What is two-step sizing for a grease interceptor?

What is two-step sizing for grease interceptors is a common question I am asked. Uniform Plumbing Code (UPC) sizes hydromechanical grease interceptors (HGI) by flow rate (pipe size) as shown in step 1 above. If you are using a 4" grease waste line on a project UPC would require a minimum 75 GPM HGI (2-minute drain time) such as the Schier GB3. The GB3 is a third-party certified 75 GPM HGI so code is satisfied, right?

Technically yes, but the shortcoming of stopping at step 1 is that it does not factor loading. Compare two similar restaurants, let's say they are similar in size, kitchen equipment, the number of meals served per day (let's say 300) but the difference is food type. Restaurant 1 is a sandwich shop and restaurant 2 is a bar and grille. Back to the GB3 that satisfied UPC. The GB3 is third-party certified to a maximum grease capacity of 175.6 lbs of grease separated and stored. The sandwich shop would be considered a category A in step 2 above and produce 0.005 lbs of grease per meal served. 300 meals served per day * 0.005 lbs per meal served = 1.5 lbs of grease produced per day. By comparison the bar and grill at 300 meals served per day * 0.0455 lbs per meal served = 13.65 lbs of grease produced per day.

See the problem with stopping at step 1? If the GB3 is used it will require maintenance every 117 days at the sandwich shop (175.6 capacity of GB3 / 1.5 lbs grease produced per day). The bar and grill every 12 days (175.6 / 13.65). Both are UPC compliant per step 1 but by not considering loading they yield two very different maintenance intervals. 

The solution? The Schier GB-250 is 100 GPM, satisfies step 1 as it's 75 GPM or larger but will separate and store 1,751 lbs or grease. By using a GB-250 in the bar and grill application we can match the maintenance interval of the sandwich shop. 1,751 lbs grease capacity / 13.65 lbs produced per day = 128 days between cleanings. 

That's the basic concept of the two step approach. To make this really easy Schier has an online sizing program Grease Monkey that will allow you to input all parameters of the job and give a recommendation. They even have a team that will pre-approve AHJ acceptance and it's free.

Try out Grease Monkey today >>