Vent distance from trap - physics explanation

[lanebm24]

I understand the need for venting as well as the distance tables by pipe size. I guess I am trying to understand the physics behind these images that are consistently depicted this way. At a certain slope, you can only have the vent so far from the trap so air can get back there to avoid a syphon; this makes sense. What doesn't make sense to me in the images is why does the water level rise like a wave the further down the pipe it goes? If it was 1" right after the trap and it flowed downhill wouldn't it still be 1" at the end of the pipe as it enters the main stack?

I am wondering is it due to the fact there is pressure building up as the water slug pushes air out of the pipe? More specifically, as the water starts to flow it pushes the air out and since its not open air at the end, there is some pressure build up, which slows the water enough to make it rise?

I often like to understand the why, as opposed to just knowing what is allowed and isn't. Thanks in advance

 

[Twowaxhack]

Study how a siphon is formed and then understand that a siphoned trap is bad because the water in the trap is what separates sewer gas and bugs from the living space.

 

[lanebm24]

Study how a siphon is formed and then understand that a siphoned trap is bad because the water in the trap is what separates sewer gas and bugs from the living space.

Thanks for taking the time. I understand the siphon is bad in most plumbing (outside the designed siphon in a toilet). I understand how a siphon works. What I dont understand is why does the water level rise as it goes down a sloped horizontal pipe like in the image?

 

[Twowaxhack]

Thanks for taking the time. I understand the siphon is bad in most plumbing (outside the designed siphon in a toilet). I understand how a siphon works. What I dont understand is why does the water level rise as it goes down a sloped horizontal pipe like in the image?

It doesn’t necessarily rise, depends on how much water you’re running down the drain.

If you run a large volume of water it fills the pipe up and that creates a siphon. The siphon is broken by sucking air through the trap.

 

[lanebm24]

That's not what every illustration seems to indicate. It is why they draw that level line across from the vent to the weir of the trap indicating the water rises as it travels down the pipe. If the full pipe after the trap filled from the jump, it doesn't matter where the vent is down the horizontal pipe as there will be a siphon regardless. The idea as I read is for air to be able to slip in from the topside.

 

[MicEd69]

Look at it this way, as water flows out of the sink, there will be a level of water that flows over the P-trap weir. The height of that water is determined by the geometry of the piping and the flow volume through the sink drain. The faster the water comes out of the sink, the higher the water level is over the weir. The codes on the distance from the weir to the vent are in place to assure that the flows will not siphon the P-trap dry.

The codes do not take into account any strainers that will slow the flow rate out of the sink which would end up lowering the height of the water flowing over the weir. Nor, in general, is the height of the sink addressed. There is a limit on how low the P-trap can be from the sink drain and there is a minimum height of the P-trap. So, you need to understand that while some things are addressed in the codes, the initial head pressure of the water going into the P-trap when the drain starts is not directly addressed by the codes. Therefore, the codes make some assumptions and provide reasonable parameters around the distances from the P-trap to the vent.

In other words, a large flow of water using the worst-case dimensions of the codes can slug flow the line from a sink and that can potentially siphon the P-trap dry.

 

[Pipe Dreams]

So codes are different everywhere so i will be speaking from my area of NC........

Take 2'' for example. The maximum distance ( in pipe) from the discharge side of the trap and the vent is 8 ft. Why you ask? (or so I think) 2'' pipe must run at 1\4' per ft of slope.....how many 1\4s are in 2''? = 8. Beyond the 8ft distance, ''hydraulic grade'' is broken and drain will not vent properly under its HIGHEST volume.

1 1\2'' pipe, 6ft
and so on.

Where I work 3'' can be layed at 1\8'' slope, but vent distance is still 12', 4'' pipe is 16'.

Drain systems work off of displacement of air by water.

Does that help?

 

[Twowaxhack]

That's not what every illustration seems to indicate. It is why they draw that level line across from the vent to the weir of the trap indicating the water rises as it travels down the pipe. If the full pipe after the trap filled from the jump, it doesn't matter where the vent is down the horizontal pipe as there will be a siphon regardless. The idea as I read is for air to be able to slip in from the topside.View attachment 41288

It only rises in the pipe if enough water is introduced into the drain or the pitch is too great. Fixture drains have size requirements and fixture arms have maximum developed lengths to insure a siphon is not created.

I don’t care what the drawings show. Go get a pipe and run some water through it and see for yourself.

The bottom of the trap outlet can’t be above the top of the vent opening. It’s that simple.