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Valves - Manual Valves - Ball, Rotary and Piston Valves


The ball valve is the most common example of a whole family of valves that include rotary valves, piston valves and other valves that use selective positioning of an inner core with one or more through-holes or cavities to direct flow from one or more inlet ports to one or more outlet ports.  I realize that the above is a mouthful so let's start with a simple example.
A rotating ball with a hole (bore) through it allows or prevents flow through the valve as it is rotated on its vertical axis as shown above.

A rotating mechanism with a hole bored through it is positioned in a valve body between an inlet port and an outlet port.  In some cases, the flow path is provided with an indent or cavity in the rotating ball.  Flow is allowed when the hole through or cavity in the rotating mechanism lines up with and connects the inlet port and the outlet port.  The rotating mechanism can be a ball or a thick, flat disc of metal or other material as shown in the illustration of a rotary valve below.
In a rotary valve, a rotating disc with a hole or indents allow or prevent flow from the inlet to the outlet.

Sounds simple, right?  Well hang on!  Now that we have this basic concept, it doesn't take much to imagine that any orientation of the inner core of a valve of this type that prevents alignment of the bore hole(s) with the inlet and outlet ports will stop the flow through the valve.   In the rotary valve, the central core with a bore through it commonly rotates on a single axis to align the bore with the ports. However, displacing the central on the vertical axis (in the illustration below) would just as effectively control flow through the valve.
If the rotating part of a rotary valve is arranged so that it can move along the vertical axis instead of rotating around it, the rotary valve becomes a piston valve as shown above.

The rotary valve becomes a piston valve activated by sliding the inner core (piston) up and down instead of rotating it. In a ball valve, rotation of the ball is commonly limited to one axis to turn the flow on and off.  However, if the ball was not confined to a single axis of rotation and could rotate freely on any axis (which is possible since it's a ball), once the bore and ports were aligned on one axis, rotation on a perpendicular axis would not change the alignment of the first bore with its associated ports.  In fact, there could be a second bore within the ball that would align once another particular orientation was achieved on a second axis of rotation. By cleverly designing the ports on the ball, several control options can be accomplished in a single valve with multiple axes of rotation. Simple Challenge - Most of you have seen valves of both of the compound types described above many, many times and may use them daily.  Where?  Click on the "Comment" link below and let me know. Advanced Challenge - What musical instruments use valves of this family?  Which instruments use which type of valve? SUPER Advanced Challenge - Where would you find valves of these types in a car? I'll "approve" all of the answers to all of these challenges for publication. This whole family of valves has almost unlimited possibilities in the world of both liquid and gas flow.  Variations abound.  The real challenge of valves of this type is providing an acceptable and durable seal between the moving part (with the bore) and the ports while allowing the required motion. Benefits -
  • Reliability!  The life of a well-designed ball, rotary or piston valve can be expected to exceed that of a globe valve with a washer (as described in a previous blog).  These valves usually rely entirely on the primary seal between the moving part and the ports and do not have a requirement for a packing nut.
  • When the bore and ports are at least equal in diameter to the inlet and outlet pipes, there is little or no restriction to flow through the valve when the bore and ports are in alignment (valve "on").
  • Usually the flow is equal in both directions through the valve.
  • These valves have the ability to rapidly transition from "on" to "off" with a minimum mechanical displacement and can, therefore, be used in applications where speed is of the utmost importance.
Coming Next - The final valve in the manual valves series is the gate valve.  An upcoming blog will describe the gate valve and its variations including the "butterfly" valve for you wannabe entomologists out there.

-  FJF  -