Purposeful Plumbing: What is a P-trap?

Purposeful Plumbing: What is a P-trap?

Purposeful Plumbing: What is a P-trap?

So many sinks are built into cabinets.  It’s such a great idea, more storage space for all your stuff!  Well that’s what you think until you open the doors and see that curved pipe hanging out right in the middle of all that storage space.  What is that U-shaped pipe all about and why is it taking up valuable storage space?

The P in P-trap

P-trap? Where did the P come from? Well, while at first glance this pipe looks like a U, if you include the pipe that connects to the drain pipe and tilt your head (or just turn the pipe on its end), there it is, it does look like a P.

Well that seemingly obtrusive pipe is called a p-trap.  The p-trap’s purpose is to stop the sewage gases from drifting from the drain pipe into your home or building.

What are sewage gases? And why would they be in my house?

Well in the plumbing systems in our buildings, all waste water and any accompanying waste go down the drain entering our Drain Waste Vent (DWV) system.  Since water is not flowing through this system constantly (which it shouldn’t), it can get a little dry in the pipes causing waste to decompose.

With this decomposition, gases are formed. These gases are considered sewage gases which not only smell like what we associate with the odor of sewage but also include gases that can be detrimental to health in highly concentrated doses.

These gases want to escape from the pipe.  The DWV system provides a vent in the roof that these gases can escape from.  However, these gases would also try to come back up through the pipes attached to our drains if it wasn’t for the p-trap.  P-traps are located at every open drain in your building, meaning they are connected to all sinks, showers, toilets, and drains.

So how does a curved piece of pipe block gases?

P-trap diagram showing the trap seal depthThe p-trap traps water in its dip, sealing off the pipe.

As water flows down the drain, the water level rises and flows down the drain pipe with accompanying waste to the sewage system.

As the water stops flowing, the water balances out and new water becomes trapped again in the dip of the p-trap.  This water eliminates space for any air or gases to flow back up through the drain.

As you can see, water is the main factor that allows a p-trap to work.  If the p-trap becomes dry, then those smells can start to seep into your building.  P-traps typically only become dry at drains that are rarely used, most often floor drains.  If you begin to smell an odor from a drain, poor some water down it to fill the p-trap back up.

For the most part, waste is able to move through the P-trap pretty successfully.  The P-trap is built to create a smooth flow and there is typically enough water to create a consistent flow.  Some things can begin to build up over time however and cause a clog in the drain.  This can be remedied fairly easily through cleaning or replacing the p-trap.

So the next time you’re ready to curse that stupid curved piper under your sink for taking up valuable space, remember your life would stink more (literally) without it there.

Categories: Plumbing

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