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The four valves introduced here are the most commonly used closing valves in the chemical industry. Their main function is to connect and cut off the medium flow in the pipeline. Although some of them can play the role of regulating the flow size, they can not be used as regulating valves completely.

Ball valve

This is a very comprehensive valve, improved on the cock valve and valve body center ball rotation control opening and closing, flexible action, small volume, light weight, medium flow resistance is small, good sealing effect.

Ball valve is divided into floating and fixed, the difference is mainly in the middle of the ball support mode, floating ball valve, ball under the side without support shaft, by the ball at both ends of the seat force support, ball work pressure will be slightly inclined to the export end seat, if the pressure is too high, it will aggravate the wear of the sealing layer. Fixed ball valve and the stem is fixed, the other end of the stem is connected with the bearing, after the work pressure, the ball will not be offset, supported by the bearing seat, seat sealing surface compression degree is not high, fixed internal structure is more complex, the price is relatively high.

Floating ball valve is generally used for DN200 below middle and low pressure pipeline, high pressure large diameter will choose fixed ball valve, although the ball valve in use can play a part of the adjustment function, but under normal circumstances is not recommended to use ordinary ball valve to adjust the flow, ball valve for a long time in part open state, will greatly reduce the service life of the ball valve, The impact of the medium will destroy the edge of the hole in the ball and the sealing layer of the seat, resulting in internal leakage.

Gate valve

As one of the most commonly used valves in the chemical industry, it is also the oldest valve with a wide range of uses. The closing key of the gate valve is the gate plate, which moves in the vertical direction of the flow of the medium in the pipeline and realizes the interrupt of the medium through the closure of the valve seat. The structure of the gate valve is simple. Wide range of application, stable and reliable sealing, durable.

But its stroke is large, opening and closing time is long, opening and closing must be fully open or fully closed, high size, need a certain space, gate valve wedge type gate valve and plate gate valve, their difference is mainly in the two sealing surface of the gate plate and valve body is in a parallel state, or there is a certain Angle wedge state.

The pressing force of the plate gate valve disc to the seat is provided by the medium pressure and the sputtering mechanism between the double gate plates. The sealing surface is always in a fit state during the opening and closing process, which can avoid the influence of particulate matter and suspended matter in the medium on the sealing surface and achieve self-cleaning. The medium pressure will affect the stress degree of the downstream sealing surface, and the high pressure and high flow rate can also seal well.

The sealing principle of wedge gate valve is achieved by combining wedge gate and sealing surface on the valve body. The sealing surface pressure comes from the downward pressure of medium and gate, because the wedge gate valve is slope, the sealing surface can be separated faster in the opening process, wear less, but it is easier to adhere to sediment, and the impurities of hard particles will damage the sealing surface. The cleanliness of the medium directly affects the valve life.

Gate valves have almost no regulating effect. Like ball valves, they can only be used as fully open or fully closed. The partially open state of the gate will cause significant vibration and impact, damaging the seat seal and reducing the service life.

Stop valve

The opening and closing keys of the globe valve are plug - shaped disc. The sealing surface is flat or conical, the valve flap up and down activity changes the distance between the valve flap and the seat, just like our water bottle plug, closed medium flow path, the valve stroke is short, the switch quickly wear-resistant, long life, single sealing surface, easy maintenance. Medium in the valve will cause changes in velocity and flow direction, fluid resistance, opening and closing process and medium pressure resistance, for large caliber, high pressure opening and closing more difficult.

The globe valve is directional, the medium flow from bottom to bottom can be easy to open, reduce the leakage of the packing, generally used for medium and low pressure pipelines. Medium flow from top to bottom will be easy to close, improve disc tightness, suitable for high pressure pipeline.

Globe valves are commonly used straight through and straight flow, their main difference is the flow of medium in the valve. The straight flow will allow the medium to lose less pressure.

The through-through type is the most common model, and its internal structure is similar to Z-shaped. They can partially adjust the flow rate in use. Compared with ball valves and gate valves, they have less wear on the sealing surface during adjustment and are suitable for small pipe diameters.

The direct flow has little influence on the flow direction of the medium, and is generally used when the pressure of the medium is required.

Butterfly valve

Butterfly valve through the center of the circular butterfly plate, with the stem rotation to open and close the fluid channel role, butterfly valve structure is simple, small size, light weight, large diameter, flexible and rapid operation, ordinary butterfly valve in low and medium pressure can achieve a good seal, relative to the other three kinds of valves, seal stability and durability slightly worse, but better adjustability.

There are many types of butterfly valves, but I generally only divide them into center butterfly valves and eccentric butterfly valves. One is adaptive seal based on the elasticity of sealing surface material, and the other is enhanced seal that can assist the application of external force, which is somewhat similar to the relationship between parallel gate valves and wedge gate valves.

Eccentric butterfly valve has a single eccentric, double eccentric, three eccentric, respectively for the sealing surface center and disc rotation center of the axial eccentric, valve flow to the center and disc rotation center of the radial eccentric and sealing surface Angle eccentric, these are design improvements, to improve the sealing, reduce wear purpose.

summarize：

1.Ball valve wide applicability, low pressure with floating, high pressure with fixed, as far as possible not to adjust the flow, unless it is a special regulator and V-shaped ball valve.

2.Gate valve is durable, wide applicability, but the operation time is relatively long, do not use the after-force lever when operating, the damage to the sealing surface is irreversible, and like ball valve, do not use to adjust the flow.

3.The globe valve is large to the fluid resistance, only recommended to the medium pressure loss is not required, small diameter use.

4.Butterfly valve, in fact, is also a more comprehensive valve, with the development of technology, butterfly valve sealing, durability, applicability have been greatly improved, flexible and adjustable operation, commonly used in low pressure large diameter pipeline.