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Valve/Fittings Pressure Losses

Valve/Fittings Pressure Losses

Valve/Fittings Pressure Losses

Greetings all,

I have gone through almost all the discussions held here and in other forums regarding the valve/fittings pressure drop calculations. I'm currently studying the Crane TP.410, and I have already accessed Hooper & Darby technical papers. I have done hydraulic calculations, and the results are shown in the attached pdf file, along with a simple sketch of the system in question.

I have a few questions, if you don't mind; first of all, I read in two different forums that the pressure loss in the reducer (expansion) is higher than those encountered in a pipe reducer (reduction). However, my calculation showed the opposite as follows:
based on Crane TP.410 for 20"x18" reducer (reduction), the K2 = 0.09228 while for the 20"x18" reducer (expansion) the K = 0.05855. Also, using Hooper's method yields resistance coefficients of 0.05444 and 0.03853, respectively. Before I read those comments, my understanding and preception were that the wall separation and turbulence and the energy utilized by the fluid to force moving through the reduced area should result in a higher K value; I'm not sure if my understanding is correct.

Also, for butterfly valves with sizes larger than 24", can I scale the K value given in Crane TP.410 to the new size? let us say if I have a butterfly valve with a 30" size, can I use K2 = K1/beta^4, where beta = (24/30) since (L/D)eq is constant for all sizes at the same flow condition?

The last question, the pipe schedule is standard, and all fittings are class 150, now in Crane TP.410, page 2-9, the fitting schedule is 40; the question is for the pipe reducer, elbows and WYE connection, which schedule should I use? in my opinion I should stick to the pipe schedule. However, still, I'm not sure if this is right?

I do appreciate your guidance. Thank you all.

RE: Valve/Fittings Pressure Losses

I recommend you to get a copy of : Pipe flow a comprehensive and practical guide by Donald C.Rennels and Hobart M. Hudson -Wiley

Good luck

RE: Valve/Fittings Pressure Losses

Thank you, Pierreick, for the recommendation. I bought the book already and delved into the subject immediately; I understand now that in expansion, excessive turbulence (eddies) caused by fluid separation from the wall result in higher head losses compared to a reduction.

However, my calculations show different conclusions, and I still don’t know why I get K for a reduction higher than an expansion in both methods; Crane & Hooper. I appreciate any comments on my calculations.

RE: Valve/Fittings Pressure Losses

Why are you puzzled that you find a higher K value in a reduction than in an expansion? Which sources suggested that it should be the other way around?

One aspect to be aware of is that there are 3 main types of reducer. These are a sudden change in diameter, a conical reduction and a standard pipe reducer. The standard pipe reducer is the one you will find overwhelmingly often in process plants, but for some reason most texts deal with sudden changes and conical reducers only. Even Crane seems to ignore standard reducers. The radiused (rounded) transitions from the cylindrical to the conical section significantly reduce the K values. Have a look at the Crane data in Appendix A for pipe entrances and note how the radiusing of the transition to the entrance affects the K value. This same effect is found in standard pipe reducers with radiused transitions when the flow through the reducer is from the larger pipe to the smaller one. The effect is much less when the reducer is used as an expander.

Katmar Software - AioFlo Pipe Hydraulics

"An undefined problem has an infinite number of solutions"

RE: Valve/Fittings Pressure Losses

From my calcs, using the traditional Crane method, referencing upstream dia, K = 0.1(similar to your result) for the DN500xDN450 reducer, while K=0.036 for the DN450xDN500 expander.
It is not clear which dia you are referencing for the expander. Based on the downstream dia of DN500, K=0.061
Pls note that for reducer and expander, this calc only gives you irreversible frictional loss. The total dp for each fitting should include reversible velocity head loss / gain also. But in your case, since there is not net change in dia (line size goes back to DN500), these reversible losses cancel out.

For the DN750 butterfly valve, you can convert published Cv value to K using formulas in Crane; no need to extrapolate as suggested

Your last query: Think "std schedule" is not the same as sch 40 - anyway, based on actual id, you can convert from Crane K value to the required K using the formula in Crane.
I dont have Hooper 2-K method calc procedures for K values.

RE: Valve/Fittings Pressure Losses

Quote (georgeverghese)

For the DN750 butterfly valve, you can convert published Cv value to K using formulas in Crane; no need to extrapolate as suggested

The Crane formula (Eq 2-11 in the 2013 edition) to convert from Cv to the K value is
K = 890 x D^4 / (Cv^2)

If it is a 30" butterfly valve and you use 30 as the diameter you will get the K value for a 30" valve in a 30" line. The problem of converting a 30" K value to one applicable to a 20" (or 18") line remains. You could use the 18" or 20" directly in Eq 2-11 to get the same lower K value in one step rather than first calculating the K value for a 30" valve and then scaling it to the new size. It boils down to the same thing in the end but it is good to understand what is being done.

Katmar Software - AioFlo Pipe Hydraulics

"An undefined problem has an infinite number of solutions"

RE: Valve/Fittings Pressure Losses

To add to @katmar's abstract from Crane eqn 2-11, D is internal dia in inches, Cv is in US gpm - see Crane for more details.

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