What is the Approach Flow System?
The Approach Flow System is the final stock preparation stage between machine chest and head box.
Its purpose is to deliver clean, air-free, pressure-stable, and uniformly mixed stock to each layer’s head box at the correct consistency, flow, and pressure.
“In a multi-layer board machine, each layer (Top / Middle / Back) has a separate approach flow, ensuring independent quality control.“
Importance of Approach Flow in Multi-Layer Board Machines:
“A well-designed approach flow system is critical for board quality and machine stability”
Key Importance:
- Ensures uniform basis weight (GSM) profile
- Improves formation and layer bonding
- Removes air, dirt, shives, and stickies
- Provides stable head box pressure & flow
- Prevents streaks, pinholes, and breaks
- Supports high-speed operation
- Enables layer-specific furnish control
“Poor approach flow = Good stock wasted at the head box”
Recommended Consistency for Each Layer:
| Layer | Consistency at Head box |
| Top Layer | 0.5 – 0.7 % |
| Middle Layer(s) | 0.6 – 0.9 % |
| Back Layer | 0.7 – 1.0 % |
Complete Equipment of Approach Flow System:
“The Complete Equipment from Machine Chest to Head box are known as Approach Flow System”
Equipment’s with Functions:
Machine Chest
Function:
- Acts as a buffer/storage for prepared stock
- Provides short-term consistency stability
- Allows continuous machine operation
Key Features: 1. Agitator, 2. Level control, 3. Separate chest for each layer
Fan Pump
Function:
- Pumps stock from machine chest to approach flow
- Maintains required flow & pressure
Importance: 1. Must deliver pulsation-free flow, 2. Oversized or unstable pump causes MD profile variation
Pressure Screen (Fine Screen)
Function:
- Removes shives, plastic, stickies, uncooked fibers
- Final cleanliness protection before head box
Typical Slot Size: 1. Top layer: 0.15 – 0.20 mm. 2. Middle/Back layer: 0.20 – 0.25 mm
Critical for: 1. Surface quality. 2. Formation. 2. Head box nozzle protection
Screen Reject Handling System
Function:
- Recovers usable fibers
- Prevents fiber loss
- Controls dirt load
Includes: 1. Reject screens. 2. Dilution water. 3. Junk trap
Deaeration System / Deaeration Tank
Function:
- Removes entrained and dissolved air
- Prevents foam, pinholes, and head box streaks
Why Critical: 1. Air causes poor formation. 2. Leads to sheet breaks and coating defects
Centrifugal Cleaners (Low Consistency Cleaners)
Function:
- Removes light and heavy contaminants
- Works on density difference
Types Used:
- Low Density Cleaner (LDC)
- Multi-stage cleaner system
Protects: 1. Screens, 2. Head box, 3. Forming wire
Consistency Control & Dilution System
Function:
- Maintains exact consistency to head box
- Enables automatic GSM control
Components: 1. Consistency transmitter. 2. Control valve, 3. White water dilution
Flow Control Valves & Basis Weight Control
Function:
- Regulates stock flow to head box
- Controls MD basis weight
Connected with: 1. QCS / DCS system
Approach Flow Piping
Function:
- Transfers stock smoothly with minimum turbulence
- Maintains pressure stability
Design Considerations: 1. Large radius bends, 2. No dead zones, 3. Stainless steel preferred
Head box Feed Header (Manifold)
Function:
- Final stock distribution before head box
- Ensures uniform pressure across width
Utilities & Instruments Used Pressure transmitters Flow meters (magnetic) Consistency transmitters Temperature sensors Control valves Automation (DCS/QCS)
Common Problems Due to Poor Approach Flow:
| Problem | Cause |
| GSM variation | Flow instability |
| Streaks | Poor screening / air |
| Pinholes | Entrained air |
| Sheet breaks | Pressure fluctuation |
| Poor layer bonding | Consistency mismatch |
Conclusion
The approach flow system is the heart of a multi-layer board machine. Even with excellent stock preparation, poor approach flow control will destroy sheet quality.
A well-engineered approach flow ensures: 1. Clean stock. 2. Stable pressure. 3. Uniform consistency. 4. Excellent formation. 5. High machine efficiency.