Trapping and Venting for Grease Interceptors

June 29, 2015

From time to time I will get an email that contains a schematic of a grease interceptor installation with the question, "does this look right?"

Usually the email-sender wants to know if the trapping and venting is shown correctly.

The answer requires a knowledge of both the applicable plumbing code and the installation requirements of the specific grease interceptor.

Model plumbing codes require each fixture discharging into a grease interceptor be individually trapped and vented, and require the installation of a vent downstream of the grease interceptor.

Standards also come into play when it comes to determining the type and location of vents upstream of a grease interceptor.

PDI G101 mandates the installation of an vented external flow control. The vent on the flow control is an air intake.  As the waste stream flows through the orifice in the flow control device negative pressure is created, drawing in air from the air intake which is intended to mix with the waste water as it enters the grease interceptor aiding in separation efficiency.

ASME A112.14.3 allows four types of ratings as follows:
  Type A - units with external flow control, with air intake (vent): directly connected
  Type B - units with external flow control, without air intake (vent): directly connected
  Type C - units without an external flow control: directly connected
  Type D - units without an external flow control: indirectly connected

Manufacturer's are required to identify which Type their interceptor is rated to when certifying to the ASME standard. 

PDI G101 and ASME A112.14.3 Type A certified interceptors are required to have a vented external flow control installed upstream of the interceptor.

Okay, so lets take a look at some drawings and see what works and what doesn't.

What's wrong with this installation?

It depends on which model plumbing code applies to the installation.

The International Plumbing Code (IPC) 1002.1 (exception 3) allows a grease interceptor to serve as a fixture trap - where it is intended by the manufacturer to serve as a trap - for a single fixture or a combination sink of not more than three compartments so long as the vertical distance from the outlet of the fixture to the inlet of the interceptor is not more than 30 inches and the developed length of the waste pipe from the most upstream fixture outlet to the inlet of the interceptor does not exceed 60 inches.

This drawing for a PDI G101/ASME Type A grease interceptor appears to be compliant with the IPC.  Assuming the interceptor is intended to serve as a fixture trap, there is no requirement to install an additional trap and vent between the fixture and the interceptor. 

The Uniform Plumbing Code (UPC) does not permit a grease interceptor to serve as a fixture trap and also prohibits the installation of a vent between the air intake on the flow control and the grease interceptor. The above diagram would not be compliant with the UPC. There needs to be a trap and vent between the fixture and the vented flow control fitting.

What's wrong with this installation?

Actually, nothing!

The drawing shows a PDI G101/ASME Type A grease interceptor connected to a the three compartment sink that is trapped and vented, an external flow control with air intake and a vent on the downstream side of the grease interceptor.

This installation would be compliant with both the IPC and the UPC.

What's wrong with this installation?

The drawing shows a PDI G101/ASME Type A grease interceptor with an external flow control with air intake, however it also shows a trap on the fixture but no vent for the trap.

This is not compliant with the IPC since the code does not allow double trapped fixtures. The code does allow the interceptor to serve as a trap - assuming that this interceptor is intended to serve as a fixture trap - thus adding a trap to the fixture upstream of the interceptor creates a double trapped fixture installation. Either adding a vent to the trap or removing the trap on the fixture altogether would solve the problem.

The only solution for compliance with the UPC is to add a vent for the trap on the fixture upstream of the vented flow control fitting.

What's wrong with this installation?

This drawing shows a semi-automatic draw-off type grease interceptor but without a vented external flow control.

Since all semi-automatic draw-off grease interceptors are certified to either PDI G101 or ASME A112.14.3 Type A (that I am aware of) an external vented flow control must be shown for compliance with the IPC.

For compliance with the UPC this drawing would have to show both a vented external flow control and a trap and vent for the fixture.

What's wrong with this installation?

Again, there is nothing wrong with this installation, though it may not be immediately obvious as to why.

The unit shown is certified to ASME A112.14.3 Type C (without external flow control) with a built-in or integral flow control and does not require an air intake.

That being the case the interceptor does not require a vented external flow control - jurisdictions unfamiliar with this type of interceptor often question drawings like this and understandably so.

This installation is compliant with both the IPC and the UPC since it shows a trapped and vented fixture discharging through an approved interceptor without external vented flow control with a vent installed downstream.

Air Admittance Valves (AAV)

The UPC does not include provisions for AAVs except as a part of an "Engineered Vent System" under section 912.0, much to the chagrin of engineers around the country. Many states that adopt or adapt the UPC have added an allowance for AAVs so you will have to check with your state to see if they are approved.

The IPC approves the use of and installation requirements for AAVs under section 918.0.

What's important to remember about AAVs is that they only allow for the relief of negative pressure in the drainage system. Therefore, when using an AAV to vent a grease interceptor, it's important that the drainage system has provision for the relief of positive pressure to ensure proper flow.

Boilerplate drawings can be trouble

To be honest, much of the confusion over whether a drawing shows a code compliant installation of a grease interceptor or not, can be the result of boilerplate drawings provided by manufacturers in submittals or installation instructions. Manufacturer's want to provide guidance for a broad range of installations without providing detailed drawings for every single installation variable that can exist. 

When it comes to traps and vents for grease interceptors and the fixtures discharging to them, it's incumbent upon an engineer or contractor to identify and comply with local code requirements regardless what a manufacturer's drawing shows. 

Hopefully after reading this post you will find it easier to identify a drawing that is right for your installation or one that needs to be corrected.

3 comments

  • A greasy guy

    May 24, 2017

    Excellent article! Very informative, thanks.

  • The Interceptor Whispererd

    Sep 13, 2016

    Mike, on Schier’s main web page you will see the option across the top for “products”, when you click on that you will be brought to the products page where you can find a row of links to each of Schier’s product offerings. When you click on a product (interceptor, accessory, etc.) you will see a black box that says “download center” – click that and you will get a pop-out window with optional links to various things. The technical data package contains all you need for specifications and drawings in either PDF or CAD (dwg) format. Cheers!

  • MIKE WALSH

    Sep 12, 2016

    do you have a cad FILE detail for the GB-75 BELOW GRADE INSTALLATION FOR CAD. SO YES PLEASE E-MAIL IT TO ME THANK YOU


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