Pass Flow-Balancing – Low Flow Shutdown
Figure 600-1 also has a low flow shutdown on each pass. Loss of charge flow has caused major fire losses in recent years as discussed in Section 300. On loss of flow, furnace tubes overheat, rupture, and dump process hydrocarbons into the firebox. Totally shutting off all fuel to the fire box on loss of flow through any furnace pass is the only sure way to prevent tube rupture on loss of flow.
Double block and bleed shutoff valves for the fuel gas, which is the recommended method, are shown in Figure 600-1.
Adding a low flow shutdown is a significant step, and it requires careful analysis to ensure that the furnace operation will not be interrupted by “nuisance” shutdowns. See Section 1300 “Process Alarms and Shutdowns,” Instrumentation and Control Manual, for additional guidance.