Venturi in domestic water supply.

[Kman1138]

I'm a new EIT in the world of mechanicl design for a firm that does building design. I'm the only mechanical guy in my firm i.e. no senior P.Eng to help me with questions.

Anyway I was looking at some mechanical drawings form another consultant. I noticed that on a cold water supply line, they had a venturi upstream of a supply header for an apartment suit. The line goes from 1/2" down to 3/8" then back up to 1/2". My guess this is to increase the water velocity and reduce the pressure. Would there be any other reason to do this?

Regards,
Kris.

 

[zdas04]

Is it a venturi or a choke or just a change in diameter? It probably doesn't matter, but precision in language is important--typically something called a venturi is designed to prevent boundary-layer separation through a vena contracta and they tend to be quite expensive. The other two don't care about boundary-layer separation.

Piping like you describe (two swedges and a short piece of reduced-diameter pipe) are usually intended as flow limiting devices. They are sized to provide a minimal dP under normal flow rates. As flow increases so does the dP until the upstream pressure is all used up. I've used that technique on free gas taps coming off of gas wells and delivering gas to the land owner where the well is located. For a normal household load there is adequate supply. If he adds a huge additional load (one guy was running a huge commercial greenhouse) then he runs out of gas. For a water line it could keep someone from filling a swimming pool as quickly as they hoped. It could also limit the flow in a leak, but the limited flow would still be a bunch of water.

David

 

[Kman1138]

Thanks zdas04,

You right I should be more precise. It must be a choke. I have attached a schemiatic.

Kris.

 

[zdas04]

Funny that it doesn't say how long the 3/8 section needs to be. I would typically make it around 10 times the ID (something like 3-4 inches) to make sure the pressure drop is permanent (a plate choke tends to recover part of the dP downstream, the amount recovered is a function of the Beta ratio).

You know, I don't know what I would call the widget in your drawing. It certainly isn't a classical manufactured "Venturi", but the definition of a "Venturi" is a "reduced diameter section in a pipe" so maybe your choice of words was right. My problem is that the things you can purchase as a "venturi" have very smooth transitions to reduce boundary-layer separation and a pair of swedges and a short length of pipe doesn't quite cut it. It isn't a "choke".

David

 

[mjpetrag]

Maybe the owner of the apartment complex pays for the tenants' water bill...I know my last one did.

-Mike