Soil/Waste Stack Offset in HR Buildings 3

[mitcham]

Hi
Although this forum is dedicated to HVAC and Refrigeration, however, I think all who are involved in design of building mechancial systems, including plumbing, cross here. So I dare to post my inquiry here.

Have you ever heard or seen of any requirement (as per code or good engineering practice) to offset soil or waste stack in high rise building, say every 10 or 15 floors, to reduce velocity or noise?

Thanks all who share their knowledge in this forum.

 

[PEDARRIN2]

Refer to the international or uniform plumbing code. I do not think there is a requirement for offsets, but there are requirements of what you do if you have to offset the pipe. Check the codes.

 

[mitcham]

PEDARRIN2

Thanks for your reply and feedback. I am aware of that part of code dealing with offsets in soil/waste stacks. However, what I am addressing is an arbitrary short offset in a stack actintg as a bumper just to reduce vertical velocity and associated noise. I am wondering about its necessity and ruling criteria here.

 

[friartuck]

In the UK we keep the stacks as straight as possible to avoid 'plugs' which would create excess local pressure fluctuations. Any change of direction should be long radius bend and any bend above the highest take off connection is ok (This part is the vent and so it does not matter)

Friar Tuck of Sherwood

 

[mitcham]

friartuck
Thanks. Now I know how it's done in UK. Can I have feedbacks from people who work as per North American or other European codes?

 

[PEDARRIN2]

There is nothing in the code that dictates using an offset for reducing velocity. Studies have been done to show that the fluid will reach a terminal velocity at a certain distance and will not increase. So there should not be a problem with velocity. At the bottom, you may want to use long turn fittings so the force from the change in direction is lessened.

What kind of piping are you using. Typically in applications above, say, two or three branch intervals, cast iron is used just because of the sound issue. If you are using PVC, you will have noise issues. Putting in an offset may help this but you would have to put them in about every 2 or 3 branch intervals.

 

[friartuck]

Take care on high rise stacks. In UK we take the last flor off a separate stack , otherwise, the @?"£ hitting the bottom slow radius bend causes a high pressure and pushes the trap out of the ground floor wc.

Friar Tuck of Sherwood

 

[mitcham]

I would like to thank both PEDARRIN2 and friartuck for their valuable tips on this inquiry.

 

[PEDARRIN2]

In the International Plumbing Code, connections to a stack going from vertical to horizontal have to connect at least 10 pipe diameters downstream of the change in direction so that the flow has time to level out so you do not blow out the traps.

 

[SAK9]

There is no need to have offsets in soils tacks because of the terminal velocity explanation given by PEDARRIN2 I have seen in some high rise buildings(40 &up) two soil stacks with alternate floors connecting to different stacks.This would mean if the main stack chokes,50% of the floors can still operate.I have always connected the last floor(1st floor in Europe and 2nd floor in most other countries) to the main stack with no problems.

 

[jherbert]

Here in Hong Kong, we don't offset stacks (40+ floors).

However, one must design around the hydraulic jump at the base, as mentioned above.

Tall stacks do create noise, but don't offset the stack, take other sound reduction measures.

cheers,