Grease Trap Sizing Calculations: Step-by-Step Guide for MEP Engineers-2025

Grease Trap Sizing Calculations for MEP Engineers

Introduction

Grease traps play a vital role in commercial kitchens by stopping fats, oils, and grease (FOG) from blocking drainage systems. For MEP engineers, it’s crucial to size grease traps correctly to meet plumbing codes and prevent expensive maintenance problems. In this guide, we’ll take you through the process of calculating the right size for grease traps, sharing helpful tips, formulas, and common pitfalls to steer clear of.

What is a Grease Trap?

A grease trap is a plumbing device designed to intercept fats, oils, and grease (FOG) from wastewater before it enters the sewage system. It works by slowing down the flow of water, allowing FOG to separate and float to the top, while solids settle at the bottom.

Why Proper Sizing Matters?

1. Code Compliance: Meets local plumbing codes (e.g., IPC, UPC).
2. Efficiency: Prevents blockages and reduces maintenance costs.
3. Environmental Protection: Minimizes FOG discharge into public sewers.

Step-by-Step Grease Trap Sizing Calculations

Step 1: Determine Flow Rate

• Formula: Flow Rate (GPM)=Peak Flow FactorTotal Fixture Units​
  
  
• Example:
  • A commercial kitchen has 50 fixture units.
  • Peak flow factor = 2.5 (for commercial kitchens).
  • Flow Rate = 50 / 2.5 = 20 GPM.

Step 2: Calculate Grease Trap Capacity

• Formula:
• Retention Time: Typically 30 minutes for commercial kitchens.
• Example:Grease Trap Capacity=Flow Rate X Retention Time
  • Flow Rate = 20 GPM
  • Retention Time = 30 minutes
  • Grease Trap Capacity = 20 × 30 = 600 gallons.

Step 3: Adjust for FOG Load

• FOG Load: Depends on the type of kitchen (e.g., high-FOG kitchens like restaurants require larger traps).
• Example:
  • High-FOG kitchen: Multiply capacity by 1.5.
  • Adjusted Capacity = 600 × 1.5 = 900 gallons.

Step 4: Verify Local Code Requirements

• Check local plumbing codes (e.g., IPC, UPC) for minimum grease trap size requirements.
• Example: Some codes require a minimum of 750 gallons for commercial kitchens.

Common Mistakes to Avoid

1. Undersizing: Leads to frequent blockages and non-compliance.
2. Ignoring FOG Load: High-FOG kitchens need larger traps.
3. Improper Installation: Ensure proper placement and slope for efficient operation.

Tools for Grease Trap Sizing

1. Manual Calculations: Use formulas and local code guidelines.
2. Software Tools:
   • AutoCAD MEP: For detailed plumbing layouts.

Case Study: Grease Trap Sizing for a Restaurant

Project: A 100-seat restaurant with 75 fixture units.
Steps:

1. Flow Rate: 75 / 2.5 = 30 GPM.
2. Grease Trap Capacity: 30 × 30 = 900 gallons.
3. FOG Adjustment: 900 × 1.5 = 1,350 gallons.
4. Final Size: A 1,500-gallon grease trap (rounded up for safety).

Conclusion

Proper grease trap sizing is essential for efficient and code-compliant plumbing systems in commercial kitchens. By following this step-by-step guide, MEP engineers can ensure optimal performance and avoid costly mistakes.