GET GATE VALVE WIKIPEDIA INFORMATION

Construction of Gate Valves

Common gate valves are actuated by a threaded stem that connects the actuator (e.g. handwheel or motor) to the gate. They are characterized as having either a rising or a non-rising stem, depending on which end of the stem is threaded. Rising stems are fixed to the gate and rise and lower together as the valve is operated, providing a visual indication of valve position. The actuator is attached to a nut that is rotated around the threaded stem to move it. Non rising stem valves are fixed to, and rotate with, the actuator, and are threaded into the gate. They may have a pointer threaded onto the stem to indicate valve position since the gate’s motion is concealed inside the valve. Non rising stems are used where vertical space is limited.

Gate valves can be actuated manually (handwheel) or electrically using a high torque motor.

The sliding gate (disc) can be wedge-shaped (tapered) or parallel-shaped. Wedge shape designs include solid, flexible, and split wedge designs.

Seat rings are used to make the replacement of a worn/leaking seat easier. Seat rings have a screw thread on the reverse side which can be screwed into the main valve body, the flat surface on the opposite side is the seating surface area that presses against the disc. If seat rings are not used, it is possible to machine a flat seat on the main valve body itself, unfortunately, this makes replacing the seat impossible and the seat can only be machined a few times before the entire valve must be replaced.

Gate valve bonnets are often constructed of cast iron. Cast iron is brittle and this makes the bonnet prone to cracking. Special care should be taken when handling and maintaining valves with cast iron bonnets.

As the stem penetrates through the valve bonnet, it is necessary to install a sealing gland to prevent leakage from occurring through the gap between the stem and bonnet; sealing is usually achieved using the fibrous packing material.

Flanges are installed on the suction and discharge side of the valve so that piping can be easily connected.

Gate Valve Types

The gate comes in various designs and technologies to produce effective sealing for differing applications.

Knife gate valve

A knife gate valve is used for thick fluids and dry bulk solids. The gate is only one piece of metal, which is typically pointed like a knife. These valves are self-cleaning as they pass the seat rings every time they open and close.

Wedge Gate Valve

A wedge gate valve has a gate in the shape of a wedge that sits on two inclined seats. In addition to the primary force created by fluid pressure, a high wedging force on the seats created by the stem tightening assists with the sealing. The wedge-shaped gate does not stick to the seat in case of high fluid differential pressure and has an increased service life due to less “rubbing” on the seats. However, wedge-shaped gate valves have an additional compression load on the seats that may result in thermal binding and restricted valve opening due to expansion.

Parallel Slide Gate Valve

A parallel slide gate valve has a flat gate and seats parallel to it. Parallel slide gate valves use line pressure and positioning to make a tight seal. Flat gates consist of two pieces and have a spring in the middle. The spring pushes the pieces towards the seats for enhanced sealing. Due to their inherent design, parallel gate valves have a safety advantage in higher-temperature applications. Furthermore, since there is no wedging action in parallel gates, closing torques are comparatively smaller, resulting in smaller, less expensive actuators or less manual effort. Due to their sliding into position, parallel gates keep dirt away from the seating surfaces.

Slab Gate Valve

Slab gates, also called through-conduit gate valves, are one-unit gates that include a bore-size hole. The bore is in line with the two-seat rings in the open state. This alignment creates a smooth flow with minimal turbulence. This unique design allows for minimal pressure loss in the system and is perfect for the transportation of crude oil and natural gas liquids (NGLs). The valve seats remain clean. However, the disc cavity can capture foreign material. Therefore, the cavity typically has a built-in plug for maintenance purposes of draining the accumulated foreign material.

Parallel Expanding Gate Valve

Expanding gate valves have two slab gates matched together that provide sealing through the mechanical expansion of the gate, as seen in Figure 5. When lifted, both of the slab gate’s cavities allow the media to flow. The upward force on one slab and the stoppage of the second slab, by a step in the valve body, allows for outward mechanical expansion for a proper seal. When closed, the slab gates block the media flow, and the downward force (stem) on one slab and upward force (step in valve body) allows for outward mechanical expansion for a proper seal.

These valves provide an effective seal simultaneously for both upstream and downstream seats. This seal makes them ideal for applications like isolation valves in power plants, block valves in process systems, and high-temperature valves in refineries.

Metal seated vs Resilient Seated Gate Valves

Before the resilient seated gate valve was introduced to the market, gate valves with a metal seated wedge were widely used. The conical wedge design and angular sealing devices of a metal seated wedge require a depression in the valve bottom to ensure a tight closure. Herewith, sand and pebbles are embedded in the bore. The pipe system will never be completely free from impurities regardless of how thoroughly the pipe is flushed upon installation or repair. Thus, any metal wedge will eventually lose its ability to be drop-tight.

A resilient seated gate valve has a plain valve bottom allowing free passage for sand and pebbles in the valve. If impurities pass as the valve closes, the rubber surface will close around the impurities while the valve is closed. A high-quality rubber compound absorbs the impurities as the valve closes, and the impurities will be flushed away when the valve is opened again. The rubber surface will regain its original shape by securing a drop-tight sealing.

The vast majority of gate valves are resilient seated, however, metal seated gate valves are still requested in some markets, so they are still part of our range for water supply and wastewater treatment.