Piping design: pipe flow v.s. open channel flow ?

[EngReno]

Hi All,

For designing a liquid flow collection and distribution network, under what condition is open channel flow better than pipe flow? Does open channel piping also need pumping or the increased elevation at one end will work? And how do you decide whether to use circular pipes or rectangular pipes?

Thanks,

EngReno

 

[cheute79]

EngReno, the only times that open channel flow is used (or generally preferable) are in "open channels", such as ditches, rivers, and streams, and in gravity drains. Open channel is used in gravity drains, as the air in the pipe system has to be able to escape in order for the liquid to flow.

Full pipe flow pretty much requires a pump (although downflow in a vertical pipe will rarely be full-pipe flow). In the event that open-channel flow is directed to a sump to pump to a higher elevation, the pump and it's discharge pipe must operate in full-pipe flow.

How often do you see rectangular pipes? There are oval culverts and other concrete "pipes", and "square" trench that is poured concrete, but I have never seen rectangular pipe. It would be enormously difficult to fit, support, transport, etc.

 

[europipe]

The advantage is ;
It's difficult to let sail a ship in a pipe

 

[EngReno]

Thanks guys.

cheute79, what about collecting low velocity stream from scattered points and transport it to a sink. The liquid is volatile, so even for free surface flow the channel (i.e.pipe) has to be completely sealed. In this case, the only chance for air to escape is at the end of the pipe when the stream drops to the sink. Like you said, pumps are only needed if the stream collected in the sink has to be elevated afterward. However, what makes free surface flow better than pipe flow in this case? On the one hand, free surface is only driven by gravity, so there is avoided operation cost (electricity for running the pumps) Are there other pros and cons?

 

[cheute79]

EngReno, my understanding of what you have described is essentially plumbing and waste flow. I can't see how the only air escape is at the point of collection, and still get channel flow. Head pressure can drive the flow, but even then the flow will be slug flow, not true channel flow, as the air/vapor still has to move. If you DO have channel flow from sources to the collection sump (the sink), you are providing gas/vapor vent in the opposite direction (upflow), and are not really sealed. You must have pressures balanced in the system. Gravity just converts potential energy to kinetic energy. Mass and energy balance is going to happen, you just might not understand the entire sytem. The laws of physics are hard to violate.

Perhaps related, look up air-lift pumps and figure out how and why they operate, and what their limitations are.