Waste Pipe Flow Rate Capacity

[MortarThePoint]

Does anyone have a good source of information about design flow rates through different sized waste pipes?

 

The closest I have found is NHBC say drainage pipe runs should be designed to maintain a self-cleansing velocity (0.7 m/s). That's drainage rather than waste, but hopefully 0.7m/s still applies (about half of walking pace). Using that and the ID of each pipe size I can work out some flow rates for various 'fill levels':

 

PIPE ID. C-A. FILL. CAPACITY

32mm. 8cm2. 50%. 17L/min

32mm. 8cm2. 20%. 7L/min

40mm. 13cm2. 50%. 26L/min

40mm. 13cm2. 20%. 10L/min

50mm. 20cm2. 50%. 41L/min

50mm. 20cm2. 20%. 16L/min

 

A pipe filled to a quarter of its depth is 20% full by area.

I've just used nominal pipe IDs.

I presume things get a lot more complicated what the pipe starts to get full, hence limiting FILL to 50%.

 

A 26L/min feels high for 40mm pipe but that's what the numbers suggest and it tallies with this McAlpine shower trap. Should mean a 12L/min shower will be absolutely fine through 40mm (1.5") waste pipe. Does that bear true through experience?

Edited Saturday at 08:00 by MortarThePoint

[Mike]

If you're planning a shower, then just choose 50mm diameter pipe. And make sure that the maximum flow capacity of the shower trap is more than the flow from the shower head(s).

[Nick Laslett]

@MortarThePoint, here is the thread I have book marked, where both NickfromWales and PeterW opined.

 

They both suggest 50mm for smoothest long term functioning. 

[Nickfromwales]

On 02/11/2024 at 10:45, MortarThePoint said:

Does anyone have a good source of information about design flow rates through different sized waste pipes?

 

The closest I have found is NHBC say drainage pipe runs should be designed to maintain a self-cleansing velocity (0.7 m/s). That's drainage rather than waste, but hopefully 0.7m/s still applies (about half of walking pace). Using that and the ID of each pipe size I can work out some flow rates for various 'fill levels':

 

PIPE ID. C-A. FILL. CAPACITY

32mm. 8cm2. 50%. 17L/min

32mm. 8cm2. 20%. 7L/min

40mm. 13cm2. 50%. 26L/min

40mm. 13cm2. 20%. 10L/min

50mm. 20cm2. 50%. 41L/min

50mm. 20cm2. 20%. 16L/min

 

A pipe filled to a quarter of its depth is 20% full by area.

I've just used nominal pipe IDs.

I presume things get a lot more complicated what the pipe starts to get full, hence limiting FILL to 50%.

 

A 26L/min feels high for 40mm pipe but that's what the numbers suggest and it tallies with this McAlpine shower trap. Should mean a 12L/min shower will be absolutely fine through 40mm (1.5") waste pipe. Does that bear true through experience?

It’s the suds/foam atop the moving water that slows the air exchange and impedes flow.

Its why I routinely put 40mm rubs to basins and 50mm to everything else, then reduce at the absolute last minute.

[saveasteading]

 

 

 

2 hours ago, Nickfromwales said:

good source of information about design flow rates through different sized waste pipes?

The building regs.

 

A long time ago i did these calculations from first principles. 50mm dia up to 2m.  I've forgotten it all now, but my point is that it is complex. 

 

Someone has done these sums for normal uses and published them. The building regs have these in chart form.

Using them is easy and slightly conservative, so do that.

Most importantly, avoid sharp turns as they severely  disturb flow.

Also note that too steep a slope and too big a pipe cause solids to be left behind....the charts cover that too.

[Nickfromwales]

1 hour ago, saveasteading said:

Most importantly, avoid sharp turns as they severely  disturb flow.

Yup. Two ways to go around a corner, a) an elbow, and b) a bend. 
Elbows are sharp 90°’s, bends have a sweeping radius. 
Bends for waste where horizontal, elbows or bends fine for vertical or where space is tight / immediately off the trap or outlet.

People don’t realise how much restriction can be caused by cheap traps (typically much lower L/p/m flow rate at gravity) particularly for Chinese wastes on basins.

Went to one job where the basin wouldn’t empty, suspected blocked trap etc, but after realising it was all relatively new pipe work I drilled out the slots in the plug hole to enlarge them and problem solved. Just not enough ‘hole’ for the water to fall through.