What is the structure of a manual gate valve?

2025-04-18 08:53:33

The structural design of Manual gate valves is centered on achieving reliable cutoff of medium flow. Its main components and structural characteristics are as follows, which can be analyzed in detail from three parts: core components, drive mechanism, and sealing system:

1、 Core structural components

1. Valve body

Function: To form the main framework of the valve, withstand medium pressure, and connect pipelines.

Structural features:

Usually cylindrical or spherical cavity with flanges, welded joints or threaded joints (small diameter) at both ends;

Materials are selected according to the working conditions, such as carbon steel (WCB) for ordinary working conditions, stainless steel (CF8/CF8M) for corrosive media, and alloy steel (15CrMo, F91) for high temperature and high pressure scenarios;

The internal flow channel adopts a straight through design, and when fully opened, the gate completely separates from the flow channel, resulting in extremely low flow resistance (resistance coefficient only 0.08~0.12).

2. Disc

Function: As an opening and closing component, it can cut off or conduct media through lifting and lowering.

Classification and Structure:

Wedge gate (most commonly used):

Single gate, the sealing surface is at a certain angle (usually 5 °~15 °) to the axis of the valve stem, and a wedge-shaped fit is used to press the valve seat to achieve sealing;

Suitable for high-pressure and high-temperature working conditions, the sealing surface can be welded with hard alloy (such as Stellite) to improve wear resistance.

Parallel gate:

Double gate structure, with two parallel gates and a spring or wedge block in the middle to provide sealing force;

Suitable for low-pressure, large-diameter pipelines (such as DN ≥ 500mm), with automatic compensation for worn sealing surfaces.

Movement mode: Lift and lower perpendicular to the fluid direction. When fully opened, the gate fully enters the chamber above the valve cover without contacting the medium.

3. Stem

Function: Connect the handwheel and gate to transmit opening and closing torque.

Category:

Rising Stem:

The valve stem has trapezoidal threads, which rotate and move up and down with the lifting and lowering of the gate during opening and closing;

The valve opening can be visually determined by the length of the valve stem extension, which is commonly used in situations where the status needs to be observed (such as pipeline main valves).

Non Rising Stem:

The valve stem thread is located inside the valve body and only rotates without lifting, while the gate moves along the guide groove on the valve stem;

Compact structure, suitable for space constrained scenarios such as underground pipelines and ship valves, but cannot directly observe the opening degree.

2、 Drive and operating mechanism

1. Handwheel/Handle

Function: Manually operate the component and drive the valve stem by rotating it.

Structure:

The center of the handwheel is equipped with an internal thread, which matches with the external thread on the top of the valve stem. When rotating, it is converted into linear motion of the gate plate;

Large diameter valve handwheels need to be equipped with extension rods or gearboxes to reduce operating torque (such as valve handwheels with a diameter of up to 400mm for DN300 and above).

2. Bonnet valve cover

Function: Seal the top of the valve body and secure the valve stem guide device.

Connection method:

Bolt connection (most common): fixed with flange bolts for easy disassembly and maintenance;

Welding type: used for high pressure and high temperature conditions (such as power station valves) to avoid the risk of bolt leakage.

3、 Sealing system

1. Seat Seal

Location: The annular surface where the valve body contacts the gate plate at both ends.

Form:

Soft seal: The valve seat or gate is covered with rubber, polytetrafluoroethylene (PTFE) and other materials, with excellent sealing performance (zero leakage), but temperature resistance ≤ 200 ℃;

Hard seal: Metal to metal seal (such as stainless steel overlay hard alloy), with strong temperature and pressure resistance (up to 570 ℃, 100MPa), allowing for trace leakage (in accordance with API 598 standard).

2. Packing Seal

Location: The annular packing box between the valve stem and the valve cover.

Structure:

Fill the stuffing box with flexible graphite, asbestos rope or polytetrafluoroethylene packing, and tighten it with gland bolts to prevent the medium from leaking along the valve stem;

Some high-pressure valves are equipped with double-layer packing or auxiliary sealing grease injection holes to improve sealing reliability.

4、 Summary of structural characteristics

Principle of opening and closing:

When fully open, the gate rises above the valve body and the flow channel is completely connected;

When closed, the gate descends to the valve seat and is tightly sealed by the medium pressure (self sealing) or handwheel torque.

Typical structural advantages:

Direct flow channel, low resistance, suitable for high flow rate and high flow medium;

The open pole design facilitates observation of the opening, while the concealed pole design saves space;

The hard sealed structure can adapt to harsh working conditions (high temperature, high pressure, granular media).

limitations:

The sealing surface of the gate and valve seat is susceptible to particle erosion and wear, and requires regular maintenance;

Large diameter valves require high manual operating torque and require auxiliary devices such as gearboxes.

5、 Structural adaptation to typical application scenarios

Low pressure water pipeline: using a rising rod wedge gate and soft seal, lightweight and reliable sealing;

High temperature steam pipeline: Use concealed alloy steel valve body and hard seal (welded with Stellite) to avoid valve stem oxidation and jamming;

Corrosive medium: All stainless steel valve body+PTFE soft seal, valve stem plated with hard chromium for corrosion prevention.

Through the above structural design, the manual Gate valve achieves the reliability and adaptability of the cut-off function. Its core is to meet the needs of different pipeline systems through the coordinated movement of the gate plate, valve seat, and valve stem, combined with the selection of materials and sealing forms.