Is there any potential danger, say, sewage gas, of having one p-trap after the other?

I wanted to add a clean out with a 1.5" plug above the p-trap. I may put in some plastic mesh in the clean out so I can inspect regularly what my wife and kids put through the kitchen sink. I don't want the clean out to be under water and have to clean up the mess every time I open the clean out.

I found out that I don't need and modification to the existing plumbing by simply splitting up the current p-trap and add another p-trap. The p-trap is standard - 1.5" slip joint and 1.5" union. The only other thing I need to add is a tee with an adapter plug, and a 90 deg bend.

I'm sure no code allow this. But if the drain cannot siphon the water in one p-trap, can both the p-trap be dry?

I own the house. But if you rent, the "device" is portable and you can revert to the original p-trap in no time.

http://i39.tinypic.com/2vif49c.jpghttp://i42.tinypic.com/12513x4.jpg

2 traps are a no-no as are S traps, they can siphon. A trap is so easy to remove I am surprised you went through all this just to add a clean out. I would have kept the original and just dropped the trap if a problem came up.

Is there any potential danger, say, sewage gas, of having one p-trap after the other?
STOP!! Chapter 7, section 703.6 of the Standard Plumbing Code reads. No fixture shall be double trapped.
Ya can't do it it's against code. Sorry, Tom

Why not just use a P trap with a drain plug?
https://www.askmehelpdesk.com/members/hkstroud-albums-+pictures-picture520-trap-drain.jpg

The usual drain plug are too small to put screen mesh in it. I expect to inspect it often so I prefer it to be dry.

It takes about two minute to glue the thing together without modifying any existing thing. I did that before I realized that it becomes a double trap. I hope to understand the science of siphoning in there and I can do experiments. I can screw in a temporary vent instead of the clean out plug, but it will overflow if the sink blocks.

Background. The clog is a bit less than 25' down the drain line, due to decades of grease and sludge build up. It opens the clog a little if I pour 128 oz of gel down the drain and leave it overnight (there's a p-trap bypass clean out outside the house). Sulfuric acid from hardware stores used to be good, but now I have to leave it down there for hours if not overnight. I doubt if it's safe leaving it for so long and I wonder if it's really pushing the clog further down the line. Anything else I will need a lot of it (and money) to get enough to reach 25' down, and in the hope it works.

Now I'm going back to the snake. But after each time, the drain will be completely blocked. I have to use the "water pressure bladder" to blast it open again. Then I have to bath the snake so it doesn't smell in the garage. And then oil it so it won't rust. And let the oil dry so it doesn't smell.

I have a classic recurring slow drain. It will be blocked again in about a month. Everybody is supposed to use a fine mesh strainer at the sink to prolong the time between blockage, until we find some solutions. But I doubt if everybody observe the rules. That's why I want to add some screen strainer down the sink to collect samples.

I hope the regular use of the hand held snake will improve the situation. If I'm desperate enough I'll hire the big gun - rent a professional snake. Any other solutions will be welcomed.

It is wrong. Just leave 1 P-trap and install Y on horizontal line. If you like Clean Out before the trap than install it before the trap ( but why ? )

Apparently your rodding is not reaching the clog completely. It could even be out at the city riser. How far from your sewer pipe where it leaves the house to the far side of the street?

There is a sewer at the entrance of the garage, about one and a half car length from the sidewalk. My hand held snake 25' long is working OK, restoring near perfect flow at the kitchen sink if I work on it repeatedly. Obviously the thick grease and sludge are not cleared completely.

This gives the idea:
http://www.plumbingsupply.com/images/no-clog-safety-trap.jpg
I'm trying to see if I can add a poorman's grease trap down there without spending hundreds of dollars, and doesn't need to get wet whenever I open the trap.

Btw, a check vent just cost a few dollars, and the perfect place for it is replacing the plug in picture, if it works sideways. Just don't want to go under the sink with my power saw.

25' is not nearly long enough to rod the entire drain line. If you are rodding outside by the garage then the whole house must be backing up or draining slow, right? I rented an electric sewer rodder with 200' of snake. I hit tree roots 190' out.

Just took a look at my house. One end is the kitchen sink drain. I thinks it's in line with the downstairs toilet, which is separated by a distance about the size of a living room. Next to the bathroom is the garage, and near the entrance is the sewer, which isn't in line with the kitchen sink and toilet. The sewer to the sidewalk is about one driveway away.

The other drains are not affected. When the kitchen is completely blocked I wash in the bathroom sink downstairs.

I can't even insert the full 25' length of the hand held snake, which is driven by a electric drill. I rodded near the sink, but there's a clean out outside the house. I don't know if the snake is blocked by bends or clogs, but it's unclogging something. I don't know if it's harder or easier to insert for bigger and thicker rodders. But sure that's the next I'll try.

You need to use proper cutter tips to clean your drain. The cutting tip should be sized for the size of your kitchen sink drain. If you use only "point" than the hole you cut in the middle of the clogged pipe will enclose as you pull the cable out of the drain.

Size of the cable for kitchen sink should be 3/8" x 50'. Fifty feet will get you all the way to 3" or 4" drain.

If you don't have the snake than go to Home Depot and rent one for couple of hours. It is a good investment...

I am enclosing 2 pics to show you what it looks like when you "clean" drain with improper size cutting tip. Even the drain was snaked, it remained clogged.