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How To Calculate Maximum Flow Through A Pipe?


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#1 prav

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Posted 05 July 2011 - 12:49 PM

Hi All,
I would like to know how can we calculate the maximum flow through a pipe. The pipe outside diameter is 1 inch and the steam is flowing at 5 barg and 180°C. What are the velocity constraints for a pipe?

Thank You

Praveen

#2 breizh

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Posted 05 July 2011 - 09:00 PM

Let you try this resource :

http://www.spiraxsar...sting-pipes.asp


Hope this helps

Breizh

#3 Padmakar Katre

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Posted 05 July 2011 - 11:00 PM

Hi All,
I would like to know how can we calculate the maximum flow through a pipe. The pipe outside diameter is 1 inch and the steam is flowing at 5 barg and 180°C. What are the velocity constraints for a pipe?

Thank You

Praveen


Hi,
Follow the link as suggested by Breizh, a very useful online steam portal. All the information related to steam system is covered in the portal. Addition to that one point I want to share is that, in case pipe hydrualics velocity is a key factor but allowable presure drop consideration is also another facor to be seriously looked into. For a general calculations based on the allowable velocity, you can calculate the feasible flowrate through the given line size, but for the optimized results you need more than that. Hope it clears you.

P.S.- Refer the allowable steam velocities as given below,
1. Exhaust steam - 20 to 30 m/s (70 - 100 ft/s)
2. Saturated steam - 30 to 40 m/s (100 - 130 ft/s)
3.
Superheated steam - 40 to 60 m/s (130 -200 ft/s)

In you case it's superheated steam. If you see the range of allowable velociy as recommended above, you will get a range of flow through 1" line. But ultimately it's your judgement to accept the final value.






#4 GS81Process

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Posted 06 July 2011 - 03:10 PM

There are a few possible answers to your question, because it not exactly clear what you are seeking:


1. Do you want to know the maximum physical possible amount of steam that can pass through a 1" OD pipe (are you referring to 3/4" NPS)? If so, the maximum flow is limited by the choked (sonic) flow condition. The mass flow rate at the choked condition is given by:

Posted Image = Posted Image

where:

where: Posted Image= mass flow rate, kg/sC= discharge coefficient, dimensionlessA= discharge hole cross-sectional area, m²k= cp/cv of the gascp= specific heat of the gas at constant pressurecv= specific heat of the gas at constant volumeρ= real gas density at P and T, kg/m³P= absolute upstream pressure of the gas, PaM= the gas molecular mass, kg/kmole (also known as the molecular weight)R= Universal gas law constant = 8314.5 (N·m) / (kmole·K)T= absolute upstream temperature of the gas, KZ= the gas compressibility factor at P and T, dimensionless


2. Do you want to know the proper size (diameter and schedule) of pipe to use for design of a steam system? If so, the criteria provided by Padmakar and Breizh is what you seek.

3. Do you want to know the flow rate of steam that will flow from a 5 barg source to atmospheric pressure through a fixed length of 1" OD pipe? If so, you will have to perform a pressure loss calculation.

4. Are you trying to estimate the maximum flow rate of steam through an existing section of pipe? If so, is there a control valve on this line? Is the condensed steam returned to a condensate header? What is happening to the steam exactly?


Please explain which you are seeking and I will be happy to assist.

Edited by GS81Process, 06 July 2011 - 11:45 PM.


#5 prav

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Posted 06 July 2011 - 03:42 PM

There are a few possible answers to your question, because it not exactly clear what you are seeking:


1. Do you want to know the maximum physical possible amount of steam that can pass through a 1" OD pipe (are you referring to 3/4" NPS)? If so, the maximum flow is limited by the choked (sonic) flow condition. The mass flow rate at the choked condition is given by:

Posted Image = Posted Image

where:

where: Posted Image= mass flow rate, kg/sC= discharge coefficient, dimensionlessA= discharge hole cross-sectional area, m²k= cp/cv of the gascp= specific heat of the gas at constant pressurecv= specific heat of the gas at constant volumeρ= real gas density at P and T, kg/m³P= absolute upstream pressure of the gas, PaM= the gas molecular mass, kg/kmole (also known as the molecular weight)R= Universal gas law constant = 8314.5 (N·m) / (kmole·K)T= absolute upstream temperature of the gas, KZ= the gas compressibility factor at P and T, dimensionless


2. Do you want to know the proper size (diameter and schedule) of pipe to use for design of a steam system? If so, the criteria provided by Padmakar and Breizh is what you seek.

