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Equivalent Length Calculation


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#1 T_bag

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Posted 19 February 2024 - 12:36 PM

Hello Everyone,

 

Recently I was working on the eq. length calculation, and I am having a very silly doubt.

I'll explain my doubt with 2 examples.

 

Example 1:- If we have an expander (Say 2X4 inch) in a line. the estimated calculated eq. length is L1 with 2 inch piping and L2 with 4 inch piping. Now to estimate the pressure drop, the expander eq. length should be consider in which piping segment (2 or 4 inch) ? 

 

Example 2 :- I have a Tee through branch from 2 inch to 6 inch. The flow is from 2 inch line to 6 inch header making a TEE. Now the eq. length calculated from Crane's handbook to be considered in 6 inch header or 2 inch line.

 

Thank you.

 



#2 latexman

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Posted 19 February 2024 - 03:52 PM

K is diameter dependent. Match K1 to d1 (and v1) to get L1 or match K2 to d2 (and v2) to get L2. This is all related by K2 = K1/Beta4. If you do it consistently the fitting should give the same pressure drop whether you use K1, d1, v1, L1 or K2, d2, v2, L2.

#3 breizh

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Posted 19 February 2024 - 06:34 PM

Hi,

Probably good to review this paper from Harvey, owner of Katmar software and member of this forum.

https://www.katmarso...essure-drop.htm

Breizh



#4 T_bag

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Posted 20 February 2024 - 11:23 AM

Hi,

Probably good to review this paper from Harvey, owner of Katmar software and member of this forum.

https://www.katmarso...essure-drop.htm

Breizh

Article says 

How come the article says the K-Value is constant for a TEE (Big concern for me). When searched through Crane's handbook , it should be a function of flows/ TEE split size.

System is not allowing me to post screenshot of the Crane Handbook section Hydraulic resistance for Tees and Wyes.



#5 latexman

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Posted 20 February 2024 - 12:58 PM

Article says 

 

 

How come the article says the K-Value is constant for a TEE (Big concern for me). When searched through Crane's handbook , it should be a function of flows/ TEE split size.

System is not allowing me to post screenshot of the Crane Handbook section Hydraulic resistance for Tees and Wyes.

 

The article is old.  October, 2012.  My 2018 version of Crane was the first I saw/remember where they changed from a constant K for tees to correlations  using variations in tee geometry and flow conditions in each flow path.  The technology has advanced from the time of Katmar's article to the current version of Crane TP410.  Or, maybe he thought the complexity of the Crane method was not justified over the simplicity of the method his article discussed.  Maybe, Katmar will reply here.

 

To get to the attachment functions, click "Use Full Editor" or "More Reply Options".  Then, the attachment functions are under the text box.



#6 breizh

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Posted 20 February 2024 - 06:11 PM

Hi, you should carefully read the document and understand the limitation of the method. Better use 2K or 3K method.
my view.
Breizh

#7 katmar

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Posted 21 February 2024 - 11:00 AM

The first thing to understand with calculations of this type is that they are approximations. I have seen reports where several "identical" fittings were purchased from a single supplier and the measured pressure drops varied significantly. So if you are using generic data for fittings from a variety of suppliers you should not expect the calculated pressure drop for a particular fitting to be within 30% of the actual (and probably worse than that).

Luckily for us the pressure drops through the fittings are usually less than through the straight pipe, and the pressure drop through the pipe can probably be calculated to within 10-15%. It amuses me to see arguments over the third significant digit in the calculation of the friction factor. The pressure drops through the fittings are commonly known as the minor losses for exactly this reason.

For your Example 1 - the table of equivalent lengths for reducers and expanders should always specify whether it is based on the upstream or downstream velocity. If the table does not specify this, be very suspicious of any data presented there.

For your Example 2 - I do not believe that it is possible to achieve good accuracy using equivalent lengths for wide ranges of merging (or diverging) flow ratios and for reducing tees with area ratios of around 10. If I was faced with a design like this I would take the 2" pipe as having a 2x6 reducer and then assume a 6" tee. I would calculate the pressure drop in sections and then I would add an appropriate safety factor.

Avoid the temptation to throw all your data into a black box and then to believe the answer.


Edited by katmar, 21 February 2024 - 11:05 AM.


#8 T_bag

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Posted 21 February 2024 - 12:00 PM

True 

 

The first thing to understand with calculations of this type is that they are approximations. I have seen reports where several "identical" fittings were purchased from a single supplier and the measured pressure drops varied significantly. So if you are using generic data for fittings from a variety of suppliers you should not expect the calculated pressure drop for a particular fitting to be within 30% of the actual (and probably worse than that).

Luckily for us the pressure drops through the fittings are usually less than through the straight pipe, and the pressure drop through the pipe can probably be calculated to within 10-15%. It amuses me to see arguments over the third significant digit in the calculation of the friction factor. The pressure drops through the fittings are commonly known as the minor losses for exactly this reason.

For your Example 1 - the table of equivalent lengths for reducers and expanders should always specify whether it is based on the upstream or downstream velocity. If the table does not specify this, be very suspicious of any data presented there.

For your Example 2 - I do not believe that it is possible to achieve good accuracy using equivalent lengths for wide ranges of merging (or diverging) flow ratios and for reducing tees with area ratios of around 10. If I was faced with a design like this I would take the 2" pipe as having a 2x6 reducer and then assume a 6" tee. I would calculate the pressure drop in sections and then I would add an appropriate safety factor.

Avoid the temptation to throw all your data into a black box and then to believe the answer.

 

It becomes tricky in tight revamps where system is hydraulically limited and people does not let you consider extra safety margins.



#9 katmar

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Posted 21 February 2024 - 01:11 PM

If there is any way to avoid such tight design constraints you should try.  Equipment degrades with time.  Lines foul up.  Marketing forecasts change.  Raw materials fluctuate in price, availability and specification.  As much as I have said that the engineering design is uncertain, it is probably the most accurately known part of the overall business.  When everyone else in the chain is uncertain of their area of responsibility it is unreasonable to expect the design engineers to be spot on with their calculations.






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