Incidents drain, down

The flow of fuel to the boiler should have been slopped automatically when the water level fell to a dangerous level. Unfortunately, the low-level trip failed to function. (Many newer boilers come equipped with backup low-level trips.) The operatingengineer investigating this incident determined that this trip had not been tested for years. He set up the following program to prevent a recurrence. First, the fuel gas control valve was locked into its normal operating position with its hand jack. Then the low-liquid-level trip pot (see Fig. 17-1) was blocked in and drained down. Finally, an operator verified that the pneumatic signal to the fuel-gas control valve fell to zero (this valve was AFC—air failure closes). 

The initial incident involved a pinhole leak on a 6-in. line located 112 feet above grade. The unit might have been shut down at this point, but a decision was made to keep it on line. Efforts to isolate the circuit by closing four block valves failed. The line remained hot and the level in the downstream stripper vessel continued to rise. Several attempts were made to depressure the circuit and free drain all the residual naphtha. At least 15 work orders were issued over a 13-day period and most of these failed to meet the company s own work standards. 

By similar considerations, it is best to shut down the reboiler and drain liquid before repressuring the column at shutdowns. This practice is imperative if the column contains hot, flammable liquids and air is likely to enter during repressuring. In one incident (169), violent combustion of liquids upon air ingress into a column at shutdown caused ii jury and column damage. 

In order to secure the facility and to ensure that the incident is not allowed to get worse, the persons present at the scene (and the emergency responders) have had to take immediate corrective actions such as draining tanks, shutting down equipment, and removing equipment that is restricting access. All such actions should be recorded in the first report because they may affect the subsequent analysis. 

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