Choke flow condition 1

[I2P]

Could any one pl explain the concept of choke flow, and mach number in relation to flow across restriction orifices? What is it's significance in designing the RO? What are the equations which explain the Choke conditions? Any good links for understanding compressible fluid flow?

 

[Montemayor]

I2P (Chemical):

Go to:

http://www.air-dispersion.com/usource.html

Milton Beychok has what is probably the best and most practical treatise on choked flow that you will ever read. His explanations are direct, down-to-earth and easily digested. He presents it in a manner commensurate with engineering applications and needs and you can readily detect the importance of the subject and the need to understand it.

You will not regret reading and downloading all the material he presents on that website.


Art Montemayor
Spring, TX

 

[zdas04]

Art,
That is an excellent description, but it has one short-fall, it assumes that the exhaust is always into atmospheric pressure. Every time the web page says "atmospheric" pressure I change it to "exhaust" pressure (in my mind) and it works. This is real important with chokes (both permanant and adjustable) and other throttled valves. 500 psia on the upstream of a globe valve and 272 psia downstream (with a k=1.3 gas) will be choked flow.

When I teach this class I tell folks that any time upstream pressure is more than about twice downstream pressure (in absolute terms) you will have choked flow. That fits very well with Baychock's 1.7-1.9 times (twice is easier to remember in a clutch and methane works out to 1.82 times).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[davefitz]

Also please note that the computed flowrate for choked flow thru a orifice is not based on a Cd =1.0 unless it is razor sharp. For tippical thin orifices the Cd=0.83, and for very thick (t/d > 1.0) Cd=0.81 .

 

[mikesahid]

I hope my friend appreciates the email increase. Try arthur@petrotec.com.au

 

[tonycola]

When gas or vapor is flowing through an orifice, a situation can exist when the flow rate would not be increased if the downstream pressure is decreased ( while the upstream pressure remains constant). When this happens, the flow rate through the orifice depends on the upstream pressure, but is independent of the pressure difference arcross the orifice. This situation is referred to as "choked flow". If when the downstream absolute pressure divided by the upstream pressure is equal to or less than a number that depends on the ratio of the constant pressure specific heat to the constant volume specific heat of the gas or vapor, then the flow is said to be "choked". For example this number is equal to 0.5283 for air.
For a given upstream pressure, the maximum gas or vapor flow rate that can be obtained through an orifice, would be the "choked flow" rate.

Have fun!

 

[clever]

The choke condition ( know as the choke performance)the the pressure loss across the section

these can be obtained be

Wellhead pressure- the separator pressure

Ther presure loss accross this sesiin in comparism with the other presure loss at each section of the system tell you the performance of the choke.