To size a grease trap in a commercial kitchen, determine the number of sinks to be drained in the trap and measure the sink dimensions. Calculate the sink capacity, convert this to gallons per minute, calculate the displacement and multiply by two to obtain the pound capacity of the trap.
To calculate the flow rate capacity in pounds or pound capacity of a grease trap for one sink, measure its internal length, width and depth in inches and multiply them to obtain the sink capacity in cubic inches. To convert the sink capacity in cubic inches to gallons per minute or GPM, divide it by 231.
Then, calculate the displacement by multiplying the GPM value by 0.75. Finally, double this figure to arrive at the trap's pound capacity. Select a grease trap of this size or the nearest available for the commercial kitchen.
If multiple sinks are connected to the grease trap, calculate the flow rates for all the sinks separately in a similar manner. Add the highest flow rate to half of the second highest and 25 percent of the rest. This gives the flow rate in GPM of the trap. Again, multiply by two to arrive at the trap's pound capacity.
As per the Plumbing and Drainage Institute's recommendations, each dishwasher must have its own grease trap, and its size depends on the machine's capacity. Accordingly, grease trap sizes range from 15 pounds for a 10 to 15 gallon dishwasher to 100 pounds for a 70 to 100 gallon dishwasher.