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Impact Of Vapour Pressure To Flashing Liquid Control Valve Sizing
#1
Posted 02 March 2015 - 07:12 AM
#2
Posted 02 March 2015 - 07:58 AM
The amount of vapor at flashing conditions seems negligible. I don't think you will have any problem with that. Just avoid to have a control valve which will operate at 2% opening most of the time - this will create problems.
#3
Posted 08 March 2015 - 10:14 AM
Dear Zauberberg,
Thank for the reply and sorry for my late reply. Cv value calculated can be quite different for each case e.g. Cv = 160 when at 25 bara vapour pressure and Cv = 130 when at 15 bara vapour pressure, based on my company sizing spreadsheet.
Consequently, we decide to go with the balanced design and go with 15 bara vapour pressure which is what we will likely to see at CV inlet although there is only very low volume of vapour. Thanks.
#4
Posted 08 March 2015 - 02:17 PM
What CV you get if you use vapor pressure of 0.2 bara? The only concern I have (and I have seen many, many similar cases in produced water service lines) that you will end up with an oversized valve, designed for maximum flow and high vapor pressure. Then in reality, when you face e.g. 20% turndown flow and no flashing at all, the valve will end up operating at 2% opening most of the time and wear itself off in a matter of weeks.
#5
Posted 08 March 2015 - 11:10 PM
tingyik90,
I think you can go with the vapor pressure of water (75%) plus MEG (25%) i.e. 0.25 bara at process temperasture. The pressure at CV downstream is 1.2 barg (2.2 bara) well above that vapor pressure and create a reasonable margin will lead to having no flashing at downstream due to the vaporization of such ppm order of magnitude of hyrocarbons...
#6
Posted 09 March 2015 - 02:14 AM
I agree with Fallah,
in case of doubts for two phase flow you may consider a HEM model such that included in Prode Properties,
for this case I would simply calculate bubble point pressure and use that as vapor pressure,
it is not the best option but consider that ISA formulation is for pure fluids, not mixtures...
#7
Posted 09 March 2015 - 12:57 PM
To be absolutely sure that the valve will work, you can look through several design cases by using ValspeQ or Fisher Specification Manager software. The latter provides you with a convenient overview of design alternatives:
http://softadvice.in...an_Valspeq.html
http://www2.emersonp...andownload.aspx
#8
Posted 10 March 2015 - 12:35 AM
Hi,
You may find good tips reading this paper .
hope this helps .
Breizh
Edited by breizh, 10 March 2015 - 12:35 AM.
#9
Posted 10 March 2015 - 02:14 AM
most manufacturers adopt ISA S75 (derived from old Masoneilan formulations)
ISA (in general) should be adopted only for vaporization of pure liquids,
for mixtures things are complex (something similar to PSV).
When (as for the case proposed by tingyik90) bubble point presssure is very low
one can adopt the formulation for liquids without considering vaporization,
another possibility (suggested in some old papers and frequently adopted by manufacturers)
is to consider separately vapor and liquid flows (i.e. Cv = Cv Vap + Cv Liq).
There are a few papers (Samson) discussing applications of control valves with liquid mixtures
at conditions where a large fraction of liquid vaporizes,
the procedures proposed for solving such cases are based on HNE-DS,
as alternative one could adopt a methodology based on HEM
I adopt HNE-DS or HEM only for difficult cases (mixture of liquids flashing)
these methods are available in Prode Properties and easily coded in Excel
#10
Posted 18 March 2015 - 02:57 AM
#11
Posted 18 March 2015 - 03:57 AM
3) "the stream will have up to 82%vol of vapour, with up to 71%mol CO2 and 29%mol H2O in the vapour"
a 82% vapor fraction with 71% mol CO2 seem quite strange considering cases 1 and 2,
are you sure about the results of isothermal (or, maybe, with specified H) flash ?
which is the feed composition ?
as said in my previous post in case of doubts for two phase flow you may consider models based on HEM methodology
#12
Posted 19 March 2015 - 03:20 AM
Serra,
The inlet stream has a lot of components which are at ppm level. Thinking that it might be due to correlation limit of the simulator, I stripped down components which are less than 50 ppm, but that practically left me with water, MEG and CO2 only. Still, the flashing is significant and I can confirm it is isenthalpic flashing via a control valve unit operation. This is tested with HYSYS using Peng Robinson, Kabadi Danner and the Glycol package.
You might want to verify my results using other simulator. The conditions are as follows.
Pressure: 25 bara --> 1.2 bara
Temperature: 77.81 degC
H2O: 74.8296%mol
EGlycol: 24.6995%mol
CO2: 0.4710%mol
#13
Posted 19 March 2015 - 04:54 AM
I think there is something strange in your result 3) see above message #10
for personal interest I solved your flash operation with Hin = Hout,
with two models (CPA-PR and Extended PR)
Pin 25 Bar.a Pout 1.2 Bar.a
Tin 77.81 C Tout about 77.8 C
feed % (mol) H2O 74.8295 MEG 24.6995 CO2 0.471
with both models I get limited vapor fractions,
with CPA-PR
Vapor Fraction (mol) 0.0053
vapor % (mol) H2O 21.9 MEG 0.06 CO2 78
if you compare this value (0.53 %) with your 82% of vapour
it seems there is some difference,
a possibility could be that you are calculating volume fraction as
Vfract = volume vapor / (volume vapor + volume liquid)
but also for this case with values for liquid volume about 0.0011 m3/kg and vapor volume about 0.63 m3/kg
the values seem different...
Edited by serra, 19 March 2015 - 06:45 AM.
#14
Posted 21 March 2015 - 12:54 PM
Hi,
Can you confirm if there is any change in MEG to water ratio and thereby resulting different loadings of these traces in the heavy phase stream from separator? How confident are you about predictions of these loadings with selected thermo model in simulator?
You can assess these things first before you make final decision. Good luck
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