Introduction

When comparing the 4 major process variables: temperature, pressure, flow, and level in a large-scale continuous process like a distillation column chain, the importance of the level loop is often over-looked. Typically, the level loop is controlled by manipulating the inflow or outflow of the operating unit, and is considered an easy loop to control.

Why Level Control Can Be Difficult

It is difficult to properly tune a PID controller for good control performance under all conditions due to the potential inflow and outflow variations of the operating unit. Overly tight level control will result in too much movement of the flow loop, which can cause excessive disturbances to the downstream operating unit. Thus, a PID level controller is usually detuned to allow the level to fluctuate; so the variations of the outflow are minimized. The detuned PID, however, cannot provide prompt control against large disturbances, which may result in safety problems during a plant upset. In addition, oscillations in level can cause the process to swing, which also results in a lower yield.

MFA Control Solution

Robust MFA controllers have been used to control the levels to allow smooth material and energy transfers between the operating units, and also protect the levels from overflowing or becoming too dry during abnormal conditions. The Robust MFA controller can be configured with the parameters as shown in the following bitmap.

1. Upper and Lower Bound - The bounds for the process variable (PV) being controlled. These are "intelligent" upper and lower boundaries that are typically the marginal values the PV should not exceed. PV is unlike the controller output (OP) where a hard limit or constraint can be set. PV is a process variable that can only be varied by manipulating the OP. Thus, the Upper and Lower Bounds for PV are very different from the OP constraints.

2. Gain Ratio - The coefficient to increase or decrease the MFA control action based on the PV. For instance, a Gain Ratio of 3 is the default setting. As PV gets closer to reaching its bounds, MFA will react like it has a gain that is 3 times higher than its setting. Notice that this is not a gain scheduling approach, although it appears to be. Gain scheduling will not be able to resolve the complex problems described.

Boiler Steam Drum Level Control

The steam drum level needs to be kept near the midline to prevent either heat stress on the boiler water tubes (level too low) or corrosion (level too high). Improper control of the level can cause system shutdown, waste of energy, and a shorter equipment life cycle.

Key variables affecting the drum level are feedwater inflow, steam outflow and fuel/mix inflow. Each variable has its own distinctive type of disturbance. Cold feedwater creates a delay time in the process. A sudden increase of steam outflow causes a distinctive “shrink and swell” response. This will confuse the controller because the process “acting type” will be temporarily changed.

CyboSoft’s three-element MFA control system can effectively control the steam drum level. The MFA level controller is cascaded with the feed-water controller to regulate the drum level and compensate for disturbances from feedwater and steam outflow. The Anti-delay MFA controller handles the large varying delay time. Feedforward MFA controllers keep feedwater supply in balance. Table 1 shows what this MFA control solution can accomplish and the benefits achieved by customers.

Table 1. MFA Control Benefits for Steam Drum Level Control

Evaporator Level Control

Evaporators are popular operating units for dewatering, concentration, and crystallization in various industries. If the density or the consistency of the product is an important quality variable, the level control will be much more difficult.

For instance, tomato processing plants use evaporators to produce tomato paste. The density of the tomato paste is the most critical quality variable. It is desirable to control the density within a small range. The manipulated variables are the inflow and outflow, which affect both the density and level at the same time. The evaporator, by its nature, is a multi-input, multi-output (MIMO) process. When using single-loop controllers to control the level and density separately, both loops can fight each other causing major problems.

A multivariable MFA controller can quickly and tightly control density and level by manipulating inflow and outflow simultaneously. An MFA constraint controller protects the evaporator level from running too high or too low. Table 2 shows what a MIMO MFA controller can accomplish and the benefits achieved by evaporator customers.

Table 2. MFA Control Benefits for Tomato Evaporator Control

Summary

Based on the core MFA control method, various MFA controllers have been developed to solve specific control problems. This applies to level control applications as well.

1. The Robust MFA controller is well suited to control the conventional level loops and provides a good protection from running too high or too low.

2. Anti-delay MFA with Feedforward MFA controllers can effectively control the steam drum level.

3. MIMO MFA can control the density and level for operating units like evaporators.

Case Studies

To read more about implementations of CyboSoft’s MFA level control solutions, click on the following case studies:

Model-Free Adaptive Control of Evaporators

Model-Free Adaptive Control of Steam Drum Level

Model-Free Adaptive Control of Air Separation Units

MFA Control and Optimization of Distillation Columns