17 replies to this topic

[stu]

Dear Sir,
In API 12J Appendix D worked Example, for converting Standard flow rate to actual flow rate , I used
the following formula ACFD=SCFDx{(Temp.@ Actual / Temp.@ STP)}X{(Press. @ STP / Press. @ Actual)} , then I convert the flow rate to flow per second.But I am not get the answer same as they worked out.Please clear the doubts where i had done wrong.
Please explain the relation they arrived to find the actual flow rate from standard flow rate by using molecular wt., volume of gas at STP.
Please explain.
Thanks & Regards.
Stu.

[Art Montemayor]

Stu:

You don't give us your student background, but I must assume you are at university level since you are dealing with an API document - something that is well beyond the realm of grade or high school levels.

If I am correct, then you should be well-prepared to logic and employ the Ideal Gas Equation, PV = Z N R T. Using this Equation of State (E.O.S.) and the ability to also employ an electronic spreadsheet such as Excel (which, if you are a university student in India, you should be well versed in) you should be able to derive, employ and demonstrate that the equation you found in API 12J is valid.

Please do me the favor of preparing an Excel spread sheet and employ the equation I cite above to find the equivalent volume of an ideal gas at "standard" conditions. Be sure to copy the EXACT EQUATION FOUND IN API 12J. You should be able to derive the same, exact equation and make the conversion in Excel without any problems. When you have the Excel Workbook ready, submit it to us in this thread. I will personally review it and work with your product to ensure it is correct and if not, will point out where you are going in the wrong direction.

If you are unable to do the above, give us your reasons (i.e., you don't know how; you don't have Excel or another spreadsheet; you don't want to; you don't have the time; etc., etc.).

I consider this an important step in your preparation towards becoming an engineer. If you agree with me, you should follow my recommendations. If not, then tell me and I can explain what it all means.

[ankur2061]

Stu,

The concept of standard cubic feet to actual cubic feet is given very precisely in the following articles found on the web. These explain in terms of time in minutes instead of day but it is only a matter of conversion from minutes to day. If you notice the underlying principle is the same as the ideal gas equation (PV = ZnRT) as mentioned by Art. To give you a further hint consider the two conditions as condition 1 for standard and condition 2 for actual and apply the ideal gas equation.

http://www.dresser.c...R-SCFM-ACFM.pdf

http://www.pdblowers...-actual-cfm.php

For % relative humidity, CAGI (Compressed Air and Gas Institute) uses 0% instead of the 36% mentioned in the articles above.

Let us see you come up with a spreadsheet.

Regards,
Ankur.

[stu]

Dear Sir,
Please find the Excel I had prepared for The conversion of SCFM to ACFM based on your guidance.
The excel results Actual flow rate as 5.42 ft3 / sec for 25MMSCFD flow rate of gas (molecular Weight= 20.3) at 800 Psig & 80 Deg F. But in API 12J it results as 4.552 ft3/sec.
Please correct me to get the right results from excel sheet.
Thanks & Regards.
Stu

Attached Files

•  SCFM_ACFM_Conversion.xls   36.5KB   916 downloads

[katmar]

stu,

As far as I can see your spreadsheet is perfect. I have written a similar conversion routine for my free units conversion program "Uconeer" (free download from www.katmarsoftware.com). In my version I have to convert from SCFM to mass flow, and then from mass flow to ACFM, but in the end I get exactly the same answer.

Your spreadsheet is more rigorous than it really needs to be for this particular example. As long as you are assuming that the ideal gas law holds, you do not need to worry about the molecular mass and the ideal gas constant (unless you need mass flows). A simple ratio gives the same answer, i.e.
Q2 = Q1 x (P1/P2) x (T2/T1) = 17361.1 x (14.7/814.7) x (540/520) = 325.3 acfm

I do not have the API 12J document so I cannot comment on why their answer should be different.

[katmar]

Stu - sorry I saw in your original post that you had already used the ratio method so you obviously understand well what you are doing.

[Art Montemayor]

Stu:

Good job of responding and showing interest in your developement.

As you may suspect, I created an Excel Workbook of my own on this problem the same day I read your query. I created and stored it in my hard drive at home. I am presently at work and will submit my explanation and workbook to this thread as soon as I get home from the gym (I put in 3.5 miles of walking daily, after work). I apologize for not having it here at work today to share with you.

Will post tonight.

[ankur2061]

Art/Katmar,

I also have prepared a spreadsheet based on the articles mentioned in my earlier post on the same topic.

Could you please review the sheet and let me know your valuable comments.


Regards,
Ankur.

Attached Files

•  SCFM_to_ACFM.xls   29KB   601 downloads

[Art Montemayor]

Stu:

Attached is the Workbook I promised.

