The Fundamentals of Gate Valves

Aug. 06, 2024

The Fundamentals of Gate Valves

Butterfly valves tals 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.

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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.

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 Photo Credit: Greg Johnson

The Difference Between a Knife Gate Valve and Slide Gate

When most people describe plumbing valves, they generally do not take the time to distinguish between the different types&#;which are often differentiated by only minute details. However, when it comes time for either construction or repairs, knowing what kind of valve to use when is essential in getting the job done right.

If you are interested in learning about the differences between a knife gate valve and a slide gate valve, read on to learn everything you need to know.

What Is a Knife Gate Valve Used For?

This kind of valve is often used in wastewater applications due to its ability to&#;as can be inferred from the name&#;slice through more solid materials like a knife. A knife gate valve is designed for use in harsh environments, so you will want to use it if you need something tough.

However, unless they are specifically designed for it, a knife gate valve should not be used to regulate flow. So, unless your goal is to destroy your expensive valve, it should only be used in entirely open or closed applications.

What Is a Slide Gate Valve Used For?

Perhaps the most significant difference between slide gate valves and knife gate valves is the kind of materials that they are designed to regulate.

Whereas knife gate valves are typically used to control wet bulk such as waste liquid, slide gate valves are meant for dry bulk materials like powders, granulates, and pellets. As far as usage and purpose go, this pretty much sums up slide gate valves.

Differences Between the Two Types of Valves

Some will venture to say that there is almost no distinct difference between the two types of valves other than their usage. Furthermore, many &#;experts&#; claim that the two are so similar that they might as well be the same valve. This is not true at all.

Granted, they are very similar. However, that does not mean they are the same valve.

For example, the most obvious difference is that knife gate valves have a sharp blade to sever unwanted solids that might prohibit the valve from closing.

Another difference has to do with both the dimensions of the valves and the on which they are constructed. Knife gate valves are built to the American National Standards Institute standards. However, slide gate valves are produced according to those set by the Technical Association of Pulp and Paper Industry.

Additionally, a knife gate valve has a lesser profile than a slide gate valve. This means that it is smaller and more compact&#;making it more helpful in managing the valve pressure of liquids rather than bulk solids. Besides this, it is also bi-directional.

How Much Does a Knife Gate Valve Cost?

Like most commodities, the cost of a knife gate valve depends on both the seller, size, location, model, and make. These factors come into play when the price of a specific knife gate valve is being negotiated.

So, when searching for the best deal for a knife gate valve (or any valve, for that matter), you will want to find the company with the best deals out there. Obviously, this will allow you to save money and get the type of valve you need for your project.

Whether you need it for home improvement, business operations, or something else along these lines, taking the time to search for a good deal is worth it and should be an integral part of your purchasing process.

How Much Does a Slide Gate Valve Cost?

When searching for the correct slide gate valve, multiple characteristics are worth your consideration. These include the following:

Material characteristics

Contact surface

Operating environment

Actuation method

Electrical classification

Taking each of these into account when searching for the best slide gate deal out there will save you loads of cash.

Interestingly enough, however, slide gate valves have a more comprehensive range than knife gate valves. This may be because slide gate valves can come in larger and smaller sizes than their knife counterparts. As a result, they can be somewhat cheaper than knife gate valves most of the time.

Where Can I Buy One of These Valves?

Finding slide gate valves or knife gate valves at your local department store should not be too challenging. However, that is not where you will find the best valves on the market. Additionally, you will not learn how to operate the valves correctly.

So, where should you go to buy the best knife or slide gate valves?

The answer is online. Because our world is gradually shifting to the digital realm, the companies selling the most efficient and cost-effective valves will sell them there. Additionally, most online websites include a blog where you can learn how to operate your new valve.

One online store stands out from the crowd&#;Anything Flows. This company is the industry leader in valves, actuators, instruments, and more. If you want quality valve equipment from a trusted company, then Anything Flows is for you.

