What I learnt on Monday is that some precast producers make concrete ovoid pipes. My fist impression was "what's the point in using such an odd section?". As fas as I know they are very rare in Spain. After flicking through an old book on hydraulics I realised that the ovoid pipes have better velocity characteristics at low flow than the equivalent circular pipes and this feature implies two great advantages:
• Better self cleaning and less risk of blockages.
• Reduced costs of excavation.
The moral of this post is: be reluctant and never trust traditional solutions blindly.
Pic: Ovoid pipes, around 100 € per metre.
"The ovoid pipes have better velocity characteristics at low flow than the equivalent circular pipes". What do you mean by "equivalent"?
ReplyDeleteI sure have seen those pipes somewhere, but I can´t remember the place.
ReplyDeleteAnyway, everybody (specially technicians) could be very surprised discovering things thought as modern, or even unknown, in old books. Things that were refused or barely used, so they went disappearing from modern books (and probably were never hanged on the net) but that could become useful today... or tomorrow.
Equivalent pipes means, in this case, with the same area.
ReplyDeleteAt high flows, the flow is pressurised and the average velocity of the water in the section must remain under some value (i.e. 2.5 m/s) to avoid erosion and problems with waterhammers in the valves.
At low flows, the flow is non-pressurised and the average value of the water velocity must be over some value (i.e. 0.5 m/s) to avoid blocking, so a narrow bottom helps.
I see. However, the ovoid pipes seem disadvantageous in pressurized flow (more losses). How much more expensive than the "equivalent" circular pipes are they?
ReplyDeleteYou are right, David. Although egg shaped pipes are the best for drainage purposes they are expensive. If you only have pressurised flow (f.ex. urban water distribution) circular shapes behave better. The approx. prices are:
ReplyDeleteConcrete ovoid pipe ------- 100€/m
Concrete circular pipe ---- 60€/m
can you please tell me the nature of the flow in egg shaped pipes..will it be a laminar or turbulent?
ReplyDeleteReynolds number (Re) is the key. For a gross number:
ReplyDeleteRe = Velocity x Hidraulic diameter / Kinematic viscosity
Kinematic viscosity of water is around 10^-6
Normal velocity in a pipe varies between 0.5 m/s and 5m/s
For shallow flows the hydraulic diameter is aprox. 2 x depth
Laminar flow occurs where the Reynolds number (Re) is less than 2000. Turbulent flow occurs for Re > 4000. As a gross number, turbulent flow will occur for depth > 1 cm.
Probably laminar flow only occurs at very small depths or at a negligible zone very close to the pipe skin.
nice work.
ReplyDeletePile Repairs
Waterproofing