3 replies to this topic

[ianmcq28]

Hello,

 

Crane TP 410 provides methods to determine the K values for various standard fittings.  A turbulent friction factor (fT) is used for doing so and the figures are tabulated for various pipe diameters in the appendix A26.  The figures have been determined based on new steel.

 

Can these figures be applied for materials other than steel piping please?

 

The system I am looking at consists of polypropylene (PP) piping.  If I calculate fT for PP piping (using method described here: http://www.pumpfunda...com/help16.html

 

Taking into account the absolute roughness for PP piping, the friction factor is reduced to approximately half vs steel piping.  The overall effect on the end calculation result (for my system) is a difference in flow capacity of ~5%, this would obviously be greater if the proportion to fittings vs straight pipe was greater.

 

I have also checked my hand-calculation using AFT Fathom software and checked the help files within the software package to determine the methodology behind the calculations.  The software appears to apply the Crane values for fT for all piping material types.

 

How do other engineers consider this when using pipe materials other than steel please?

 

Kind Regards,

 

Ian

Edited by ianmcq28, 05 February 2014 - 05:50 AM.

[ankur2061]

Ian,

 

For plastic pipe, generally referred as 'smooth pipes' which includes Polypropylene, the absolute roughness value is 0.000005 ft (0.0015 mm) which is 30 times less than the absolute roughness of commercial steel pipe of 0.00015 ft (0.045 mm).

 

It is obvious from the above that the friction factor for PP pipe will be lower than the friction factor for commercial steel pipe. Why is this an issue? Use the absolute roughness values and use any explicit equation for friction factor such as the Swamee-Jain equation mentioned in your link to calculate the friction factor.

 

Regards,

Ankur.

Edited by ankur2061, 05 February 2014 - 02:23 PM.

[katmar]

The Crane K-values are a source of enormous confusion when applied to anything other than commercial steel pipe (and even when applied to steel pipe!). A lot of very heated discussion has taken place over the years. I will give you some links below for your edification and amusement.

AFT Fathom is doing the correct thing by using fT for all materials. Let's take as an example the Crane data on page A-29 for standard 90 degree threaded elbows. The K-value is given as
   K = 30 x fT

You have already explained what the fT is, but where does the "30" come from?  In early versions of Crane 410 the fitting pressure drops were given in terms of equivalent length. This value of 30 is the equivalent length of commercial steel pipe that would give the same pressure drop as the elbow. Equation 3-15 on page 3-4 explains the conversion between L/D and K.  It is therefore wrong to use a value of 30 here if you want to work with PP pipe. The elbow would have very much the same pressure drop whether it was made of steel, PP or any other material because the pressure drop is due to the change in flow direction and NOT because of friction in the fitting.

The friction losses in PP pipe are lower than in steel pipe (all else being equal) and this makes the equivalent length of the elbow IN TERMS OF PP PIPE greater than 30. The increase in equivalent length would be inversely proportional to the friction factor for the PP pipe. The equivalent length increases and the applicable fT decreases and the effects cancel out and the K-value for a PP fitting is very much the same as for a steel fitting of the same dimensions.

I have written an article with more detail than above if you want to dig a bit deeper.  See
http://www.katmarsof...essure-drop.htm

If you need a bit of reassurance that other engineers have faced the same confusion see
http://www.eng-tips.....cfm?qid=173164
 

Edited by katmar, 06 February 2014 - 02:22 AM.

[ianmcq28]

Gents,

 

Thank you for your responses.

 

Ankur, I have no issue with the pipe friction factor for various materials.  I was concerned with the applicability of the f_T (Crane A-26) used for determining K values for fittings for materials other than commercial steel pipes.

 

Harvey, thank you for the link to your article; this has cleared the matter up for me beyond any doubt.  I take comfort that I am not alone in falling into the confusion caused by Crane - judging by the lengthy debate in eng-tips!

 

Much appreciated,

 

Ian