Globe valve

Flow control device

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Globe valve

A globe valve, different from ball valve, is a type of valve used for regulating flow in a pipeline, consisting of a movable plug or disc element and a stationary ring seat in a generally spherical body.[1]

Globe valves are named for their spherical body shape with the two halves of the body being separated by an internal baffle. This has an opening that forms a seat onto which a movable plug[2] can be screwed in to close (or shut) the valve. The plug is also called a disc.[3] In globe valves, the plug is connected to a stem which is operated by screw action using a handwheel in manual valves. Typically, automated globe valves use smooth stems rather than threaded and are opened and closed by an actuator assembly.

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Although globe valves in the past had the spherical bodies which gave them their name, many modern globe valves do not have much of a spherical shape. However, the term globe valve is still often used for valves that have such an internal mechanism. In plumbing, valves with such a mechanism are also often called stop valves since they don't have the spherical housing, but the term stop valve may refer to valves which are used to stop flow even when they have other mechanisms or designs.

Parts of a typical globe valve

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Body

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Internal parts of a typical globe valve

The body is the main pressure containing structure of the valve and the most easily identified as it forms the mass of the valve. It contains all of the valve's internal parts that will come in contact with the substance being controlled by the valve. The bonnet is connected to the body and provides the containment of the fluid, gas, or slurry that is being controlled.

Globe valves are typically two-port valves, although three port valves are also produced mostly in straight-flow configuration. Ports are openings in the body for fluid flowing in or out. The two ports may be oriented straight across from each other or anywhere on the body,[4] or oriented at an angle (such as a 90°).[5] Globe valves with ports at such an angle are called angle globe valves. Globe valves are mainly used for corrosive or high viscous fluids which solidify at room temperature. This is because straight valves are designed so that the outlet pipe is in line with the inlet pipe and the fluid has a good chance of staying there in the case of horizontal piping. In the case of angle valves, the outlet pipe is directed towards the bottom. This allows the fluid to drain off. In turn, this prevents clogging and/or corrosion of the valve components over a period of time. A globe valve can also have a body in the shape of a "Y". This will allow the construction of the valve to be straight at the bottom as opposed to the conventional pot type construction (to arrange bottom seat) in case of other valves. This will again allow the fluid to pass through without difficulty and minimizes fluid clogging/corrosion in the long term.[citation needed]

Bonnet

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The bonnet provides a leakproof closure for the valve body. The threaded section of the stem goes through a hole with matching threads in the bonnet. Globe valves may have a screw-in, union, or bolted[4] bonnet. Screw-in bonnet is the simplest bonnet, offering a durable, pressure-tight seal. Union bonnet is suitable for applications

Plug or disc

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The valve's closure mechanism involves plugs that connect to a stem, which is adjusted either by sliding or screwing it up or down to regulate flow. Plugs come in balanced or unbalanced types. Unbalanced plugs, typically solid, are suitable for smaller valves or those with low pressure drops. They offer advantages such as simpler design, with potential leakage only at the seat, and usually lower cost. However, they are limited in size, as larger unbalanced plugs may require impractical forces to seal and control flow. On the other hand, balanced plugs feature holes through the plug itself. They offer advantages such as easier shut-off due to reduced static forces required. However, they introduce a second potential leak path between the plug and the cage, and they tend to be more expensive.

Stem

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Long-stemmed globe valve

The stem serves as a connector from the actuator to the inside of the valve and transmits this actuation force. Stems are either smooth for actuator controlled valves or threaded for manual valves. The smooth stems are surrounded by packing material to prevent leaking material from the valve. This packing is a wearable material and will have to be replaced during maintenance. With a smooth stem the ends are threaded to allow connection to the plug and the actuator. The stem must not only withstand a large amount of compression force during valve closure, but also have high tensile strength during valve opening. In addition, the stem must be very straight, or have low run out, in order to ensure good valve closure. This minimum run out also minimizes wear of the packing contained in the bonnet, which provides the seal against leakage. The stem may be provided with a shroud over the packing nut to prevent foreign bodies entering the packing material, which would accelerate wear.

