Mastering Drum Level Control: PID, Two-Element, and Three-Element Strategies

~ Brandt Boulden

Maintaining stable water level within a boiler drum is crucial for safe and efficient operation. Too low, and boiler tubes risk overheating (or worse - rupturing). Too high, and water can carry over into the steam system, reducing efficiency or causing significant damage. PID (Proportional-Integral-Derivative) control loops are widely used to regulate drum level by adjusting feedwater valves based on feedback. This article will explore how PID control works, delving into advanced strategies of two-element and three-element control, which offer improved performance in dynamic environments.

Understanding Control Loops

A control loop is a feedback system that continuously measures a process variable (e.g., temperature, pressure, or level) and compares it to a desired setpoint. The system then adjusts its outputs to minimize the error between the measured value and the setpoint.

In drum level control, the process variable is the water level in the drum. A sensor measures this level and sends the data to a controller, which compares it to the setpoint (the desired water level). The controller then sends a signal to the feedwater control valve, adjusting its position to bring the water level back to the setpoint.

PID Control Loop Basics

A PID control loop consists of three core components:

• Proportional (P) Control: The proportional term adjusts the feedwater valve position based on the current error (difference between setpoint and measured level). A larger error results in a greater valve adjustment. Proportional control directly reduces the error, but it may leave a small steady-state error, where the level is close to but not exactly at the setpoint.
• Integral (I) Control: The integral term addresses the steady-state error by considering the accumulated error over time. It adjusts the valve based on the sum of past errors, ensuring that even small deviations from the setpoint are eventually corrected. Integral control eliminates the steady-state error, bringing the level closer to the setpoint.
• Derivative (D) Control: The derivative term predicts future behavior by analyzing the rate-of-change of the error. If the error is changing rapidly, derivative control dampens the output, reducing overshooting and oscillations. Derivative control adds stability to the system, especially when dealing with sudden system changes.

Note that the derivative term is typically avoided in drum level control applications as it is designed to react to rapid changes. This can amplify small noise or brief fluctuations in the level measurement due to the “shrink or swell” effects caused by variations in steam demand. This can lead to excessive or erratic valve corrections by the controller, ultimately destabilizing the system. The relatively slow response of the drum level to changes in steam flow or feedwater flow makes the derivative term less useful in this application.

Drum Level Control Using a Feedwater Control Valve

The PID controller monitors the drum level and adjusts the feedwater valve accordingly. The proportional term handles immediate deviations, the integral term ensures long-term accuracy.

Example: During a sudden increase in steam demand, the water level might drop quickly. The PID controller opens the feedwater valve wider to compensate.

Two-Element Feedwater Control: Using Steam Flow as a Feed-Forward Signal

While a basic PID loop works well for drum level control, it can struggle to keep up with rapid changes in steam demand, especially in dynamic environments. Two-element control addresses this by adding a feed-forward signal – steam flow. This signal anticipates changes in steam demand and makes preemptive adjustments to the feedwater flow.

Why Use Two-Element Control?

In a single-element system, the controller only reacts after the drum level has changed. However, steam demand fluctuations can cause “shrink and swell” effects:

• Swell: When steam demand increases, pressure in the drum drops, causing water to flash into steam, making the level appear higher when it’s actually going down.
• Shrink: When steam demand decreases, pressure in the drum rises, causing steam to condense, making the level appear lower when it’s actually going up.

These effects can confuse the single-element system, leading to incorrect adjustments. The feedwater valve may temporarily do the exact opposite of what is needed. Two-element control compensates for this.

How It Works:

1. Drum Level (Primary Feedback): The drum level measurement remains the primary input to the PID loop, controlling the overall water level.
2. Steam Flow (Feed-Forward Signal): The steam flow is used to anticipate changes in steam demand. When steam flow increases, the control system opens the feedwater valve preemptively, even before the drum level drops, ensuring a faster and more accurate response.

Advantages of Two-Element Control:

• Faster Response: Two-element control anticipates demand changes, preventing large drum level fluctuations due to sudden load shifts.
• Improved Stability: The feed-forward signal smooths the control process by compensating for shrink and swell effects, resulting in more stable water level control.
• Better Performance During Load Changes: Two-element control is particularly useful in systems with variable loads, where steam demand can change rapidly and unpredictably (e.g. during a sudden increase in hot water usage in a boiler used for heating application).

