1 reply to this topic

[alex.gutu]

Dear Gents,

 

I would like attention of any (the best, more) experience engineer who dealt with this.

 

All of us knows that in the internet you are able to find all kind of information and more recently, online calculation features. In any situation, in order to proceed further with all this information (calculation results), we, as engineer are mandatory to check and recheck them. Here I just want to discuss about a small inconsistency that I found out, and also I would like to get you opinion, because I got confused.

 

The subject is related to choke flow conditions and a tool for control valve sizing.

 

As far as I know, choked flow conditions occurs whenever downstream pressure/upstream pressure ration (Pds/Pus) is less than critical pressure, and critical pressure being defined as Pcr=[ 2 / ( k + 1 )] ^{k / ( k - 1 )}, Pds/Pus <= Pcr (i.e Pus/Pds >= [( k + 1 ) / 2] ^{k / ( k - 1 )}), where k is heat capacity ratio Cp/Cv

 

I will give an example to be easier to realize and feel all this. Let's assume for air, k=1.4, ==> Pcr is 0.528.

Basically if Pds/Pus <= 0.528 (Pus/Pds >= 1.893) => choked flow,

 

One thing I just want to point here, in all this calculation are based on absolute pressure

 

I would like any of you to confirm above theory (or infirm it).

 

On this website http://www.chemkb.co...ion---gas-vapor , there is a control calculator online. I just wanted to check something quick and there one thing jump into my eyes: FLOW CONDITION. Based on this online calculator i got confused concerning choked and unchoked flow condition. It seems that based on this calculator it works the other way around (i.e. when the flow is supposed to be choked, it is unchoked and vice-versa). I have noticed over there PRESSURE DROP RATIO (i.e deltaP/Pus), and I was thinking whether flowing condition are based on pressure drop ratio rather on Pds/Pus ratio.

 

Please I would like to get opinion from some experience engineers to find out whether that online calculator defines flow condition wrong, or there is something that I miss and do wrong. I filled in data for air (k=1.4, MW=29) and for a ratio of Pds/Pus=0.9 (Pus=200kPa, Pds=180kPa), I should get unchoked flow (Pds/Pus=0.9> 0.528), but the calculator gives it choked. Is there something wrong in the theory that I have stated above, or the calculator has a mistake incorporated in it?

 

As a conclusion I just want to give a message to those who uses online tools. Before proceeding farther with results, we must check and recheck results and the best is to get a database with those online tools which are not reliable.

 

I'm looking forward for your replay.

Edited by alex.gutu, 19 October 2013 - 01:12 PM.

[PingPong]

Pus/Pds >= [( k + 1 ) / 2] ^{k / ( k - 1 )}

Indeed, if that is the case the flow is choked.

 

Best never use online tools for calculations, certainly not for calculations that you can easily do yourself.