【2026 Interlock System】Interlocked shutdown of the heating furnace: The surge in negative pressure is not merely due to exhaust effects
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During the operation of refining and chemical processing units, the interlocked shutdown of heating furnaces is an extremely important safety protection measure. When critical process parameters exceed limits or equipment malfunctions occur, the system automatically shuts off the main fuel to prevent further expansion of the accident. However, a phenomenon occurs during interlocking: the negative pressure in the furnace often soars instantly ； And the ever-burning lamps, which should have remained lit, were sometimes extinguished.
I. Actions taken by the system after interlock shutdown: Upon activation of the interlock, the heating furnace does not simply shut down with a single button press; instead, protective actions are carried out in stages in accordance with established safety logic. First, the main fuel cut-off valve closes rapidly, stopping the supply of fuel to the main burner and eliminating the heat input at its source. Secondly, the blowers and exhaust fans usually do not stop operating immediately, but rather maintain a certain air flow to purge the furnace. Its purpose is to quickly displace any combustible gases that may remain in the furnace chamber, thereby preventing the formation of explosive mixtures. Finally, the treatment methods for process media are not entirely consistent. For reactive furnaces such as hydrogenation furnaces and cracking furnaces, it is often necessary to simultaneously cut off the feed after interlocking ； For equipment such as atmospheric and vacuum distillation units and reboilers, which rely primarily on heat exchange, the flow of material is usually maintained to allow a medium to carry away the excess heat from the furnace tubes, thereby preventing dry burning and coking. Therefore, interlock shutdown is essentially a comprehensive control process that takes into account safety, equipment protection, and process stability.
II. Why does the negative pressure in the furnace increase suddenly? Many people believe that the increase in negative pressure is simply due to the reduction in smoke after combustion stops, while the exhaust fan continues to draw air out. In fact, this is only part of the reason. The core mechanism that truly causes the occurrence of the negative pressure peak is the \"thermal contraction effect\". During normal operation, the furnace is filled with high-temperature flue gas at 700–800°C. After the main fuel is cut off, the heat source disappears instantly, while the blower continues to supply air at room temperature. Since the temperature of the furnace gas drops rapidly within a few seconds, according to the ideal gas law, a decrease in gas temperature in a fixed volume necessarily leads to a decrease in pressure. This pressure drop caused by the sudden drop in temperature can be understood as a strong \"thermal draft\" being created inside the furnace.
III. Why is a permanent light still needed after interlocking? Since the main fuel supply has been cut off, why not shut down the perpetual light as well? It’s mainly for safety reasons. For many conventional distillation and reboiler units, the pilot light serves not only as a source of fire for restarting the process but also as an important mechanism to prevent the accumulation of flammable gases. If there is a slight internal leakage in the main fuel shut-off valve, or if a small amount of unburned gas remains in the furnace, the pilot light can ignite it promptly, preventing the combustible gases from reaching their explosive limit. For some high-risk units, such as hydrogen production furnaces and hydrogenation feed furnaces, a design approach that involves simultaneously shutting off the main fuel supply and the pilot light is also employed, in order to reduce risks by completely isolating the fuel source. Therefore, there is no absolute uniform answer as to whether to retain the perpetual light; it is determined by the device’s risk level and design philosophy.
IV. Why does extreme negative pressure cause the perpetual light to go out? The most common issue on site is that the design requires a constant-lit lamp, but it gets extinguished by negative pressure after interlocking is activated. The root cause is the disruption of the stable combustion conditions. Most modern furnace pilot lights adopt a premixed design. Under normal conditions, the fuel gas and air are mixed in a certain ratio, then ejected from the nozzle to burn steadily. When the negative pressure in the furnace suddenly increases, the pilot light air duct is subjected to a strong suction effect, causing the amount of primary air to increase sharply and thus raising the injection speed of the mixture significantly. For the flame to remain stably attached near the nozzle, it is necessary for the velocity of the mixed gas stream to be in balance with the speed of flame propagation. When the jet velocity far exceeds the flame propagation speed, the flame root cannot remain stable, resulting in \"flame detachment\" and ultimately leading to extinguishment. Sometimes, when the main flame of the burner goes out instantly, it also causes the pilot light to go out, for the same reason.
V. How to prevent the everlasting light from being extinguished? From the perspective of on-site adjustment, the following principles should be followed: First, prioritize controlling the air volume. Adjust the damper of the permanent light to a lower setting in order to limit the excessive intake of air caused by negative pressure, thereby reducing the injection speed of the mixture. This needs to be done during normal operation in order to prevent the use of permanently lit lamps under high-speed mixing conditions. Of course, closing the long-flame air damper also has its disadvantages; one is the insufficient rigidity of the flame and inadequate combustion ； Second, the small air damper is prone to clogging.
Second, optimize the interlock logic. Increase negative pressure control at the moment of main fuel tripping, such as automatically reducing the load on the exhaust fans or adjusting the flue dampers, to suppress the peak value of negative pressure. The advantage of doing this is that it also slows down the cooling rate of the furnace chamber, thereby reducing thermal shock damage to the refractory lining and furnace tubes.
Third, improve the structure of the eternal light. Manufacturers of perpetual lamps are required to conduct hot-test under high negative pressure conditions to ensure that the lamps can still operate stably in such conditions.
The interlocked shutdown of a heating furnace may seem like a simple fuel cut-off action, but in reality it involves multiple specialized fields such as combustion, thermal fluids, and safety engineering. The surge in negative pressure is not merely a suction effect, but rather the result of the thermal contraction of the furnace combined with the inertia of the exhaust fan ； The extinguishment of the everlasting lamp is not accidental; it is the result of the stability of combustion being disrupted by an instantaneous negative pressure.
The following, “Differences and Connections between SIS and DCS”, is provided for reference. Functional role: The SIS system is focused on safety interlocks, and is used for safety protection functions such as emergency shutdown and monitoring of hazardous conditions, in order to ensure the safety of personnel, equipment, and the environment. For example, a shutdown is triggered immediately when temperature or pressure exceeds safe limits. DCS system: Used for conventional process control (such as temperature and flow regulation) to ensure the continuity and stability of production, with an emphasis on the dynamic adjustment and monitoring of process parameters.