What is the working principle of manual gate valve?

2025-04-18 09:16:39

A Manual gate valve is a device that uses a handwheel (or handle) to drive the valve stem, which in turn drives the gate to move up and down in a direction perpendicular to the fluid channel (axial direction), achieving valve opening, closing, and sealing. Its working principle can be analyzed from three aspects: structural drive, opening and closing process, and sealing mechanism:

1、 Core structure and power transmission

1. Drive components

Handwheel/Handle: By rotating the handwheel (large diameter handwheel can increase torque) or pulling the handle, human labor can be converted into rotational motion;

Valve stem:

The upper end is connected to the handwheel through a key or thread, and the lower end is hinged to the gate (such as T-shaped groove, dovetail groove or pin shaft connection);

The surface of the valve stem is processed with trapezoidal threads (in accordance with GB/T 5796 or API standards), which are matched with the internal threads on the valve cover to convert rotational motion into linear motion of the gate (when the handwheel is rotated, the valve stem rises and falls along the axis).

2. Guidance and Support

Valve cover: equipped with a guide sleeve or guide rail to limit the radial displacement of the valve stem and gate, ensuring the vertical lifting and lowering of the gate (guide error ≤ 0.1mm);

Packing box: The valve stem passes through the sealing structure at the valve cover (such as graphite packing+gland) to prevent medium leakage.

2、 Opening and closing process: gate movement controls the flow channel opening and closing

1. Opening process

Rotate the handwheel counterclockwise → the valve stem drives the gate to rise → the gate disengages from the sealing surface of the valve seat → the fluid channel is fully opened;

Fully open state: The gate is fully raised above the valve body (or embedded in the valve cover), and the flow path is unobstructed (such as the wedge Gate valve gate rising to the upper edge of the valve seat, and the parallel gate completely leaving the sealing surface of the valve seat). At this time, the flow resistance is minimized (resistance coefficient β ≈ 0.1~0.3).

2. Closing process

Rotate the handwheel clockwise → the valve stem pushes the gate down → the sealing surface of the gate is in contact with the sealing surface of the valve seat → the fluid channel is cut off;

Fully closed state: relying on the axial force (or medium pressure assistance) applied by the handwheel, the gate and valve seat generate sufficient sealing pressure (usually ≥ 1.5 times the medium pressure) to prevent medium leakage.

3、 Sealing mechanism: precise fit between gate and valve seat

1. Sealing secondary structure

Wedge seal (most commonly used):

The sealing surface between the gate and the valve seat is wedge-shaped (with an inclination angle of 3 °~6 °, as in GB/T 12234 Wedge gate valves). When closed, the gate wedges into the valve seat, and the inclined surface is used to convert axial force into radial sealing pressure;

Elastic wedge gate (with a middle cut or elastic arm) can adapt to slight deformation of the valve seat (such as valve body expansion caused by temperature changes), compensating for non parallelism of the sealing surface.

Parallel seal:

Double gate structure (two parallel gate plates), which compresses the valve seats on both sides through springs or medium pressure, suitable for low-pressure and large-diameter working conditions, relying on the parallelism of the valve seats on both sides (tolerance ≤ 0.02mm).

2. Sealing surface material

Hard sealing (metal to metal): such as valve seat welding with hard alloy (cobalt chromium tungsten alloy), gate surface quenching (HRC ≥ 55), suitable for high temperature and high pressure (such as above 300 ℃, PN ≥ 16MPa), reducing wear by relying on material hardness difference;

Soft seal (metal+non-metal): The valve seat or gate is embedded with rubber or polytetrafluoroethylene (PTFE), and when closed, the soft material deforms to fill the micro gap. The sealing ratio is low (only 0.5-1MPa), but the temperature resistance is ≤ 200 ℃.

4、 Key characteristics and adaptability to working conditions

1. Advantages

Low flow resistance: When fully opened, the gate completely separates from the flow channel, and the fluid is almost unobstructed (better than a Globe valve, whose butterfly valve disc generates vortices);

Bidirectional sealing: Wedge gate valves can achieve bi-directional pressure sealing (subject to API 598 bi-directional sealing test), while parallel valves typically have unidirectional sealing (subject to marking of medium flow direction);

Wide applicability: from low-pressure water pipelines (PN1.6) to high-pressure steam pipelines (PN160), with diameters ranging from DN15 to DN1200, can be designed.

2. Disadvantages

Long opening and closing stroke: The gate needs to be fully raised and lowered (stroke ≈ nominal diameter), and the opening and closing time is long (DN100 valve requires 50-100 turns of the handwheel rotation);

Easy to get stuck: Impurities in particulate media (such as sewage and mud) can easily embed into the sealing surface or guide gap, causing the gate to get stuck (it is necessary to design a sealing surface chamfer of ≥ 5 ° or add a blowing hole);

Not suitable for regulating flow: When the gate is half open, it is subject to high-speed fluid erosion, and the sealing surface is prone to wear (only for fully open/fully closed use).

5、 Examples of typical work scenarios

Tap water pipeline: manual wedge gate valve (soft seal), slowly opened and closed by hand wheel to avoid water hammer effect;

Steam boiler: hard sealed wedge gate valve (material WCB+welded STL), maintains sealing by wedge force at high temperatures, and the handwheel needs to be equipped with a locking device to prevent vibration loosening;

Wastewater treatment: Parallel double gate valve (with flushing structure), with a guide hole at the bottom of the valve seat. When closed, impurities are discharged with the fluid to reduce jamming.

Summary: Working principle, core logic

Human driven → Valve stem thread drive → Gate lifting → Sealing surface fitting/separation → Control flow channel on/off

Manual gate valves convert manual labor into sealing surface loads through mechanical transmission, and achieve leak free cutting through the precise fit between the gate and valve seat. The design core is to ensure that the gate moves vertically and the sealing surface is uniformly stressed during the opening and closing process, while adapting to the influence of working conditions such as medium pressure, temperature, and impurities.