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Well, there you have it. This article answers some of the most common questions regarding a knife gate valve or a slide gate valve. The only step left is to shop around for the most suitable valve for your needs at the highest quality and best price you can find.

What are you waiting for? Contact Anything Flows today and purchase the valve you need at the best price out there. We guarantee you won&#;t be disappointed with our quality, costs, or customer service.

Difference Between Gate Valve and Globe Valve

Gate valves and globe valves are two of the most common valves used in industrial applications. Both contribute greatly to any industrial services. They may look alike but these valves function differently. In this article, you will know what makes the gate and globe individually unique as well as the major differences between the two valves.

Gate valves are linear valves belonging to the shutoff valve family. Characterized by its wedge-like disc, the gate valve is commonly used for isolating media because of can tightly seal off media. This kind of valve can be used in applications where tight sealing is required. A special kind of gate valve, knife gate valve, is used in thicker and more viscous media.

Gate valves provide little pressure drops since no obstruction comes in the way of media flow. The gate operates by the gate disc moving upwards to open the valve. It closes by moving the disc downwards to meet with the seats. The disc of the gate valve can be formed like a wedge, knife or parallel.

While it does have some throttling capacity, the gate valve is not used to regulate the flow of media. Its disc is thin so the vibration from the media causes this thin disc to misalign from its seat. This condition causes damage and shortens its service life. To lengthen the valve life, the gate valve should fully open or fully close when in use.

The globe valve is also a member of the linear motion valve family. Its functions include stopping and starting of media as well as regulate the flow of media. Because of its design, the globe valve provides a tight seal with fewer chances of leakage.

The globe valve is aptly named as such because of its traditional body design similar to that of the globe. But as one gate valve China manufacturer would tell you, however, modern globe valves may adapt to other body shapes. A plug-like disc allows or prohibits the flow of media.

The seats of the gate valve are parallel to the flow of the media so there is less seat erosion when the valve is on. This also makes the globe valve a great throttling valve. The downside is, there is a high-pressure drop because of its design.

If you look at the image above, you&#;d notice that the valve forces the media to change its flow direction exactly inside the valve. Additionally, the media flow inside the valve depends on the space between the seat and the disc. The farther these are, the more fluid comes in.

Globe valves highly suitable for applications where safety and leaks are the main concerns. These valves are often in top-entry design, meaning, access to the internal components of the valve is from the top.

Gate Valve vs. Globe Valve

There is always a comparison between the gate valve and the globe valve. Members of the linear valve family, you&#;d find these two in tandem in pipe systems. How would you use these two? Look at the differences between the two below:

Structure

One of the main differences between the gate valve and the globe valve is how these look. The gate valve construction is much simpler. Much of its internal components are located on the top part of the valve body. When it is turned on, its body becomes hollow. That&#;s why there is a little pressure drop. The gate valve may have a rising or a non-rising stem.

On the contrary, many globe valve parts are inside the cavity itself. Since its primary purpose is to throttle and provide positive shut-off, the design allows obstructions that aid the globe valve in this purpose. The design also permits changes in flow direction and rate that contributes to a high-pressure drop. The globe has a rising stem.

Applications

The gate valve is suitable for applications where low-pressure drops are of utmost importance. It&#;s a multidirectional valve. The globe is best used in applications where massive changes in pressure are not an issue. This valve is unidirectional.

Function

The gate valve is not designed for flow control; it is for isolation of media. The gate valve cannot handle the strength of the media flowing in a partially opened position. On the other hand, the globe valve is more of a control valve.

How They Work

In the gate valve working mechanism, the disc lodges between the seats. Its movement is at the right angle in relation to the media flow. It provides either full flow or no flow at all. If you open the valve, the flow space is as big as the valve cavity with little extra where the stem and the seats are located as you&#;d notice in the first image.

Seat for the Type FLS Slab Gate Valve

Seats for Cameron-Style FLS Slab Gate Valves

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Product Information

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We have seats for type FLS slab gate valves for sale in a wide variety of sizes. Available sizes include:

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The Fundamentals of Gate Valves