Ball valve
Ball Valves ( English ball valve ) are fittings with a pierced ball as a shut-off and, particularly for larger pipe diameters, as spherical valves , respectively. Just like the similarly constructed plug valves, ball valves are mostly used as shut-off valves . It is characteristic of a shut-off valve that it closes completely by turning the axis by just 90 ° (in contrast to valves and slides, for example). Ball valves can be equipped with actuators to automate opening and closing.
Ball valves are sometimes misunderstood as ball valves ( English ball check valve designated). A ball valve, however, usually means a check valve with an axially movable ball that only allows flow in one direction.
Design features
A distinction is made between ball valves with full bore or reduced bore. With full passage, the bore in the ball has the same inside diameter as the connected pipeline. This results in low flow losses. The ball is actuated - as with other shut-off valves - by turning the handle by up to 90 °.
As a rule, soft-sealing ball valves with sealing rings made of plastic (often PTFE ), which are suitable for temperature ranges up to 220 ° C, are used. Metallic seals are used at higher operating temperatures.
Simple ball valves have a floating ball (see sectional diagram on the right), which is usually held in position by the seat in sealing rings arranged on both sides.
In the case of ball valves with a guided ball , the ball is provided with a pin on two opposite sides that is mounted in the housing. Only one sealing ring is then required.
The sealing rings are positioned in such a way that the pressure of the medium on the ball or sealing ring increases the contact pressure between the ball and sealing ring. With floating bearings, the ball is pressed into the opposite sealing surface. When the ball is guided, the fluid presses the seal against the housing and ball.
For large nominal sizes (e.g. pipelines ), a shut-off device that deviates from the precise spherical shape is often used. A slight ovalization of the ball increases the pressure on the sealing seat rings when actuated.
The surface pressure between the ball and sealing rings required for sealing requires relatively high working torques to open and close the valve, in large nominal sizes (e.g. DN 1200) up to 80,000 Nm. The breakaway torque increases even further over time due to deposits and adhesion effects between the ball and the seal seat . If ball valves are not operated regularly, the required operating force can become so high that, in simple designs, the handle or drive shaft break or the sealing rings are damaged.
The actuation force also depends on the pressure of the medium. For large models and high pressures, it makes sense to bypass ( use to put) the main valve. In this way, the differential pressure can first be reduced in order to facilitate subsequent opening. A worm gear can also be used to reduce the actuation force .
With a three-way ball valve, the ball has an L-hole or a T-hole instead of a through hole. This means that the flow of the medium can be distributed over one or two outlets. Four or five-way taps are also available.
particularities
When a normal ball valve is operated, part of the medium gets into the dead space between the ball and the housing. This can cause hygienic problems, since the dead space is only partially flushed through by the medium and can usually hardly be cleaned. For critical applications, the dead space is therefore reduced or eliminated by filling bodies.
Changes in the volume of liquids due to cooling or heating can generate such a high hydraulic pressure in the volume inside the ball when the ball is shut off that the ball bursts. This can be remedied by a predetermined breaking point for volume compensation or a small relief hole.
A hole through the housing and the axis of rotation of the ball also allows the tightness of the ball valve to be checked and temperature and pressure sensors to be inserted and replaced without the pressure having to be released from the connected line.
A special method allows nozzles to be attached to containers or pipelines under operating conditions. The socket is butt welded. The ball valve is mounted on this and a hole is then drilled through the wall through the open ball. This method can also be used with flammable gases and liquids.
Compared to other fittings, a ball valve has the following advantages:
- Ball valves with a full cross-section enable large K vs values
- Ball valves with a full cross-section allow a pig to pass through to clean the pipe
- Ball valves are generally small in size
- Depending on the construction and choice of material, sufficient tightness and rapid actuation can be achieved even at high pressure.
Disadvantages are:
- dead space touched by the medium; Only special designs and similarly constructed plug valves are free of dead space
- increased wear on the sealing surfaces depending on the operating conditions
- Complex construction with large nominal widths, which require a double bearing and gear reduction of the spindle drive in order to reduce the operating force
- Can only be used to a very limited extent for controlled control of the flow rate, as the opening cross-section does not change proportionally to the angle of rotation of the ball
- Due to the design, closing a ball valve causes pressure shocks in the connected lines, the strength of which depends on the closing speed of the valve and on the mass and compressibility of the medium flowing in the connected lines; Pressure surges load the sealing rings of the ball valve and can lead to leaks
Types of ball valves
Usual types of execution:
- One-piece ball valve with reduced passage and internal thread
- 2-part ball valve with full passage and internal threads (I / I)
- 2-part ball valve with full passage and internal / external thread (I / A)
- 3-part ball valve with full bore, internal thread and ISO-TOP (actuator)
- 3-way ball valve with full bore, T or L bore, internal thread and ISO-TOP 5211 for actuator
- Mini ball valve I / I or I / A
- One-piece compact ball valve
- 2-part flanged ball valve
- 3-part ball valve with cutting ring or press connection
- Ball outlet valve ( KFE cock )
Lever or wing handles which are parallel to the flow direction in the open position are common.
Web links
Individual evidence
- ^ [1] Konrad Bergmeister , Frank Fingerloos, Johann-Dietrich Wörner (eds.): Concrete Calendar 2011: Focus: Power plants, fiber concrete (Concrete Calendar (Vch)) Ernst & Son, Berlin 2010, ISBN 978-3-433 -02954-1 .