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astreyu (Marine/Ocean)
13 Oct 11 10:15
Hello everyone,

How can I compute approximately the flexible pipe head loss?
I read in this forum that I should be using a roughness of ID(mm)/250 but how can I use this to have the pressure drop.

thank you  
waterpipe (Mechanical)
13 Oct 11 11:03
I hope that you use the term "flexible" for a pipe in its engineering terminology (in this context, steel pipe is "flexible"). So unless the cross section of a pipe considers to be round, and you are not referring to  the pipe cross section changes, then the pressure drop only depends on the surface roughness and flow regime. It doesn't depend, whether the pipe is flexible or rigid. For head loos calculation take a look here:

MiketheEngineer (Structural)
13 Oct 11 17:04
But if you flex to make a corner (or other weird shape) - that will need to be considered.
astreyu (Marine/Ocean)
17 Oct 11 4:43
Thanks for your answer,

However, for me, the issue is that you use a ID-dependent absolute roughness that begets a ID-independent relative roughness since in Moody diagram, we use the e/D relative roughness.
For example,

You have a flexible pipe with ID=8"
The absolute roughness is e=ID/250
To use Moody you will have to get the relative roughness defined by e/D with D=ID so the relative roughness is 1/250 whatever the inner diameter of the flexible pipe ...

You could say that in this theory a pipe with ID=6" is equivalent to a pipe with 8" etc..
ione (Mechanical)
17 Oct 11 4:57
I suggest you to read this thread , taking particular care to katmar's post.
astreyu (Marine/Ocean)
17 Oct 11 5:08
Thank you but I don't really see the relation to what I'm asking. If I understand right, he says that there is a low variation between different roughness, so an error in the roughness does not have that much impact. However, in my case, using a formula like that ID/250 for absolute roughness seems like a complete nonsense. I cannot believe that a flexible pipe of 6inches features the same pressure drop coefficient of a pipe of 10 inches.
The danger here is that you could underestimate the pressure drop and find yourself with an undersized pipe...

ione (Mechanical)
17 Oct 11 6:03
What you need to calculate the pressure drop is to apply the Darcy-Weisbach equation, which asks for the relative roughness of your hose through the friction factor. The relative roughness is equal to e/D, where:
e = absolute hose roughness
D = ID
astreyu (Marine/Ocean)
17 Oct 11 8:14
I know ! I think I have written it before, in the post before your first one ...

Like I said and like you said, the relative roughness pipe is e/D with e = absolute roughness (in the case of flexible pipe it's apparently ID/250 see: Mark62 and Handbook of subsea engineering)
and d=ID
so relative roughness is equal to (ID/250)/ID = 1/250 which means that whatever the ID , darcy friction factor is independent of your geometry...

I find it surprising, that's all .
I'm not asking about calculating pressure drop in general but only  about the relevance of this formula (e=ID/250) !!
Helpful Member!  BigInch (Petroleum)
17 Oct 11 8:29
It looks like it was supposed to be
absolute roughness = 1/250 = 0.004
rel roughness = 0.004 / ID

but I'm guessing there.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

ione (Mechanical)
17 Oct 11 8:45
To me the relevance of the formula quoted is equal to zero.
BigInch (Petroleum)
17 Oct 11 9:21
or maybe i-one

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

astreyu (Marine/Ocean)
17 Oct 11 9:49
Thanks BigInch, I also thought it was e=1/250 but since I found this exact formula in this forum and in the subsea engineering handbook (Q.Bai & Y.Bai) and in other places as well. So I am at my wit's end.
What do you mean by i-one ? ID-1?

To ione thanks for your answer, I guess now you understand what my problem is... I am in front of a formula that I must use while I know it's irrelevant.
Maybe someone could enlighten me on how Pressure drop is assessed in a flexible pipe like NKT's flexible pipe . What darcy friction factor to use to take into account the carcass geometry.
StoneCold (Chemical)
17 Oct 11 9:55
I think this is what he is looking for
astreyu (Marine/Ocean)
17 Oct 11 10:34
Well since I don't really like to use a program that does not give me informations on how it calculates , it's not what I'm looking for ...
Is it a flexible pipe, is it a flexible hose, how does it calculate those results ?

katmar (Chemical)
17 Oct 11 10:55
astreyu - there have been several discussions here on the friction in flexible hose. If you can specify what type your hose is then we could give you better answers. "Flexible hose" varies from very smooth rubber hose used for hydraulic machines right through to corrugated spiral wound pipe used by farmers and lay-flat hose used by fire fighters. Give us details - we will give you answers. From my tagline below you will see that I feel quite strongly about this.

Katmar Software - Engineering & Risk Analysis Software

"An undefined problem has an infinite number of solutions"

astreyu (Marine/Ocean)
17 Oct 11 11:05
By the way using this website, try calculating your pressure drop with same reynolds for two different ID. Using the resulting pressure drop, and the data input, calculate the darcy friction factors ... you will find out that watever the ID, it's the same friction factor which is not right.
astreyu (Marine/Ocean)
17 Oct 11 11:18
Hello Katmar,

I am talking of a flexible pipe used in the subsea oil and gas industry like this one
The carcass profile is the problematic geometry here. I wanted to know if there is another way of calculating flexible pipe pressure drop than using an absolute roughness of ID/250 since like I said this leads to an ID-independent darcy friction factor.
In the link provided by SToneCold (thx btw), it seems a similar formula is used, i.e a formula that leads to an ID-independent darcy friction factor.
katmar (Chemical)
17 Oct 11 11:37
Astreyu - you have caught me out, that is a type of hose that I have not used before. Probably the closest that I have seen discussed here was in a reference by vzeos in thread378-212758: Pressure loss in hoses and connectors for air regarding the book "Fluid Power Design Handbook" by Frank Yeaple.

Katmar Software - Engineering & Risk Analysis Software

"An undefined problem has an infinite number of solutions"

astreyu (Marine/Ocean)
17 Oct 11 11:55
Ok thank you Katmar for the provided link.

The closest geometry being the corrugated pipe, it seems corrugated pipe correlations should be used instead ...

So nobody can explain why the formula e=ID/250 for a flexible pipe is used?

BigInch (Petroleum)
17 Oct 11 13:22
All I can tell you is that 0.004 would "roughly" correspond to an absolute roughness of 0.0018 inches, a typical value for wrought steel pipe converted into centimeters.  Sorry, I don't know anything more and couldn't possibly tell you where the original formula came from, or its correct form.

However I'm sure that it wouldn't be the first time that an error was propagated time and time again, especially across the internet.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

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