Ratio Control System Fault Diagnosis: Black Liquor Oxidation with ABB AC500 and Yokogawa CENTUM VP

ABB AC500, Black Liquor Oxidation, Flow Transmitter, Industrial Automation, PID Tuning, Pulp and Paper Automation, Ratio Control, Square Root Extraction, Yokogawa CENTUM VP
เมษายน 14, 2026

The Wild Flow Problem in Kraft Pulp Mills

Kraft pulp mills generate black liquor as a byproduct. This liquid contains volatile sulfur compounds such as hydrogen sulfide and mercaptans. Mills must oxidize this black liquor with pure oxygen to stabilize the sulfur and reduce emissions. The control challenge lies in maintaining a precise oxygen-to-black-liquor ratio at all times.

In this ratio control architecture, the black liquor flow acts as the wild variable. The oxygen flow serves as the controlled variable. The ABB AC500 handles the secondary control loop. The Yokogawa CENTUM VP operator station manages the ratio calculation and display.

First, identify the wild flow transmitter. In a kraft pulp mill, the black liquor flow transmitter operates on differential pressure measurement. The transmitter sends a 4-20mA signal proportional to the square root of the differential pressure. The Yokogawa CENTUM VP AAI141-S40 analog input card receives this signal.

Second, verify the square root extraction setting. The Yokogawa CENTUM VP includes a square root function block (ARITH-S) for linearizing the flow signal. Open the function block properties in CENTUM VP. Check that the SQRT ENABLE parameter is set to ON. Failure to enable square root extraction produces a non-linear flow signal. The controller then responds incorrectly to flow changes.

Step-by-Step Fault Isolation Procedure

Follow this 6-step procedure to isolate ratio control faults:

Step 1: Record the wild flow transmitter reading on the Yokogawa CENTUM VP FACEPLATE. Note the PV value in mA and the converted flow value in GPM.
Step 2: Perform loop calibrator check. Connect a Fluke 754 process calibrator to the 4-20mA loop at the transmitter terminal. Inject a 4mA signal. Verify the Yokogawa shows 0% flow. Inject 20mA. Verify the Yokogawa shows 100% of range.
Step 3: Check the multiplier block configuration. In the Yokogawa CENTUM VP BCDL function, locate the multiplier block (ARITH-M). Verify the inputs: wild flow PV enters IN1. The manual controller output enters IN2. The multiplier output provides the setpoint to the ABB AC500 via Modbus TCP.
Step 4: Verify Modbus TCP communication. Use the ABB AC500 CM577-EP Ethernet module to check Modbus register 40001. This register holds the ratio setpoint from Yokogawa. Confirm the Modbus poll returns valid data within 100ms.
Step 5: Check the ABB AC500 configuration. Open Automation Builder software. Navigate to the PID loop configuration for the oxygen control valve. Verify the PV source is set to Modbus TCP. Set the PID mode to AUTO after verifying Modbus data integrity.
Step 6: Perform step test on the oxygen valve. Issue a 10% step change in setpoint via Yokogawa CENTUM VP. Observe the ABB AC500 PID output response. The oxygen valve should reach new position within 15 seconds for a characterized control valve with 5-second travel time.

Transmitter Failure Modes and Safety Consequences

This section details four critical failure modes in the black liquor oxidation ratio control system.

Wild flow transmitter reads low: If the black liquor flow transmitter drops to 8mA (50% of span) due to plugged impulse lines, the ratio control system interprets this as low black liquor flow. The multiplier block reduces the oxygen setpoint accordingly. The ABB AC500 PID loop closes the oxygen valve. Oxidation efficiency drops below 85%. Sulfur compounds remain unstable in the effluent stream.
Wild flow transmitter reads high: If the differential pressure transmitter diaphragm fails open, the signal exceeds 20mA. The ratio control system opens the oxygen valve wider. Oxygen concentration in the reaction vessel rises above 25%. This creates a fire and explosion hazard in the oxygen-enriched environment.
Oxygen flow transmitter fails low: If the oxygen transmitter shows 4mA (zero flow) due to coil failure, the ABB AC500 PID loop drives the oxygen valve fully open. The multiplier block cannot correct this because it receives the black liquor signal correctly. The operator must intervene immediately.
Oxygen control valve fails fully closed: If the valve actuator loses air supply, the ABB AC500 PID output saturates at 0%. No oxygen enters the reaction vessel. The oxidation reaction stops completely. The HIMA HIMatrix F-GAS system must trigger an emergency shutdown within 30 seconds.

PID Tuning for the Captive Oxygen Loop

The ABB AC500 PID controller requires tuning after any configuration change. Follow this tuning sequence for the oxygen control loop.

Step 1: Set the PID to MANUAL mode. Set the output to 50%.
Step 2: Perform a step response test. Change the output from 50% to 60%. Record the time until the PV reaches 63.2% of the final value. This is the open-loop time constant (Tau). For a typical oxygen control valve, Tau equals 8–12 seconds.
Step 3: Calculate initial tuning parameters using the Ziegler-Nichols method. Set Proportional Band (PB) to 3 times Tau divided by Dead Time. Set Integral Time (Ti) to 2.67 times Dead Time. Set Derivative Time (Td) to 0.
Step 4: Enter the calculated values into the ABB AC500 PID function block. Enable the integral term last. Monitor the loop for oscillations. If oscillations exceed 3 cycles, increase PB by 20%.
Step 5: Verify performance under load. Change the black liquor flow by 25%. Observe the oxygen flow response time. Target settling time is 45 seconds or less. Verify the ratio stays within +/- 3% of setpoint during transients.

Conclusion and Action Advice

Ratio control systems in kraft pulp mills demand rigorous fault diagnosis and preventive maintenance. The combination of ABB AC500 and Yokogawa CENTUM VP provides robust secondary and primary control respectively. However, engineers must understand the multiplier block configuration, Modbus TCP communication, and PID tuning procedures.

First, verify square root extraction on the wild flow transmitter signal at least once per turnaround. Second, check impulse lines for plugging every 6 months using differential pressure comparison. Third, calibrate the oxygen control valve positioner quarterly to ensure accurate positioning.

Finally, document all ratio control setpoint changes in the Yokogawa CENTUM VP alarm log. This documentation supports IEC 61511 compliance for SIS integration with HIMA HIMatrix. Engineers who follow this structured approach will maintain oxidation efficiency above 95% and prevent dangerous oxygen-enriched conditions in the pulp mill.