Observations on testing valve packing systems with methane or helium

摘要

Fugitive emission testing using both methane and helium as the test gas may exhibit results where the data seems to "flip flop". In one case methane leakage is greater than helium leakage and in another case helium leakage exceeds methane leakage. The laws of fluid mechanics may explain the difference. For a leak path that exhibits a "molecular flow mode" helium leakage is expected to exceed methane leakage. When a leak path exhibits a "laminar flow mode" methane leakage is expected to exceed helium leakage. The data can be further complicated during fugitive emission testing due to dimensional changes in the leak path geometry caused by a higher test pressure. A leak path that initially supports a "molecular leak" can be physically changed by a higher test pressure and support a "laminar leak" during later test cycles. This article explains the fluid mechanics of leaks and why the "flip flop" in helium/methane leak rates may be merely a change in the flow mode across the leak path.