Diaphragm valves use a flexible diaphragm to shut off flow. The center of the diaphragm is pushing down into a seat. Use these valves for fluids that are dirty or have a high particulate content. Available in plastic or metal. In general, metal valves withstand higher temperatures and pressure than plastic valves. Choose the valve based on your application. Determine the temperature, pressure, and flow rate under which your valve will be operating.