Note there are actually two types of piping involved here.� Items 1-3 are "drainage system" components, and they typically are 1-1/2" OD, either chrome-plated brass or white plastic, with very thin walls and connected together with compression fittings.� The line going out, item 5, is Schedule 40 or DWV PVC piping, which is 1-1/2" ID, fairly thick wall, and glued together with PVC cement.� Item 4 is an adapter to connect these otherwise incompatible types of piping.� The adapter is glued into the end of the PVC pipe and provides a compression fitting for connecting the P-trap outlet.
This drainage arrangement sucks.� It works, it just doesn't work very well.� The problem is with the "end outlet tee", item #2 in the illustration above.� Here's what the innards of this part look like:
There is another type of drain for a pair of sinks called a "center outlet tee", in which the drains from both sinks turn towards each other and there's a tee in between with an outlet downward.� It has the same problem.
Fortunately, it's really easy to fix this bodge.� The idea is to provide each sink with its own P-trap, then plumb the drain lines together back at the heavier pipe into the wall using a wye connector instead.� Wye connectors have big open passages through them.� To illustrate:
Item 2 is a tailpiece -- but you'll need a longer one than the one that was in there, so you need to buy a new one.� They typically come in 6" and 12" lengths, so make sure the one you buy is long enough; they are intended to be cut to length anyway.� Alternatively, you can keep your old tailpiece and buy an "extension", but as long as you're in there the new tailpiece is the neater way to go.
Item 3 is a new P-trap assembly, essentially similar to your old one.
Item 4 is a 1-1/2" wye.� It doesn't look like it in
this illustration
because it needs to be installed in a flat plane so all connections to
it are horizontal.
Here's a photo of the finished assembly:
Remember that you will need two adapters for this assembly; you might be able to reuse your old one, but they're cheap and buying two new ones while you're shopping might be a good idea.� Note that some adapters are made to be glued onto the end of a pipe, while others are made to be glued directly into a fitting such as the wye used here.� If you get the first type, you will need two short pieces of 1-1/2" pipe to attach them to the wye.
So, while you're at the store, you'll need to buy a disposer elbow if you don't have one, a tailpiece, a P-trap, two adapters, a small can of PVC primer and a small can of PVC glue.� You will also need a hacksaw if you don't have one.� All of this adds up to something like ten bucks.
The real challenge, of course, is making sure it will all line up.� Whenever you cut off a PVC pipe, you need to make sure you're leaving about an inch for gluing a new fitting on.� And when connecting up P-traps, you need to make sure that all the parts can reach each other.� Note that the outlet end of a P-trap is also intended to be cut off if it's too long.� Also remember that it's a simple matter to loosen a disposer's mounting lock ring and rotate the disposer around, which will realign its outlet connection.
When you get all this connected up, the drains will work
much better
since there is a full pipe diameter drain from each sink.
As an aside:� The reason that the drain from the sink
to the right in the photo has two 45� bends in it rather than a
straight drop to the P-trap is to reduce noise.� Water falling
more than a foot into the standing water in a P-trap makes a splashing
sound; by putting these jinks in the drop, the water runs down the side
rather than falling straight, and the sound is somewhat baffled from
coming back up the pipe.� Functionally it does nothing, however; a
straight pipe will work just fine.
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