Fuel Gas Firing – Dewpoint
Dewpoint is the temperature at which liquid starts to condense from a vapor mixture. This can happen all too easily in a fuel gas system; the pipe wall temperature need only drop below the dewpoint of the gas. Frequently process gas is saturated with water vapor, and the water dewpoint can be more troublesome than the hydrocarbon dewpoint. There are four different approaches to avoiding slugs of liquid in the fuel gas system:
1. Heat trace the pipe wall to keep it hotter than the gas maximum dewpoint of the gas.
2. Keep the gas dewpoint below the pipe wall temperature. This might be done by limiting the specific gravity of the various fuel gas sources.
3. Slope the fuel gas piping for draining and install condensate pots or knockout drums at the low spots. High level alarms should tell the operator when to drain it.
4. Some combination of the above three approaches will give the most reliable fuel gas system.