3 replies to this topic

[Seonwoo]

Hello,
I have a question regarding the resistance K values for reducers and enlargers, as the calculated values appear to be excessively high.

Based on my research, I found that there are three commonly referenced sources for resistance K values:

 

1. Crane

 

2. 3K-Method

 

3. API 521

 

Among these, the Crane and 3K-Method approaches tend to produce very high K values, which result in significant pressure drops.
In contrast, although the K values from API 521 are quite low, they are similar to the exit and entrance K values found in Crane.

Additionally, I am unsure whether the resistance K values represent only frictional losses, or if they also account for losses due to changes in pipe diameter (i.e., expansion or contraction effects).

I am currently performing discharge line sizing in order to calculate the backpressure.

Could you please clarify this?

Edited by Seonwoo, 20 March 2025 - 09:35 PM.

[breizh]

Hi,

Let you dig into the document issued by Katmar (our expert in fluid mechanic)

https://www.katmarsoftware.com/

 

Breizh

[latexman]

If you would show your conditions, calculations, and final results for the different methods, we could check your results and comment further.

[katmar]

The more specific link than that which Breizh gave above to the article that discusses pressure drop at changes in diameter is

www.katmarsoftware.com/articles/pipe-entrance-and-exit-losses.htm

On that page there is also a link to a downloadable PDF version.

 

In every case that I have seen, the published K values apply to the frictional losses only and ignore the Bernoulli effects.  The main point of the article referenced above is that both the frictional and Bernoulli effects must be taken into account to get accurate results.

 

An important point to note when comparing K values for changes in diameter is which diameter (and therefore which velocity) the K value refers to.  The K value based on the larger diameter will be very different from that based on the smaller diameter.

 

A strange anomaly in the world of reducer K values is that almost all sources look only at sudden and conical reducers, while in the real world of piping 99% of reducers are specially formed pipe reducers with radiused corners.  The only source that I have found for K values for real world reducers is Hooper, Chemical Engineering Magazine, Nov 7, 1988, pgs 89-92.  Even the 3rd edition (2017) of Darby's book refers to Hooper for these values (although Darby still ignores the pipe reducer formulas).