Fuel Gas Firing – Testing Minimum Fire Bypasses
Minimum-fire bypasses must be checked regularly because firebox safety depends on them. After each shutdown, the bypasses should be tested for flame stability at minimum-fire conditions with all burners lit and full draft.
To test a minimum flow bypass, the operator:
• Opens the valve enough to see the burner header pressure change or to see the firing control valve close a bit
• Return the valve to its set position
Minimum-pressure fuel bypasses should be installed so that their output can be blocked off from the burner header and instead vented to relief. Downstream of the vent valve there should first be a pressure gage and then a restriction orifice.
To test the minimum pressure bypass (refer to Figure 600-10), the operator:
1. Pulls the restriction orifice to verify that it is clean (if the orifice is drilled in a union blind, his task is easier), then reinstalls it
2. Closes the valve that isolates the bypass discharge from the main fuel line
3. Opens the valves that vent the bypass to relief through the restriction orifice
4. Reads the resulting pressure upstream of the restriction orifice, which should equal the chosen minimum-fire pressure
5. Closes the vent valve that vents to relief
6. Opens the valve that isolates the bypass discharge from the main fuel line