What is a Globe Valve? Working Principle & Function
A Globe Valve is a type of pipeline valve that consists of a movable plug or disc and a stationary ring seat enclosed in a normally spherical body. Globe valves are common industrial tools in plant pipes. They can be manually or mechanically operated. Globe valves can be used to control flow, pressure, or entirely stop the flow. Globe valves can be useful in a variety of situations, including where flow needs to be controlled but not necessary at a constant rate. When choosing the correct globe valve for your business, think about the application. Use a globe valve intended to have pressure flowing under the seat when regulating the flow of a low-temperature and low-pressure system. Choose a valve that permits pressure to travel above the disc if you need a valve that can endure more severe conditions.
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What is a Globe Valve?
It's critical to choose the proper valve because different types have distinct characteristics and applications. There are only two statuses for several of them: open or closed. Others allow for the control of fluid flow and pressure. Varying valves result in different levels of pressure loss. Specific qualities are required depending on the context.
The globe valve is one of the most frequent forms of the valve. We'll go over how globe valves function, as well as their benefits and drawbacks, in this post. Then we compare the various shapes of globe valves to other types of valves, such as gate valves and ball valves.
Consider the three main properties of a globe valve to see if it's suited for your application. They are, first and foremost, lateral movement valves, which open and close in response to the up-and-down movement of a stem. Second, they control fluid flow by allowing, stopping, or throttling it. Some valves simply have two states: open and closed, however, globe valves can control flow without completely stopping it. Third, compared to other valves, they create large head losses, which is a tradeoff for the throttling services.
Globe Valves Working Principle
Globe valves are made up of three parts: a handwheel, a bonnet, and a body. The stem is housed in the bonnet, and it screws up and down in the bonnet when the handwheel is cranked. A disk or plug is a small component at the end of the stem that can be metallic or non-metallic and come in a variety of shapes, depending on the use.
A seat is housed in the globe-shaped body for which the valve is named. The seat is a metal frame with a hole that splits the body into two chambers and allows fluid to pass through. Fluid flows up through the opening in the seat when the valve is open. The stem is screwed down until the disk blocks the opening in the seat.
When a globe valve's plug or disk is put into the seat, fluid flow is stopped. When the valve is open, the fluid must change directions twice to pass through the seat, resulting in a significant head loss.
Globe valves have the capacity to throttle or control flow, which is one of their key advantages. They might be partially open in addition to being closed or open. This allows you to modify the flow without having to stop it completely.
The most major downside of globe valves is the tremendous head loss they produce. The amount of resistance a liquid faces as it travels through pipes is referred to as head loss, also known as pressure loss. The more resistance there is, the more pressure is released. This loss is caused by gravity, friction (of the fluid against the pipe walls), and turbulence. Turbulence is the primary cause of pressure loss in valves and fittings.
Fluid passing through globe valves is forced to change directions, causing loss and turbulence. The precise quantity of loss is determined by variables such as fluid velocity and friction factor. However, a statistic known as the L/D coefficient can be used to compare the pressure losses from different valves.
The L/D coefficient of a valve allows us to compare it to a straight pipe section with the same friction factor. The letters L and D represent length and diameter, respectively. All other things being equal, a valve with L/D=5 provides the same friction loss as a pipe section with a length-to-diameter ratio of 5. The greater the L/D of a component, the more head loss it causes. A 90-degree pipe elbow has an L/D=30, but a normal globe valve has an L/D=340.
Globe Valve Shapes & Types
T-pattern/z-pattern, y-pattern, and angle pattern globe valves are available. Each type works in the same way to stop or throttle fluid flow, but the flow path—and hence the head loss—varies.
T-shaped or z-shaped globe valves are the most common. The profile of these valves is designed as a "t-pattern" because the intake and exit are in a straight line. However, "Z-shaped" is a better description because it relates to the water's passage through the valve rather than the device's exterior design. The water changes direction twice in a manner that mimics the letter Z in a normal z-shaped globe valve. Standard Z-shaped valves, with an L/D=340, cause the most substantial head loss.
Modifications of z-pattern globe valves are y-pattern globe valves. The intake and outlet are still in a straight line, but the bonnet and stem are angled (rather than perpendicular) to the body, resulting in a y-shaped profile. This gives the water a straighter course than the z-shape, reducing head loss. L/D=55 for y-pattern globe valves.
Alternatives to the normal z-pattern globe valve include y-pattern and angle-pattern globe valves. While performing the same function, they produce less pressure loss.
Angle valves have a 90-degree inlet and outlet, and they can be used as both elbows and valves. They produce less head loss than z-shaped valves with L/D=150 because they only change fluid direction once.
Considerations of Using Globe Valves
When you need to modify flow but don't want to worry about pressure loss, globe valves are the way to go. The following are some examples of applications:
- Fuel oil systems
- Cooling water systems
- Turbine lubricating oil systems
- Feedwater and chemical feed systems
- In fire sprinkler or other water-based fire defense systems, drain and trim applications.
Note: In fire sprinkler systems, where pressure is a concern, globe valves aren't the greatest choice for control valve applications. Butterfly valves, on the other hand, are very prevalent.
When constructing a plumbing system, it's critical to select the appropriate components for the job, which is especially true when it comes to valves. Globe valves cause high-pressure losses because they force water to change directions even when fully open. Globe valves, on the other hand, can control water flow as one of their main advantages. They might be completely open, slightly open, or completely closed.
Gate valves and ball valves, which are two alternatives to globe valves, have very low-pressure losses. They do not, however, offer throttling services like globe valves. Gate or ball valves are preferable choices to globe valves in applications where pressure is a concern.