9 replies to this topic

[Frya_R]

Hello,

 

So I'm having this problem in designing a simple pipe segment, in hysys,

its a pipe segment of,

 

od 24" and id of 22.8699" that is 25 miles long with inlet conditions of 60barg, 86F, and 652MMSCFD, with friction factor of 50 microns, and I cant manage to get it it right.

 

I'm a relatively new process engineer, I know I can increase the diameter or increase inlet pressure and all that, but my supervisor says its a problem with the heat transfer.

 

can you guys help me figure this out I've been at it for two days I feel like I'm missing something very simple.

 

the following is the attached hysys file.

 

thanks for your time.

[katmar]

I was unable to see your data file, so I have assumed the gas to be pure methane to determine the physical properties. This will give a ballpark estimate and an indication of the problem.

The attached image shows the pressure drop and exit velocity calculated in AioFlo using the isothemal compressible model. If there are heat losses along the pipe the density of the gas will be slightly higher and the pressure drop lower by a couple of percent, but the overall picture will not change significantly. At these high pressure drops and Reynolds Numbers the calculated pressure drop is very sensitive to the pipe roughness and the margin of error is higher then usual.

The reason for Hysys to not solve the problem may be that the gas properties are significantly different from pure methane, or they may have put checks into the solution algorithm that do not allow such a high pressure drop relative to the inlet pressure.
 

Attached Files

•  Frya_R Cheresources 20 July 2025.gif   101.01KB   1 downloads

[Frya_R]

I was unable to see your data file, so I have assumed the gas to be pure methane to determine the physical properties. This will give a ballpark estimate and an indication of the problem.

The attached image shows the pressure drop and exit velocity calculated in AioFlo using the isothemal compressible model. If there are heat losses along the pipe the density of the gas will be slightly higher and the pressure drop lower by a couple of percent, but the overall picture will not change significantly. At these high pressure drops and Reynolds Numbers the calculated pressure drop is very sensitive to the pipe roughness and the margin of error is higher then usual.

The reason for Hysys to not solve the problem may be that the gas properties are significantly different from pure methane, or they may have put checks into the solution algorithm that do not allow such a high pressure drop relative to the inlet pressure.
 

if i may ask why would it be a problem if the gas properties differ from pure methane? i mean it is not purely just methane but the composition is taken from a real natural gas that is more than 90% mthane

[katmar]

The relevant properties are the viscosity and the molecular weight.

 

Changes in viscosity will have a very minor effect.  The viscosity affects the Reynolds number which in turn affects the friction factor.  But at high Reynolds Numbers the friction factor is almost independent of the Reynolds Number - and therefore independent of the viscosity.

 

The molecular weight affects the density and the mass flow rate (for a constant flow expressed at standard volumetric conditions).  So if the MW is higher than that of methane the density will be higher than that of methane and the mass flow rate will be higher than the 320 lb/s we had previously.  This will result in a higher pressure drop than I showed in my previous post.  Conversely, if the MW is lower than that of methane the pressure drop will be lower.

 

What viscosity and molecular weight did you enter into Hysys?

[breizh]

Hi,

With data supplied, I got a Delta P of about 51 bars considering Pure methane at 30C and 61bars Abs with a flow rate of 12455 m3/h at the inlet or about 149 kg/s, friction factor is 0.01175.

EDIT: I used 0 C and 1 bar as standard reference.

 

If you use 15 C, the Delta P is lower (42 bar) and the flow rate 143.44 kg/s

 

 

A paper & calculators to support your work,

CheCalc ‐ Properties

CheCalc ‐ Fluid Flow

 

Breizh

Attached Files

•  GasPipelineHydraulics.pdf   2.97MB   3 downloads

[katmar]

The results from Breizh confirm what I said earlier.  He has used either a different set of standard conditions from what I used or he has used a higher molecular weight.  Either way, the result is that his flow rate is 149 kg/s compared with my 145 kg/s (320 lb/s) and this results in a significantly higher pressure drop.

[Frya_R]

The results from Breizh confirm what I said earlier.  He has used either a different set of standard conditions from what I used or he has used a higher molecular weight.  Either way, the result is that his flow rate is 149 kg/s compared with my 145 kg/s (320 lb/s) and this results in a significantly higher pressure drop.

 

 

Hi,

With data supplied, I got a Delta P of about 51 bars considering Pure methane at 30C and 61bars Abs with a flow rate of 12455 m3/h at the inlet or about 149 kg/s, friction factor is 0.01175.

 

A paper & calculators to support your work,

CheCalc ‐ Properties

CheCalc ‐ Fluid Flow

 

Breizh

thank you both for helping me out but we use panhandle equations so when doing the calculations with excel the pressure drop is not remotely close to that of the hysys simulator, in hysys sometimes the temperature comes out as negative, I've even tried keeping the temp constant throughout the pipe segment it just doesn't solve properly

[breizh]

Hi,

You can try this calculator specific to Natural gas with different models:

https://checalc.com/...PipeSizing.html

 

You will need to iterate on P2 until you reach the flowrate expected then you will calculate the Delta P.

 

Good luck 

Breizh 

[twell]

I presume (as Katmar and Breizh did) that at all conditions allong the pipeline the stream is all vapor, 

if that is true the values provided by Katmar and Breizh shouldn't be too far (with the discussed corrections) from a rigorous calc. (i.e. subdivide the pipeline in many segments, solve heat balances and adjust fluid properties)...

Note that at 60 Barg.g small amounts of heavy HC can reduce the dew point to 304 K  (88F) or below...

I have not access to Hysys (I have Prode Properties which includes Panhandle  equation) I have observed that this equation calculates (generally) lower pressure drops but not completely different  so I suspect the reported error could have different causes...

[breizh]

Hi,

Reviewing the examples in the attached link may help you to find the reasons of your problem.

Comparison of Compressible Flow Equations and FluidFlow - Part 1

 

The calculations I did were based on Isothermal model using excel and Newton Raphson to get the pressure at the discharge point.

EDIT: No news from OP, he may have found clue to his problem?

 

Good luck

Breizh