9 replies to this topic

[KeJoSa2010]

I have the Cv of a valve and I need to calculate the maximum flow rate through the valve. If this were a 1 phase situation I think the calculation would be straight forward. For example, I would start by using the general flow equations.

 

My issue is that, the valve I am trying to calculate is near the inlet of the plant and, there is 3 phases through this valve during normal operation. How do I calculate the maximum flow through a valve (I have the Cv) that passes multiple phases?

 

Thanks

Edited by KeJoSa2010, 20 February 2013 - 01:07 PM.

[Bobby Strain]

Ask the valve supplier for help.

 

Bobby

[SSWBoy]

Doesn't sound like particularly good design to have multiple phases through a control valve. I hope you're not actually trying to control anything!

[KeJoSa2010]

It is a level control valve upstream of a free water knock out drum. I need to know the max flow through this valve in order to determine the size of the PSV in the blocked outlet case.

Edited by KeJoSa2010, 20 February 2013 - 04:34 PM.

[sheiko]

The calculation should be based on the maximum Cv as per the data sheet (100% opening) and maximum deltaP across the valve (= maximum pressure upstream the valve - minimum pressure downstream).

Google Emerson Control Valve Handbook.
Pick the right equation.

[Pan Nata]

KeJoSa2010,

 

its quite complex problem to size a control valve for three phase fluid, as bobby suggested (post 2) is better for you to contact control valve vendor or maybe you can try following approach for preliminary, assume water and heavy hydrocarbon in one phase (liquid) then you only facing two phase condtion (vapor-liquid). you can use Instrucalc Software to perform calculation, it provide two-phase control valve sizing.

 

I already check emerson-fisher control-valve handbook, the formula is only available for liquid and compressible fluid.

 

good luck for you,

 

rgds,

 

wild

[saeed]

Dear  KeJoSa2010,

First of all what do you mean by "Three Phases" you mean Liquid phase and gas/vapor phase and solid phase? i believe you don't mean it as it does not happen in reality (As far as i know), Please describe how many substances ( for example water and Acetone) and how many phases (For instance, liquid and vapor).  

Second, please note that In order to calculate maximum (volumetric) flow rate  you need to have Max CV value (which is nominal valve CV value) that you say you know it and also Differential Pressure (Look post No. 5). I just don't like to say Max. DP as  Maximum DP is: P1-P atm (don't consider vacumm condition) and for your case it might not be applicable. So i just want to say beside having valve nominal CV value you need to have working Differential pressure. then for NORMAL cases you can easily calculate the Max. flow rate from simplified equation (for incompressible fluids)  Q=CV * ( DP/(R/R0) )^1/2  where 

R is the specific mass of fluid in Ib/ft3

R0 is the specific mass of water in Ib/ft3

 

 

Dp is the static pressure drop from upstream to downstream expressed in psi.

 

 

But the problem is that you are talking about probably two phase flow, if this is so, The only help i may can do is that you can send me the CV value, Differential pressure, Temperature, pipe size, valve size, substances, their vapor pressures (if they are liquid), The viscosity of each of them together with Z value and Cp/Cv value (If they are gas), so at that case i think i can calculate the max Flow rate by using software.

 

Have a good day.

[paulhorth]

Some important questions here, before you worry about the flowrate.

(1) As stated in Post 3 (SSWBoy), a control valve acting on a twophase (liuid-gas) mixture is unlikely to work properly. The third phase (water) won't change that. The reason is that the mass flow through the valve in two phases is not a simple function of the valve opening, So - why is the valve there at all? Secondly, a valve which sees continuous twophase flow will be subject to erosion damage.

(2) In Post 4 you tell us that it is a level control valve upstream of a free water KO drum. I don't understand this. Are you trying to control the level in the drum by controling the feed? This is better done by a valve on the outlet liquid (single phase) not on the inlet stream.

(3) where is the feed coming from? If it is from another vessel, then if the valve goes open then there could be gas blowby, when all the liquid from the upstream vessel has gone through. This will be a different flow from the three phase case.

(4) In Post 5, I think Sheiko is mistaken to use the Minimum downstream pressure. For the relief case, the downstream pressure should be the PSV set pressure plus accumulation.

 

These questions should be considered and answered before you start running any software.

 

Paul

[sheiko]

Paul, I have indeed mistaken, but I would rather use the KO drum normal operating pressure, as the pressure at the outlet flange of the control valve. I don't remember if API recommends something about it but I believe this is a more conservative option.
Anyway, the final user of the plant shall have its say.

Edited by sheiko, 10 March 2013 - 09:07 PM.

[KeJoSa2010]

Thanks everyone for your help, it is very much appreciated! I was confused in my understanding of the client P&IDs. It looked to me at first like the level control valve was controlling feed into the tank (as paulhorth mentioned) and I was confused by it. Indeed, this valve is actually controlling the level of the liquid in the vessel upstream. Sorry for confusing everyone! I think it makes sense to me now.

 

Thanks again,