Gate Valve - How They Work

Dec. 16, 2024

Gate Valve - How They Work

Learn About Gate Valves

Figure 1: Gate valve

A gate valve is designed to control fluid flow by either fully opening or closing the passage. It's called a gate valve because it uses a flat or wedge-shaped gate to block or allow flow. The gate moves perpendicular to the flow path, similar to how a gate opens and closes to control passage. A gate valve's primary advantage is the straight, unobstructed path it provides, minimizing pressure loss and allowing for pipeline cleaning with pigs, unlike butterfly valves. While slower than quarter-turn valves, gate valves are ideal for applications requiring infrequent operation and should not be used for throttling. They are available in manual versions, which are cost-effective, or automated with electric or pneumatic actuators for more convenience.

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Gate Valves

Diagram of a gate valve and its parts

As seen in Figure 2, a gate valve has seven main parts: handwheel (A), stem (B), gasket (C), bonnet (D), valve body (E), flange (F), and gate (G). A flanged gate valve or threaded gate valve is the most common connection type to connect the valve to an application. In addition, depending on the specific design and application, the handwheel, stem, bonnet, and gate can have different designs to accommodate different applications. However, the main function of the gate valve parts remains the same. Continue reading to find those sections.

Figure 2: Gate valve parts: handwheel (A), stem (B), gasket (C), bonnet (D), valve body (E), flange (F), and gate (G).

Gate valve symbol

The gate valve symbol has two triangles pointing towards the center of a vertical line, as seen in Figure 3. This figure is commonly used in piping and instrumentation diagrams (P&IDs). Read our valve symbols article for more information.

Figure 3: Gate valve symbol

Operation

A gate valve operates similarly to other valves. To open the valve, turn the handwheel (Figure 2 labeled A), which moves the gate (Figure 2 labeled G) up or down on the stem (Figure 2 labeled B) via the threads. A gate valve requires more than one 360° turn of the handwheel to open or close the valve fully. When the gate is lifted up, it opens the inlet to the outlet allowing an unobstructed passageway for the media to flow. When the gate is lowered, it closes and blocks the media flow.

The relationship between the gate's vertical travel and the flow rate is nonlinear for a gate valve, with the greatest changes occurring near complete closure. When used to regulate flow, the relatively high velocity of the flow at partial opening results in gate and seat wear, which, along with possible vibrations of the gate, shortens the valve's service life. Therefore, a gate valve should only be used for on/off control.

A gate valve is best for on/off control with minimal pressure drop when fully open. It is not suitable for throttling. In comparison, ball valves are excellent for on/off control with low-pressure drop and quick operation. They can be used for regulating flow but are not as precise as other valve types, such as needle valves. Read more in our gate valve vs. ball valve article.

A globe valve is best for precise flow control and throttling, but has a higher pressure drop. More about the differences between globe valves vs. gate valves can be found in our article.

Finally, butterfly valves are versatile for on/off and throttling applications, with a compact design and low-pressure drop. For more information, read our article on gate valve vs. butterfly valve. Each type of valve has its own advantages and is suited for different applications based on the system's specific requirements.

Gate valve actuation method

There are three main types of gate valve actuation methods:

Manual gate valve: A manual gate valve has a manually spun handwheel to open or close the valve, as shown in Figure 2. This requires a user on-site to spin the wheel. A manual gate valve is the most economical, especially since the usage of gate valves, once installed, is typically low.
Pneumatic gate valve: A pneumatic gate valve uses a pneumatic actuator instead of a handwheel. By utilizing compressed air, the actuator can spin the stem to raise or lower the gate. This allows it to be operated remotely with no user on-site but requires a pneumatic system on site.
Electric gate valve: An electric gate valve uses an electric motor instead of a handwheel. By utilizing electrical power, the actuator spins the stem to raise or lower the gate. This allows it to be operated remotely with no user on-site but requires electrical power on-site. These are also referred to as motorized gate valves.

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, as seen in Figure 4, Labeled A. In addition to the primary force created by fluid pressure, a high wedging force on the seats created by the stem's 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.

Figure 4: Wedge gate valve (A) vs parallel gate valve (B)

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 valves

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 gates

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) allow 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.

Figure 5: Expanding gate functioning with the closed position (A) and the open position (B)

Bonnets

The bonnet of a gate valve protects its internal parts by creating a leak-proof seal. Therefore, it is removable for repair or maintenance purposes. Depending on the application, gate valves can have screw-in, union, bolted, or pressure seal bonnets.

