Fiberglass Relief Valves -Fiberglass valves are used the same way as their counter parts manufactured in metal, primarily on above ground storage tank installations. Fiberglass construction can be used where highly corrosive and toxic liquids are being stored. The Fiberglass series design will protect the tank from damage created by overpressuring or excessive vacuum. Costly product evaporation losses due to normal tank "breathing" are greatly reduced. Retention of product vapors reduces the possibility of atmospheric contamination.