Below is a list of Diaphragm Check Valve manufacturers from Nashua, New Hampshire. Diaphragm check valves are process stream regulation devices that maintain the unidirectional flow of a system through the use of a flexible membrane. Also known as non return diaphragm or one-way directional diaphragm valves, these devices are commonly employed in a number of industrial, commercial and even domestic applications that require reliable back flow prevention.