flow direction

[amrfd]

if you a flow inside a pipe ,how to know the direction of
flow? and how to know the type of flow compressible or incompressible?

 

[Latexman]

Flow is usually from the high pressure side to the low pressure side.

One rule of thumb I use is if the pressure drop < 10% of the inlet pressure, flow is incompressible. If pressure drop is between 10 - 40% of the inlet pressure, flow is compressible, but I use average properties and treat it like it is incompressible. If pressure drop > 40% of the inlet pressure, flow is compressible.

Good luck,
Latexman

 

[racookpe1978]

In any given pipe, flow can go either direction.

Which end has the higher pressure?

 

[zdas04]

Latexman,
What kind of results do you get in the 10-40% ramge with average properties? I've found it to be pretty crummy. I usually break the pipe down into chunks that result in less than 10% dP and recalculate compressibility, density, average pressure, etc. for each segment. It is a pain, but the results usually match measured data very well.

David

 

[Latexman]

David,

Unfortunately, I couldn't get any results today. I tried to fire up my favorite compressible flow program and got an error, "Your Security Key is invalid or missing". It's a time date thing to protect intellectual property. I got the software when with Co. #1. We (the software and I) got sold to Co. #2. Co. #2 bought Co. #3 and the FTC made Co. #2 sell us to Co. #4. The reason for the song and dance is, I fear it may take some time to get this sorted out, but when I do I'll come back to you.

It's been quite a while since I've used this 10-40% rule personally. I probably haven't used it since I got my handy dandy compressible flow software back in 1997 (I think). It's easier to run this software once than iterate on average properties with a incompressible flow spreadsheet, but back in the day when hand calcs ruled it was a time saver.

In the meantime, anyone else got some data to share on this?

Good luck,
Latexman

 

[BigInch]

1) Flow often goes down hill as well.

2)If you don't know the pressures at both ends of the pipe, bet on rule #1.

3) You can also look for an arrow painted on the pipe. Many times that is correct. 100% correct if you see it on a valve. It will even overrule rule #1.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[btrueblood]

"100% correct if you see it on a valve."

We make a valve with a flow direction arrow on it. Doesn't keep it from getting installed backwards, though.

 

[btrueblood]

Should have added, which is why we also provide the valve standard with Pete's plugs, so that you can check the drop across the valve element(s) and confirm it's installed and operating correctly. Some people grumble about the cost, until they see the reason.

 

[BigInch]

Very thoughtful of you guys.

I would have thought that the plugs would only tell you that pressure was higher on one side than on the other.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[racookpe1978]

Well, sort of. But the worst plugs in a valve really only stop flow ... from either direction (can't drain out, can't flow in, can't flow through.) 8<)

 

[BigInch]

Guess that's why they call them plugs.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[Latexman]

David,

SNAFU on the old software. It's fate will soon be in the hands of the lawyers, so I'll probably never see it again.

Using a new tool for me. 200 feet of 2" sch. 40 pipe with 75 F (isothermal) air at 25 psig inlet and 10 psig outlet (38% loss). Using average properties method, it says 575 scfm. Using compressible flow method, it says 563 scfm. A 2% difference. Not bad.

Good luck,
Latexman

 

[BigInch]

I'd get rid of rule #1

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[Artisi]

When all other test and suggestions fail, looking at the pipe ends should give you a fairly accurate indication of flow direction.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[btrueblood]

BigInch,

Typical service call:

customer: my valve isn't flowing.
tech: what is the valve position, open or closed?
customer: um, let me get back to you.

Artisi, I like that suggestion.