Design of Constant Pressure Water Supply Systems with VFD Pumps

Constant pressure water supply system is designed to maintain a steady water pressure throughout the distribution network, regardless of variations in demand. This is achieved by using Variable Frequency Drive (VFD) pumps, which adjust the pump speed to match real-time water demand, ensuring consistent pressure and energy efficiency.

 

Introduction to Constant Pressure Water Supply Systems

Constant pressure water supply system is designed to maintain a steady water pressure throughout the distribution network, regardless of variations in demand. This is achieved by using Variable Frequency Drive (VFD) pumps, which adjust the pump speed to match real-time water demand, ensuring consistent pressure and energy efficiency.

 

Design Considerations for Constant Pressure Water Supply Systems

✅ 1. Water Demand and Flow Rate

• Determine the peak and average flow rates required.
• Consider daily water consumption patterns.
• Use flow meters or historical data for accurate demand analysis.

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✅ 2. Required Pressure Settings

• Identify the target pressure needed for the distribution system.
• Consider factors like:
  • Building height (for multi-story buildings).
  • Friction losses in pipes.
  • Fixture pressure requirements.
• Typically, residential systems require 40–60 PSI, while industrial systems may need 80–120 PSI.

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✅ 3. VFD and Pump Selection

• Choose pumps with VFD compatibility.
• Ensure the pump capacity matches the maximum flow and pressure requirements.
• Consider efficiency curves to optimize performance.
• For larger systems, consider multiple VFD pumps operating in parallel.

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✅ 4. Pressure Tank Sizing

• Prevents rapid cycling of the pump.
• Helps maintain consistent pressure.
• Size the tank based on:
  • Flow rate.
  • System pressure.
  • Pump on/off cycles.
• Formula:

Tank Size (gallons)=Flow Rate (GPM)×Run Time (minutes)4\text{Tank Size (gallons)} = \frac{\text{Flow Rate (GPM)} \times \text{Run Time (minutes)}}{4}Tank Size (gallons)=4Flow Rate (GPM)×Run Time (minutes)​

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✅ 5. Pipe Sizing and Layout

• Proper pipe sizing minimizes friction losses and ensures consistent pressure.
• Undersized pipes create excessive resistance and pressure drops.
• Oversized pipes lead to inefficiencies and reduced velocity.
• Use the Hazen-Williams formula to calculate pressure loss:

Pressure Loss (psi)=4.52×Q1.85C1.85×d4.87\text{Pressure Loss (psi)} = \frac{4.52 \times Q^{1.85}}{C^{1.85} \times d^{4.87}}Pressure Loss (psi)=C1.85×d4.874.52×Q1.85​

Where:

• QQQ = Flow rate (GPM)
• CCC = Pipe roughness coefficient
• ddd = Pipe diameter (inches)

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✅ 6. Energy Efficiency and Optimization

• VFD pumps reduce energy consumption by adjusting speed to match demand.
• Install energy-efficient motors with high IE ratings.
• Use soft-start features to reduce mechanical stress and power spikes.