Common chemical valve use method and principle


Common chemical valve use method and principle

The valve is an important accessory for controlling the flow of the medium on the chemical pipeline. This lecture focuses on the practical application of the valve. For the detailed principle and structural cross-sectional view, please refer to the relevant information books. The valve consists of three parts: the valve body, the opening and closing mechanism, and the valve cover. This time It focuses on the structural characteristics of the company’s commonly used valves and the precautions for correct use.
1. The role of the valve
1. Opening and closing function – cut off or communicate the flow of fluid in the pipe;
2. Adjustment function – adjust the flow and flow rate in the pipe;
3. Throttle effect – make the fluid pass through the valve to produce a large pressure drop;
4. Other functions – a. Automatic opening and closing b. Maintaining a certain pressure c. Steam blocking and drainage.

Second, the type of valve
1. According to the purpose: cut-off valve, regulating valve, diverter valve, check valve, safety valve; *z1i9N:r’W%}!j.~2, according to the force: other action valve , Automatic valve.

Third, the main parameters of the valve
PN nominal pressure (the larger pressure that allows fluid to pass);
DN nominal diameter;
TN temperature range (temperature range of allowable fluid).

Fourth, the valve inspection before use
Inspection items include:
1. Whether there are blisters, cracks and other defects on the inner and outer surfaces of the valve body;
2. Whether the connection between the valve seat and the valve body is firm, whether the valve core and the valve seat match, and whether the sealing surface is defective;
3. Whether the connection between the valve stem and the valve core is flexible and reliable, whether the valve stem is bent, whether the thread is damaged or corroded;
4. Whether the packing and gasket are aging and damaged;
5. Whether the valve opening is flexible, etc.

5. Problems that often occur during the use of valves
1. Flange and thread leakage at the connection with the pipeline;
2. Packing culvert leakage, waist pad leakage and valve stem can not be opened;
3. The valve core and valve seat are not tightly closed to form internal leakage.

6. Characteristics of valves commonly used in chemical industry and precautions for their use
(1) Plug valve (cock)
Features: TN is less than 150 degrees, PN is less than 1.6Mpa, it has the advantages of simple structure, rapid opening and closing, convenient operation, small fluid resistance, etc.

Precautions for use:
1. The outer end of the valve stem is square, the straight line marked on the diagonal is perpendicular to the direction of the valve body, it is in the closed state, and it is in the open state when it is consistent with the direction of the valve body;
2. Cork’s special wrench is used for normal switching valves to avoid safety accidents caused by slipping with the valve stem; try not to use adjustable wrenches to cause slipping;
3. Open the valve and check according to the previous inspection items. After the inspection, slowly open the valve. When opening, try not to stand in the direction of the sealing surface. Wear an acid-proof mask when encountering acid and alkali fluids;

The ball valve and the plug valve are the same type of valve, but the opening and closing part is a sphere with a through hole, and the sphere rotates around the center line of the valve stem to achieve the purpose of opening and closing.
Insulated ball valve with jacket
Stainless steel ball valve
Stainless steel ball valve quick opening ball valve
Features: The valve is simple in structure and reliable in operation. It is used in the pipeline of bidirectional flow medium, with low fluid resistance and good sealing performance; Disadvantages: The medium is easy to leak from the valve stem.

Precautions for use:
The same as the plug valve;
Valve with handle, the handle perpendicular to the flow direction of the medium is in the closed state, and the direction consistent with the direction is in the open state;
If you encounter a ball valve with jacket insulation, you should pay attention to the following matters:
The jacket insulation steam should be turned on to melt the easy-to-crystallized medium in the valve before opening and closing the valve. Do not force the valve to open and close before the medium is completely melted;
When the valve cannot be opened, the method of lengthening the lever arm cannot be used to forcibly open the valve, because this will cause the valve stem to be greatly resisted and the valve core to fall off, resulting in damage to the valve or damage to the wrench, thus causing unsafe factors. .

