Understanding Rated Grease Capacity Versus Maximum Grease Capacity

July 03, 2014

The grease interceptor revolution is about challenging the status quo.

It's about asking tough questions and thinking radically different about the answers.

It's about listening to the people who are affected by the products we make.

The revolution is about making High-capacity grease interceptors because contractors, engineers, restaurant owners and jurisdictions are demanding superior performance.

As more manufacturers see the light and join the revolution it's important that you understand what to look for to make sure that an interceptor that claims to be high-capacity really is high-capacity.

The journey to enlightenment begins with an understanding of the testing and rating requirements of both nationally (US) recognized and plumbing code approved standards and ends with an understanding of some of the tricks of the trade used to disguise an interceptors performance to make it appear to be something that it is not.

Testing and rating requirements

In most jurisdictions across the US, hydromechanical grease interceptor's must be tested and rated to either PDI G-101 and ASME A112.14.3.

Both standards are similar in most respects including the same testing and rating protocol, however there is a key difference between them; PDI allows for either Rated Grease Capacity (RGC) or Maximum Grease Capacity (MGC), while ASME currently only allows for MGC.

What's the difference between RGC and MGC?

An interceptor certified to MGC has actually been tested all the way to its breakdown point while an interceptor tested to RGC has not.

Let me quote from PDI G-101 so you can see the difference between the two ratings:

7.6.2 Determination of Test Breakdown Point (Maximum Grease Capacity)

The test failure, or breakdown point of the interceptor, shall be established at the increment preceding two (2) successive increments in which either the average efficiency is less than ninety (90) percent or the incremental efficiency is less than eighty (80) percent.

7.7 Efficiency Determinations (Rated Grease Capacity)

The grease shall be removed from the skimming tank and the efficiency of the interceptor shall be computed at the (13) thirteenth increment. This provides at least a twelve and one-half (12.5) percent safety factor on the ratio of the rated grease retention capacity to flow rate as indicated in Table 1:



To put the difference between RGC and MGC into perspective lets review the actual test data from NSF for Schier Product's GB-250:

 

You can see at the 13th increment the GB-250 had an average efficiency of 95.2% and had separated 247.46, which under PDI would allow it to be certified at 200 lbs grease storage capacity according to Table 1.

Under ASME A112.14.3 (which this unit is certified to) there isn't an option for RGC so you have to test to failure (prior to 2007 this wasn't an option under PDI either).

As you can see the GB-250 suffered two successive increments (59 and 60) with incremental efficiencies below 80%. The test breakdown point (the increment that is to be used for certification of the interceptor) is increment 58 at which the GB-250 had an average efficiency of 92.8% and separated 1076 lbs of grease at 100 gpm.

The revolution is gaining momentum

As I said earlier, other manufacturers are apparently starting to see the light.

As new products emerge to compete in the arena of high-capacity grease interceptors though, it's important not only to know how grease interceptors are rated, but also how a manufacturer reports their ratings.

Tricks of the trade

How do you know if the grease capacity shown on a specification or submittal sheet is the interceptor's actual certified capacity or an unsubstantiated claim?

To be honest it can be difficult.

One manufacturer uses the term Grease Design Capacity to confuse or imply that this is the interceptors actual certified capacity. An investigation into the testing and rating of the interceptor revealed that it was certified as RGC, which means the interceptor wasn't tested to failure and therefore their posted 'design' capacity is uncertified and unsubstantiated.

One manufacturer uses the term Greasy-Sludge Capacity. What does that mean? I'm not sure. The amount listed is more than their certified grease storage capacity. The number appears to be a combination of grease and solids capacities combined. But since neither PDI nor ASME test for solids there is no way this number can be substantiated.

Another manufacturer doesn't use pounds at all, instead opting to show their grease capacity in gallons. When you take the gallons and convert it into pounds, you end up with a lot more claimed grease capacity than their apparent certification.

When in doubt ask for the test report.
At the end of the day there is really only one way to know what the actual grease storage capacity of an interceptor is and that is to ask the manufacturer for the test reports.

In case you were wondering, testing agencies are not obligated to share the certification reports for any of their manufacturer's (I was informed of this in a very lawyer-like manner when I asked). That means you have to ask the manufacturer themselves.

Both PDI and ASME require interceptor tests to be documented on Test Form #1, which records each and every test increment along with incremental and accumulated efficiencies. The test report will tell you whether the interceptor was tested only to 13 test increments and is therefore rated at RGC or if the interceptor was tested to failure and you can see for yourself the interceptors MGC.

Schier Products believes wholeheartedly in the interceptor revolution and as such is working on posting the NSF test reports for each certified unit on its website. In the mean time, if you would like to see a test report for any of Schier's certified units, just send me an email and I'll get them to you right away.

Competition is a good thing and manufacturers who want to join the revolution are welcome, but now you know what to look for to be sure that the interceptor that claims to be high-capacity really is high-capacity.
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