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Domestic Sewer pipe Material, Sizing and Fall.
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Some basic pipe science.Please note that I am refering to domestic systems only. The moment you have high density complexes or buildings the number of waste generating units multiplies and things get complicated. Dont use these numbers to build a system at home - they are for comparison only!! Hazen-Williams formula for full pipe used. The purpose of this page is to discuss sewer pipe material and to show the different flow carrying capacities of sewer lines and the effect of slope or "fall". Fall is critical because without it the sewerage does not move! Fall also determines the load carrying capacity of the pipe and the pipes ability to carry solids. Although not generally used in domestic sewer systems, I have included 150mm pipe for the sake of comparison. Material. The most common material used for domestic sewer pipe is 110mm PVC. This has a number of advantages over other types - it has a good strength to weight ratio. It is chemical resistant. It is resistant to fungi and bacteria. It is easily joined. It comes in 6m lengths so its easier to lay. Thats the good parts, now for a few negatives - It's not strong enough in demanding applications. It is vulnerable to impact damage. It cannot easily be used for slip lining or pipe bursting - it's too rigid. Another lesser used pipe is HDPE (High density polyethelyne). This is a truely remarkable material. It has been used on sewer systems since the 1940's. It's very strong, flexable, chemical resistant and resistant to bacteria and fungi. In fact it has all the attributes of PVC pipe and more. It is however more expensive. In fact there are only two disadvantages in using this pipe - the cost and the fact that it's nor generally straight. When it comes to lining or bursting the cost of the pipe is not important and the fact that the pipe is set into the old pipe makes it straight. So there are no real disadvantages. HDPE pipe is stronger than PVC. It is very resistant to impact and it can survive ground movement much better than PVC. It comes in rolls so it can be installed in long continuous sections. We use HDPE for slip lining/pipe bursting because its tough, resistant and comes in rolls! For more info on HDPE pipe used in sewer applications look at the Plastic Pipe Institute's PDF document on HDPE for Sanitary Sewers. Sizing. Lets start with a typical example - a 40m length of 110mm PVC sewerage pipe running at a common slope of 1 in 40 or 2.5%. This means that the pipe drops 1m for every 40m in length. So the total drop in this example just happens to be 1m. If we feed this length of sewer with as much water as it can handle (it runs "full pipe") and vary the diameter of the pipe as follows - 150mm pipe discharges 38.79 litres per second at a speed of 2.19 m/s 100mm pipe Discharges 13.35 litres per second at a speed of 1.70 m/s 90mm pipe discharges 10.12 liters per second at a speed of 1.59 m/s
Okay - you will notice that the 150mm pipe moves 3x more water than a 100mm pipe! Also the 90mm pipe loses 3 l/sec on the 100mm pipe at a reduced velocity. What is a 90mm pipe you may ask? Its a 100mm pipe with about 5mm of slip lining inside it. In other words our 100mm OD slip lined pipe is actually a 90mm effective diameter pipe. As you can see it does reduce the capacity slightly - but only at "full pipe" and it's only by three litres per second! Now lets play with the slope (or Fall). The above example was at the common 1:40. Lets make that 1:60. Remember - the bigger the number - the less the slope! 150mm pipe discharges 31.15 litres per second at a speed of 1.76 m/s (about 22% reduction in flow) 100mm pipe Discharges 10.72 litres per second at a speed of 1.36 m/s (also 22% less - surprise!) 90mm pipe discharges 8.12 liters per second at a speed of 1.27 m/s (Also 22%)
Our 150mm pipe is still a mover and shaker at 31.15 litres per second. The 110mm pipe lost 3 litres and the 90mm pipe 2 litres. This shows that the relationship between flow, speed and slope are tightly related. Change one and the others change. Remember that a slope of 0 makes the other numbers 0. You end up with a 40m long tank! This often happens when pipes have to be re-routed around an obstruction - like a building. The re-routed sextion ends up with little or no fall and gives trouble for ever after!
Right - now lets look at a partially obstructed santar type pipe running at 50% of it's capacity due to partial collapse and root intrusion - 100mm 50% blocked pipe discharges 5.22 litres per second at a speed of 1.35 m/s No surprise here - you halve the pipe area, you halve the flow. This is commonly found in fibre pitch (santar) pipe systems. In fact - you are lucky to get 50%. The calculated speed of 1.35m/s is also very optimistic. Because of the multiple obstructions in the pipe you are probably never going to see the water move this fast. The calculation assumes a clean unobstructed pipe of reduced diameter only. A quick note on the speeds given here. Speed obviously refers to how fast the water is going. If it's too slow the solids will tend to settle out and not move properly. If its too fast the system will erode. Limits vary from area to area. Commonly .6 m/s min to 3.6 m/s max. Why is there an upper limit? If the material moves too fast is will aerode/brade the pipes and otherwise stress the system. There may also be problems with the movement of solids - but I dont buy the idea often mentioned that the water "leaves the solids behind". Maybe, but I have never seen it :-). Another factor to consider is the percentage of the pipe used for normal or average flow. Sewers are not intended to run "full pipe". When they do it's a pressure system rather than a gravity system and you pick up problems with venting and possible overflows. Sewer pipes run best at 80% full - the maximum velocity occurs at 80% at a very slightly reduced flow. Most systems are designed to run at around 50-55% full. This allows for another factor to come into play - two actually. Infiltration and Inflow. Infiltration - is water that is in the ground that finds its way into the pipe through cracks and holes in the pipe. This is normally not severe in a domestic system but can add to the load. Inflow is the discharge of storm/rain water directly into the sewer pipe. This can be a big problem and it's prohibited by ALL municipalities for very good reasons.
So what do the above numbers tell us? 1. 150mm pipe is generally overkill in a domestic system! 2. A standard 110mm pipe can carry a respectable amount of water when it's running full. 3. A slight reduction to 90mm (slip lining) does make a difference but it's not that much. 4. A big reduction like 50% makes a 50% difference to the flow. 5. 110mm sewer pipe at the correct slope is adequate to convey the sewerage for up to 20 large houses! 6. Flow speed (m/s) must be within limits.
Contact me if you need more info on this or if you have any additional info on this type of pipe.
BLOCKBUSTER DRAIN CLEANING - THE DRAIN BRAINS!
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