Comprehensive explanation and definition knowledge of gate valves

1.Definition of gate valve

It is a kind of valve which is widely used in pipeline. It mainly plays the role of connecting and cutting off the medium. It is not suitable for regulating the flow rate of medium, but it can judge the flow rate according to the rise and fall of the stem (e.g. fire-fighting elastic seat gate valve with opening and closing scale). Compared with other valves, gate valves have a wide range of application for pressure, temperature, caliber and other requirements.

2. Gate Valve Structure

Gate valves can be divided into wedge type, single gate type, elastic gate type, double gate type and parallel gate type according to their internal structure.

According to the difference of stem support, it can be divided into open stem gate valve and dark stem gate valve.

3. Valve body and runner

The structure of gate valve body determines the connection between valve body and pipeline, valve body and valve cover. In terms of manufacturing methods, there are casting, forging, forging, casting and welding, and pipe plate welding. Forging valve body has developed to large caliber, while casting valve body has gradually developed to small caliber. Any kind of gate valve body can be forged or cast, depending on the user’s requirements and the manufacturing means owned by the manufacturer.

The flow path of gate valve body can be divided into two types: full-diameter type and reduced-diameter type. The nominal diameter of the flow passage is basically the same as the nominal diameter of the valve, and the smaller diameter of the flow passage than the nominal diameter of the valve is called the reduced diameter type. There are two kinds of shrinkage shapes: uniform shrinkage and uniform shrinkage. The tapered channel is a non-uniform diameter reduction. The aperture of the inlet end of this kind of valve is basically the same as the nominal diameter, and then gradually reduces to the minimum at the seat.

The advantages of using shrinkage runner (whether conical tube non-uniform shrinkage or uniform shrinkage) are the same size of valve, which can reduce gate size, opening and closing force and moment. The disadvantages are that flow resistance increases, pressure drop and energy consumption increases, so shrinkage hole should not be too large. For tapered tube diameter reduction, the ratio of the inner diameter of the seat to the nominal diameter is usually 0.8-0.95. Reduction valves with nominal diameter less than 250mm generally have a seat inner diameter one gear lower than nominal diameter; Reduction valves with nominal diameter equal to or greater than 300 mm generally have a seat inner diameter two gear lower than nominal diameter.

4. Movements of gate valves

When the gate valve closes, the sealing surface can be sealed only by the medium pressure, that is to say, only by the medium pressure to press the sealing surface of the gate to the seat on the other side to ensure the sealing surface, which is self-sealing. Most gate valves are forced to seal, that is, when the valve closes, the gate must be forced to the seat by external force to ensure the sealing surface.

Motion mode: The gate of the gate valve moves in a straight line with the stem, also known as the open bar gate valve. Usually there are trapezoidal threads on the lifting rod. Through the nut at the top of the valve and the guide groove on the valve body, the rotary motion is changed into linear motion, that is to say, the operating torque is changed into the operating thrust. When opening the valve, when the gate lifting height is equal to 1:1 times the valve diameter, the flow passage is completely open, but when running, this position can not be monitored. In practical use, the vertex of the valve stem is used as the sign, that is, the position of the valve stem that is not moving is used as its full open position. In order to consider the locking phenomenon of temperature change, the valve is usually opened to the vertex position and reversed to 1/2-1 turn as the position of the fully open valve. Therefore, the full open position of the valve is determined by the position of the gate (i.e. stroke). Some gate valve stem nuts are set on the gate plate. Handwheel rotation drives the stem to rotate, which lifts the gate plate. This kind of valve is called rotary stem gate valve or dark stem gate valve.

5. Performance advantages of gate valves

1. Valve fluid resistance is small, because gate valve body is straight-through, medium flow does not change direction, so the flow resistance is smaller than other valves;

2. Sealing performance is better than globe valve, and opening and closing is more labor-saving than globe valve.

3. Wide range of applications, in addition to steam, oil and other media, but also suitable for medium containing granular solids and high viscosity, also suitable for use as vent valve and low vacuum system valves;

4. Gate valve is a valve with double flow direction, which is not limited by the flow direction of medium. Therefore, gate valve is suitable for pipelines where medium may change the flow direction, and it is also easy to install.

6. Shortcomings of gate valve performance

1. High design dimension and long starting and closing time. When opening, it is necessary to lift the valve plate to the upper part of the valve chamber, and when closing, it is necessary to drop all the valve plates into the valve seat, so the opening and closing stroke of the valve plate is large and the time is long.

2. Because of the friction between the two sealing surfaces of the valve plate and the valve seat in the opening and closing process, the sealing surface is easy to scratch, which has an impact on the sealing performance and service life, and is not easy to maintain.

7. Performance comparison of gate valves with different structures

1. Wedge type single gate valve

A. The structure is simpler than the elastic gate valve.

B. At higher temperatures, the sealing performance is not as good as that of elastic gate valve or double gate valve.

C. Suitable for high temperature medium which is easy to coke.

2. Elastic gate valve

A. It is a special form of wedge type single gate valve. Compared with wedge gate valve, the sealing performance is better at high temperature, and the gate is not easy to be jammed after being heated.

B. Suitable for steam, high temperature oil products and oil and gas media, and for frequent switching parts.

C. Not suitable for easily coking medium.

3. Double gate gate valves

A. Sealing performance is better than wedge gate valve. When the inclination angle of sealing surface and seat fit are not very accurate, it still has good sealing performance.

B. After the sealing surface of the gate is worn out, the metal pad at the bottom of the top of the spherical surface can be replaced and used without surfacing and grinding the sealing surface.

C. Suitable for steam, high temperature oil products and oil and gas media, and for frequent switching parts.

D. Not suitable for easy coking medium.

4. Parallel gate valves

A. The sealing performance is worse than other gate valves.

B. Suitable for medium with lower temperature and pressure.

C. The processing and maintenance of sealing surface of gate and seat are simpler than other types of gate valves.

8. Cautions for Gate Valve Installation

1. Before installation, check the valve chamber and sealing surface. No dirt or sand is allowed to adhere.

2. Bolts in each connecting part should be tightened evenly.

3. Checking the filler position requires compaction, not only to ensure the sealing of the filler, but also to ensure that the gate opens flexibly.

4. Before installing forged steel gate valves, users must check valve type, connection size and media flow direction to ensure consistency with valve requirements.

5. When installing forged steel gate valves, users must reserve the necessary space for valve driving.

6. The wiring of the driving device shall be carried out according to the circuit diagram.

7. Forged steel gate valves must be maintained regularly. No random collision and extrusion are allowed to affect the sealing.

Post time: Sep-25-2019

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