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.
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!