3. Do you want to know the flow rate of steam that will flow from a 5 barg source to atmospheric pressure through a fixed length of 1" OD pipe? If so, you will have to perform a pressure loss calculation.

4. Are you trying to estimate the maximum flow rate of steam through an existing section of pipe? If so, is there a control valve on this line? Does it condense someone and is it returned in a condensate header? What is happening to the steam exactly?


Please explain which you are seeking and I will be happy to assist.


Thank you all for the solutions and especially GS81Process.... It is the 4th option that you have suggested... We already have a pipe of 1 inch diameter ... The steam is used for fluidization of catalyst... The thing is that we want to increase the flowrate of fluidization steam for better circulation of catalyst.... There is a control valve in the pipe also with installed Cv of 2.5....The fixed pressure drop through the valve is 1.1 bar... I was wondering how far can I go to increase the steam flowrate so that there is no problem with the design of pipe... thats why i wanted to calculate the maximum flowrate of the steam in the pipe... The maximum flowrate through the control valve is nearly 120 Kg/h (Can you tell me exactly how to calculate maximum flowrate through the control valve also at 100% opening because I roughly estimated the maximum flow rate through the valve).... The normal flowrate through the pipe is nearly 28 kg/h.... How much can I increase the flowrate of the steam... We want nearly 200 Kg/h of steam flowing through the pipe.. the pipe is not very long (20 m nearly).... I know for 200 Kg/h, if it is permissible by the pipe, I have to replace the control valve... what Cv should I use for the new design of control valve also ??

Once again:

Desired steam flow rate: 200 kg/h
Normal steam flow rate: 28 kg/h
Maximum flowrate through the standard control valve: 120 kg/h (rough estimate)
If i can have 200 kg/h of steam in the pipe without any mechanical problems, I would change the control valve.... In that case, please tell me the calculation for Cv....

Thank you very much

#6 GS81Process

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Posted 06 July 2011 - 04:33 PM

Prav,

You are looking at a very detailed problem which requires specialized engineering expertise. Someone may have asked you to do this but you should question whether you are able to make a proper recommendation. You speak of using steam to fluidize the catalyst- is this a licensed process? Should you be contacting the licensor/vendor for advice?

There are many detailed questions to which I would require answers before providing a reasonable answer to your query:

1. What process is this? Provide a sketch of the system if you can (i.e. are you allowed to?)
2. Is it licensed technology? Should you be contacting the licensor?
3. You say "better circulation of catalyst"- what does this mean exactly?
4. What is the desired steam velocity for proper fluidization of the catalyst recommended by the licensor or original designer.
5. How did you determine the desired steam flow rate or where did this information come from?
6. How do you know that the desired steam flow rate is correct? Why is it greater than the maximum rate estimated for the existing control valve? Are you sure the current control valve is undersized? If so, was there a design error? What is wrong with the current operation exactly?
7. Are you debottlenecking the process? If so, why not contact the fluidized bed vendor/licensor for support?
8. Where did you obtain your control valve information- from the datasheet?
9. By "maximum flowrate through the standard control valve" do you mean what is listed on the datasheet for maximum flowrate or the flowrate when the valve is fully open? How did you estimate this?
10. Where did you get the installed Cv of 2.5? Is this listed on the datasheet as the rated Cv or is it listed as the Cv under maximum flow?
11. What model of control of control valve is this, what trim size, etc?
9. You state that "The fixed pressure drop through the valve is 1.1 bar". How is this possible- the pressure drop through the control valve will vary depending on its position (i.e. what % open)



There are many good resources for sizing of control valves for steam service. The Fisher Control Valve Handbook is one:

http://www.documenta.../book/cvh99.pdf


Spirax Sarco also has information on its website:

http://www.spiraxsar...eam-systems.asp



My biggest advice to you though is to make sure why you are doing this or are being asked to do this. I look forward to your response.

Edited by GS81Process, 06 July 2011 - 06:25 PM.


#7 prav

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Posted 07 July 2011 - 01:44 PM

Hi GS81Process,
Thank you very much for analyzing the situation so carefully. I got all the information from the datasheet but you are right that to design a pipe, we need to contact the license provider. it's a licensed technology, so I am not allowed to discuss it or show you some sketches. But your links can give me a rough idea and your questions can solve to determine the best scenario is our case.

Praveen




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