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Gate Valve A Brief Introduction
Datetime: 2016/7/14 14:20:42  Hits: 14755

In this article you will find overall description of the gate valve, its design and application. 

The gate valve name is derived from the appearance of the disk in the flow stream.

Gate valves are mainly used to start or stop flow and when a minimum flow restriction is needed with straight-line flow of fluid. In general, this valve in service is either fully open or fully closed. 

WFE Cast Steel Gate Valves

The gate valve types and design

Gate valves are available with different disks or wedges. Four main types of discs are available in gate valves: solid-wedge disc, double disc - parallel faced, split-wedge disc, flexible-wedge disc. While parallel disks are used in valves having parallel seats, other three: solid, flexible and split wedges are used in valves having inclined seats. 

Figure 1. Gate valve

The solid wedge shown on Figure 2.1 is the most commonly used disk by its simplicity and strength. 

Flexible wedges (Figure 2.2) were developed specially to overcome sticking in high-temperature service with extreme temperature changes. The flexible wedge is a one-piece disk with a cut around the perimeter to improve the ability to match error or change in the angle between the seats. The cut varies in size, shape, and depth. A shallow, narrow cut gives little flexibility but retains strength, on the other hand, deeper and wider cut allows more flexibility but compromises strength.

Figure 2. Types of wedges:  

2.1. Solid wedge         2.2. Flexible wedge        2.3. Split wedge

The split wedge (Figure 2.3) is a two-piece disc that seats between matching tapered seats in the body. The spreader device for pressing the discs against the body seats is simple and integral with the disc halves. As the valve is opened, pressure on the disc is relieved before the disc is raised, thus avoiding friction and scoring of the seat. Other type of split wedge is one with a ball and socket joint that forces each disc to align itself against the body seat for tight closure. Gate valve with this wedge, should be installed with the stem in the vertical position

The double - parallel faced disc makes closure by descending between two parallel or tapered seats in the valve body. After parallel-faced double discs are lowered into position, they are seated by being spread against the body seats. A disc spreader makes contact with a stop in the bottom of the valve and forces the discs apart. 

Figure 2.4 Double disc – parallel wedge

Bonnets for gate valve come in different designs to satisfy various requirements: union bonnet, screwed bonnet, bolted bonnet, U-bolted bonnet, pressure seal bonnet.

Other classification of the gate valves is by the stem configurations, most common designs are: rising stem, outside screw and yoke (OS and Y), rising stem, inside screw, no rising stem, inside screw. 

Figure 3, rising stem OS and Y

For the OS (Outside Screw) and Y (Yoke) construction, the stem threads are outside the valve. When the hand wheel is rotated to open the valve, the stem-threading mechanism causes the stem to rise while the hand wheel remains in the same location.

Figure 4, nonrising stem inside screw

Operating principle

A gate valve is a linear motion valve. The function of the stem in a gate valve is to raise and lower the disc. 

On opening the gate valve, the flow path is enlarged in a highly nonlinear manner in relation to percent of opening, meaning that flow rate does not change evenly with stem travel. Also, a partially open gate disk tends to vibrate from the fluid flow. Most of the flow change occurs near shutoff with a relatively high fluid velocity causing disk and seat wear and eventual leakage if used to regulate flow. For these reasons, gate valves are not used to regulate or throttle flow.

Application, benefits of gate valve

A gate valve can be used for a wide variety of fluids and provides a tight seal when closed, however, it does not regulate or throttle flow.

Depending on wedge and bonnet type application of the valve vary:

- Most common solid wedge gate valve may be installed in any position and it is suitable for almost all fluids. It is practical for turbulent flow.

- Selection of the double disc is indicated where the application requires a tight seal to assure a leakproof shutoff. Valves with double discs are widely used in the waterworks and sewage fields and in the oil and gas industries.

- Gate valves with flexible wedges are mainly used in steam systems. 

- The simplest and cheapest design is the screwed bonnet. It finds its greatest application in low-pressure installations where shock and vibration are not present. It should not be used where frequent disassembly of the valve is required 

- When frequent dismantling for replacement or maintenance is anticipated the union bonnet configuration is a three-piece construction and is the preferred choice over the screwed bonnet.

- Bolted bonnets are easily disassembled with conventional wrenches. Bolted bonnets are suitable for rugged service and all pressures and temperatures. They are practical for larger valves but can be find on smaller valves too, whereas the screwed and union bonnets are generally found on small valves only.

Major disadvantages of gate valves

The major disadvantages to the use of a gate valve are:

- It is not suitable for throttling applications.

- It is prone to vibration in the partially open state.

- It is more subject to seat and disk wear than a globe valve.

- Repairs, such as lapping and grinding, are generally more difficult to accomplish.

Gate valve in service or during overhaul

This article is just a brief introduction to gate valves, if you have any question or need further information about plug valves or other valves, please contact us via mail sales@wfecn.com or direct call +86 (577) 5775 5599.

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