(3) Butterfly valve
The butterfly valve uses a disc that can rotate around the shaft to control the opening and closing of the pipeline, and the size of the corner reflects the opening degree of the valve.
According to different transmission modes, butterfly valves are divided into three types: manual, pneumatic and electric. The most commonly used is manual. The rotary handle drives the valve stem through gear transmission to open and close the valve.
Features: Butterfly valve has the advantages of simple structure, quick opening and closing, small fluid resistance, convenient maintenance, etc., but it cannot be used in high temperature and high pressure occasions, PN is less than 1.6Mpa, t is less than 120 degrees for large-diameter water, steam,
Air, oil and other pipelines.

Precautions for use:
1. The valve core can only be rotated 90 degrees. Generally, the valve body will indicate the direction of the CLOSE and OPEN arrows. Turn the handwheel clockwise to close, and vice versa to open;
2. If there is a certain resistance to opening and closing, the valve can be opened with a special F wrench, but it cannot be forced to open or close, otherwise the valve stem gear will be damaged;
3. It is forbidden to remove the handwheel and use the adjustable wrench to move the valve stem;
4. When opening and closing, gradually open and close, observe whether there is any abnormal situation, and prevent leakage.

(4) Globe valve
Globe valve is a type of cut-off valve widely used in chemical production. Compared with the above three cut-off valves, it does not use the rotation of its closing parts to open and close the valve, but uses the lift of the valve stem to drive the circular valve connected to it. Valve disc (valve head), change the distance between the valve disc and the valve seat to control the opening and closing of the valve.
Streamlined globe valve American standard globe valve globe valve is a kind of cut-off valve widely used in chemical production.
Features: The upper part of the globe valve has a handwheel and a valve stem, and the middle part has a thread and a packing culvert sealing section. The thread on the valve stem of a small valve is in the valve body. It can be judged from the height of the valve stem protruding from the valve cover
The structure of the globe valve is more complicated, but the operation is simple and effortless, it is easy to adjust the flow and cut off the channel, and the opening and closing are slow and there is no water hammer phenomenon, so it is widely used.
When installing the globe valve, pay attention to the fluid direction. The pipeline fluid should flow through the valve seat port from bottom to top, and the so-called “low in and high out”, the purpose is to reduce the fluid resistance, so that the opening and closing state of the valve stem, packing culvert Some parts are not in contact with the medium to ensure that the valve stem and packing culvert will not be damaged and leaked.
The globe valve is mainly used for pipelines of water, steam, compressed air and various materials. It can accurately adjust the flow rate and cut off the channel tightly, but it cannot be used for materials with high viscosity and easy crystallization.

Precautions for use:
1. Check whether the valve is defective before opening, especially whether the packing culvert is leaking;
2. When the valve stem cannot be rotated directly by hand, it can be opened and closed with a special F wrench. When it still cannot be opened and closed, please do not lengthen the lever arm of the wrench to force the opening and closing, which will cause damage to the valve or cause safety accidents;
3. When it is used for medium pressure steam pipeline valve, the condensed water in the pipe should be drained first when opening, and then the valve should be opened slowly with 0.2 to 0.3Mpa steam to preheat the pipeline to avoid sudden pressure increase. If the sealing surface is damaged, adjust the pressure to the required state after the inspection is normal;

(5) Gate valve
Gate valve is also known as gate valve or gate valve. It controls the opening and closing of the valve through the lifting and lowering of the gate. The gate is perpendicular to the direction of the fluid, and the size of the channel can be changed by changing the relative position between the gate and the valve seat.

Rising stem gate valve Dark stem gate valve
According to the different movement of the valve stem when the gate valve is opened and closed, the gate valve is divided into two types: open rod type and dark rod type.
The stem thread of the rising stem gate valve is exposed outside the valve body. When the valve is opened, the stem extends out of the handwheel. Affected by medium corrosion, the disadvantage is that the height of the overhang space is large.
The stem thread of the concealed stem gate valve is to be matched with the inner thread of the gate plate inside the valve stem. When the valve is opened, the valve stem only rotates without lifting up and down, and the gate plate rises along the valve stem thread. The advantage of the concealed stem gate valve is that the extension space is small, and the disadvantage is that the opening of the valve cannot be judged according to the condition of the valve stem, and the thread of the valve stem is easily corroded in contact with the medium for a long time.
The gate valve has the advantages of small fluid resistance, constant medium flow direction, slow opening, no water hammer phenomenon, and easy flow adjustment.
1. When the valve stem opens and closes in place, no more force can be used, otherwise the internal thread or bolt screw will be broken and the valve will be damaged;
2. When opening and closing the valve, the F wrench can be used to open and close it when the hand cannot directly open it;
3. When opening and closing the valve, pay attention to observe the sealing surface of the valve, especially the packing gland to prevent leakage.

