Valves 101: Two Types of Ball Valves
Valves 101: Two Types of Ball Valves
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Publicado por : tagacefe
Publicado en : 26-08-21
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Valves 101: Two Types of Ball Valves
Valves 101: Two Types of Ball Valves
Small-size valve balls are a type of piping equipment that shuts off or controls the flow in a pipeline by using a hollow sphere and round seats held in a valve body. There are two basic technologies for the design of ball valves, floating ball design and trunnion mounted ball design.
The major components of a ball valve include:
Valve body – A pressure vessel that contains the components needed to control or shut off the flow through a pipe. It is designed to connect two or more sections of pipe or tubing to each other.
Ball – A sphere with a flow path (hole or tunnel) through the center of it and a connection point for a shaft to rotate it.
Seats – Round donut-shaped discs that form a seal between the body and ball.
Stem – A shaft that connects the internal ball to the outside of the valve to facilitate rotation of the ball.
Packing – Flexible seals that fit around the shaft and prevent the media traveling through the valve from escaping externally.
Bonnet – The part of the valve body that houses the stem and packing.
Operator or Actuator – An external device designed to rotate the stem of the valve. This can be a lever, a gear, a motor-operated gear (electric actuator), or a pneumatic/hydraulic actuator.
Floating Ball Design
A floating design ball valve sandwiches the hollow ball between two or more cupped seats that form a tight seal between the body and the ball at the inlet and outlet ports of the valve body.
The floating valve balls are rotated (usually 90 degrees) by a shaft that protrudes outside of the valve body and is connected to some form of operator. For this reason, most ball valves are referred to as quarter-turn valves. This shaft is not rigidly attached to the ball in order for the ball to pivot on the end of the shaft as it rotates on the axis of the ball. This is usually accomplished with a slot on top of the ball, perpendicular to the ball’s flow path. The cupped portion of the seats cradle the ball prevents it from moving down in the valve body.
The valve packing prevents the media from escaping the valve body through the stem opening in the bonnet. The most common type of packing today is chevron v-ring type packing. If you look at the cross section profile of this packing, it resembles the letter “V”. The outer diameter of the packing matches the bore of the bonnet. The inside diameter matches the outer diameter of the stem. Multiple rings of this packing are stacked on top of each other and the stem is inserted through the packing. A packing gland on top of the bonnet pushes down on the top of the “V” and causes the packing to expand and seals agains the stem and bonnet.
When the ball is in the closed position, the flow path in the ball is perpendicular to the flow path in the valve body. The solid parts of the ball covers both upstream and downstream cupped seat openings. The pressure in the upstream pipe pushes against the solid part of the ball which moves on its pivot point and is forced tightly against the downstream seat. This shuts off flow.
As the operator rotates the ball from the closed position, the ports in the body in relation to the ports in the ball create a variable size orifice which, as it gets larger, will increase the flow through the pipe. When moving from open to close, this orifice gets smaller and decreases flow.
Floating ball design valves are the most economical valves, but are limited by the amount of pressure the seats can handle.
Trunnion Mounted Ball Design
The Trunnion Valve Balls work nearly the same way as the floating ball except the seats are spring loaded against the ball and the ball does not pivot. The ball only rotates on its axis in this design.
In the trunnion mounted ball valve, the ball utilizes a second shaft and bearing on the bottom of the ball. This stem or “post” holds the bottom of the valve in place. The top of the ball is not slotted and the upper stem is rigidly attached to the ball. This prevents the ball from moving into the downstream seat.
Since the ball does not move into the seats, the seats must move towards the ball. Springs behind the seat push them tightly into the ball in order to make the seal.
Trunnion valves are very effective at sealing off very low pressures that would not be strong enough to move a floating ball into the downstream seat. They are also required on large diameter valves and high-pressure valves. The reason for this is the contact area of the seats in a floating ball is relatively small. From physics, we know force = Pressure x Area. The force on the downstream seat in a floating ball is derived from the process pressure in the upstream pipe pushing on the solid area of the ball in the closed position. If the pressure is high or the area is large, the downstream seat will be destroyed.
The tradeoff is that trunnion valves are much more expensive than floating ball designs.
Both floating ball and trunnion designs are available as multi-port valves by using elaborate flow paths in the ball and additional ports in the body.
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HOLLOW VS. SOLID BALL VALVE
The ball valve is available with many different options, and one of the most important distinctions is in how the ball is made. Manufacturers either join two hollow hemispheres or machine a hole through a solid ball. Machining takes more work, but it produces a valve with better flow characteristics. Here’s why.
