Designing a Validated Water for Injection (WFI) Control System: Lessons from the Field

Introduction

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:

• Tank level management under variable demand
• Temperature control challenges and optimization
• Lessons learned from real-world system performance

Key Control Challenges in WFI Systems

1. Tank Fill Dynamics and Demand Variability

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.

2. Temperature Control and Thermal Stability

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:

• Localized boiling
• Oscillation (“hunting”) in control valves
• Excessive energy consumption

Heat exchanger performance and control valve behavior play a major role in achieving stable operation.

3. Interlocks and System Protection

Control system interlocks are just as important as physical safety devices.

Effective strategies include:

• Pump and valve shutdown based on rupture disc activation
• High-temperature interlocks to prevent excursions (>90°C)
• Integrated monitoring of pressure, flow, and temperature

These safeguards ensure both system integrity and regulatory compliance.

Project Overview: Recirculating WFI Distribution System

This project involved a recirculating ambient WFI loop (25°C) supplying hot WFI (80°C) to two destination tanks.

Key system components included:

• Dual VFD-driven pumps operating in matched speed mode
• Plate-and-frame heat exchanger using low-pressure steam
• Modulating control valves tied to destination tank levels
• High-accuracy instrumentation (pressure, flow, control valves)

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.

The Control Challenge: Why PID Didn’t Work

During testing, a standard PID control loop caused instability.

The issue:

• Cooler inlet water required increased heating
• The system overcompensated
• Temperature continued to rise beyond the setpoint
• High-temperature alarms were triggered

This created a runaway condition instead of a stable loop.

Solution: Simplifying to Proportional Control

To stabilize the system, the team simplified the control strategy:

• Removed Integral (I) and Derivative (D) terms
• Implemented proportional-only control

The result:

• More predictable response
• Reduced oscillation
• Improved steady-state control

In this case, a simpler control approach outperformed a more complex PID loop.

Unexpected Issue: Air in a Closed Loop

After extended operation, new issues appeared:

• Flow fluctuations without changes in pressure or pump speed
• Cyclical anomalies during long run times

Troubleshooting steps ruled out:

• Pump cavitation
• Valve-induced flashing
• Instrumentation errors

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.

Lessons Learned

1. Define System Criticality Early

A system initially classified as “backup” was later treated as “primary,” impacting expectations and performance requirements.

2. Simulation ≠ Reality

Using an end-of-line valve to simulate process conditions did not accurately replicate real tank filling behavior, leading to adjustments during operation.

3. Expect the “Invisible” Variables

Factors like air entrainment, thermal inertia, and valve dead-band can significantly impact performance, even when instrumentation is calibrated and validated.

Final Thoughts

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.

Frequently Asked Questions

What is a WFI control system?

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.

Why is temperature control critical in WFI systems?

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.

Why might PID control not work well in WFI systems?

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.

When is proportional-only control a better option?

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.

What causes flow instability in closed-loop WFI systems?

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.

What are common challenges in WFI system design?

Common challenges include:

• Managing variable demand from downstream systems
• Maintaining stable temperature control
• Preventing biofilm through proper flow conditions
• Accounting for system dynamics like thermal inertia and air entrainment

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. 

Read    My Hallam Story 

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.