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Floating ball valve and Trunnion ball valve

November 30, 2017

Ball Valves are a type of piping controller that shuts off or controls the flow in a pipeline by utilizing a hollow ball and round seats held at a valve body. There are two fundamental technologies for the layout of ball valves, floating ball design and trunnion mounted ball layout. 

  • The Significant components of a ball valve would be: 
  • The Bonnet – The Region of the valve system where install the stem and packing. 
  • The Stem – A rotating shaft that joins the inner ball to the exterior of the valve to facilitate rotation of the ball. 
  • The Valve Body – A pressure vessel which contains the components required to control or shut off the flow through a pipe. It is designed to connect two or more sections of pipe or tubing. 
  • The Ball – A ball using a flow path (hole or tunnel) through the center of it and a connection point for a rotating shaft to rotate it. 
  • The Seats – Round donut-shaped discs that form a seal between the body and ball. This can be a lever, a gear, an Electric Actuator, or even a pneumatic/hydraulic actuator.

Floating Ball Valve

When the ball is in the closed position, the flow path in the ball is perpendicular to the flow path from the valve body. The parts of the ball covers both the upstream and seat openings that are cupped. The strain in the pipe that is upstream pushes against the region of the ball that goes on its point and is forced tightly against the seat. Medium flow shuts off. 

The ball is rotated (usually 90 degrees) by a shaft that protrudes out of the valve body and is connected to some kind of operator. For this reason, most ball valves are known as quarter turn valves. This shaft isn’t rigidly attached to the ball in order for the ball to pivot on the end of the shaft as it moves on the axis of the ball. This is usually accomplished using a slot perpendicular, on top of the ball to the circulation route of the ball. 

The part of the seats cradle the ball keeps it from going down in the valve body. They are rigidly held into place in the valve body to make a tight seal. The seat does not move in the ball design. The medial side of the seat which sees the ball is generally cupped along with the inner diameter of the seat is larger than the hole in the ball. 

Floating ball design valves would be the cheaper valves, but are restricted by the amount of pressure the seats can handle. A floating design ball valve sandwiches the hollow ball between a couple of cupped seats that form a tight seal between the body and the ball at the inlet and outlet ports of the valve body. 

The press is prevented by the valve packaging from escaping the valve body. The most common type of packaging now is chevron v-ring type packing. When you look at the cross-section profile of the packaging, it resembles the letter “V”. The diameter of the packing matches the bore of the bonnet. The inside diameter fits valve stem. Several rings of the packing are stacked together and the stem is installed through the packing. A packing gland on top of the bonnet pushes down on the very top of the “V” and causes the packing to enlarge and seals against the stem and bonnet. 

The flow path in the ball is consistent with the flow path within the body when in the open position. The openings at the ball are aligned with the openings in the body’s ports. This allows flow in the upstream port to journey through the ball’s flow path and outside to the downstream pipe(s). Since the ball rotates the vents in the body concerning the ports in the ball create a varying size orifice which, as it becomes bigger, will increase the flow through the tube. This orifice reduces circulation and gets smaller, when going from open to close. 

Trunnion Mounted Ball Valve

Both floating ball and trunnion layouts are available as multi-port valves by using flow paths in the ball and ports within the body. Trunnion valves are extremely effective at sealing off very low pressures that wouldn’t be strong enough to move a floating ball to the seat. They are also needed on large diameter valves and high-pressure valves. The cause of this is that the contact area of the seats in a ball is tiny. The force on the seat in a floating ball comes from the process pressure in the upstream pipe. The area is large or if the pressure is large, the seat that is downstream will be destroyed. 

In the trunnion mounted ball valve, the ball uses a second shaft and bearing on the bottom of the ball. This stem or “post” holds the base of the valve in place. The cover of the ball is not slotted and the top stem is attached to the ball. This prevents the ball from going to the seat. The seats must move towards the ball, considering that the ball does not move into the seats. In order to insure the seal, Springs behind the seat push them closely into the ball. 

The tradeoff is that floating ball designs are not more expensive than trunnion valves. The trunnion layout ball valve works almost the same way as the floating ball except the seats are spring loaded against the ball and the ball doesn’t pivot. 

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