A weir-type diaphragm valve is shown in Fig. 11.8a. The valve is shown open;
closure is achieved by forcing a flexible membrane down onto the weir.
Diaphragm valves are good for slurries and liquids with suspended solids, are
low cost devices, but tend to require high maintenance, and have poor flow
characteristics.
Aone-piece ball valve is shown in Fig. 11.8b. The valve is a partial sphere that
rotates. The valve tends to be slow to open. Other than the one shown in the
figure, the ball valve is available in other configurations also with spheres of
various shapes for different flow characteristics. The valve is good for slurries
and liquids with solid matter because of its self-cleaning operation. Ball valves
have tight turnoff characteristics, are simple in design, and have greater capacity
than similar-sized globe valves.
An eccentric rotary plug valve is shown in Fig. 11.8c. The valve is of medium
cost but requires less closing force than many valves and can be used for forward
or reverse flow. The valve has tight shutoff with positive seating action,
has high capacity, and can be used with corrosive liquids.