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Grease Trap Sizing Guide

Grease Trap Sizing Guide

A grease trap is designed to prevent greasy substances from entering plumbing systems, septic tanks, and waste water treatment facilities where they are difficult to process and may create a number of environmental problems. Understanding how to size a grease trap is an important way to keep your establishment up to code and ensure that your plumbing is running smoothly.

How Grease Traps Work

A restaurant grease trap is often specified for a pot washing stainless steel sink or a dish machine. A flow restrictor on the inlet side of the trap slows the incoming flow and redirects it through baffling inside the trap. This slowing and baffling process allows lighter-than-water substances like fats, oil, and grease to accumulate inside the trap above the static water line while the water flows out below. Essentially, a grease trap works by using gravity to separate heavier water from lighter fats.

Local plumbing codes typically determine design and installation criteria, but they typically follow the guidelines set forth by the Plumbing & Drainage Institute (PDI). If you're wondering how to install a grease trap, contact your local municipality to learn more about installation criteria for your area.

Sizing a Grease Trap*

If you operate a restaurant or other commercial facility, you may have wondered "what size grease trap do I need?" Commercial grease traps are sized according to the rate of incoming flow, in gallons per minute (GPM). Associated with this incoming flow rate is the trap's capacity. This rated capacity, in pounds, is listed at twice the flow rate. For example, a 10 GPM trap has a rated capacity of 20 pounds.

*All calculations, formulas, and charts used to determine grease trap sizing and capacities are widely accepted by most plumbers and regulatory agencies. Always check with your local authority and plumber for specific regulations and recommendations on grease trap sizing.

For Pot Washing Sinks

The Plumbing and Drainage Institute (PDI) recommends that you round up to the next available size when determining the correct grease trap for your application.

Grease Trap Sizing Calculator

Determine the flow rate of your sink:
  1. Calculate the capacity of the sink in cubic inches (measurements of one compartment), and multiply that total by the number of compartments:

    x x =

  2. Convert the capacity from Total cubic inches to gallons per minute (GPM):

    ÷ 231 =

  3. Adjust for displacement (displacement takes into consideration the actual useable capacity of your sink):

    x 0.75 = x 2 =
  4. * The resulting GPM is the required flow rate to drain the sink in one minute. You would then choose a grease trap with a GPM flow rate close to this number.

If Draining Multiple Sinks Into One Grease Trap

  1. Determine the flow rate for each sink to be serviced by the grease trap using the same calculations as a single sink.
  2. Add together 100% of the largest flow rate, 50% of the second largest, and 25% of all others.
  3. The result is the recommended flow rate (in GPM) of the grease trap.

Calculating Grease Trap Requirements For Dish Machines

The Plumbing & Drainage Institute (PDI) recommends that all dish machines have their own grease trap. Here are some recommendations:
  • 10-15 Gallon Capacity Dish Machine Tank: 15 Pound Grease Trap
  • 20-30 Gallon Capacity Dish Machine Tank: 20 Pound Grease Trap
  • 30-50 Gallon Capacity Dish Machine Tank: 25 Pound Grease Trap
  • 50-70 Gallon Capacity Dish Machine Tank: 35 Pound Grease Trap
  • 70-100 Gallon Capacity Dish Machine Tank: 100 Pound Grease Trap

Grease Interceptor Maintenance Suggestions

Noble Chemical Drain Maintainers We suggest that you use a drain maintainer or cleaner like EDM+ Enzymatic Drain Maintainer from Noble Chemical, or Noble Chemical Orange Peel Citrus Solvent Cleaner as part of a regularly scheduled cleaning and maintenance routine for your grease trap.

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