While a wide variety of materials and part shapes are available, diaphragm valves use the same basic mechanism. A solid, movable piece (the "actuator") presses into the middle of a thick yet flexible barrier (the "diaphragm"), forcing material into the conduit and blocking the flow of liquids.
The actuator consists of a wide loop or disk on top of the valve. At the center of the disk is a threaded rod that enters the valve and attached to the "head" piece, right above the diaphragm.
Essentially a screw, the threaded rod moves up and down in its housing as the disk is turned counterclockwise and clockwise, respectively. The large diameter of the disk allows the user to generate considerable leverage/torque.
Therefore, even at high pressures, pipes can be shut off with enough work from the user. Note: there are also actuator designs where, instead of a manual control, the position of the head is controlled by hydraulic press or pneumatic tool.
The shape of the actuator head corresponds to the size and shape of the pipe inside the valve. For "A"-type valves, the pipe narrows to a small gap right below the diaphragm.
As such, the head consists of a shallow wedge. "KB"-type valves feature a full-sized pipe diameter, requiring the head to be a tall, slab with gently sloping edges; this allows the head to distribute pressure across a large area of the diaphragm.
Finally, "WFB"-type valves have a wedge-like head that, in addition to squeezing the pipe closed, can "pull" the diaphragm up beyond the normal diameter to increase flow.
Durable and thick, the purpose of the diaphragm is to protect the material inside the valve from the contents of the pipe. For example, in a chemical manufacturing facility that producing hydrofluoric acid (which dissolves virtually every substance except for certain plastics), a special diaphragm would seal the metal actuator head and valve housing.
Otherwise, the acid could melt the actuator, burst from the pipe to kill workers and surge uncontrollably. Either riveted to the ceiling of the pipe or squeezed along the edges between two stacked pieces, the diaphragm is flat when at rest.
The center is considerably thicker and often reinforced with mesh fibers. Meanwhile, the edges are thinner for easier stretching.
The shape of the valve's conduit (ie. the cross section) has important ramifications on the pressure within the pipes downstream.
For example, the "A"-type's narrowing conduit slows the flow rate at the valve. This slow rate is good for abrasives in that it helps protect the interior of the pipe.
In the same way, the "WFB"-type works well for hydrants in that the narrow conduit allows for hoses to fill as a controlled rate, preventing the "whip-cracking" effect caused by the rapid expansion of a flat, folded tube. Once the water has displaced all the air in the hose, however, the conduit can be expanded to increase the flow of water.
This increases the pressure at the nozzle of the hose, allowing water to shoot farther. Finally, the "KB"-type's large conduit opening pairs the high-speed, high-volume flow of a regular pipe with the convenience and safety of an emergency stop valve.