Check-All Valve Mfg. Co.”s Flange Insert Valve (seen here) fits between Class 150, Class 300 or Class 600 flanges and uses the pipe as the valve body, making it lightweight and cost effective. It has a supply-side spring so the valve trim is upstream of the mixture. It also has tight seals available in common or exotic elastomers for special conditions. A variety of spring settings are available from 1/8 PSI to 100 PSI settings.