Shut-off valve

Shut-off valve

A shut- off valve (or shut-off valve ) is a fitting for the controlled opening or closing of flow openings or pipelines . It regulates the flow of fluids in liquid and gas installations .

Executions

Pressure loss coefficients of shut-off and control valves according to nominal size. Ordinary valves cause significantly higher pressure losses than taps and slides. With larger designs, the ratio worsens.

Shut-off valve heating circuit

Shut-off valves are manual or motorized valves with 2 or 3 connections. In most cases, the flow is not shut off with a single movement of the hand, as is the case with ball valves , but is continuously throttled by turning the spindle several times, during which the cross-sectional area of the valve opening is gradually reduced.

Depending on the "shape of the flow opening", a distinction can be made between lift valves and ring valves . In the case of the former, the shut-off body rises vertically from an opening. With the ring valve, the opening is only free on the outside, for example because an axis is in the middle.

The shut-off body can be designed, among other things, as a plate, cylinder, cone or truncated cone, regulating cone (English cone, truncated cone, regulating cone) or ball.

Ordinary straight or angle valves have two connections, while with three-way valves the flow can be opened in two different directions.

The flow resistance of shut-off and control valves primarily depends on how much the medium is deflected, whether the cross-section narrows and whether eddies occur.
If the pressure loss coefficient of the valve is known, the formula    with applies    ${\displaystyle \Delta p=\zeta \cdot \rho \cdot {\frac {v^{2}}{2}}}$

${\displaystyle \Delta p}$: Pressure loss (derived SI unit : Pa )

${\displaystyle \zeta }$: Pressure loss coefficient (dimensionless)

${\displaystyle \rho }$: Density (SI unit: kg / m ³ )

${\displaystyle v}$: Flow velocity (SI unit: m / s)

Examples

• A traditional faucet is a shut-off valve with a plate-shaped shut-off body.

• Johnson valves (named after their inventor, the railway engineer Harley Sherman Johnson , * 1894, † 1996, USA ) belong to the range of adjustable shut-off valves. They are used in a variety of ways and in many sizes, but are particularly suitable for nominal widths of several meters, such as those found on dam drains. The cone slide can be driven mechanically or hydraulically and can be located outside or inside the closure element. They are characterized by low hydrodynamic losses and cavitation damage .

• Howell Bunger Valves, also suitable for outlets of dams, shut off water with a sliding ring that releases a flow opening at one end.

• 

  Johnson valve, schematic diagram.
  Moved to the right, the shut-off body closes.

• 

  Howell Bunger Valve, schematic diagram. The shut-off body is a sliding cylinder that is open on both sides. Moved to the right the shut-off body closes.

Advantages and disadvantages

Compared to other control devices, a shut-off valve has the following advantages:

• constant characteristic for reproducible and continuous control,
• Seat and plug are exchangeable and accessible from above (K _vs value adjustment, characteristic curve),
• task-specific linear equal-percentage characteristic,
• can be tightly closed if required,
• high differential pressures are possible,
• Noise-reducing installations are possible,
• Spindle sealing possible via bellows.

Disadvantage:

• relatively small K _vs values ​​related to the nominal size,
• large space requirements,
• relatively large dead spaces,
• with simple designs, high pressure loss and risk of cavitation .

See also

• Valve
• Safety valves are not operated manually, but open when a certain pressure is exceeded
• check valve
• Throttle valve

Web links

Commons : Ball valves  - Collection of pictures, videos and audio files