SIZING INTERCEPTORS ACCORDING TO PDI STANDARD PDI-G101
The recommendations for sizing, installation and maintenance of grease interceptors contained in this section are based on experience of the industry and were taken from PDI-G101.

Sizing Considerations
A grease interceptor conforming to Standard PDI-G101 is designed and certified to operate efficiently at its rated capacity. The larger the interceptor the higher the flow rate it will handle efficiently with a greater quantity of grease retained before cleaning is required. While a small interceptor, undersized, can accommodate a flow of waste water well in excess of its rated capacity, it will not intercept grease efficiently under such overload conditions.

Fixture drainage period, in combination with the service required and the quantity of waste water involved, establishes the rate of flow through the grease interceptor. Flow rate is therefore the primary gauge which establishes interceptor size or capacity.

The link between flow rate and drainage load to produce satisfactory grease interceptor operation is a Flow Control Fitting. Without it, a correctly sized grease interceptor will not regulate the flow of water discharged from the fixture it is serving. Therefore, to ensure that the flow rate does not exceed the grease interceptor’s rated capacity, a flow control fitting is essential for protection against overloading the grease interceptor which could occur from sudden surges from the fixture. The flow control fitting will control the flow of waste water at all times, enabling the interceptor to operate at its certified capacity.

Table 1 : Certification Standard Flow Rates and Grease Retention Capacity Ratings for Grease Interceptors

Table 2: Sizing and Rating

Table 3: Procedure for Sizing Grease Interceptors

Table 4: Selection Chart

 

Size Symbols
It has been determined through the testing and rating procedure that eight (8) different sized grease interceptors are required for normal domestic, commercial, and institutional installations. These sizes are based on certification standard flow rates and grease retention capacity ratings for grease interceptors, see Table 1. Table 2 lists the PDI size symbol for each of the standard rated grease interceptors.

Sizing Procedure
 Table 3 is provided to show the standard formula in steps for sizing grease interceptors to suit requirements of specific fixtures.

Selection
Table 4 is included as a selection chart for standard PDI Certified grease interceptors applicable to various size fixtures commonly used in domestic, commercial and institutional installations. The selections listed are based on the sizing formula covered in Table 3.

Dishwashers
A separate grease interceptor is recommended for each commercial dishwasher. The size of the interceptor is determined by the GPM discharge rate of the dishwasher. Select proper interceptor of equivalent or next higher rate from Table 2.

Multiple Fixtures
When multiple fixtures are served by a single interceptor, calculate the total capacity of all fixtures, establish the maximum number of fixtures that may be drained simultaneously and apply this factor to the total capacity to determine the maximum simultaneous capacity. Then proceed with sizing and selection of interceptor using sizing formula, Table 3.

 

Alternate Sizing Method Based on Drainage Fixture-Units
Most plumbing codes list drainage Fixture- Unit values for plumbing fixtures and for fixtures not listed, these values are given for drain outlet or trap size. Fixture-Unit values are converted to discharge rates on the basis of one Fixture-Unit equaling 7.5 GPM. See Table 5 for recommended PDI size grease interceptor based on drainage Fixture-Unit sizing method.

 

Table 5

 

Additional Links:
Example for Sizing Grease Interceptors
Typical Single Fixture Installations
Multiple Fixture Installations
Dishwasher Installation
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