The key to ball valve performance is the sealing design, regardless of whether the seats are made of metal, plastic or other material.
The key to ball valve performance is the sealing design, regardless of whether the seats are made of metal, plastic or other material. Another important function of ball valve performance is the stem seal design. A typical floating ball valve seals off the line pressure using soft seats such as PTFE (a plastic material called polytetrafluoroethylene and otherwise known as Teflon)
Pressure on a closed valve acts not only on the upstream side of the ball, but also on the back side of the upstream seat. The outcome of this force pushes the ball and the upstream seat in the direction of the downstream seat. This force can result in both elastic deformation and plastic deformation of the downstream seat. The onset of extreme plastic deformation results in the limitations of the valve seats.Elastic deformation is the intended temporary deformation engineered in the seat design that uses the stored energy in the design to temporarily change its shape to improve sealing performance while under pressure and temperature swings.Plastic deformation is the permanent deformation of the seat due to compression or extreme pressures and temperatures. This is sometimes referred to as "cold flow." Again, this defines the limitations of the seat design and material.
There are two different types of seating technology used in floating ball valve designs: jam seat and flexible seat, which is also referred to as an energized seat. Many of the early commercial ball valve designs, as well as some used today, depend on the jam seat design. This type of sealing is created from using excess material in the seat design, thereby "jamming" the ball (during assembly) slightly into the seat, which causes the seat to be compressed against the ball and inside body wall. The compression, or preset of the seat, is thereby compromising the seat due to plastic deformation. While this plastic deformation is slight, the compression of the seat is needed for tight sealing. This allows sealing in low-pressure applications, and as the pressure increases, the ball is forced into the downstream seat, causing a tighter seal.
There are many different designs of the energized seat. Again, the energized seat has a way of using the stored energy engineered in the design to compensate for changes in pressure, thermal compensation and wear by taking advantage of the elastic deformation built into the design. For example, a popular design used to achieve built-in energy in the seat is to shape the internal diameter of the seat so it is flexible. Along with this design, a flexure zone is included between the valve body and the outside edge of the seat. An initial preset, which takes place during final assembly of the valve, or compression is applied to the seat to cause the seat to bend into the flexure zone, thus storing elastic energy. This design takes advantage of the elastic deformation built into the design, therefore decreasing the chance of plastic deformation due to increases in pressure, thermal expansion and wear. As long as there is some clearance between the seat and the body in the flexure zone, the seat will flex rather than cold flow.
With both the jam seat design and the flexible seat design, in a new valve, upstream sealing will occur as long as upstream pressure does not move the ball past the effective preset. As upstream pressure increases, it forces the ball and the upstream seat to move in the direction of the downstream seat, causing the ball to press tighter against the downstream seat, which gives the valve a tight shutoff. When this happens, a gap will form between the valve body and back of the upstream seat. This allows upstream pressure to enter the valve cavity, which is exactly what is intended.
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