Understanding the “Flow”

There are very strict rules when it comes to drain waste lines.  These rules dictate such things as the locations, slope, size, type of fittings that must be used, ventilation, accessibility for maintenance, and how traps must be used.

The easiest way to think about it is like this: Once waste water enters a drain, it must flow as easily, and smoothly as possible, all the way from the fixture, to the waste water treatment facility (or septic tank).  So, if you think about the path of the water, if it is traveling along a horizontal pipe, the pipe must slope downward, so gravity can take it where it needs to go.  The code says that the minimum slope is 2% (or 1/4” of downward slope per ever foot of horizontal length).

If it must then transition to a vertical pipe, the drop off should be fast (so a “short sweep” fitting must be used).  If the vertical pipe transitions to a horizontal pipe, the transition should flow smoothly (so a “long sweep” fitting must be used).  If the horizontal line needs to turn (and it should turn as little as possible) it must do so gradually (so a “long sweep” fitting must be used here as well).

The example above is but a small part of the equation, but it does illustrate the following point – Unlike potable water supply lines, where there can be quite a bit of leeway, with waste lines, there is virtually no room for error.  If a plumber is running a water line, or a gas line somewhere, and something is in the way, there are options.  Maybe whatever is in the way can be drilled through, or removed.  Maybe the plumber will decide that it is easier to just plumb around it.

With a waste line, there are really no options.  Most of the time, whatever is in the way needs to be removed.  There is no “plumbing around it” with drain lines.

Why Size Matters

Drain pipe sizing is another aspect of the equation that is often not fully understood, even by some plumbers.  There are actually rules, set within the plumbing code, for how big a drain pipe must be, depending on the likely flow of waste water through them (measured in fixture units).  Although, those guidelines are for the minimum sizes that must be used, is going a little bigger better?

Surprisingly, the answer is “no”.  Here is why…

If you flush 1 gallon of water down a 4” sewer line, the water flow will be shallow.  If you flush a gallon of water down a 3” sewer line, it takes up more of the pipe, so the water flows more deeply.  This is better for two reasons:

1 – Deep water travels faster than shallow water, and with less turbulence.  In essence, it travels more smoothly.

2 – Deep water does a better job of carrying solids along with it, where with more shallow water there is more opportunity for solid waste to drag along the bottom of the pipe and disrupt the flow.

Still, once the water fills up the pipe to a little over half volume, there begins to develop diminishing returns.  If the water fills the pipe to full capacity, obvious problems begin to develop.  So, we want to make sure that the drain pipe is small enough to be as efficient as possible without ever being so small that it reaches the point of diminishing returns.

We hope that this article was not too terribly technical.  We just wanted to give you a little insight into the world of drain line replacement.  You can always feel free to call an Allstar Plumbing experienced plumber to help you out with any drain line issues or questions you might have.  In fact, we hope that you do.