15 replies to this topic

[Root]

Hi,
To calculate a velocity in pipeline, equation is given in Rules of thumb Page 359 and here in this equation Q is in million ft³/hr . my question is why Q used in million of ft³/hr why not ft³/ min or nm³/hr or min?
Can any one explain this point.
Thanks
Toor

[Zauberberg]

I believe it is for convenience reasons only. As you probably know, gas transmission pipelines usually handle flows ranging from a couple of millions to couple of hundreds of millions standard cubic meters of gas per day. This is sort of equivalent to having the flow expressed in "millions of cubic feet per hour", instead of "cubic feet per hour".

For your own purpose, you can always modify the equation into cubic feet per hour, by applying the 1E+06 factor inside the equation.

[acer_asd]

Hi Toor

Most of the equation used in chemical engineering are empirical equations specially in fluid dynamics. For this equation also someone would have conducted some experiments and noted down the flow in million of cubic ft/hr and found the correlation using regression. That's why he/she reported it in millions of cubic ft/hr.

One thing you should always keep in mind while using this type of correlations that these correlations are developed for some range of parameters and are valid in that range only. So also check the valid range for any empirical correlation you use.

hope this answers your question

have a nice day

[paulhorth]

Toor,

You don't need a rule of thumb to calculate the velocity of gas or liquid in a pipeline.

Velocity = actual volume flowrate/cross sectional area

Hence velocity = mass flowrate/ (density x cross sectional area)

It couldn't be much simpler could it? It's your responsibility to input the data in consistent units. It doesn't matter if the volume flow is given in cubic angstroms per century, provided you include the required unit conversions. Obviously in the case of gas flow you should use ACTUAL volume flow not Standard volume flow.

Paul

Edited by paulhorth, 30 April 2011 - 04:43 AM.

[ankur2061]

Paul,

I beg to differ on the issue of using standard or actual conditions. While performing engineering calculations it is always more convenient and practical to represent gas flow in pipelines in terms of standard conditions simply for the reason that in long distance pipelines ascertaining actual temperature conditions based on ambient conditions over a wide geographical area if not an impossible task is certainly quite a difficult one. Additionally, due to pressure drop in the pipeline the actual volumetric flow increases across the flow direction which would lead to utter confusion about what quantity of gas is being transported.

In fact all gas flow measurement in pipelines is based on defined standard conditions (e.g. OPEC countries use a standard condition of 1.0156 bara and 15 deg C) and it is the norm for all custody transfer applications. The idea of using standard conditions is to avoid confusion in measurement due to pressure and temperature variations during gas transportation.

Hope I have been able to put my view across.

Regards,
Ankur.

[paulhorth]

Ankur,

I think there has been a misunderstanding.
I agree with your comments concerning use of standard volume units for custody transfer and gas flow reporting. However, Toor's question was about how to calculate VELOCITY. For this, only the actual volume flowrate will do. Of course, the actual volume flowrate changes with pressure, so the velocity at the outlet of a gas pipeline is higher than at the inlet. To avoid all ambiguity, I prefer to work with the mass flowrate and the density at the point of interest. That's why I don't like rules of thumb where the units are given arbitrarily and the conversions are all rolled into an arbitrary constant.

Paul

[Root]

Paulhorth,

I agree with you and I got confused when I read 1440constant in this equation and if some one know about this constant from it come please explain further.
Toor

[katmar]

Toor, not everyone has access to this book, so you should give the equation you are querying in full. Also, at least the author's name and preferably the edition should be given as well.

Anyway, the answer has already been given to you by paulhorth. The formula to convert from an actual volumetric flow rate to a velocity is simple mathematics - and was given in detail by Paul. And as Paul has said, any units conversion can simply be rolled up into the constant. You could use any units you want for the flow rate, the pipe diameter and the velocity - you just have to adjust the constant to suit.

[ankur2061]

To All,

For single phase gas pipeline calculations a spreadsheet is available on the online store of "Cheresources" using both SI & English units. Prominent fearures are:

1. Volume flow calculation given the pipe size and pressure drop using 4 different equations: AGA, Panhandle A, Panhandle B and Weymouth

2. Goalseek can be used to calculate pressure drop by providing volume flow rate

3. Actual and Erosional Velocity calculation to verify suitability of the pipe size for a given volume flow.

Following is the link:

http://www.cheresour...low_preview.pdf

Regards,
Ankur.

[Root]

katmar -

Dear Sir,
File is attached for further discussion, please explain all terms.

Thanks
Toor

Attached Files

•  Velocity calculations.xlsx   115.67KB   206 downloads

[breizh]

Toor ,

Take a look at this paper , page 32 and followings .

Hope this helps
Breizh

[katmar]

Toor, including the actual equation completely changes this discussion. One of my favourite sayings is that unless you ask the right question you will not get the right answer. The inclusion of the pressure, temperature and compressibility factor is to allow the volumetric flow rate at standard conditions to be converted to the actual volumetric flow rate.

Once you have the volumetric flow in actual terms then all the explanations by Paul and myself (and everyone else) above apply.

If you need to brush up on the conversions between standard and actual conditions I suggest that you spend some time on Milton Beychok's air-dispersion site, starting with
http://www.air-dispe...las.html#volume

[Silo Ment]

There's an Excel spreadsheet that implements the Panhandle B equation here. It allows you to convert from field units to SI (and even mix the two). There's another spreadsheet that implements the earlier Weymouth equation here.

Silo Ment

Edited by Silo Ment, 18 June 2011 - 08:34 PM.

Hi Everyone,

Let me add my 2 cents here. Mr Toor wanted to know why the units of Q are Mft3/h in 'rules of thumb' . I would ask Mr Toor what his favourite units for Q are and what he would like to see but really that is not the solution. What is the velocity criteria to settle your design problem should be the right question. In process engineering one cannot wait or expect to get the units in the most convenient way he/she want it to be. Process engineers should be able to convert one to the other in order to arrive at the objective.

Mr Toor please state why you want the units to be different when all you want to do is find the velocity. When T and P are not stated it’s ok to provide standard conditions and ask you to determine the velocity. Usually velocity selection criteria is a thing of practice or company procedure. Designer have to ask various question such as

pipe buried or above ground

Liquid or gas (solids present in the medium yes or no)

Short length or long length

Well supported or not

Near to erosion velocity or not

Affects the upstream and downstream units or not



There could be more .....



Maditha

[sheiko]

There's an Excel spreadsheet that implements the Panhandle B equation here. It allows you to convert from field units to SI (and even mix the two). There's another spreadsheet that implements the earlier Weymouth equation here.

Silo Ment

Thanks Silo,

Are you the owner of this blog?

Good job.

Edited by sheiko, 20 June 2011 - 08:36 PM.

[Silo Ment]

Yes, I own it and write all the posts

Silo