Cage

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The cage is a part of the valve that surrounds the plug and is located inside the body of the valve. Typically, the cage is one of the greatest determiners of flow within the valve. As the plug is moved more of the openings in the cage are exposed and flow is increased and vice versa. The design and layout of the openings can have a large effect on flow of material (the flow characteristics of different materials at temperatures, pressures that are in a range). Cages are also used to guide the plug to the seat of the valve for a good shutoff, substituting the guiding from the bonnet.

Seat

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The seat ring provides a stable, uniform and replaceable shut off surface. Seat are usually screwed in or torqued . This pushes the cage down on the lip of the seat and holds it firmly to the body of the valve. Seat may also be threaded and screwed into a thread cut in the same area of the body. However this method makes removal of the seat ring during maintenance difficult if not impossible. Seat rings are also typically beveled at the seating surface to allow for some guiding during the final stages of closing the valve.

Economical globe valves or stop valves with a similar mechanism used in plumbing often have a rubber washer at the bottom of the disc for the seating surface, so that rubber can be compressed against the seat to form a leak-tight seal when shut.

Electrically automated 3/4" flanged globe valve

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Additional reading:
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Globe valves are operated with a handwheel and modulate water flow. But they also cause higher pressure loss.

Picking the right valve is important, as different types have different features and uses. Some of them only have two states: open or closed. Others allow fluid flow and pressure to be modulated. Distinct valves also cause different amounts of pressure loss. Depending on the situation, specific features are needed.

One of the most common types of valves is the globe valve. In this article, we explain how globe valves work, including their advantages and disadvantages. Then, we look at the different shapes of globe valves and compare them to other types of devices, like gate valves and ball valves.

QRFS stocks a wide range of valves, including globe valves. Shop T-shaped globed valves and angle valves now.

What is a globe valve, and how does it work?

To decide if a globe valve is right for your application, consider its three core features. First, they are lateral movement valves, which means they open or close based on the up-and-down movement of a stem. Second, they allow, stop, or throttle fluid flow. Some valves have only open and closed states, but globe valves can throttle flow without totally stopping it. Third, they cause significant head losses compared to other valves, a tradeoff for the throttling services.

How globe valves work

From the outside, globe valves have three parts, a handwheel, a bonnet, and a body. The bonnet houses a stem, and when the handwheel is turned, the stem screws up and down in the bonnet. The end of the stem has a small component called a disk or plug, which can be metallic or non-metallic and may come in different shapes, depending on the need.

The globe-shaped body for which the valve is named houses a seat. The seat is a metal frame that divides the body into two chambers and has a hole to allow fluid to pass. When the valve is open, fluid flows up through the opening in the seat. To close it, the stem is screwed down until the disk blocks the seat&#;s opening.

One of the main advantages of globe valves is their ability to throttle or modulate flow. Besides being closed or open, they can also be partially open. This allows you to adjust flow without totally stopping it.

The main disadvantage of globe valves is the comparatively significant head loss they create. Head loss, also called pressure loss, refers to the amount of resistance a liquid encounters as it flows through pipes. The more resistance, the more pressure that is lost. Gravity, friction (of the fluid against the walls of the pipe), and turbulence all cause this loss. Valves and fittings cause pressure loss mainly through turbulence.

Globe valves force fluid to change directions as it passes through, creating loss and turbulence. The exact amount of loss depends on factors like fluid velocity and friction factor. However, it&#;s still possible to review the pressure losses from different valves using a metric called the L/D coefficient.

A valve&#;s L/D coefficient lets us compare it to a straight section of pipe with the same friction factor. L and D stand for length and diameter, respectively. A valve with L/D=5 creates the same friction loss (all else being equal) as a section of pipe with a length-to-diameter ratio of 5. The larger L/D is for a component, the higher the head loss it creates.

A standard globe valve has an L/D=340, compared to an L/D=30 for a 90-degree pipe elbow.

