: The stem of the shut-off valve stem is outside the valve body. The advantage of this is that the valve stem is free of media and is easy to lubricate. This type of structure is more common.

2) Lower threaded stem shut-off valve: The stem of the shut-off valve stem is in the valve body. This type of valve stem thread is in direct contact with the media and is susceptible to erosion and cannot be lubricated. This kind of structure is used in places with small diameter and low temperature.

Globe valves have the following advantages:

1, simple structure, manufacturing and maintenance more convenient.

2, the work schedule is small, opening and closing time is short.

3, good sealing, sealing surface grinding

Low friction, long life.

The disadvantages of the globe valve are as follows:

1. The fluid resistance is large, and the force required when opening and closing is large.

2, does not apply to particles, viscosity, easy to coke the medium.

3, poor regulation performance.

The type of globe valve is divided into male thread type and female thread type according to the position of the stem thread. Divided by the flow direction of the medium, straight-through, direct-flow, and angular. The cut-off valve is divided according to the form of the seal. There are packing seal external thread shut-off valve and bellows sealed shut-off valve.

Shut-off valve structure:

The body of the valve body has straight-through, direct-flow and right-angle types. Straight-through is a common structure, but its fluid resistance is large. The DC type fluid has small resistance, and it is mostly used for fluids containing solid particles or with high viscosity. The right-angle valve body is mostly forged and is suitable for smaller pass-by valves and higher pressure shut-off valves.

For the installation and maintenance of globe valves, please note the following:

1. The handwheel and handle operated stop valve can be installed anywhere on the pipe.

2. Handwheels, handles and vigor mechanisms shall not be used for hoisting.

3, the direction of flow of the medium should be consistent with the direction of the arrow shown in the valve body.