Three-Element Feedwater Control: Introducing Feedwater Flow Control

Three-element control takes precision to the next level by adding a second PID loop for direct control of feedwater flow. This adds a third process variable – feedwater flow – to the control system, allowing for precise balancing of water input and steam output. This helps manage variations in feedwater supply pressure, which can fluctuate due to factors like pump operation or changes in feedwater tank levels. It also addresses the differential pressure across the valve, which can change depending on the flow rate and pressure drop across the valve. These variations can influence feedwater flow even when the valve position remains constant.

How It Works:

1. Drum Level (Primary Feedback): The primary PID controller monitors the drum level and steam flow. It then calculates the required feedwater flow rate to maintain the desired level. This calculated setpoint is sent to the second PID loop.
2. Steam Flow (Feed-Forward Signal): As in two-element control, steam flow anticipates demand changes.
3. Feedwater Flow (Secondary Feedback): The second PID controller receives the flow setpoint from the primary loop and directly controls the feedwater valve to achieve the desired flow rate. It measures the actual feedwater flow and adjusts the valve accordingly, ensuring the water input matches the demand.

Why Use Three-Element Control?

By directly controlling feedwater flow, three-element control compensates for system fluctuations, ensuring a more consistent water level in the drum. Three-element control is especially beneficial for large, high-pressure boiler systems where precise control is essential. It ensures a more accurate and stable drum level, especially during startup, shutdown, or load changes.

Advantages of Three-Element Control:

• Improved Accuracy: By directly controlling feedwater flow, three-element control ensures a better match between water input and steam output, reducing the chance of overshoot or undershoot.
• Enhanced Stability: The secondary PID loop provides additional feedback, resulting in smoother and more stable drum level control.
• Better Response to Changes: The system can respond more effectively to fluctuations in both steam flow and feedwater supply (such as feedwater supply pressure and the differential pressure across the valve), offering improved control under variable load conditions (e.g. during a sudden increase in electricity demand at a power plant).

Example: During a rapid increase in steam demand, the primary PID loop calculates the necessary increase in feedwater flow. The secondary PID loop then adjusts the valve position to deliver the exact flow rate required, ensuring a fast and stable response to the demand.

Tuning a PID Controller

Proper tuning of a PID controller is crucial for optimal system performance. Regardless of the application, the goal is to find the right balance of proportional, integral, and derivative action to ensure fast response without overshooting or oscillations. While the derivative component is typically avoided in drum level control, finding the right balance between proportional and integral terms is critical for a fast, stable response without overshooting. This requires careful consideration of the system dynamics and the specific challenges presented by the boiler application.

Tuning should be performed by qualified personnel who are familiar with the system and its interactions with other processes. Common tuning techniques include Ziegler-Nichols, relay feedback, and auto-tuning.

Challenges:

• Noise Sensitivity: The derivative term can amplify small fluctuations or noise in the level measurement, leading to unnecessary adjustments if not managed properly. This is less of a concern for drum level control as the derivative term is typically not used due to the relatively slow system dynamics.
• Interaction with Other Processes: Careful integration of the PID loop into the overall system is essential. For drum level applications, other factors like steam and feedwater temperatures and pressures, pressure differentials, and flow rates affect drum level control.

Conclusion

Precise drum level control is essential for safe and efficient boiler operation. While basic PID control can provide a foundation for stability, the advanced two-element and three-element control strategies offer superior performance in dynamic environments. Two-element control, by incorporating steam flow as a feed-forward signal, anticipates demand changes, preventing large fluctuations and improving overall stability. Three-element control, with its direct control of feedwater flow, provides even greater accuracy and responsiveness, especially when dealing with variations in feedwater supply pressure and differential pressure across the valve. By understanding the fundamental principles of PID control and the advantages offered by two-element and three-element strategies, operators can effectively manage drum level, maximizing boiler efficiency and minimizing the risk of costly downtime.