Globe valve shapes

Globe valves come in three shapes&#;t-pattern/z-pattern, y-pattern, and angle pattern. Each form stops or throttles fluid flow in the same way, but the path of flow&#;and thus the head loss&#;differs.

Most globe valves are t-shaped globe or z-shaped. They are called &#;t-pattern&#; for the profile of the valve&#;the inlet and outlet of these valves are in a straight line. &#;Z-shaped&#; is arguably more descriptive, however, because it refers to the path of the water as it flows through the valve rather than the device&#;s external shape. In the standard z-shaped globe valve, the water changes direction twice in a pattern that resembles the letter Z. As mentioned, standard Z-shaped valves cause the most significant head loss, with an L/D=340.

Y-pattern globe valves are modifications of z-pattern valves. The inlet and outlet are still in a straight line, but the bonnet and stem are placed at an angle to the body (as opposed to perpendicular), forming a y-shaped profile. This provides a straighter path for the water than the z-shape, reducing head loss. For y-pattern globe valves, L/D=55.

The inlet and outlet of angle valves are oriented at 90 degrees from one another, and these devices serve both as elbows and valves. Because they only make fluid change direction once, they create less head loss than z-shaped valves, with L/D=150.

When to use globe valves

Globe valves are ideal whenever you need to modulate flow, but you don&#;t have to worry about the amount of pressure loss. Some applications include:

• Cooling water systems
• Fuel oil systems
• Feedwater and chemical feed systems
• Turbine lubricating oil systems
• Drain and trim applications in fire sprinkler or other water-based fire protection systems

Globe valves aren&#;t the best choice for control valve applications in fire sprinkler systems, where pressure is at a premium. Instead, butterfly valves are commonly used.

Globe valve vs. gate valve

There are several alternatives to the globe valve, and understanding the different options can help you pick the right valve for the job. Two major alternatives to the globe valve are the gate valve and the ball valve. First, let&#;s consider the differences between globe and gate valves.

Gate valves, like globe valves, are linear-motion valves. They are opened and closed by turning a handwheel that screws a stem in and out of the bonnet. When a gate valve is closed, an obstruction is pushed down by the stem perpendicular to the path of the fluid.

Unlike globe valves, gate valves don&#;t require fluid to change directions&#;the flow path is straight. As such, gate valves cause very little head loss (L/D=8). However, they do not throttle as well as globe valves&#;fluid flow rate in a gate valve is not proportional to how open the valve is at any moment.

Globe valve vs. ball valve

Unlike globe valves and gate valves, ball valves are rotary-motion valves. This means that ball valves open and close when the obstacle rotates rather than opening and closing with the lateral movement of an obstruction. Specifically, a ball-shaped component with a hole through it spins in its housing, either stopping the fluid or letting it pass.

Instead of turning a handwheel, a ball valve is typically opened or closed with a wrench handle. Only a quarter-turn (90 degrees) is needed to open or close it&#;much faster than turning a handwheel multiple times. Quick closing can actually be a problem because of water hammer. When fluid flowing through a pipe is suddenly stopped, its momentum has nowhere to go because water is non-compressible. Water hammer occurs when a pressure wave propagates back through the pipe, potentially causing damage.

Since ball valves don&#;t force water to change direction, they create very little head loss (L/D=3). As with gate valves, this low pressure loss comes with the tradeoff of little throttling capability. Ball valves are either open or closed, with little in-between.

Globe valves adjust flow, but be careful of the pressure application

When building a piping system, it&#;s important to choose the right components for the job, which is especially true with valves. Because globe valves force water to change directions even when fully open, they create comparatively large pressure losses. One of the core advantages of globe valves, however, is their ability to modulate water flow. They can be open, partially open, or closed.

Gate valves and ball valves, two alternatives to globe valves, cause minimal pressure losses. However, they don&#;t provide throttling services like globe valves. For applications where pressure is at a premium, gate or ball valves are better choices than globe valves.

Shop our inventory of standard (z-pattern) globe valves and angle valves. For other options, shop our full selection of valves, including ball valves.  If you don&#;t see the item you need, contact us, and our customer service team will help you out.

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