In a solid ball valve, the hole is the same diameter all the way through. That helps the fluid flow smoothly at a constant velocity. Hollow valve balls are, as the name implies, hollow inside. That creates a bigger space for the fluid to fill, which changes its velocity and creates turbulence.
Turbulence is almost always undesirable. It creates noise and it’s inefficient. The Cv drops, and it takes more energy to pump fluid through the system.
In a few applications, valve weight is an issue. In such cases, the hollow ball may have an appreciable advantage, especially in larger valves.
Solid ball, better flow
When evaluating ball valves, check the Cv values and be sure to compare identical port formats. In most cases, the solid ball will give a higher Cv than the hollow ball.
Advantages of V-Ball Valves
Ball valves are designed to be used in a variety of applications and markets, but V opening valve balls offer the added ability to be used as a control valve where necessary. The main differentiator is a contoured V-port in the ball, most commonly available with a 15°, 30°, 45°, 60°, or 90° angle, which produces an equal percentage flow characteristic for better control. V-ball valves are known to offer a variety of advantages in flow control applications, especially when compared to larger and more expensive control valves.
Low Pressure Drop
The V-port design allows material to flow straight through the valve, minimizing pressure drop across the valve. V-ball valves also have higher flow coefficients (Cv) compared to other control valve types of a similar size; this means that a smaller V-ball valve can be used to achieve the same flow rates as a comparable control valve of a larger size.
Bubble Tight Shut-Off
V-port ball valves use the “floating ball principle, which will provide a bubble tight shut-off in addition to the modulating flow control. When the valve is closed, upstream pressure pushes the ball against the downstream seat, thus enhancing the seal. These can control or shut off bi-directional flow, which proves to be especially beneficial in gas applications. Comparable control valves often require additional shut-off valves; V-ball valves eliminate the need for these additional valves, saving the user cost and reducing the number of overall components in the system.
Precise Flow Control
Precision machined V-ball valves are available with 60° and 90° V openings and offer equal percentage flow characteristics. As the ball rotates, desired flow rates can be achieved by positioning the ball anywhere between 0° and 90°. Response time is also increased due to the ability to use smaller actuators with these valves.
High Shut-Off Pressure
V-ball valves feature a quarter turn (90°) ball rotation, allowing them to operate and shut off at much higher working pressures. Because of this, smaller lower torque actuators can be used, which results in cost savings and higher efficiency in the user’s application.
Ball Valves Defined
Ball valves use a handle to turn a ball in the valve, with a hole or port through it. The ball is used to allow or prevent flow through the valve depending on its position. If the hole is aligned with the pipe, flow continues through the valve; if the hole is perpendicular to the pipe, flow will stop at the valve. It’s really that simple!
Not all ball valves look the same and some have special uses, but they all have the same concept.
Did you know? Ball valves are considered ? turn valves because of the quarter turn that is required to close the valve.
The Benefits of Ball Valves
Easy to see if it is open or closed. When the handle is in line with the pipe you can easily tell that the valve is open; and when the handle is perpendicular to the pipe the valve is closed. This is a great visual indication of whether there is flow through the valve or not.
Operation after years of disuse is easy. The durability of ball valves allows them to work and completely shut off flow after years of disuse. This provides extra peace of mind.
Quick shut off. The motion that is required to operate a ? turn the valve is quick and minimal compared to gate valves. Water hammer is a possibility if the flow through the valve is strong enough to vibrate the line when it comes to an abrupt stop as the valve is quickly shut off.
A more compact and economical valve. Thanks to the design of ball valves they are more compact than gate valves. Gate valves are taller and require more space, as the gate needs a place to go to allow the flow to travel through the valve. By using a rotating ball, ball valves are smaller and require less material.
Different Types of Balls
There are three different types of balls that can be found in ball valves. For each of these three types, the outside of the ball needs to be smooth and the surface gets ground down to remove any dips or imperfections on the surface. As the material cools, the shape may change, resulting in more or less grinding. The three types are hollow, solid, and cored, so let's take a closer look at each of them.
The hollow ball has a cavity that goes from an open hole in the bottom of the ball to the top where the valve stem is inserted. Hollow balls require less material to construct, making them more economical, but may require more grinding to get the dips out.
The solid ball is molded as one solid ball, larger than required, to account for any shrinkage and grinding. Solid balls are usually more expensive to manufacture as they require more raw material and grinding to get them to the perfect round shape.
The cored ball is cored through the center, leaving a consistent wall thickness around the whole ball. This ensures that the ball cools more evenly, again decreasing the amount of grinding required once it has fully cooled. These types of balls are becoming more popular due to the cost savings compared to solid balls.