3 replies to this topic

[Trey]

To calculate pressure drop through expansions and contractions, you need to calculate the K value.  To calculate the K value of expansions and contractions, you need to know the angle inside the fitting.  The angle is not straight forward to calculate because at the entrance and exit of the fitting it is curvy.

 

Per page 121 of this book (full version downladable on Kindle for $51):

 

http://books.google....n angle&f=false

 

The angle of ANSI expansions/contractions should be calculated with this excel formula:

 

=2*ATAN((larger diameter-smaller diameter)/(1.2*Length per ASME B16.9))*180/PI()

 

Also, the Rennels-Hudson method is generally more conservative then the Crane equations; however, the two methods are in the same ball park for calculating K values for expansions.

[ankur2061]

Trey,

 

A revised excel workbook related to equivalent lengths of ANSI reducers / expanders based on the reference you have provided will soon be available in the Download section of Cheresources.

 

The formula for included angle that you are quoting as per the reference is only applicable when calculating the resistance or loss coefficient for "expanders". For "reducers" the factor remains '2' and not '1.2' to calculate the resistance or loss coefficient . This again is as per the reference you have provided and which I could go through.

 

Regards,

Ankur

[Trey]

Ankur,

 

The only time the factor of "2" should be used is for conical reducers where the length is calculated (see equation 10.9 in the link to the book) and not taken from ASME B16.9.  For ANSI reducers, the K value will be near zero with only friction contributing (see section 10.7 in the link to the book).  I just don't want people using the Crane equation for ANSI contractions with an angle using the "2" factor.  Also, why would the angle of the fitting be different if it was installed as an expander or a reducer?

 

Trey

[ankur2061]

Trey,

 

My apologies. You are right, an ASME B16.9 reducer can also be used as an expander, hence the conclusion that the included angle using the factor '1.2' will be the same for either a reducer an expander is logical.

 

However, if yoi read section 10.6 - 'Smooth Contraction', the last paragraph explicitly mentions that the effective angle alpha to be used in equation 10.16 should be calculated as per equation 10.10. If you see equation 10.10, the effective or included angle has a factor '2' and not '1.2'. This creates confusion regarding the effective or included angle for reducers. For expanders it is very clear that the factor '1.2' has to be used considering that this equation for divergence angle with the factor '1.2' has only been shown in Chapter 11 of the book which is for expansions.

 

Also, please note that the equation for the divergence or inclusive angle in section 11.5 - 'Pipe Reducer: Expanding' has an error in it. It uses 'd1-d2' instead of 'd2-d1' when seen with reference to figure 11.10. In figure 11.10, 'd1' is the smaller diameter and 'd2' is the larger diameter.

 

The book provides no explanation on how the factor '1.2' has been arrived for effective or divergence angle equation. Do you have an explanation for that in terms of the math involved in arriving at the factor '1.2'?

 

Regards,

Ankur.

Edited by ankur2061, 10 December 2013 - 10:36 AM.