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Grease Interceptor Sizing

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VLADIMIR LEVIN

Registered User
Joined
Jun 11, 2019
Messages
113
Location
NY
I'm in NY, relevant code is 2020 PCNYS. I'm trying to size a grease interceptor for a 3 compartment sink.
Is there anything in the plumbing code other than the grease interceptor being based on the flow rate?
Reason for the question is, I'm calculating a flow rate of 50 GPM which would require a 100 lb capacity interceptor.
I can't fit that in my space.
My calculation is based on the flow rate of all 3 sinks, but in use they wouldn't all be draining at the same time, right?
Is it valid to size the interceptor based on the flow rate from one sink?
 
Not sure on NY, but in WA under the UPC, you have to size for all three sink compartments.
 
ClassicT, are you a plumbing inspector?
I'm wondering if the inspector will ask for the sizing calculations?
 
I'm a plans examiner.... but yes, I ask for the sizing calculations when you submit your plans for permit.
 
I wonder if sizing it by drain pipe diameter will yield the same result. Do you know what the flow rate for a 2" diameter drain pipe is?
 
classicT, as a plan examiner would you accept an interceptor based on the following: 2" pipe diameter, sloped 1/4", has a flow rate of 20 GPM (I found this in a chart)
 
Vlad, I am not a plumbing inspector, however I am going to just theorize a little,

Question number
  1. Does each sink have its own faucet to fill it, or is there 1 faucet or 2 faucets
  2. IF all 3 sinks are in a row and connected to flow through one 2-inch pipe, then maybe the pipe flow is correct
  3. However, if one faucet fills all three sinks, what is the flow rate of the faucet.
  4. IF the flow rate is based on the mass of water filled in all 3 sinks at one time,
    1. Well if you had a 10,000 gallon pool, with one 2-in drain pipe and pulled the cork, what would the flow rate be?
Not sure but, my guess is that the inceptor is sized for what the potential gallons per minute is, as thus what is the worst case to establish max flow.
 
tbz, it's based on the GPM flow rate. If only someone could tell me what that is for a 2" diameter drain pipe.
 
Most hydromechanical grease intercepts have a flow control device pre-installed, so the flow rate of the inbound drain line is not going to impact the capacity of the intercept.

Here is what I found for the NY code. Note the sections highlighted in red.

1003.3.5 Hydromechanical Grease Interceptors, Fats, Oils and Greases Disposal Systems and Automatic Grease Removal Devices
Hydromechanical grease interceptors; fats, oils, and greases disposal systems and automatic grease removal devices shall be sized in accordance with ASME A112.14.3, ASME A112.14.4, ASME A112.14.6, CSA B481.3 or PDI G101. Hydromechanical grease interceptors; fats, oils, and greases disposal systems and automatic grease removal devices shall be designed and tested in accordance with ASME A112.14.3, ASME A112.14.4, CSA B481.1, PDI G101 or PDI G102. Hydromechanical grease interceptors; fats, oils, and greases disposal systems and automatic grease removal devices shall be installed in accordance with the manufacturer's instructions. Where manufacturer's instructions are not provided, hydromechanical grease interceptors; fats, oils, and greases disposal systems and automatic grease removal devices shall be installed in compliance with ASME A112.14.3, ASME A112.14.4, ASME A112.14.6, CSA B481.3 or PDI G101.
 
classicT, so in other words if my interceptor is to small for the flow rate of my 3 compartment sink, the only thing that will happen is my sinks will drain slower.
 
VLADIMIR LEVIN said:
classicT, so in other words if my interceptor is to small for the flow rate of my 3 compartment sink, the only thing that will happen is my sinks will drain slower.
Click to expand...
Most likely, yes... it will cause the water to back up out of the floor sink and make a mess on the floor. It could also lead to grease being pushed through the intercept.

If you are going down the road of saying that you would accept it not draining as fast, good luck. It is still a code violation and the AHJ should tell you no.
 
classicT, correct me if I'm wrong but the code only says that drainage from fixtures with grease laden food have to discharge into an inteceptor, and that the retention capacity of the interceptor is based on the flow rate. But it doesn't say anywhere how to calculate the flow rate. In other words it doesn't say that it must be based on the maximum flow rate; which in my case would be all 3 sinks filled to capacity and discharging at the same time. So what would be the code violation that you are referring to?
 
VLADIMIR LEVIN said:
classicT, correct me if I'm wrong but the code only says that drainage from fixtures with grease laden food have to discharge into an inteceptor, and that the retention capacity of the interceptor is based on the flow rate. But it doesn't say anywhere how to calculate the flow rate. In other words it doesn't say that it must be based on the maximum flow rate; which in my case would be all 3 sinks filled to capacity and discharging at the same time. So what would be the code violation that you are referring to?
Click to expand...
classicT said:
1003.3.5 Hydromechanical Grease Interceptors, Fats, Oils and Greases Disposal Systems and Automatic Grease Removal Devices
Hydromechanical grease interceptors; fats, oils, and greases disposal systems and automatic grease removal devices shall be sized in accordance with ASME A112.14.3, ASME A112.14.4, ASME A112.14.6, CSA B481.3 or PDI G101.
Click to expand...
 
Thanks for your help classicT.
PDI G101 is the only free publication and says for 2" pipe interceptor size is 20gpm
Problem solved.
 
After reading DPI G101 which is a reference standard for sizing grease interceptors, and a crash course in the Plumbing Code here is my conclusion:
There are 3 different ways to size it (section 8.5) and each yield a different size interceptor.
The most conservative way is to calculate the capacity of each fixture and add it up. In my case (3 compartment sink) would require an interceptor rated for 50 GPM ( which incidentally would require a 3" diameter pipe.)
Another allowable methods is to size the interceptor based on pipe size, which for a 2" pipe would be 20 GPM. The question here is whether a 2" pipe is allowed by plumbing code. My 3 compartment sinks have a 1 1/2" outlet (each), which equals to 6 DFU (table 709.2 2020 PCNYS), and a max. discharge of 6 DFU is allowed for a 2" diameter pipe (table 710.1(2) 2020 PCNYS).
My conclusion is that a grease interceptor rated for 20 GPM (40 pound retention) would be code compliant.
And that's for a 1 minute drainage. For 2 minute drainage it could be 10 GPM (section 8.3.1 DPI G101) 20 pound retention.
 
Alternate materials and methods used to calculate size just have to be approved by the AHJ. Should be a simple hoop to jump through
FYI Our public works department over sees all grease interceptor sizing and approval in our jurisdiction because their requirements are more restrictive than the plumbing code because of some federal grant monies they received for expansion of our sewer plant and establishment of a FOG's program to bring older uses into compliance.
You might want to check and see if you have all the bases covered, sizing, location and sampling port requirements are just part of PW requirements in my area
 
mtlogcabin said:
Alternate materials and methods used to calculate size just have to be approved by the AHJ. Should be a simple hoop to jump through
FYI Our public works department over sees all grease interceptor sizing and approval in our jurisdiction because their requirements are more restrictive than the plumbing code because of some federal grant monies they received for expansion of our sewer plant and establishment of a FOG's program to bring older uses into compliance.
You might want to check and see if you have all the bases covered, sizing, location and sampling port requirements are just part of PW requirements in my area
Click to expand...
Thanks for the insight
 
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