When bad things happen to good drains

I'VE BEEN CLEANING drains for 36 years; you'd think by now I'd have a pretty good idea of what causes drain lines to clog. In all those years, I've worked with a number of sewer machines from different manufacturers: thick and thin sectional cables; spooled cables; manual feed; automatic feed; and I'm old enough to have used hand-cranked cable rigs — a job for those young and fit! Push tapes, too, but they were rapidly fading away while motorized machines were gaining traction as I entered the trades.

House traps are used throughout our area and, while they don't clog often, the clogs typically occur for good reasons: caked grease; powdered laundry detergent that has precipitated out of solution and formed a crusty ring; softballs that swell up like a bloated toad; and just about anything you can imagine that doesn't belong in the sanitary sewer line — including a 2-by-4 shoved down a drain during new construction by kids. Believe me, getting a 2-by-4 — or a tree branch, for that matter — out of a line 8 ft. below grade is no picnic!

Feminine hygiene products have been single-handedly responsible for more clogged sewers than any other single item. "Flushable" evidently refers to the fact that the products make it through the water closet's trap. From toilet-to-street it's a crap-shoot and that caused me to begin asking why?

Video proof sells more work than anything I've witnessed.

The obvious reasons for clogs aside, why do bad things happen to good drains? Prior to our purchase of a sewer-line video inspection system, we had to rely on how the cables reacted and/ or sounds that we could detect while cables were pulled/pushed/run up or down the line and also when a water closet was flushed. That slug of water moving down a correctly sloped drain line will give audible clues regarding any bellies, additional clogs or that the line is clear as it cascades into the sewer main — the Niagara Falls effect!

But that doesn't tell us if there are any defects in the piping itself. That's where our video inspection system comes into play. What we once believed was happening can now be documented and owners can clearly (no pun intended) see that our suspicions were correct. Video proof sells more work — effortlessly — than anything I've ever witnessed.

But in spite of our having video equipment, I still had unanswered questions and video equipment is blind in drains full of sewage. For example, why is it that one bathroom will back up and overflow when another bathroom is conjoined with the same line, somewhat lower in elevation and the primary clog is downstream of their junction?

Or, for that matter, why will a water closet overflow while nothing shows up in the tub's lower drain with the clog downstream of their junction? Or, a bathroom sink overflows while the water closet and tub exhibit not a trace of being affected from a downstream clog? Or, for that matter, why a floor drain line will spew raw sewage during the crisis yet is solidly clogged the second you clean the blockage from the main line?

So what does a plumber have to do to get some answers? Research, of course!

Now you'd think that reams of papers, both technical and anecdotal, would detail how stoppages occur and why, but I couldn't locate anything written about this all-too-common issue. Take a perfectly good drain with proper fall and explain why it clogged when nothing more than human wastes and toilet paper have been flushed. Then, just to complicate things a wee bit, explain why one branch line will overflow sooner than another one when both are tied together and the clog is downstream of their connection.

For starters, the Reynolds Number calculation will accurately predict if we'll have smooth laminar flow to carry along the paper and solids, or if we'll see turbulent flow that will thin out and fail to transport solids. If you want to check out the Reynolds Number calculation, go to www.engineeringtoolbox.com/hot-water-systemst_29.html.

But quite frankly, Osborne Reynolds' formula, while an important design consideration, doesn't come into play.

If a line has a partial blockage, there is a reduction of velocity at that point — think of it as if a boulder were rolled into a stream. The boulder partially blocks flow and causes a pool to form upstream. Solids being carried along come upon the pool of slower flow and drop out of suspension. If a feeder stream (side branch in the sewer line) is directly affected by the pooling water (sewage), then the side branch can quickly back up, causing the one bathroom to overflow sooner than one farther upstream.

The two lines effectively become two separately clogged lines that will behave differently. The branch line closest to the boulder will see what plumbing engineer (and former CONTRACTOR plumbing columnist) Julius Ballanco calls "surge pressure" while the longer line won't see any issues, providing the blockage allows enough liquid to seep through without pooling farther up the line.

At 1/4-in. per foot fall, it will take 16 ft. of run to completely fill the lower end. Anything farther downstream (and upstream of the clog) will be submerged and see rapidly escalating settlement of solids while anything upstream of that same 16-ft. run will not see any surge pressure and life, for all remaining upstream fixtures, will continue as if there was no clog.

Dave Yates owns F.W. Behler, a contracting company in York, Pa. He can be reached by phone at 717/843-4920 or by e-mail at [email protected].

More plumbing articles by David Yates