Screw-in bonnets

Screw-in bonnets are the simplest in construction, as shown in Figure 1. They are common in small valves and provide a durable leak-proof seal.

Union bonnets

Union bonnets are held in place by a union nut. The union nut sits on the lower edge of the bonnet and screws into the valve body threads. This type of design ensures that the leak-proof seal created by the nut does not deteriorate by frequent removal of the bonnet. Therefore, union bonnets are common for applications that require regular inspection or maintenance.

Bolted bonnets

Bolted bonnets provide sealing in larger valves and higher-pressure applications.

Figure 6: Bolted bonnet gate valve

Pressure seal bonnets

Pressure seal bonnet gate valves are ideal for high-pressure applications (more than 15 MPa). They have a downward-facing cup inserted into the valve body. When internal fluid pressure increases, the cup is forced outward, improving the seal.

Stem design

The gate is raised and lowered by the spinning of a threaded stem (Figure 2 Labeled B). As discussed, a manual wheel or actuator spins the stem. Depending on the design, it is either considered a rising stem gate valve or a non-rising stem gate valve. So, as you spin the stem, it either raises or stays in place with the spin.

Outside Screw and Yoke (OS&Y), also referred to as rising stems, are fixed to the gate. Therefore, the threads are on the actuation side. So, as the gate is raised or lowered, the stem moves with it up and down. Consequently, they have built-in visual indicators of the state of the valve and are easily lubricated. Since they have moving components, they cannot be used with bevel gears or actuators. Therefore, rising gate valves are suitable for manual actuation.

On the other hand, a non-rising stem is fixed to the actuator and threaded into the gate. An indicator is often threaded onto the stem to show the valve's open or closed state. Non-rising gate valves are common in underground installations and applications with limited vertical space.

Applications

Gate valves have numerous industrial and residential applications.

Slurries: Gate valves are often used in applications that contain slurries as their media. This is because a gate valve has an unobstructed passageway for the fluid, so the slurry can easily pass through the valve. In addition, knife gate valves can easily cut right through the slurry to close

Viscous media: Gate valves are commonly used for viscous media like light grease and oils. The unobstructed passageway allows these media to flow easily. The valve can be pigged, a common cleaning method for these applications. In addition, valves for these applications are typically on or off for long periods of time.

Water gate valves: For water applications, water gate valves are commonly used since flow control is typically not done for these applications. The valve can be fully open or fully closed, allowing for proper water control.

FAQs

What is a gate valve?

A gate valve is a control valve that either allows media to flow through unobstructed or stops the fluid flow.

How does a gate valve work?

A gate valve works by rotating the stem (manually or with an actuator) to raise or lower a gate. The gate either allows unobstructed fluid flow or stops it.

What are gate valves used for?

Gate valves are used to allow for unobstructed fluid flow or to stop the fluid flow.

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Gate Valves

The Fundamentals of Gate Valves

Gate valves are a product of the industrial revolution. While some valve designs such as the globe and plug valve have been around longer, the gate valve dominated the industry for many decades, only recently ceding substantial market share to ball and butterfly valve designs.

The gate valve differs from ball, plug and butterfly valves in that the closure element, called the disc, gate, or obturator, rises on the base of a stem or spindle out of the waterway and into the valve top, called the bonnet, by means of multiple turns of the spindle or stem. These valves that open with a straight-line motion are also called multi-turn or linear valves and differ from quarter-turn styles, whose stems rotate 90 degrees and generally don&#;t rise.

An example of the original wedge-style gate valve designed by James Nasmyth in . This one dates from -. Photo credit: Greg Johnson

Gate valves are available in dozens of different materials and several pressure classes. They range in size from fit-in-your-hand NPS ½ inch, through big-as-a-truck NPS 144 inch. Gate valves are constructed of castings, forgings or weld-fabricated assemblies, although casting designs dominate.

One of the most desirable aspects of gate valves is their ability to open fully and leave the flow bore virtually free of encumbrances or friction. An open gate valve offers about the same amount of resistance to flow as a section of pipe of the same port size. As a result, gate valves are still strongly considered for blocking or on/off applications. In some valve nomenclature, a gate valve is called a block valve.

Gate valves are generally bad choices for regulating flow or operating in any orientation other than fully open or fully closed. Using a partially open gate valve for throttling or regulating flow can result in either damage to the disc or body seat rings, due to the seating surfaces banging against one another in the partially open, turbulence-inducing flow environment.