Note that I have presented the entire example - word for word - as given in API 12J. I did this on purpose for several reasons:

I wanted to show students how easy it is make a mistake by simply believing everything you read without applying logic and deriving the equations given when you have the opportunity. That way, you can confirm the validity of the equation as given.

I have always regarded this specifc example as one of the worst examples of a formal engineering calculation. Please read through my presentation of the example and my comments.

The API is a great and marvelous engineering organization - but it isn't GOD. Engineering students, in my opinion, should respect the effort and information given in the API Recommended Practices and Standards; but they should also feel free to challenge or question what they consider as not being correct or what they feel is flawed. This is not being critical for the sake of being contradictory; it is critical of weak or possibly erroneous engineering. We all, as engineers, stand to profit from seeking accuracy and truth in published material. We are all human, so any peer check we can obtain is all in our favor.

I don't know why you were not getting the correct answer. You have not revealed what the problem was. My effort has been concentrated on showing you and other students how to approach another engineer's efforts at solving a problem and learning from the success and the failure of such attempts. I hope this helps you and other out.
 API_12J_ExampleRev1.zip   133.36KB   579 downloads

[stu]

Dear Art Sir,
Thanks for your reply & guidance regarding this conversion. I am also used the same equation as you mentioned in your excel for the conversion of flow rate from one condition to another
V1=V2(T1/T2)*(P2/P1).But I am not able to get the answer as mentioned in API for the flow rate.
Please clear me whether I had done the wrong thing in my excel.
Please explain. Awaits your valuable reply.
Thank & Regards.
Stu

[katmar]

Art,

Thanks for posting your spreadsheet. As I mentioned before, I do not have the API 12J spec so I was not in a position to work out why Stu's answer was different. But now it is clear.

In Stu's worksheet he calculated the gas density and got a value of 2.854 lb/ft3. My Uconeer gives the same value. However, in the API document the gas density seems to be an entered value rather than a calculated value. They use 3.4 lb/ft3 for the gas density.

If we adjust Stu's answer for the gas flow rate of 5.42 ft3/sec by the ratio of these 2 different densities we get the API value, i.e.
5.42 x (2.854/3.4) = 4.55 ft3/sec

Now the question becomes, "where does the value of 3.4 lb/ft3 in the API 12J spec come from?"

Harvey

[JoeWong]

Harvey,

I have done same excercise as you did... while I gone through the workbook by Mr. Montemayor & stu, I found one simple point that explained the differences.

One of the main ASSUMPTIONS by Mr. Montemayor, stu and yourself is assuming compressibility factor at actual condition (Za) same as compressibility factor at Standard condition (Zs).

For Natural gas i.e MW = 20.3, the Z is close to 1. However, as the pressure increases, whilst maintaining the temperature low, the z factor starts to drop. Refer to GPSA, 23-6, the Z factor at 800 psig (55.17 barg) @ 86 degF (26.67 degC) is about 0.84-0.85.

So this is very sigficant to density and actual flow.

At Z = 1, density = 2.854 lb/ft³ (just take your value)
At Z = 0.85, density = 2.854 / 0.85 = 3.36 lb/ft³ (approximately taken as 3.4 lb/ft³)

This explained why API 14J has taken 3.4 lb/ft³.



Stu,

Hope this helps.

[katmar]

Joe,

Thanks for the good work. The combination of low temperature and high pressure should have alerted me to this. Problem solved, thanks to you.

Harvey

[JoeWong]

Harvey,
Welcome.

I am pretty happy with this discussion as this thread contains necessary basic information, effort by all, step by step approaching to conclusion...

Good works for all.

[stu]

Dear Sirs
Art / Harvey / Joe wong / Ankur
Thanks for your guidance in Directing me in a right way.I herwith attached the Z-chart as Mr.Joe Wong indicated one, which will be usefull for others also to check .
Thanks Once Again.
Thanks & Regards,
Stu.

Attached Files

•  Z_Chart.pdf   196.37KB   375 downloads

[abc420]

Thank you all of you for providing such an expensive concept

[Art Montemayor]

Stu/Ankur/Harvey/Joe/All:

Please forgive my oversight in assuming that the Compressibility Factors would cancel out in my derivation of the required equation. I wanted so much to show Stu (& other students as well) the importance of knowing and understanding the origin of these relationships that in my haste I simply did the easiest step and cancelled out the "N", the "R", and the "Z" - as I normally do in most cases. However, as has been pointed out in this valuable thread, NOT IN ALL CASES! While it is obvious that the number of moles and the Universal Constant remain the same and must cancel out, the Compressibility Factor - as noted by Joe - does not.

That was my mistake and I thoroughly apologize for the oversight. I am revising the spreadsheet derivation.

[Qalander (Chem)]

Dear Art.

As always, I admire your greatness in accepting the inadvertant ommission.

regards
Qalander