Questions regarding the fuel gas cascade in fuel gas heating furnaces? ? ?

Generally, the outlet temperature of a heating furnace is controlled in cascade with the fuel gas pressure. Since the flow rate of the fuel gas is prone to fluctuations, it has a significant impact on the furnace temperature, whereas the pressure of the fuel gas is relatively stable, making its flow rate easier to control.

The single-loop control system can be used when high requirements are not placed on the exit temperature of the material being heated, and when the pressure in the fuel pipeline remains relatively stable; in this case, the fuel supply is adjusted directly based on the exit temperature of the material from the heating furnace. The advantage of this approach is simple control and low investment ； The disadvantage is that the heating furnace raises the temperature of the material from dozens of degrees to several hundred degrees, resulting in a high heat load; any slight variation in the pressure and calorific value of the fuel causes a significant change in the temperature at the furnace outlet ； At the same time, when the heating amount changes, due to the lag in heat transfer and temperature sensing elements, the adjustment does not occur in a timely manner, resulting in large fluctuations in the furnace outlet temperature. The principle of a single-loop temperature control system is shown in the figure (Automatic Control System for Oil Outlet Temperature). In this figure, transmitters are used to measure various parameters such as temperature, pressure, and flow rate; the measured values are then converted proportionally into air pressure or current signals, which are sent to instruments for display or to regulators for adjustment. Upon receiving a signal, the regulator compares it with the set value and issues commands in accordance with certain rules to activate the actuator, thereby maintaining the controlled parameter at the set value. The actuator typically uses a pneumatic diaphragm control valve, which receives commands from the regulator to change the opening degree of the control valve, thereby functioning in a manner similar to manually opening or closing the valve. The control process of a single-loop temperature control system is relatively long, and the furnace responds slowly to adjustments; therefore, it is less commonly used in actual production. The following types: 1. Temperature control using a single parameter – an automatic control system for the temperature of the oil product at the time it leaves the furnace. 2. Cascade temperature control system: For heating furnaces where strict requirements are placed on the exit temperature of the material being heated, and where various input parameters remain stable, the main controllable factor is the calorific value of the fuel. For fuels with stable properties, controlling the calorific value can be equivalent to controlling the fuel flow rate. Therefore, a control loop based on the outlet temperature of the heated material is used as the main loop, while fuel flow regulation serves as the secondary loop. The secondary loop is utilized to detect disturbances caused by fluctuations in fuel flow in a timely manner and to control them. By shortening both the control and feedback paths, the overshoot of the outlet temperature of the heating furnace is reduced, thereby improving the quality of control. From a control theory perspective, the secondary loop of a cascade control system has a much greater ability to overcome disturbances compared to a single-loop system. As for disturbances affecting the primary loop, although the secondary loop cannot eliminate them directly, it reduces the time constant, improves the dynamic characteristics of the system, and accelerates the adjustment process; as a result, dynamic deviations are also reduced. At the same time, the advantage of using fuel flow as a secondary loop is that it allows for the measurement of fuel consumption, which is useful for testing and accounting during the operation of the device. Series control of the heating furnace outlet temperature and fuel flow (pressure). The series control system uses a main regulator to control the oil outlet temperature, while a secondary regulator is used to adjust the fuel flow. The output of the main regulator is used as the “setpoint” for the secondary regulator, and this setpoint is variable. In this way, when there is a change in the fuel flow rate, due to the significant lag of the furnace, the temperature of the oil exiting the furnace does not change immediately. As a result, the output of the main regulator, which is also the setpoint for the secondary regulator, does not change during this time, allowing the secondary regulator to quickly restore the fuel flow rate to its original value and thus preventing temperature fluctuations. Series control of the material outlet temperature in the heating furnace and the furnace temperature: Influenced by factors such as the calorific value of the fuel oil, feed flow rate, and feed temperature, these factors first cause changes in the furnace temperature, which in turn affect the temperature at the furnace outlet; the lag associated with the former is much smaller than that associated with the latter. The furnace temperature control can be used as a secondary loop, while the outlet temperature control of the material being heated serves as the primary loop, thus forming a cascade loop. In this way, the originally lagging object is divided into two; the secondary circuit acts in a leading manner, allowing for rapid adjustment once disturbance factors affect the furnace temperature, thus maintaining stability in the outlet temperature of the material being heated. It should be noted that, to protect the equipment, the furnace temperature should not experience large fluctuations; therefore, when adjusting the parameters, the secondary controller should not be set to be too sensitive, nor should a differential action be applied. This control scheme is more effective in the following situations: (1) when the heat load is high but the heat intensity is low. That is, large fluctuations in furnace temperature are not allowed to prevent damage to the equipment. (2) When the main disturbance is the change in the calorific value of the fuel (i.e., changes in its composition), the secondary loops of other cascade control schemes cannot detect it. (3) There are two sets of furnace tubes in the same furnace, used to heat two different materials simultaneously. At this point, although only one set of temperatures is controlled, it is required that the other set also remain relatively stable. (4) Double-inclined-roof box-type tubular furnace, the key point being to identify a temperature measurement point that is fast in response and capable of representing the furnace temperature.

In my opinion, the stability of furnace control is greatly influenced by the calorific value of the fuel gas. And flow rate makes it easier than fuel gas pressure to facilitate reactions and monitor the calorific value of the fuel.

So, what you’re saying is that the flow rate can indicate the heating condition of the furnace, in other words, it shows the heating capacity of this furnace, right?

To implement cascade control, it is first necessary to adjust the PID settings properly. Start by making small adjustments to the PID parameters to stabilize the operation. Given the significant lag in temperature control in heating furnaces, a relatively long differential time value is required

It can be said that flow rate provides a more accurate indication; if pressure control is used alone, it is difficult to take into account changes in the density of the fuel. Using flow rate, this can be reflected in the mass flow rate, and temperature and specific gravity can be taken into account to comprehensively calculate the calorific value of the fuel. In this way, the calorific value of the fuel is also incorporated into the control process, resulting in better temperature control.

The actual effect of being connected in series with pressure is also achieved by changing the flow rate of the fuel gas entering the furnace; it seems to me that connecting it directly in series with the flow rate would make control easier for us； However, many devices nowadays, including those that have been newly installed, use a configuration in series with pressure; this approach must have its own advantages as well. It’s just that those of us working at the operational level don’t understand it. I hope someone knowledgeable can explain it to us!

For the gas phase, pressure is generally controlled, while for the liquid phase, flow rate is controlled. There are many factors that affect the flow rate in the gas phase, making it difficult to control. As for the temperature at the reactor inlet, precise control is required; therefore, pressure control is used

The calorific value of the fuel gas and the flue gas at the outlet of our furnace fluctuates day by day, and all adjustments are done manually