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#1 Narayanan.u

Narayanan.u

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Posted 10 February 2010 - 02:53 AM

Hi all,

I am trying to figure out how a 6" size has been selected for a gas injection pipeline (above Ground) with a max operating flow rate 15.5 to 56.5 mmscfd.

Design data is as below.

Nominal size = 6"
Design Pressure = 280 barg
Design Temp. = 134
Fluid Catagory(ISO 13623) = E
Density = 183 - 257 Kg/m^3
Viscocity = 0.013 - 0.014 cp
Dew point = 71.91

The velocity is much higer than the recommended standards that I have access. I do'nt have the basic engineering documents for this completed project.

Would be of much help if anyone could guide me to find the criteria applied for above sizing.

Regards,

U.Narayanan

#2 demank

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Posted 10 February 2010 - 07:34 AM

There is a missing data, which is compressibility factor.
If I assume compressibility factor is 1, that 6 in pipe seem normal which have the velocity around 5 m/s.
And momentum (roV2) around 7000 kg/m-s2 which have the criteria is 20000 kg/m-s2 for very high pressure.

#3 Narayanan.u

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Posted 10 February 2010 - 12:33 PM

There is a missing data, which is compressibility factor.
If I assume compressibility factor is 1, that 6 in pipe seem normal which have the velocity around 5 m/s.
And momentum (roV2) around 7000 kg/m-s2 which have the criteria is 20000 kg/m-s2 for very high pressure.


Thanks for the guidance. The flow rate need to be corrected to the operating conditions, if I understood it correctly.

Regards,

Narayanan

#4 ankur2061

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Posted 10 February 2010 - 03:52 PM

Hi all,

I am trying to figure out how a 6" size has been selected for a gas injection pipeline (above Ground) with a max operating flow rate 15.5 to 56.5 mmscfd.

Design data is as below.

Nominal size = 6"
Design Pressure = 280 barg
Design Temp. = 134
Fluid Catagory(ISO 13623) = E
Density = 183 - 257 Kg/m^3
Viscocity = 0.013 - 0.014 cp
Dew point = 71.91

The velocity is much higer than the recommended standards that I have access. I do'nt have the basic engineering documents for this completed project.

Would be of much help if anyone could guide me to find the criteria applied for above sizing.

Regards,

U.Narayanan


U Narayanan,

A lot of inputs are missing for any evaluation to be done:

1. What is the composition or molecular weight of the gas?

2. You mention design pressure as 280 barg. What is the operating pressure of the pipeline?

3. What pressure drop are you alllowing from source to destination (starting point to termiantion point)?

4. What is the length of the pipeline?

Other points to take care of:

1. Compressibility factor (Z) at high pressures would never be 1. It would be always less than 1 and you need to calculate the 'Z' value at your operating pressure & temperature using an 'Equation of State' such as R-K (Redlich-Kwong), Soave-Redlich-Kwong (SRK), Peng-Robinson (PR).

2. Once you have the 'Z' factor, you can calculate the density of the gas at the given operating temperature & pressure by the equation
rho = P*M / R*T*Z

where:
rho = density @operating Pr/Temp, kg/m3
P = operating pressure, bar (abs)
R = universal gas constant, 0.08314
T = operating temperature, K
Z = compressibility factor, dimensionless

3. You cannot calculate the gas velocity based on the volume flow given in standard conditions (SCFD). Your velocity has to be calculated based on the volume flow at the operating pressure & temperature.

4. First you need to convert your volume flow in standard conditions to mass flow by multiplying with the gas density at standard conditions. Gas density at standard conditions (1.013 bara & 15 deg C) is calculated by dividing the molecular weight of the gas by 23.7 (MW/23.7).

5. Once you have the mass flow rate & the gas density calculated as per step 2, actual volume flow can be calculated as "Mass/Density'.

6. Once you have calculated the actual volume flow as per step 5 with the units as m3/h, then the velocity in the pipeline can be calculated as below:

v = 3.5368E-4 * Q / d2

where:
v = velocity, m/s
Q = volume flow rate, m3/h
d = Internal diameter of the pipe, m

7. For long pipelines carrying gas you can allow normal operating velocities of 30-35 m/s without any problems. Some earlier guidelines used to restrict the gas velocities to a maximum of 18 m/s (60 ft/s).

8. Where pressure drop is critical, the velocities would be restricted by the pressure drop criteria. You may refer section 6.4 of 'Norsok' Std. P-001 which has a table giving the recommended maximum pressure drop per 100 m for the operating pressure range of the gas. The standard is available as a free download.

Hope this helps.

Regards,
Ankur.

#5 Narayanan.u

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Posted 10 February 2010 - 11:24 PM

Thanks Ankur. It is quite clear now.

FYI

Max operating pressure is 260 barg.

Fluid type (Liquid, Gas, Polyphasic) : Gas ( as given in the document I have).


The error was that I calculated the velocity using flow rate at standard conditions. I did right by asking above question to the experienced friends in this forum .

Thanks again and regards,

U.Narayanan

#6 Gerardo Villamizar

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Posted 11 February 2010 - 09:36 PM

Please forget all these contributions,
Forget also the Z and other parameters, what you need is the criteria and nothing else. The detailed calculations in a suitable software must be carried out by yourself.
Check also possible hydrate formation. The maximum velocity suggested for high pressure (above 80 barg) pipelines is about 10 m/s allowing a rhov2 of about 15000 Pa.

Important: allowable delta P vs. delivery pressure

Best Regards,

GERARDO VILLAMIZAR P.

#7 ankur2061

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Posted 12 February 2010 - 02:57 AM

Please forget all these contributions,
Forget also the Z and other parameters, what you need is the criteria and nothing else. The detailed calculations in a suitable software must be carried out by yourself.
Check also possible hydrate formation. The maximum velocity suggested for high pressure (above 80 barg) pipelines is about 10 m/s allowing a rhov2 of about 15000 Pa.

Important: allowable delta P vs. delivery pressure

Best Regards,

GERARDO VILLAMIZAR P.


Gerardo,

Two questions:

1. What if you don't have a suitable software as you call it? Does it mean that you cannot calculate the velocity by hand calculations? I am certainly not dependent on 'suitable software' as you call it & I suggest any young engineer not to depend on so-called 'suitable softwares'. Calculating velocity by using 'suitable software' would be an insult to my engineering skills.

2. What is the source of your information for maximum velocity of 10 m/s (above 80 barg)? You are not giving refernce to any source. Per Norsok std. P-001, for single-phase gas lines with continuous flow where pressure drop is not critical the maximum allowable velocity can be calculated as follows:

v = 175 * (1/rho)0.43

or 60 m/s whichever is less

where: rho = density of gas in kg/m3

In practice, the above velocity values are a little too high for my comfort but I have seen single-phase gas lines operate at 100-115 ft/s (30-35 m/s) without any problems provided that they are well designed (proper routing, well supported & without any solid particle contamination).

Regards,
Ankur.




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