What is a gate valve?
Gate valves are commonly used across a wide range of applications and are suitable for both above-ground and underground installations. In particular, selecting the correct type of valve for underground use is crucial, as improper choices can lead to costly replacements and maintenance challenges.
Gate valves are intended for use in either fully open or fully closed positions. They are primarily installed in pipelines as isolation valves and are not suitable for flow regulation or control purposes. The valve is operated by rotating the stem, either clockwise to close (CTC) or clockwise to open (CTO). As the stem is turned, the gate travels up or down along the threaded section of the stem, allowing or blocking the flow of fluid through the pipeline.
Where Are Gate Valves Used?
Gate valves are commonly chosen in applications where minimal pressure loss and an unobstructed flow path are required. When fully open, the gate valve offers a clear passage with no flow restriction, resulting in very low pressure drop. This design also allows for the use of pipe-cleaning pigs to maintain pipeline cleanliness. As a multi-turn valve, gate valves operate via a threaded stem, requiring several rotations to move between open and closed positions. This gradual operation helps reduce the risk of water hammer, protecting the system from sudden pressure surges.
Gate Valves with By-Pass
By-pass valves are typically used for three main purposes:
To balance the differential pressure across the pipeline, reducing the torque needed to operate the main valve and enabling easier, single-person operation.
To allow continuous flow when the main valve is closed but the by-pass valve is open, preventing fluid stagnation.
To enable controlled, delayed filling of pipelines.
Parallel vs. Wedge-Shaped Gate Valves
Gate valves are generally classified into two main types: parallel and wedge-shaped. Parallel gate valves feature a flat gate that moves between two parallel seats. A common example is the knife gate valve, which has a sharp edge at the bottom of the gate for cutting through media. In contrast, wedge-shaped gate valves use two angled seats with a slightly tapered gate that fits between them, providing a tight seal through the wedging action.
Metal-Seated vs. Resilient-Seated Gate Valves
Before the introduction of resilient-seated gate valves, metal-seated designs were widely used. These valves rely on a conical metal wedge and angular sealing surfaces, which require a groove or recess at the bottom of the valve to achieve a tight seal. However, this design tends to trap sand, pebbles, and other debris in the flow path. No matter how thoroughly a pipeline is flushed during installation or maintenance, some impurities always remain. Over time, these particles can damage the metal sealing surfaces, causing the valve to lose its ability to maintain a drop-tight seal.
A resilient-seated gate valve features a flat valve bottom, allowing sand and pebbles to pass through freely. If impurities are present during closure, the flexible rubber surface compresses around them, maintaining a seal while the valve is closed. A high-quality rubber compound absorbs these particles during the closing process, and when the valve is reopened, the impurities are flushed out. The rubber then returns to its original shape, ensuring a reliable, drop-tight seal with each operation.
Gate Valves: Rising vs. Non-Rising Stem Design
Rising stem gate valves have the stem attached directly to the gate, so the stem moves up and down with the gate as the valve is operated. This provides a clear visual indication of the valve’s open or closed position and also allows easy access for lubricating the stem. In this design, a fixed nut rotates around the threaded stem to move it. Rising stem valves are suitable only for above-ground installations due to their space and visibility requirements.
Non-rising stem gate valves feature stems that are threaded into the gate assembly, rotating to drive the wedge-shaped disc vertically within the valve body. This design confines the stem entirely within the valve housing, significantly reducing vertical clearance requirements. These valves demonstrate exceptional versatility, with approved applications spanning both above-grade and buried installations. The threaded stem mechanism ensures reliable operation while maintaining compact dimensions, making them particularly suitable for space-constrained environments.