POLMAC AUTOMATIC SPRING-LOADED MAXIMUM PRESSURE SAFETY VALVES. These valves are made of a plastic (nylon and fiberglass) which is particularly resistant to liquid chemical products and even abrasive liquid chemical products. If they are used as safety valves, they guarantee a constant pressure whenever it is not necessary to continuously regulate the working pressure. Any excess liquid bypasses the valve and allows it to maintain a constant line pressure. Available in the membrane version (for better regulating accuracy) and the piston version (to deaden the effect of vibrations).