1 Definitions and terms of gate valves

1. Definition of gate valve

Gate valve refers to a valve whose opening and closing parts (gate) move along the vertical direction of the medium flow direction (channel axis) to realize opening and closing.

Among various types of valves, gate valve is the most widely used one.

Gate valves are mainly used as cut-off media in pipelines, that is, fully open or fully closed. They can be applied to low temperature and low pressure, high temperature and high pressure, and can be used for various media according to different materials of valves.

2. Part name terms

The gate valve is mainly composed of valve body, bonnet, bracket, valve stem, valve stem nut, gate, valve seat, stuffing box, sealing packing, upper sealing seat, packing gland, transmission device and other parts.

The names of main parts are shown in the following figure:

3. Characteristics of gate valve

Gate valves have the following advantages:

? Low fluid resistance: the medium channel inside the valve body is straight, and the medium does not change its flow direction when flowing through the gate valve.

? The torque required for opening and closing is small;

? It can be used in the loop network pipeline where the medium flows in two directions, that is, the flow direction of the medium is unlimited.

? The shape structure is relatively simple and the manufacturing process is good.

? Good sealing performance: the sealing surface is less eroded when it is fully opened.

Disadvantages of gate valve:

? The required opening and closing time is long, the overall dimension and opening height are large, and the installation space is large.

? It takes a long time to open or close.

? During opening and closing, there is relative friction between the sealing surfaces, which causes great wear and tear, and it is even easy to cause scratch at high temperature.

? Generally, gate valves have two sealing pairs, which increases some difficulties in processing, grinding and maintenance.

2 classification of gate valves

According to the structural form, gate valves can be divided into the following types:

According to the valve stem structure, it can be divided into: rising stem gate valve (lifting stem gate valve) and Non rising stem gate valve (rotating stem gate valve).

Rising stem gate valve (lifting stem gate valve):

A gate valve that realizes the opening and closing of the ram by rotating the valve stem nut to drive the valve stem to rise and fall (without rotation).

The gate valve with this structure is beneficial to the lubrication of the valve stem. The opening of the gate valve is clear. The thread of the valve stem and the nut of the valve stem are not in contact with the medium, and are not affected by the temperature and corrosion of the medium.

Examples are as follows:

Non rising stem gate valve (rotary stem gate valve)

The valve stem nut is in contact with the medium in the valve body, and the gate valve can be opened and closed by rotating the valve stem.

The gate valve with this structure has a small height and size, but its opening and closing stroke is difficult to control. It needs to increase the indicator. The valve rod thread and valve rod nut are in contact with the medium, which is affected by the medium temperature and corrosivity. Therefore, it is suitable for non corrosive media and occasions with poor external environmental conditions.

Examples are as follows:

According to the structure of the gate, the gate valve can be divided into parallel gate valve and wedge gate valve.

Parallel gate valve:

Gate valves with sealing surfaces parallel to each other and perpendicular to the center line of the valve body channel. The hoist (RAM) can be a single ram or a double ram with a supporting mechanism.

The flow resistance of flat gate valve is small, and the flow resistance of flat gate valve without necking is similar to that of short straight pipe. The flat gate valve with diversion hole is installed on the pipeline and can also be pigged directly with a pig. At the same time, because the ram slides on the sealing surfaces of the two valve seats when opening and closing, it is suitable for media with suspended particles.

In addition, the flat gate valve will not overload the sealing surface due to the thermal elongation of the valve stem.

However, the flat gate valve has the following disadvantages:

a. When the medium pressure is low, the sealing force of the metal sealing surface is insufficient to achieve satisfactory sealing performance;

b. When the medium pressure is high, frequent opening and closing may cause excessive wear on the sealing surface;

c. The RAM will produce violent vibration when cutting off high-speed and high-density media;

d. The circular ram moving laterally on the circular channel is sensitive to the flow control only when it is at 50% of the closed position of the valve.

Wedge gate valve:

Gate valve with wedge-shaped sealing surfaces. The purpose of using wedge is to improve the auxiliary sealing load and realize the sealing of both high and low medium pressure.

The valve body of wedge gate valve is equipped with a guide mechanism (guide rib), which can prevent the ram from rotating when opening or closing, and ensure that the sealing surface of the ram and the valve seat are aligned accordingly.

At the same time, the wedge gate valve will not rub with the valve seat before closing, so as to reduce the wear of the sealing surface.

However, wedge gate valves have the following disadvantages:

a. The sealing surface is easy to contain solid particles in the flowing medium;

b. The thermal expansion of the valve stem may overload the sealing surface;

c. The ram cannot be set with a diversion hole.