What are ball valves?
Ball valves are rotary motion actuated valves; they have found widespread applications due to their simple design, low manufacturing cost and low maintenance requirements. This type of valve is ideally suited for open/close cycling and can be used for low to very high flow rate systems.
Ball valves are classed as fast acting as only a quarter turn (90 degree) of the valve disc is necessary to completely open or close the valve.
Ball valves are one of the most common valve types, the other types being the butterfly, gate, globe and plug valves.
Construction of a Ball Valve: Body, Ball, Stem etc...
A ball valve consists of a ball, body, packing, stem, bonnet, actuator and seat.
It is possible to mount a planetary gearbox between the actuator and stem; this adjustment makes it easier to change the position of large ball valves without applying significant torque.
The seat is manufactured from materials with similar properties to rubber. Typical elastomeric materials used for seat construction include Teflon, Nylon and Neoprene, although there are others. The use of non-metallic materials for the seat prevents ball valves being used in high temperature systems.
The gap between the bonnet and stem is sealed by packing or O-rings.
The stem usually connects to the ball using a simple square shaped key. The square shape allows a high level of torque to be applied whilst minimizing the probability of rounding the edges.
Flanges are installed on the suction and discharge side of the ball valve so that piping can be easily connected.
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How Ball Valves Work
The below video is an extract from our Introduction to Valves Online Video Course.
A ball with a hole through the middle is installed within the body. If the hole is in-line with the flow, the valve is said to be in the open position and there is little resistance to flow. If the hole is rotated 90 degrees and is perpendicular to the flow direction, the valve said to be in the closed position and there is no flow. It should be noted that it is often possible to rotate a ball valve through 360 degrees although the open and close orientations remain the same.
The valve handle is often used to indicate the position of the valve. If the valve handle is in-line (parallel) with the piping, the valve is in the open position. If the valve handle is not in-line (perpendicular) with the piping, the valve is in the closed position. Note, there are exceptions to this rule (see below)!
Fuel transfer/bunkering valves often utilize ball valves with the handle indicating the opposite of what is normally expected e.g. an in-line handle means the valve is closed. The reason for this oddity is not obvious, but the reason relates to accidental movement of the valve handle. If the valve is fully open (perpendicular to the pipe) and someone accidentally moves the valve handle, the valve would move towards the closed position (parallel to the pipe). If however someone accidentally moves the valve handle on a normal ball valve that is closed (perpendicular to the pipe), they would accidentally open the valve slightly and this could lead to a fuel spill. This scenario might seem unlikely but bunkering hoses are often manually moved and sometimes dropped, a valve with the handle perpendicular to the hose is thus not ideal and would be more difficult to handle.
Ball Valve Advantages
This type of valve is considered fast acting as it can move from the fully open to fully closed position by rotating the valve stem only one quarter of a turn (90 degrees).
Ball valves allow flow in either direction and have a very low pressure drop.
Ball valves seat very well and are not prone to leakage.
Ball valves are well suited for direct local manual operation, although they can be remotely actuated using a motor and planetary gearbox arrangement.
Ball valves are usually the cheapest of all the common valve types and maintenance costs are also low.
Ball valves seat well without needing to apply significant torque from the actuator.
Disadvantages
Ball valves are not suitable where flow control (throttling) is required as the flow rate cannot be accurately controlled. Throttling (any valve position between fully open and fully closed) also creates significant turbulent flow across the ball, which leads to frictional losses and potential damage to the ball itself. For these reasons, ball valves should not be the first valve design chosen when flow-control is required.
Ball valves should not be used in high temperature systems because the valve seating materials could become damaged and the valve will no longer seat correctly (there are exceptions to this rule if special high temperature ball valves are required).
Ball valves should not be used in any flowing mediums with a high abrasive particle count. Abrasive particles could easily damage the valve seat and prevent the valve seating correctly.
3D Model Details
This 3D model shows all major components associated with a typical ball valve, these include:
- Valve Handle
- Valve Stem/Spindle
- Ball
- Seat
- Casing/Body
- Gasket
- Nuts and Bolts
Additional Resources
https://en.wikipedia.org/wiki/Ball_valve