Let’s face it ball valves are everywhere, but being one of the most commonly used valves in industry they raise a lot of questions.
In this article we will answer some of those questions and dispel some of the myths around ball valves including:
What is the difference between a floating ball valve and trunnion mounted ball valve?
How are ball valves made and what are the typical materials of construction?
What exactly Fire-Safe and Anti-Static mean?
The difference between 1 piece, 2 piece and 3 piece ball valves
The History of Ball Valves
The humble ball valve is so widely used in industry today that it is considered to be a valve type in its own right, distinct from linear and rotary valves as main groups. However, when it was first introduced in the 1950’s it was known as a ‘spherical plug valve’.
The use of a ball valve has developed rapidly since the 1950’s because of the development of machines that are able to accurately produce the spherical polished surface of the ball and the availability of different ball and seat materials. In more recent years the continual development of seat materials together with metal seats made from low wear materials has extended where these valves can be used particularly where abrasive and high temperature media are involved.
How are ball valves made?
Ball Valves can be made from bar stock, forgings or castings with screwed, socket weld, butt weld or flanged connections.
What services can ball valves be used on?
The versatility of ball valves is what makes them so popular and they can be used on everyday services like water, solvents, acids and natural gas to more arduous services like oxygen, hydrogen peroxide and other gases.
What materials can ball valves be made from?
The most common materials for ball valve manufacture are carbon steel grades WCB and LCB for the body and stainless steel grade 316 or CF8M for the ball and shaft. In special applications the body can be made from stainless alloys in corrosive or low/high temperature applications.
The standard seat material for ball valves is PTFE (Polytetrafluoroethylene) because it is chemically inert to a wide range of fluids and has a coefficient of friction at less than 0.1.
PTFE loses rigidity at high temperatures which is why Ball valves have a pressure/temperature curve supplied by the manufacture showing what temperature is achievable at what pressure.
To overcome the limitations of standard PTFE, special Nylons, Polyether-etherketone (PEEK) and powder filled PTFE’s are used to gain a higher rigidity over the seating face and an increase in maximum service temperature. When temperatures are over 280°c metal seated ball valves are required.
Floating Ball Valve Design
With this design the ball is held between two ball seats by the compression of the seats against it. To turn the ball 90° or quarter turn the ball is driven by a shaft which connects to a slot in the top of the ball. The slot allows a little lateral movement of the ball due to the influence of upstream line pressure. The design principle around floating ball valves is that they are loaded by line pressure against the downstream seat which is why this seat is always regarded as the primary seat. In some cases the upstream seat can also give a secondary seal if the design has a pre-loading or a spring in the the upstream seat design.
Trunnion Mounted Ball Valve Design
On this design the ball is supported by a trunnion and not the valve seats which allows much higher pressure and temperature ratings. The concept behind this is that the shaft and trunnion are intended as being a one piece construction. The shaft and trunnion are held in bearings to prevent the ball from moving against the downstream seat. To facilitate loading the upstream seat is designed to move forward against the ball and thus in a trunnion mounted ball valve the primary seat is formed at the upstream side. A spring mechanism is also incorporated behind both seats to ensure even seat loading even at low pressures and to provide a secondary seat on the downstream side. As with floating ball valve designs, trunnion mounted ball valves are capable of bi-directional shut off. One of the main benefits of trunnion mounted ball valves over floating ball valves is a lower operating torque and consequentially this means that actuation costs can be lower.
Different Ball Valve Body Designs
One Piece Ball Valve Design – The ball is placed into the body through an entry in one of the pipe flanges and secured with a gasket forming part of the raised face. This has inherent strength and minimises leak paths.
Split Body Ball Valve Design – This is also known as a 2 piece or 3 piece ball valve and the body is split in one or two places in the same plane as the valve flanges. The body is then bolted around the ball. 3 Piece ball valves are common among smaller sizes due to lower manufacturing costs and trunnion mounted ball valves due to component weight. The big advantage of split body ball valves is the ease of maintenance.
Top Entry Ball Valve Design – This is a rarer design of valve where the ball is inserted through a bonnet in the top of the valve. The main advantage of this is that the critical parts of the valve can be maintained while the valve remains in the pipeline. It’s often used in welded pipe systems.
Fully Welded Ball Valve Design – This is where the body of the valve is an all welded construction and completely non-maintainable. These are often found used in gas pipelines.
The difference between Reduced or Full Port Ball Valves (also known as Full or Reduced Bore)
One of the most important considerations with a ball valve is port size. Full port generally means the ball port is the same as the inside diameter of the pipe. One of the advantages of Reduced Bore Ball Valves is that they are lighter in weight and cost less but the disadvantage is a reduced flow and in some cases a 1 piece construction rendering them non-maintainable.
Anti-Static Ball Valves
If the ball is held between seats without any metallic contact between it and other components the flow of fluid can lead to a build up of static electrical charge on the surface of the ball. To prevent this static forming a spark, igniting fluid or causing an explosion valve design standards require a mechanism to ensure electrical continuity between ball, body and downstream pipework.
In the floating ball valve design the shaft is not attached to the ball. It is essential to make sure that the shaft cannot be blown out past the packing if the gland bolting is removed or slackened.
Fire Testing of Ball Valves
As the majority of ball valves rely on soft seats in order to seal properly, engineers are often concerned about whether a ball valve will still operate in the event of a fire which has partially damaged or destroyed the ball seats. For certain industries like oil, chemical and petrochemical industries it’s a requirement that valve can still operate . Ball valve fire safe standards stipulate the details of fire to which a ball valve must be subjected to before it can be certified as ‘fire tested’.
Ball Valve Cavity Relief
When a ball valve is closed the body cavity of a ball valve is full of line fluid which can always escape if the pressure is not sufficient to load the ball onto the downstream seat. In this case an increase in temperature may render the valve inoperable due to the increase in temperature, and as a solution to over pressure relief, a hole can be drilled in the upstream side which renders the valve uni-directional. Cavity relief is part of the design in a trunnion type ball valve but not inherent in a floating ball valve so required adding.
Lined Ball Valves
These are a type of ball valve which is manufactured for corrosion resistance on services like acid. The wetted internal contact parts of the valve usually have a PFA or PTFE lining inside a metal body with the ball being fully lined to or made from ceramics. If the lining is damaged in any way the less corrosion resistant body will end up being exposed to the line fluid.
Low Temperature Ball Valves
Specific low temperature ball valves are used for low temperature and cryogenic applications like LPG, and LNG. These are usually low temperature carbon steel but more frequently stainless steel has been standardised by manufacturers for this service, going as low as -196°c. Cryogenic valves often suffer from higher operating torques due to the PTFE ball seats contracting at this temperature and another standard feature is an extended bonnet. This isolates the gland seals from the low temperature to avoid ice formation and maintain the integrity of the seal.