GATE VALVE STYLES

From the outside, most gate valves look somewhat similar. However, inside there are a host of different design possibilities. Most gate valves consist of a body and bonnet that contains a closure element, called a disc or a gate. The closure element is attached to a stem that passes through the bonnet of the valve, ultimately interfacing with a handwheel or other actuation device to operate the stem. Pressure around the stem is contained with a packing material that is compressed into a packing area or chamber.

Gate valves generally have a low total cost of ownership. They are relatively easy to manufacture and are easy to repair. Photo credit: Greg Johnson

The motion of a gate valve&#;s disc upon the stem dictates whether the stem rises during opening or threads into the disc. This reaction also defines the two major stem/disc styles of the gate valve: the rising stem or the non-rising stem (NRS). The rising stem is the overwhelmingly popular style of stem/disc design for the industrial market, while the non-rising style has merited longtime favor with the waterworks and plumbing industry segments. Some marine applications where gate valves are still used and space is tight, also utilize the NRS style.

For more information, please visit large size gate valve.

The most common stem/bonnet design in use on industrial valves is the outside screw and yoke (OS&Y). The OS&Y design is preferred for corrosive environments because the threads are outside the fluid containment area. It also differs from other designs in that the handwheel is attached to a bushing at the top of the valve yoke, and not to the stem itself, thus the handwheel does not rise as the valve is opened.

GATE VALVE TRIM

The word &#;trim&#; is often overheard when valve professionals are talking about industrial gate valves. Trim has nothing to do with how slim and fit a valve is; rather, it refers to the internal components of a valve that are exposed to great stress or subject to a harsh combination of erosion and corrosion. In a gate valve, the trim components are the stem, disc seating area, body seats and backseat, if applicable. Common utility bronze or brass valves usually have trim parts of the same material as the body and bonnet. Cast and ductile iron valves have either all iron trim components or occasionally bronze trim. The term for an iron valve with bronze trim is &#;iron body, bronze mounted&#; (IBBM).

This is one of the eight 90-inch gate valves located in the bowels of Hoover Dam. Photo credit: Greg Johnson

Steel valves can be furnished with a number of different trims. Stellite, Hastelloy, 316ss, 347ss, Monel and Alloy 20 are some of the materials regularly used for gate valve trim.

DISC DESIGN

The heart of the gate valve is the closure element, which can be of two designs, either the wedge or the parallel seat. The wedge design is the most popular and has been around since invented by famous British engineer James Nasmyth in . The wedge style utilizes the slightly angled disc mating with the same angled valve body seats to affect a tight closure. These valves are seated by applying torque to push the disc firmly into the seats. Three types of wedge disc are available:

The solid wedge has been around the longest and at one time virtually all wedge gates were the solid type. The drawback to a solid design is that it does not have any flexibility and if there is any valve body/seat distortion due to extreme temperature fluctuations or pipe stresses, the solid disc can become jammed in the seats. The solid disc is still standard on bronze, cast iron, water service and compact carbon steel valves (API 602 type).
The flexible wedge type is just that: flexible. By the addition of a groove or slot around its periphery, the flexible disc can adapt to temperature changes and adverse piping stresses without binding. The flexible design also is a little easier to manufacture in that minor imperfections in the seating surface angles can be compensated for by the disc&#;s flexibility. The flex-wedge design is by far the most common type seen on commodity gate valves used in industrial applications.
The split wedge type consists of a two-piece design with mating surfaces on the back side of each disc half. These mating surfaces allow the downward stem thrust to be uniformly transferred to the disc faces and onto the seats. This flexible design also provides protection against jamming due to thermal expansion. A disadvantage to the split design is that in dirty services, residue or debris can cake in between the disc halves, causing the valve to improperly seat or even jam. Split wedge designs are commonly found on stainless steel and high alloy valves, as well as many small bronze valves.

Space is at a premium on ships and NRS gate valves have been used in these applications for decades because they require less room than OS&Y gate valve designs. Photo credit: Greg Johnson

Wedge gates are guided by grooves or ribs cast or welded into the body of the valve. These wedge guides keep the disc in alignment as it opens or closes and also keeps the disc from sliding against the downstream seat during opening and closing.

The other gate valve disc style is the parallel seat design. Parallel seats may be spring loaded to provide for a tighter seal or create positive sealing in the upstream direction. Parallel seated valves are position seated, in that the position of the disc dictates the sealing efficacy, and not the amount of force (torque) applied to the disc by the stem.

BODY/BONNET DESIGN

Gate valves generally are made of two principal parts: the body and the bonnet. These comprise the pressure-containing envelope of the gate valve. There are a variety of designs for the interface of these two components.

