Replace grease trap installed with threaded copper drain lines

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Old 11-13-17, 07:59 PM
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Replace grease trap installed with threaded copper drain lines

I'm trying to replace a corroded grease trap under my sink with a very similar model. The existing grease trap has a threaded connection on each end. My problem is that my 2" copper drain pipes are attached to the grease trap with copper male threaded adapters, and thus it is not possible to simply unscrew the drain lines from the existing grease trap and then screw them into the new one, which was what I originally expected. See pictures of my situation below:

Left side of grease trap:


Right side of grease trap:



I have a few questions for the forum:
1) Can I simply cut the copper pipe about 1" from each threaded connection, then unscrew each adapter end (plus 1" of copper drain pipe) from each side of the old grease trap, and then screw the threaded adapters (plus 1" of copper drain pipe) to the new grease trap, and then connect the new grease trap/male threaded adapters plus 1" copper pipe ends assembly to the existing copper drain pipe with 2" flexible couplings? Do you see any major issues/drawbacks to this method?

I proposed this method because I expect it might be a challenge to remove/separate 2" copper drain lines, and I'm not sure how to go about removing the threaded male adapters to unscrew them from the existing grease trap. I would also prefer the ease of maintenance/future replacement of having flexible couplings that can be removed with a screwdriver.

2) If you recommend an alternative method to the one I proposed above, please share details of how I should go about removing the existing grease trap (heat male threaded adapters and then use wrench to unscrew? Tricks to getting these off? How long will I have to heat? Should I use Mapp gas?

Any tips are appreciated. I've done one or two very small plumbing jobs with 1/2" and 3/4" copper, so this is the largest pipe I've ever worked on, and the first time i've ever potentially had to desolder existing pipe. Thank you very much for your help!
 
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Old 11-13-17, 08:41 PM
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Welcome to the forums.

Your pictures didn't quite make it...... How-to-insert-pictures
 
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Old 11-14-17, 05:26 AM
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The missing pictures

I hear the pictures didn't make it the first time. Please see the attached.
 
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Old 11-14-17, 08:22 PM
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bump...any advice out there?

thanks

Kyle
 
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Old 11-15-17, 09:57 AM
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I would cut the copper in the longer horizontal section, then solder the new one back with two couplings. I don't know if there's a copper x copper coupling. And if the new grease-trap isn't identical to the old one, there may need to be some minor adjustment (with fittings).

And it turns out that there is a 2" copper x 2" copper coupling:

https://missionrubber.com/product/ba...-part-0802595/

Note: don't expect the inside of the pipe to look like the outside. Might want to put down a plastic drop cloth before cutting.
 

Last edited by steve_gro; 11-15-17 at 09:59 AM. Reason: added words.
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Old 11-15-17, 05:20 PM
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Thanks so much for the reply Steve, I appreciate it.

So if I cut the longer copper section, I will be able to unscrew the piping on the right side of the grease trap, but I don't see how I would unscrew the piping on the left at that point. Should I cut that side too?

And I'm not sure I fully understood the rest of your reply, are you saying that when I put it back together I should use regular copper couplings and solder, or can I use the flexible copper to copper couplings?

Thanks!

Kyle
 

Last edited by overhols; 11-15-17 at 05:20 PM. Reason: name twice
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Old 11-15-17, 10:07 PM
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You're going to need a minimum 2" of clear copper to use a coupling, preferably a little more, like 2-1/2" or 3". Can you cut the copper, pull out the old trap, unscrew/screw, then put back? If not, it looks like there's enough room on the right side for a coupling between the male adapter and 90. And it looks like the left side would unscrew if you cut the copper a couple of inches past the 90.

I thought perhaps you were trying to avoid soldering. The couplings would come in handy for future service. I don't think this would cause any issues with your local inspectors. If that's a concern, you might give them a call first.

Keep in mind that if the new trap isn't identical to the old one, it may need more than just the coupling to connect.
 
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Old 11-19-17, 05:32 PM
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I'm sorry, I see what I misunderstood in your first response. For some reason I had always pictured cutting the very short copper pipes just to the left and to the right of the grease trap, while I think you had meant for me to cut the pipe further away from the trap (between each copper elbow and the back wall of the cabinet). There is certainly nothing to prevent me from cutting both right and left pipes between the elbow and the back of the cabinet. Then I could pull the entire assembly out of the cabinet, thus leaving me plenty of room to unscrew the adapter plus elbow from each side of the trap. My only concern with this approach is whether I'll be able to screw the adapters plus elbows into the new grease trap and into the correct orientation to reattach to the existing pipe while still ensuring the connections at the grease trap are water tight. If you think this is likely to work fine, then I'll go with this approach.

Regarding cutting the very short pipe sections instead, the length of the clean copper pipe is only 2 1/8" or so on the right side, and more like 1 1/4" for the left pipe. Unless its possible to still get a tight seal on the flexible copper connections when including the beginning part of the elbow, this isn't likely to work on the left side pipe, and will be very tight on the right side based on the rule of thumb you gave (2-3" of clear copper pipe).

Thanks for all of your help!

Kyle
 
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Old 11-20-17, 11:55 AM
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That's the approach I would use. Cut a couple of inches past the elbow, pull the trap out, make sure they're the same, then unscrew from the old and screw into the new. Then put back with the couplings I mentioned -- torqued to spec (approx 60 inch-pounds). Use a few wraps of teflon on the threads and a good pipe dope (I'm partial to Rectorseal Blue). In the event that it leaks, which does happen from time to time, you pull it out & try again.
 

Last edited by steve_gro; 11-20-17 at 11:57 AM. Reason: clarification
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