2019-12 - Sealing of S-trap and P-trap under the sink with strong siphon effect

Simulation of S-trap and P-trap under the sink has been carried out in OpenFOAM.
The trap under sink in our household is to use water to seal up the trap to prevent sewer gas from entering our home. In US, it appears P-trap is commonly used. In HK, bottle trap is used in household instead.
The use of S-trap has been banned in most plumbing codes in US because on rare occasions the water can be sucked out of the trap by siphon effect allowing sewer gas to enter the home. There are suggestions to change the S-trap to a P-trap by extending the trap arm to retain water in the trap. This P-trap design is slightly better, however, water can still be sucked out by siphon effect. Moreover, P-trap only design is also not allowed by most plumbing codes for new construction. Provision of proper vent for P-trap is the recommended solution.
These three simulations are created with strong siphon effect to suck water out of the trap. It is clearly seen that provision of air ventilation solves the problem and keep the P-trap sealed off by water.

[@thomasreedy4751]

It’s an s-trap if you aren’t connected to the vertical vent within 5 ft. So two s-traps and a p-teap

[@pukeschannel6882]

If the house had s traps when it was built (and a long time ago they were quite legal), then you are allowed to replace a bad s trap with a good s trap (as a repair), I have never heard of an inspector requiring more than that. That being said,..I've worked on a few houses that old (some of them have lead plumbing pipes up to 2 inches, but usually 1 1/4 to 1 1/2). My experience has been that a full kitchen sink with an s trap drain,.. can and will sometimes suck an s trap dry. I don't recall a vanity sink ever doing that. It is very easy to put an air admittance valve to deal with an s trap,..you can run a drain pipe through the roof at some point in the future,..when and if you do a complete remodel,..removing sheetrock and all.

[@ElevatorWasher5000]

That’s what they did at my house, added a vent to the trap.

[@user-kf5hp8lp6k]

The animation in the middle is wrong. It's a P trap connected as a S trap.

[@SomeDumUsrName]

The P trap IS part of the S trap. 50% of it. I mean, without the P trap, then where is the S? It doesn't exist. See? So it is NOT a P trap connected to an S trap. P traps and S traps are not independent of each other, they BOTH MAKE the S trap. And the middle is not an S trap because of the length of the trap arm. What this is (yes the animation is correct) is simply an unvented trap arm, which can siphon just like an S trap, just as the animation shows.

[@WhomAmI]

It's not wrong, it's to demonstrate the difference the vent makes. The first has the most loss in seal, the second is less, and the third still yeilds a functional seal.

[@Tisiko2000]

What program did you use to draw your geometry for Openfoam simulation? I started to use Openfoam and I only use Salome for drawing geometry but it isn't friendly :') I like your simulations

[@jjalandoon]

What if you elevate the end part of the S trap?

[@user-ry3qc3cc9r]

Idk

[@burtony3]

I think the short answer is that the problem is a siphon, so it will just continue to siphon.

[@saltypipefitter4618]

That would be a crown vent on an S strap still not to code

[@dorayocelin3259]

Ah TTS dmsj duren fhazxd ah ditegur si si si eh sf eh