6 replies to this topic

[Mathi]

I am supposed to review a control valve datasheet from the vendor. The line is for condensate flow. The inlet pressure is 220 kPag and the outlet pressure is 100 kPag. Usually when I fill the datasheet I consider the operating pressure to be the vapor pressure. In this particular datasheet the vapor pressure was given as 100 kPa. 

 

My question is can the vapor pressure be lesser than the operating pressure of the line? If so, what are its implications on control valve sizing? 

[fallah]

Mathi,

 

If you would want having no flashing at downstream, the vapor pressure should be lesser than operating pressure of the downstream line...

[ahmadikh]

 Fallah is absolutely right. Since this is a condensate line and I would assume it is not a pumped condensate, so for sure you will have two-phase flow downstream of the valve. So, two phase flow indication must be reflected on the valve datasheet by giving the percentage of flashing calculated based on the back pressure right from the header pressure.

[Mathi]

Fallah and Ahmadikh

 

Thank you for your response.

 

I agree that the vapour pressure should be less than the operating pressure.

 

I usually give the operating pressure as the vapour pressure for preliminary control valve sizing. What are your comments on this practice?

 

When I checked out the method of finding the vapour pressure in HYSYS. I obtained the operating pressure value for vapour pressure.

 

The method I followed was as such

 

1) I defined the stream's composition and temperature

2) I gave the vapour fraction as zero

 

What are your comments on this method of identifying the vapour pressure.

 

[serra]

when sizing a control valve you should consider that internally (plug) conditions can be different from outlet flange,
this explains the difficult to size correctly a control valve when there is vaporization,
if the fluid is a mixture, to identify the different (liquid, liquid+vapor, vapor) states in a range of temperatures and pressures I would suggest to calculate a phase envelope, in some cases you may observe a complex behaviour, see some examples here

'http://www.prode.com...seenvelope.htm'

for conditions below bubble point line (the value which you calculate setting vapor fraction = 0.00000 as specification) there is only liquid and if your operating point is below bubble point you can size the valve utilizing the formulations for liquids (see ISA S75),

for two phase area (mixture) things are difficult and in practice there are not accurate methods (apart, perhaps, CFD) anyway there are procedures which you can adopt, if that is your case you may consider some specific flash operation.

Edited by serra, 27 February 2015 - 08:54 AM.

[chourouk]

Hello,

 

I actually have the same problem while sizing a control valve for separator's oil outlet line.

HYSYS gives a vapor pressure of 6.2 Barg, which is equal to the operating pressure 6.2 Barg.

This means that oil will flash.

 

Is this VP correct? or should i use another method?

[Mathi]

Chourouk

 

The vapor pressure (VP) should be less than the operating pressure. You should use some other method to calculate the VP