Designing a Water for Injection (WFI) control system is not just about moving liquid from point A to point B. It requires precise control of temperature, flow, and system stability in an environment where the margin for error is extremely small.
For controls engineers, many of the biggest challenges are not obvious. Maintaining turbulent flow to prevent biofilm, stabilizing temperature control loops, and managing dynamic demand all require careful planning and tuning. As these systems become more complex, leveraging advanced SCADA systems for real-time monitoring and control becomes essential.
In this post, we explore key WFI control strategies, including:
One of the most complex aspects of WFI systems is balancing steady generation with unpredictable demand.
Understanding how these demand fluctuations impact system performance often requires the ability to collect, standardize, and visualize plant-floor data effectively.
Sudden “snap” demands from downstream systems can disrupt loop stability, making it difficult to maintain consistent pressure and flow. A well-designed control strategy must absorb these fluctuations without destabilizing the system.
Temperature control is critical for both compliance and system performance.
For hot WFI systems, maintaining temperatures above 80°C is required, but this must be achieved without:
Heat exchanger performance and control valve behavior play a major role in achieving stable operation.
Control system interlocks are just as important as physical safety devices.
Effective strategies include:
These safeguards ensure both system integrity and regulatory compliance.
This project involved a recirculating ambient WFI loop (25°C) supplying hot WFI (80°C) to two destination tanks.
Key system components included:
Each instrument followed a detailed calibration and testing process aligned with site standards and validated control system practices.
Destination tank fill rates were controlled proportionally; lower tank levels resulted in more open valves, increasing flow demand dynamically.
During testing, a standard PID control loop caused instability.
The issue:
This created a runaway condition instead of a stable loop.
To stabilize the system, the team simplified the control strategy:
The result:
In this case, a simpler control approach outperformed a more complex PID loop.
After extended operation, new issues appeared:
Troubleshooting steps ruled out:
The root cause was air entrainment in the closed loop.
When the system returned to ambient temperature, the issue disappeared, confirming the hypothesis. In sanitary closed-loop systems, degassing options are limited, making this a difficult challenge to fully eliminate.
A system initially classified as “backup” was later treated as “primary,” impacting expectations and performance requirements.
Using an end-of-line valve to simulate process conditions did not accurately replicate real tank filling behavior, leading to adjustments during operation.
Factors like air entrainment, thermal inertia, and valve dead-band can significantly impact performance, even when instrumentation is calibrated and validated.
WFI control systems require more than standard control strategies. Success depends on understanding system dynamics, simplifying where necessary, and adapting to real-world behavior.
While perfect startup conditions are ideal, the true value of a controls engineer lies in troubleshooting, collaboration, and continuous improvement.
If you’re designing or optimizing a WFI or high-purity system, our team can help you navigate control strategy, validation, and system performance challenges, from concept through implementation.
Below are answers to some common questions about WFI control systems.
A Water for Injection (WFI) control system manages the flow, temperature, and distribution of high-purity water used in pharmaceutical manufacturing. It ensures compliance with strict regulatory requirements while maintaining system stability and performance.
WFI systems must maintain elevated temperatures (typically ≥80°C for hot systems) to prevent microbial growth. Poor temperature control can lead to compliance risks, energy inefficiencies, and unstable system performance.
In some WFI applications, PID control can cause instability due to thermal lag, varying inlet conditions, and nonlinear heat exchanger behavior. This can result in overshoot or runaway temperature conditions.
Proportional-only control can be more effective when system response is predictable and proportional to demand, such as in heat exchanger-driven temperature control. It simplifies tuning and can improve stability in certain WFI loops.
Flow instability can be caused by factors such as air entrainment, pump performance differences, or valve behavior. In closed sanitary systems, trapped air can be particularly difficult to remove and may lead to fluctuating flow readings.
Common challenges include:
About the author
Sam Lacasse is a Senior Process Controls Engineer at Hallam-ICS with over 20 years of experience in industrial automation. His background includes life sciences, food and beverage, and water/wastewater systems, with expertise in control system design, toxic gas monitoring, and advanced automation technologies.
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About Hallam-ICS
Hallam-ICS is an engineering and automation company that designs MEP systems for facilities and plants, engineers control and automation solutions, and ensures safety and regulatory compliance through arc flash studies, commissioning, and validation. Our offices are located in Massachusetts, Connecticut, New York, Vermont, North Carolina, and Texas and our projects take us world-wide.