Virtual Sizing for Grease Trap and Removal Systems

The purpose of this sizing procedure is to give the specifying designer a simple method for determining the proper size Big Dipper unit for various food service facility applications. Many different methods are in use today by the major code bodies for sizing grease traps. However, the sizing formula utilized by most codes becomes inaccurate whenever the facility contains a large number of fixtures. Using such sizing methods can lead to calculations that indicate enormous flow rates being generated. In reality this never occurs because:

The building's portable water supply cannot produce this great of a flow rate.
All of the fixtures cannot be in use at the same time, and
The only fixtures that can produce high rates of flow are those that "store and release" large volumes of water such as deep multi-compartment sinks.

The key factor to be considered when sizing an automatic grease/oils removal unit is the peak flow rate. The method shown on the Single Fixture Sizing Chart takes the most commonly used sizing method and adapts it for use in single and multiple fixture service applications. An example of a single fixture is a two or three compartment sink. An individual Big Dipper unit could be plumbed to this type of fixture.

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The multiple fixture sizing formula takes points 1, 2 and 3 above into consideration. As an illustration, the multiple fixture example describes common kitchen wash equipment and drainage requirements for a typical large full-service restaurant. Using the older multiple fixture unit values, the peak flow rate is calculated to be 240 gallons per minute. Using field experience data and the combined method, we find that the peak flow rate for this "typical" restaurant is actually 48 gallons per minute. At 48 gallons per minute maximum peak flow rate, a 50 or 75 gallon per minute Big Dipper system could easily handle the effluent discharges of this restaurant.

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