Flow Control, Flow Control April 2019

Features Valves Functionality and seating technology for fl oating ball valves A typical floating ball valve seals off the line pressure using soft seats By David M Schmitt Control Tech Inc A floating ball valve is a ball valve with an unsupported ball other than the two seats it rests on so that the ball moves in the valve cavity to help seal off the line pressure 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 or TFM a modified version of PTFE and also known as TFM 1600 TFM has a slight pressure temperature advantage over PTFE Both known for their fairly high temperature and pressure capabilities PTFE and TFM are ideal choices for seat material However neither has a good memory once the seat is compressed or misshaped in any way the material will not return to its exact original shape How the floating ball valve works 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 see Figure 1 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 Seating technology 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 see Figure 2 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 Also with this type of design the seat has already formed some slight plastic deformation due to compression so if upstream pressure is substantially increased it will cause more plastic deformation because the ball pushes harder into 2 inch ANSI 300 pound fl anged automated ball valves All images courtesy of David M Schmitt 20 April 2019 Flow Control