(6) Throttle valve
Throttle valve, also known as needle valve, its shape is similar to globe valve, its spool shape is different, it is cone or parabola, it is often used in chemical instrumentation, often with threaded connection.
Precautions for use:
1. Because of the threaded connection, first check whether the threaded connection is loose and leaking when opening and closing;
2. When opening and closing the valve, proceed slowly, because the flow area is small and the flow rate is large, which may cause corrosion of the sealing surface. Pay attention to the change of pressure.

(7) Check valve
The check valve is a valve that automatically opens and closes by using the pressure difference of the medium before and after the valve to control the one-way flow of the medium, also known as a check valve or a one-way valve.
Check valves are divided into two types: lift type (beating type) and swing type (rocker type) according to different structures.

lift swing
Precautions for use:
Pay attention to the direction of the valve, the arrow is consistent with the flow direction of the medium, if the medium is easy to crystallize, it may cause the valve plate to be unable to be pressed down, and it will not play the role of command and non-return.
(8) Safety valve
The safety valve is a valve that automatically opens and closes according to the medium pressure. When the medium pressure exceeds a fixed value, it can automatically open the valve to discharge the pressure, so that the equipment pipeline is not damaged, and can be automatically closed after the pressure returns to normal.
According to the different ways of balancing the internal pressure, the safety valve is divided into two types: lever weight type and spring type.
Precautions for use:
1. The use of safety valve must be within the validity period of validity;
2. The safety valve control valve installed on the pipeline and equipment is usually a stop valve, which must be opened to ensure that the safety valve can work effectively;
3. Regularly lift the valve disc slightly, and use the medium to blow the impurities in the valve.
4. If the safety valve cannot work within the set pressure, it must be re-validated or replaced

(9) Traps
Trap valve is a valve that can automatically and intermittently remove condensed water in steam pipelines, heaters and other equipment systems, and can also prevent steam from leaking. Commonly used are bell float type, thermodynamic type and pulse type.

Precautions for use:
1. Use the pipeline bypass valve to remove the condensed water before use. When there is steam, close the bypass and use the steam trap to the right path. Otherwise, the water in the valve will be closed and the drainage effect will not be achieved;
2. Be careful not to be scalded by steam when opening and closing the valve.

(10) Various sampling valves
Sampling valve, as its name implies, is a valve used to obtain medium samples for chemical analysis. Generally installed on equipment or pipelines, it can be roughly divided into the following types:
Double open valve, flanged clamp valve, and sampling valve with insulation jacket;

Use caution method:
1. The double-opening valve is generally composed of two ball valves, which can achieve the purpose of sampling safety and through linkage in the negative pressure device; when sampling, first close the second valve near the equipment and pipeline, and open the first valve. , let the medium flow into the space between the two valves; then close the first valve, open the second valve, and place the sampling vessel in the sampling port to hold the medium;
2. The flange clamp valve is generally sealed by the cone at the top of the valve stem and the cone hole of the valve seat. When sampling, the valve stem and the cone hole are separated by rotating the handwheel, so that the medium can flow from the cone hole to the external sampling vessel;
3. The sampling valve with jacket insulation should pay attention to the following matters:
I should open the jacket insulation steam to melt the easy-to-crystallize medium in the valve before opening and closing the valve, and do not force the valve to open and close before the medium is completely melted;
Π When the valve cannot be opened, the method of lengthening the lever arm cannot be used to forcibly open the valve, because this will cause the valve stem to be greatly resisted and the valve core to fall off, or cause the valve stem and the sealing surface of the cone hole to be damaged, resulting in The valve is damaged or the wrench is damaged, causing unsafe factors.

(11) Fluoroplastic rubber-lined valve
The fluoroplastic rubber-lined valve is mainly used in corrosive media such as acid and alkali. Its structural principle is similar to that of the non-fluoroplastic valve, except that the valve stem, valve core and valve seat are all fluoroplastic substrates, and its use method is similar .

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