5 replies to this topic

[US_ChemE]

Dear Forum Members,

For calculating the K factor for a reducer/ expander and friction factor for materials other than steel, I have below two questions:

• For any reducer, Crane TP-410 says that the K value is independent of size ie. It doesn’t depend on the friction factor. From the formulae it appears that it depends only on the ratio of the bigger to smaller diameters ie. Beta Ratio. The formulae for any other fitting like for a valve are given as product of a f_t and Leq/D. The schedule of the pipe remains as schedule 40 for class 300 and lower and is independent of the actual schedule of the pipe in which valve is fitted. However, if we want to estimate the loss through a reducer, say 6”x4” in a schedule 80 pipe (class 150), do we need to consider the actual internal diameters of the reducer ends (6” sch. 80 and 4” sch. 80) or based on the permitted schedule considering class 150 (6” sch. 40 and 4” sch. 40) for estimating Beta Ratio and subsequent theta angle? If we use pipe internal diameters based on actual schedule, do we need to convert the K back to that based on schedule 40 internakl diameter so that it will be equivalent to the ones for other fittings whose K is based on schedule 40 and arithmetic addition can be done for estimating total K?
• All the formulae for estimating the K factor consider the f_t for clean commercial pipe. If we have material different from steel, I understand that Leq/D values remain same but the value of f_t estimated from graph in A-23 shall be used based on the absolute roughness (epsilon) for the material and the diameter (D) under consideration. Is this understanding correct?

I would be grateful to have your opinion on the above queries.

Warm Regards,
US_ChemE

[breizh]

http://www.pipeflowc...cecoefficientK/

• Consider this resource (link above) :
• Additional info are available using the search button in this forum.
• material change is taken account through the absolute rugosity ( as stated) used to calculate the friction factor ( document attached or chart ).

Hope this helps
Breizh

Edited by breizh, 28 February 2012 - 01:23 AM.

[vikramltv]

1. K value calculation is done based on actual internal diameters for expander/reduser. no need to do conver it back via linearization..Refer Ludvig Vol-1 Chapter-2 (Fluid flow) for more details.
2. Your understanding is correct.

[katmar]

1. Reducer
There are two things that are important to keep in mind here. Firstly, the way the K-value is used is to multiply it by a velocity squared term to get the pressure drop. This means that the K-value and the velocity must apply to the same diameter. If you are calculating the velocity based on the pipe diameter then you should correct the reducer's K-value to the same diameter. This is explained on page 2-10 in my metric version of Crane 410. If you are applying Crane Equation 2-2 to the reducer alone then you would use the K-value and velocity based on the reducer's smaller diameter end, but if you want to add the K-Values for all the fittings in the line and apply the total to a single velocity then the individual K-values must be corrected using Eq 2-5 where applicable.

The second thing to remember is that this is not an exact science. If you are correcting for a fitting that is only slightly different in diameter from the rest of the line then the correction could well be less than the uncertainty in the K-values.

2. Relevance of f_T for other fittings
Note that in Crane Eq 2-16 the only portion of the pressure drop through a bend that is affected by the pipe roughness in h_L. For standard elbows h_L is a small part of the overall pressure drop and it is usual to disregard the roughness of the elbow material in calculating the pressure drop through the bend. With the pressure drop through a valve nobody ever considers the roughness of the valve material.

What I am getting at here is that the pressure drop through these fittings is a function of the size and shape of the fitting only, and is hardly influenced at all by the roughness of the fitting. (Very long radius bends are an exception to this.) This means that if you have two geometrically identical fittings - one made of smooth plastic and the other of commercial steel - the pressure drops through the two fittings will be identical (for the same flow rate of course). BUT the pressure drop through a metre of smooth plastic pipe is not the same as the pressure drop through a metre of rough steel pipe. If you want to express the pressure drop through the fitting as being equivalent to a certain length of pipe then the eqivalent length of plastic pipe is not the same as the equivalent length of steel pipe. So it is wrong to say that Leq/D values remain the same for different materials.

If you want to read my (and others) full rant on this subject see http://www.eng-tips.....cfm?qid=173164

[ankur2061]

The Crane TP 410 method for equivalent length calculations of concentric reducer / expander and sudden reduction / expansion is very well dealt with in the spreadsheet posted at:

http://www.cheresour...pes-and-valves/

The spreadsheet takes into account factory made wrought buttwelding fittings for concentric reducers / expanders according to ASME B16.9 and the materials include
carbon steel, austenitic stainless steels, Alloy Steels and low temperature service CS and Alloy steel.

From that viewpoint the reducers/ expanders as given per Table 11 of ASME B16.9 include all materials and the same is reflected in the spreadsheet.

Hope this helps.

Regards,
Ankur.

[latexman]

I use K = 0.1 (based on smaller internal diameter) for all commercial, gradual-reduction reducers.