UPC grease interceptor sizing, round 3

 

By Timothy Allinson, P.E.

Murray Company, Long Beach, Calif.

Twice in the past few years - in October 2004 and April 2005 - I wrote about UPC grease interceptor sizing. In 2005 I participated in a task group that was ultimately successful in scripting a code revision that is reflected in the 2006 UPC Tables 10-2 and 10-3, which rationalize grease interceptor sizing.

In November I learned that, while the lion slept, a group of manufacturers formed a task group that scripted major revisions to the new tables, dramatically increasing the grease interceptor sizing that we had finally put into a rational framework. IAPMO's Technical Committee accepted these changes, and they were slated to become code in the 2009 UPC.

With the invaluable assistance of John Shaffer of EEC, an environmental consultant here in Orange County, Calif., we convinced IAPMO to reconvene a task group to evaluate the changes proposed by the manufacturers' task group. This new task group was more balanced, including sewer agency representatives, building officials, environmentalists and yours truly. Had this intervention not occurred, the proposed and accepted changes would likely have gone unopposed.

As the meeting began, I feared that our efforts would be in vain as the task group, while more balanced than the one at the previous meeting, still had a majority representation of manufacturers. (It's a shame that more engineers don't take the time to get involved in these important issues.)

There were two issues at hand: revisions to Grease Interceptor Sizing Tables 10-2 and 10-3.

You will see that the primary differences between the existing Table 10-2 and the proposed revision were the deletion of fixture unit references and the deletion of many of the intermediate size interceptors. If this revision had gone through, the use of 25- to 50-gpm interceptors would have been disallowed. As a point of interest, the listed price of a 25-gpm unit is $3,185; the cost of a 75-gpm unit is $11,675 - quite a difference to be forced to pay. The argument for making this change was that the interceptor had to be able to handle the worst case scenario of a fully loaded pipe, regardless of how many DFUs were connected. The claim was that the intermediate sizes were "dangerously undersized." Why have they made them for so many years? I wondered.

There was a great deal of debate over the gpm value of a fixture unit; the only correct answer was "it depends." A 2 DFU sink will flow about 2.5 gpm while the faucet is running and the drain is open, but it will flow far more if the sink is filled and then dumped.

There was also a great deal of debate over inserting a sizing example from the PDI classic standard using one-minute versus two-minute drain down times. This is shown in the proposed table. I argued that you cannot control how long a sink takes to drain, and that the volume of the sink is irrelevant except for its depth and tailpiece size. A 12" wide sink will drain at the same rate as a 30" wide sink as long as the depth and tailpiece are the same. Further, the PDI sizing technique is not valid today, since virtually all health departments require the indirect connection of food service sinks.

To prove my statement I went to the IAPMO kitchen during lunch, filled the sink with water and pulled the plug. The sink was 15" x 12" x 6" deep; it held 4.7 gallons. It drained in 29 seconds, making the average discharge rate 9.3 gpm. If the sink were twice as wide, the flow rate would have been the same, and it would have taken twice as long to drain. If this same sink were indirectly connected, rather than hard-piped with a P-trap, the drainage rate would have been significantly faster, due to fewer restrictions. This obviates the fact that the PDI sizing technique cannot be used for today's kitchens.

After hours of debate, to my surprise, the group voted in favor of keeping Table 10-2 as it presently reads in the 2006 UPC. Hopefully, the technical committee will agree with this recommendation and repeal its previous acceptance of the change.

Next, we spent a few hours debating the proposed changes in Table 10-3 for gravity grease interceptors (GGI's). These changes were more dramatic in nature, in that they significantly increased the required size of the interceptor. When the current Table 10-3 was created it was calculated based on a pipe flowing half-full for a 30-minute retention time. Surely there are peaks and valleys of drainage flow in any sanitary line, but the code sizing is intended to achieve an average maximum of a half-full pipe. (This same logic was not applied to the HGI's in Table 10-2, since those units function on a one-minute rather than a 30-minute basis, and the probability of momentary full pipe flow is much greater.) The proposed revision to Table 10-3 was based on full pipe flow for 30 minutes, which is not a rational assumption for a central sanitary system. Here again, DFU values were deleted, as were the intermediate size grease interceptors.

In a previous article, I pointed out that one of the prime motivators for changing the grease interceptor sizing that was previously in Appendix H of the UPC was that the interceptor sizes generated from that calculation were enormous, and that many of the large units were turning septic and generating hydrogen sulfide gas. Recently, three workers who were cleaning a large interceptor in Las Vegas were killed by H2S gas. Further, many sewering agencies have found that the large interceptors have caused their downstream mains to corrode badly, and, in some cases, the bottoms of the concrete interceptors have dissolved entirely.

At this point in the meeting it was time to hear from the sewering agency representatives. Several testified that they began implementing Table 10-3 as soon as it was published, even though it didn't become code in California until this month. They stated that they removed many oversized interceptors and replaced them with the smaller units in the new table. The reduced size resolved their H2S problems and facilitated more frequent cleaning. Since a vacuum truck holds 3,000 gallons, it is much more practical to clean a 1,500-gallon unit monthly than to clean a 12,000-gallon unit. The latter requires four vacuum trucks or four trips of the same vacuum truck, and if anyone thinks that was done monthly they are kidding themselves.

After these testimonials, the proposed Table 10-3 was quickly rejected. The group spent a few more hours scripting some minor code change proposals, but nothing of significance. All in all, it was a very successful meeting. Hopefully, the IAPMO technical committee will agree.

I hope you all have a happy and healthy 2008!

Timothy Allinson is a Senior Professional Engineer with Murray Company, Mechanical Contractors, in Long Beach, Calif. Prior to entering the design-build industry he worked for Popov Engineers, Inc. in Irvine, Calif, and JB&B in New York City. Tim holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed Accredited Professional. Tim is a past-president of ASPE, both the New York and Orange County Chapters, and sits on the board of the Society of American Military Engineers, Orange County Post.