17 replies to this topic

[Skyline]

Dear all...

I did a simulation of gas flow in a pipe using HYSYS. When i checked the velocity profile over the length of pipe, i noticed that the profile is a bit "strange".
I found that the velocity goes down and then goes up again. But the outlet velocity is lower than the inlet.
Is it normal? Isn't it supposed to be higher than the inlet (because of the pressure drop)?
How do i explain this phenomenon?

Note: there is no change in pipe size.

Thank you very much...

[latexman]

There is either something going on you have not described, the equation of state for the gas has an input error, the simulation has an input error, the simulation didn't converge, there is some two phase thing going on, or you don't know what you are doing, yet. Are you experienced on HYSYS or a newbie? Anyway, what you describe for pure gas flow is wrong. Review your inputs and the outputs very carefully.

[JoeWong]

Skyline,
You did not descibed in detail the pipe size change, piping configuration, etc. It is imposible to provide any advice...

[Padmakar Katre]

Dear,

Check the heat loss i.e. the inlet and outlet enthalpy. Perhaps this is because of the lower temperature value (Here I am not taking the temperature as calculated only by the expansion i.e inlet pressure to outlet pressure but the heat loss to surounding) what are diffferent assumptions you have taken e.g. for heat loss modeling. Waiting for your comments.

[Skyline]

To latexman:
Yes, i'm a newbie in HYSYS, that's why i need some advices from the experts here...

To JoeWong :
I use 10" pipe schedule 20. The pipe is fully straight with some valve between both ends. I simulate those valve as ball valve (open). The length is 35.3 km with elevation +38 m. The pipe is CS (i change the roughness of the pipe into 0.05 mm).

To Padmakar S Katre:
Yes, there is a decrease in mass enthalphy of the streams. In inlet: -1982 and in outlet: -1997 both in btu/lb. I use profes correlation with ambient temp 28 C while the inlet temp is 49 C and outlet temp is 32 C.

Note: I use Peng Robinson fluid package, and the components are

H2S
Nitrogen
CO2
Methane
Ethane
Propane
i-Butane
n-Butane
i-Pentane
n-Pentane
n-Hexane

Is it because of the fluid package?If that's the problem, what fluid package should i use then?

Thank you very much. Sorry if i ask too much. I'm just confused about this...

[latexman]

Heavies condensing out? Run a flash calculation with inlet concentrations and outlet conditions to see if you have two phases.

[Padmakar Katre]

Dear,
Just do an additional excersize i.e. the same inlet stream add a Valve and specify the same Delta P as in pipe across the valve and check for the temperature (As valve is an isenthalpic opertaion) and velocity. As well addition to Mr. Latexman is to check the condensation of the heavies at 32 Deg C and the prevailing pressure at pipe outlet.
May I know the temperature at pipe outlet as 32 deg C where it has come from. Had you mentioned the duty or detailed heat model giving the insulation details and soil details.If possible please give summary of your peoblem in excel sheet perhaps we can check at our side. Mention inlet conidtions like T,P. Composition and Flow and Pipe details as minimum. Additional data pertaining to the specific requirements if any will be helpful to analyse the results and validate the model. Waiting for your comments.

[Skyline]

Dear Latexman and Padmakar,

I checked the phase of the stream. No condensation occur, so the flow is gas only.
For a better insight, i attach the excel file of my work.
Thank you very much.

[Padmakar Katre]

Dear, just attach the excel file.

[Skyline]

Sorry, i forgot to attach the file.

Now, I successfully attached it. Take a look...

Thank you very much

Attached Files

•  Book1.xls   20KB   107 downloads

[herrani]

Hi Skyline

I just had a quick look at your problem, and I believe that your calculations are correct, the "strange" behaviour you can see is due to the heat transfer from the gas to the sorroundings.

If you take an adiabatic pipe (just turn off all heat transfer calculations, and specify 0 kW in the heat stream) then you can see that your velocity profile will increase over distance: at lower pressures, you have a lower density and the velocity in the pipe will increase (vel. = mass flow/(area*density)).

However, when you add the heat transfer from the environment, then the temperature in the pipe will change as well, and a lower temperature means that your density will be higher, and so the velocity will increase.

At the beginning, the temperature effect is higher than the pressure one, and that explains the strange shape you mentioned.

The sharp increase in velocity you see at the end is because of the way you have specified the order of your segments in the rating tab.

Hope it helps!

Regards

Ferran

[Skyline]

Dear Ferran,

Thank you for your explanation. Now I understand the cause of this "strange" behaviour. So, basically, it is a trade off between the effect of temperature and pressure on the density variable, am I correct?Again, thank you very much for your explanation.

Best regards,


Erwan

[herrani]

Hi Erwan

Yes, you are correct!

It is a tradeoff in the density between the temperature and pressure of your gas stream.

Ferran

[fallah]

..... and a lower temperature means that your density will be higher, and so the velocity will increase.

.....the velocity will decrease.

[xtianchem]

<!--quoteo(post=31475:date=Aug 3 2009, 11:49 AM:name=herrani)--><div class='quotetop'>QUOTE (herrani @ Aug 3 2009, 11:49 AM) <a href="index.php?act=findpost&pid=31475"><{POST_SNAPBACK}></a></div><div class='quotemain'><!--quotec-->..... and a lower temperature means that your density will be higher, and so the velocity will increase.<!--QuoteEnd--></div><!--QuoteEEnd-->

.....the velocity will <b>decrease.</b>

sure
hey man if tempearture is down the density is up then the veloc. decrease.
be careful the effect of the tmeperature and the heat tranfer is considerable but i think it is not the cause for your results because you enter a un changeable temperature and rough.. then i think this is not the correct explanations sorry...


best regards
xtian

[herrani]

<!--quoteo(post=31475:date=Aug 3 2009, 11:49 AM:name=herrani)--><div class='quotetop'>QUOTE (herrani @ Aug 3 2009, 11:49 AM) <a href="index.php?act=findpost&pid=31475"><{POST_SNAPBACK}></a></div><div class='quotemain'><!--quotec-->..... and a lower temperature means that your density will be higher, and so the velocity will increase.<!--QuoteEnd--></div><!--QuoteEEnd-->

.....the velocity will <b>decrease.</b>

sure
hey man if tempearture is down the density is up then the veloc. decrease.
be careful the effect of the tmeperature and the heat tranfer is considerable but i think it is not the cause for your results because you enter a un changeable temperature and rough.. then i think this is not the correct explanations sorry...


best regards
xtian

Hi there

I havent been looking at the post lately, so I missed your replies.

Yes, fallah you are right the velocity will decrease at lower temperatures due to the density effect. I was confused.

Here is the formula I am using, assuming this is an ideal gas:

v = (mass flow * R * T)/(Area * Mol. Weight * P)

Xtian: the point of my argument is that the temperature is changing in the calculations. You enter the inlet temperature, and the ambient temperature, and you obtain one curve. If you assume that there is no heat transfer with the environment, then you get another curve.

[sheiko]

Dear,

Check the heat loss i.e. the inlet and outlet enthalpy. Perhaps this is because of the lower temperature value (Here I am not taking the temperature as calculated only by the expansion i.e inlet pressure to outlet pressure but the heat loss to surounding) what are diffferent assumptions you have taken e.g. for heat loss modeling. Waiting for your comments.

I give a "+" to Padmakar for showing the right direction.

[Padmakar Katre]

I give a "+" to Padmakar for showing the right direction.

Dear,
Many Thanks for the compliment and indeed its nice to recieve an appreciation that I could hit the issue at right direction.