The screwed joint is the simplest design. However, it is only used for inexpensive, low-pressure bronze valves.
The union joint is also primarily used on bronze valves, but the union design allows for easier disassembly for repair and maintenance.
The bolted-bonnet joint is the most popular joint and it is used on the vast majority of gate valves in industrial use today. Unlike threaded and union bonnet valves, the bolted-bonnet connection requires a gasket to seal the joint between the body and bonnet.
The pressure-seal joint is energized by the fluid pressure in the valve body acting upon a wedge shaped, soft iron or graphite gasket wedged between the body and bonnet. On a pressure-seal valve, the higher the body cavity pressure, the greater the force on the gasket. Pressure-seal bonnets are used extensively for high-pressure high-temperature applications, such as the power industry.
Welded bonnets are a very popular body-bonnet joint for compact steel valves in sizes ½ inch through 2 inches and pressure classes 800 through , where disassembly is not required.

OTHER GATE VALVE DESIGNS

Also in the gate valve family are knife and sluice gates. The bonnetless knife gate is especially suited for use in slurries such as in pulp and paper mills.

This small gate valve is a common design in sizes NPS 1/2-2 inches. The tapered disc is clearly visible. Photo credit: Greg Johnson

Knife gates are very thin, only slightly wider than their closure element (disc). Because of their unique geometry and thin cross-section, knife gates are limited to low pressure applications.

In appearance, the sluice gate doesn&#;t look like it even belongs in the gate valve family; however, based upon its sliding disc design, it is characterized as a gate valve. Sluice gates are limited to very low pressures &#; in most cases, simple head pressure. They are used primarily in wastewater and irrigation systems.

GATE VALVE MARKET SEGMENTS

While the quarter-turn valve has achieved a large chunk of the gate valve market share over the past 50 years, there are still industries that rely heavily on them, including the oil and gas industry. Crude or liquid pipelines are still the home to parallel seat gate valves, despite the inroads that ball valves have made on the gas pipeline side.

In the larger sizes, the gate valves are still the primary choice for the refining industry for most applications. The robustness of design and total cost of ownership (which includes the economics of repair) are points that make this legacy design desirable.

An NPS 36 pipeline-style parallel seat gate valve is unloaded at one of the U.S. Strategic Petroleum Reserve locations. Photo credit: Greg Johnson

Application-wise, many refinery processes utilize temperatures above the safe operating temperature of Teflon, which is the primary seating material in floating ball valves. The high-performance butterfly valve and metal-seated ball valve are beginning to see more use in refinery applications, although their total cost of ownership is often higher than that of the gate valve.

The waterworks industry segment is still dominated by iron gate valves. They are reasonably inexpensive and long-lasting, even in buried applications.

The power industry utilizes alloy gate valves for applications involving very high pressure and very high temperature. Although some newer Y-pattern globe valves, and metal-seated ball valves designed for blocking service are found in power plants, gate valves still find favor for plant designers and operators.

A trio of NPS 36, NRS gate valves are seen in manifolds at a water treatment facility. Photo credit: Greg Johnson

MATERIALS OF CONSTRUCTION

Steel and iron are the most popular materials for gate valve construction, with steel being the choice for most industrial applications and iron for water, wastewater and heating, ventilation and air-conditioning (HVAC). Other materials popular for gate valve construction include stainless steel, bronze and high alloys such as Hastelloy and Inconel.

Standards for the design and construction of gate valves are published by the American Petroleum Institute (API), Manufacturers Standardization Society (MSS), American Waterworks Association (AWWA) and American Society of Mechanical Engineers (ASME).

STILL POPULAR

Gate valves are still the primary choice for many service applications. Their cost of manufacture to value ratio is still very high. On typical petrochemical and refining projects today, the percentage of gate valves on the requisition is about 60%.

Mark Twain once said, &#;The rumors of my death have been greatly exaggerated.&#; Although the ball, plug and butterfly valve segments have been gaining market share for decades, the venerable gate valve can respond the same way &#; the rumors of its demise have been exaggerated.

ABOUT THE AUTHOR

Greg Johnson is president of United Valve. He is a contributing editor to VALVE Magazine and a current Valve Repair Council board member. He also serves as chairman of the VALVE Magazine Advisory Board, is a founding member of the VMA Education and Training Committee and is past president of the Manufacturers Standardization Society. Reach him at .

Landscape Source: Greg Johnson

Gate Valve - How They Work