The control of drum level in heat recovery boilers has often followed traditional guidelines, such as:
- The proportional control action for drum level should not be too aggressive due to the presence of false level indications.
- To address significant fluctuations in steam load, three-element drum level control is often considered the preferred method for conventional boilers.
However, applying these standard practices directly to heat recovery boilers can lead to issues. Let’s examine the differences in drum level control between conventional and heat recovery boilers and explore why three-element control may not be necessary.
Heat Recovery Boilers: A Different Approach
In chemical plants, heat recovery boilers (HRBs) are commonly used for generating steam from the heat of exhaust gases. These boilers operate differently from conventional power boilers. One key difference lies in how steam production is determined: for heat recovery boilers, the amount of steam generated is directly related to the available waste heat. Therefore, the steam output is somewhat fixed and not influenced by fluctuations in steam demand in the same way as a power boiler.
1. False Level Indications in Heat Recovery Boilers
In conventional power boilers, false drum level indications can arise when steam pressure fluctuates due to load changes. These fluctuations affect the pressure in the steam drum, leading to the phenomenon of false level readings. Consequently, to maintain stable level control, a three-element control system, which takes into account drum level, feedwater flow, and steam flow, is often necessary.
However, in heat recovery boilers, the situation is different. The steam pressure in the drum is not directly tied to the fluctuations in the steam header pressure, which is common in power boilers. The pressure in the drum of a heat recovery boiler is typically stable, because it is controlled independently. Even when the steam load varies, the steam pressure in the drum remains stable. This greatly reduces the impact of false level readings.
2. Why Three-Element Control is Not Necessary for Heat Recovery Boilers
The primary role of a three-element drum level control system is to compensate for large variations in steam load. This is not as relevant in heat recovery boilers, where the steam output is relatively constant and directly proportional to the heat available from the exhaust gases. In this case, the need for feedforward control—often used in conventional boilers to adjust feedwater flow in response to changes in steam load—becomes unnecessary.
The steam output in a heat recovery boiler is essentially dictated by the waste heat being recovered, and as such, feedwater flow does not require the level of dynamic adjustment typical of power boilers.
3. Control Schemes for Heat Recovery Boilers
In practice, two control schemes are commonly used in heat recovery boilers, depending on the presence of a flowmeter for the feedwater:
Single-element Control: In cases where a feedwater flowmeter is not available, a single-element drum level control system is sufficient. This system adjusts the feedwater flow based solely on the drum level, which, due to the stable pressure in the drum, results in effective control.
Cascade Control: When a feedwater flowmeter is present, a cascade control system, where the drum level controller works in series with the feedwater flow controller, can be used. This type of control is more sophisticated and ensures that any minor deviations in the drum level are compensated by adjustments to the feedwater flow rate.
In both cases, the control systems have proven to be stable, with both single-element and cascade control achieving satisfactory results. This demonstrates that, contrary to the standard approach for conventional boilers, three-element control is not necessary in heat recovery boilers.
4. PID Tuning for Drum Level Control
A key aspect of improving drum level control in heat recovery boilers lies in the proper tuning of the PID (Proportional, Integral, Derivative) controller. In many cases, increasing the proportional gain can significantly improve control performance by making the system more responsive to changes in drum level.
For example, in a synthesis ammonia plant with a heat recovery boiler, the PID parameters for drum level control were set to a proportional gain (P) of 6, integral time (I) of 20 minutes, and derivative time (D) of 0. While the initial proportional gain was set too low at 2, it was later increased to 6, resulting in improved disturbance rejection and control stability. This parameter set was similarly effective in controlling the drum level in the heat recovery boiler of an ethylene cracking furnace.
Key Insights:
Proportional Control for Heat Recovery Boilers: Given that false level indications are not a major concern, increasing the proportional gain in the PID controller enhances control performance. While the integral and derivative actions also contribute to fine-tuning, the primary focus should be on the proportional gain, especially in systems with stable drum pressure.
Adjusting PID Parameters: When setting PID parameters, avoid making small adjustments around the default values, as this often yields no significant improvement. Instead, make more substantial changes based on the system’s response. In situations where control output jumps due to setpoint changes, consider implementing proportional-derivative anticipatory control to smooth out transitions.
Avoid Overcomplicating Control Systems: It’s important not to overcomplicate the control strategy by defaulting to a three-element control system or assuming that false level indications will always be a major issue. A simple, well-tuned PID system can be just as effective for heat recovery boilers.
Conclusion:
In summary, the drum level control for heat recovery boilers can be handled effectively without the need for a three-element control system. By recognizing the unique characteristics of heat recovery boilers—particularly the stable drum pressure and the direct correlation between waste heat and steam production—simpler control strategies can be adopted. Single-element or cascade control systems, combined with properly tuned PID parameters, are often sufficient to maintain stable drum level control.
Understanding these differences between conventional and heat recovery boilers allows for more efficient and tailored control solutions, improving both the reliability and efficiency of heat recovery systems.