What can a valve do? It can prevent flow (on/off), allow variable control of the flow and provide a safe working environment. What sort of questions should we be asking when selecting a valve?
Will the valve type do the job we want it to do ?
Is it suitable for the type of operation we have ?
What is the pressure drop across the valve ?
Does it need (or can it) to be controlled either manually or pneumatically ?
What does it cost ?
The various types of valve available are summarised below. Illustrations of each are at the end of thos section.
1. Gate Valve. General purpose valve (utility lines), low pressure drop and coarse control, can be used as an isolation valve. A sliding disc moves in and out of the flow path by turning the valve stem.
2. Globe Valve. General purpose valve with a high pressure drop and subject to leaks in the flow. A horizontal disc or plug is raised or lowered across a seat in the flow path. Use is mainly in water or steam lines as it can be adjusted quickly.
3. Piston Valve. Similar to globe valve, good aseptic quality and high pressure drop, can be used with pneumatic control. A piston situated between two packing rings is raised or lowered in the flow path.
4. Plug Valve. Good course control, low pressure drop, good for steam lines but subject to seizures and leaks. A tapered plug is rotated a quarter turn to open or close flow through the tapered plug.
5. Needle Valve. Similar to globe valve, gives fine control of liquid flows, not good for aseptic use. A tapered plug is raised or lowered into a seat in the flow path.
6. Butterfly Valve. Large diameter pipes, absolute seal impossible, not-aseptic use. A disc, the same diameter as the inside of the pipe is turned so that it is perpendicular to the flow (allowing flow) or across the pipe (closing flow).
7. Ball Valve. Similar to plug valve, suitable for high temperatures, has scouring action on opening/closing. Opened or closed by a quarter turn, hole in ball can be the same size as the pipe diameter.
8. Pinch Valve. Good control in the range 10 - 95% open, sleeve is subject to wear but suitable for aseptic use. The valve can be opened/closed pneumatically by pinching the flexible sleeve within the valve.
9. Diaphram Valve. Similar to pinch in quality, uses a weir which gives accurate control, most common in biotechnological applications, can also provide product containment (i.e. in line). The valve can be opened/closed pneumatically by pinching the flexible diaphragm within the valve.