Choked flow through restrictive venturi calculation 1

[joedirt9]

Unable to find an equation that will let me calculate if the venturi is experiencing choked flow. I have inlet pressure, pressure drop and all the other useful info. but all of the formulas i have found require, FL^2, the recovery coefficient. Which I do not have

any help?

 

[hydtools]

See section 6. Calculate the critical pressure ratio.

http://www.freestudy.co.uk/thermodynamics/t7201.pdf

Ted

 

[zdas04]

A bit closer to home faq378-1201

Ever wonder why people go to the trouble to write FAQ? I write them to get information that people need to know available in a peer-reviewed format.

One thing you can be certain of is that if someone puts BS in an eng-tips.com FAQ (or a post for that matter)someone will call them on it pretty quickly--this is the best peer-reviewed site in the world. I don't know if freestudy.co.uk can say that.

David

 

[davefitz]

I think the ventrui choked flow may be much different than choked flow thru an orifice- the convergent-divergent design of a venturi suggests supersonic flow is possible- may need to review a compressible flow text book or review NASA papers.

 

[joedirt9]

all the equations i have found require Fl which requires the pressure at the vena contracta. i do not know this pressure so not sure what will get me that information. any ideas? since this is a specality made venturi there is no manufacture to get the information from.

 

[joedirt9]

More Info

I need to go through two restrictive venturi's with an inlet pressure of 135 psi and outlet of 30 psi. SG is 1.2, Vapor pressure 4.2 psia, 1.5 inch pipe and 85 gpm.

venturi's are required and i am not sure if i will need two or three and not sure if i am designing this properly.

My initial estimate is the first venturi has a dP of 70 and the second a dP of 35

thoughts?

 

[joedirt9]

Informed they would like to use an existing venturi.
New pressure drops are 62.33 psi on the first and 43.32 for the second. calculations use a vena contracta pressure of -9.5 psi

 

[hydtools]

What is the fluid?

Ted

 

[joedirt9]

Crude Slurry:
Water: 25.9%
Nitric Acid: 32.92%
Adipic Acid: 13.6%
Glutaric Acid: 10.92%
Succinic Acid: 3.93%
Va: 0.07%
Cu: 0.66%

And 12% Solid Adipic Acid

 

[StoneCold]

You can correct me if I am wrong but.
I believe that venturi's are used where you can not tollerate the permenant drop of an orfice plate. (ie flow meters)

I also believe that they are mostly used on gases.

I could be way off base because of the slurry aspect but would you not be better off with a simple orfice plate?

For that matter I prefer to use a VFD on the pump to slow things down and not waste the energy in a large pressure drop if I can make that work.

This advice is probably worth what you paid for it.

Regards
Stonecold

 

[joedirt9]

well, this is used on recirculation piping so the pump does not deadhead. the alternative is vfd's but that gets expensive when installing on multiple systems. orifice plate was the initial idea but have clogged in the past.

i agree, wouldn't be dealing with this if vfd's were installed

 

[zdas04]

joedirt,
In your original post you said

Unable to find an equation that will let me calculate if the venturi is experiencing choked flow

Since only gases can experience choked flow, several of us assumed that you were using gas. The reason you can't find an equation for liquid choked flow is that there isn't one.

David

 

[Yuze]

Please refer to ANSI/ISA-75.01.01-2002. I think that you should find answer in this standard. If you don't know the flow coefficient for the ventri, you can find the coefficient in Crane 410 according to the configuration of the ventri.

The pressure recovery coefficient for ventri is normally low amonge 0.1~0.3 still depending on the configuration of the ventri meter.

Hope this help.

 

[StoneCold]

joedirt9
Ok that makes sense. I have the minimum flow problem as well with some process pumps. They are a clean service so I use the orfice plate.
Sounds like Zdas has the answer for you on making the calculations.

Regards
StoneCold

 

[ione]

If you stay above the vapour pressure.... no bother

 

[joedirt9]

I know that there is no problem if i stay above the vapor pressure, I just don't know how to determine if the vapor pressure threshold is crossed or not.

 

[hacksaw]

a true venturi with entrance and exit profiles can only exhibit a pressure drop associated with frictional losses, and that is minimal,

unless, and that word should be emphasized, unless the fluid undergoes a phase change or in the case of compressible flows reaches sonic velocity at throat conditions.

the situaton is a bit different in the case of flow nozzles, or venturis with a exit angles greater than 45-60Degs. The latter would give you an abrupt pressure drop once you exited the venturi throat.

in your application is is more common to use really thick restriction plates or machined pipe sections, but not venturis

 

[Latexman]

Why -9.5 psi? Are you inducing something into the stream?

Good luck,
Latexman

 

[ione]

Knowing the upstream pressure, the coefficient of discharge (ISO 5167), the throat area and the vapour pressure you could calculate the flow rate that would lead to cavitation

 

[hacksaw]

for a venturi with a bore of 0.525", you get a 27 psi permanent loss per element w/fluid gravity 1.2, flow of 85 gpm and pipe velocities of 15 ft/s(not good with slurries).

so you'd need four venturi's in series assuming that they are widely separated

if you only used two elements, the flow would increase to 120 gpm's with pipe velocities exceeding 20 ft/s....pretty soon you'd have no pipe, oh well...that's a minor detail