Furnace fuel shutdown system

Figure 10-23 A chemical process operator relies on a furnace fuel shutdown system. Class 3 Normal Consequences Instrument Loops...

Car-Sealed Closed Valve - In certain cases it may be advantageous to use car sealed closed valves, such as in a bypass around a fuel gas control valve used for furnace flameout protection. The bypass is provided so that the automatic shutdown system can be periodically checked for operation. Where CSC valves are used for other purposes, they are also limited to appUcations where inadvertent opening of the CSC valve would not overpressure the equipment by more than 1.5 times design pressure. 

For example, in furnace operation it is important that there be a check for the continued presence of a flame at the burners. If the flame goes out but fuel continues to flow into the hot furnace, an explosive gas cloud may be created in the furnace. Were this cloud to ignite on, say, a hot surface in the firebox, the furnace could be destroyed by the resulting explosion. To prevent such an explosion from occurring, it is common to install a fire eye in the furnace. This is a device that detects the presence of a flame at the burners. If the fire eye does not see a flame it sends a signal that cuts off the flow of fuel. Since it is vital that this shutdown system works properly, a second fire eye is often installed. Then, if the first fire eye does not work for some reason, the second fire eye will detect the problem and shut down the system. 

During normal operations, checklists and samples are collected as advanced instrumentation monitors the process. The types of problems a fired heater or furnace system typically encounter include flame impingement on tubes, coke buildup inside the tubes, hot spots inside the furnace, fuel composition changes, burner flameout, control valve failure, and feed-pump failure. Other problems may include incorrect temperature indicator readings, failure of oxygen analyzers, oxygen leaks on the furnace, and the unexpected shutdown of downstream equipment. A fired heater system is designed to run almost continuously, 24 hours a day, 7 days a week. The operational team is in place to ensure that the equipment and systems operate safely, effectively, and produce a quality product that meets or exceeds customer expectations. 

A malfunctioning high-temperature furnace trip caused a major tray upset in a large ethylbenzene fractionator. Figure 17-3 shows the tower control system. When the circulation pump briefly failed, the flow of hot vapor to the tower was interrupted. The tower temperature controller was called for more heat, and furnace fuel firing consequently increased. At this point, the automatic furnace shutdown system should have tripped off the furnace fuel due to the excessive furnace outlet temperature. This trip failed to function. 

Flue gas temperatures of waste heat boilers are only 100 to 150 F lower than from regenerative systems thus, fuel savings may be marginal. Waste heat boilers have proved effective with stainless-steel annealing catenary furnaces. They have adjacent steam requirements all year for cleaning their product after annealing. Their firing rates, flue gas temperatures, and heat stored in refractory are moderate, so water problem shutdowns are fewer. 

Fixed heaters are extensively used in the oil and gas industry to process raw materials into unstable product in a variety of processes. Fuel gas is normally used to fire the units that heat process fluids. Control of the burner system is critical in order to avoid firebox explosions and uncontrolled heater fires due to malfunctions and deterioration of the heat transfer tubes. Microprocessor computers are used to manage and control the burner systems. Principle functions of the burner management system are provided with programmed ignition of niters and burners at lightoff, flame monitoring during boiler operation, and proper furnace shutdown. 

Burner management systems are another example where the fuel/air ratio controls provide a safety role and where the start up ignition and shutdown purge sequences protect against possible explosions of unbumt fuel in the furnace. 

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