# Minimum Thickness of a duct

1. Sep 23, 2011

### buytree

How to calculate the minimum thickness of a duct which hold vacuum inside at 150 deg c? And whats the basic to select the stiffer size and distance between each stiffener.
Thanks.

2. Sep 23, 2011

### SteamKing

Staff Emeritus
How big is your duct?

3. Sep 26, 2011

### buytree

It is 16000 mm long, 1100 mm OD.

4. Sep 27, 2011

### buytree

I was reading the ASME B31.3 used the minimum thickness formula under 304.1.1, 304.1.2. Now I am not sure if I used the right vacuum pressure, I used 14.65 Psig? Can anyone comment if this is right? The material I used A 285 Grade B or A 515 Grade 60. I arrived at 0.56896 mm + 3 mm (corrosion allowance).

5. Sep 27, 2011

### Travis_King

Despite the fact that you will never get a true vacuum, it is sensible to use true vacuum pressure as your design parameter. Also, I would use at least 10 percent (though I like 20% for PV's) factor of safety in your design parameters. ie. (14.65*1.2 = operating_psi)

Now, you have a 52 ft x 42 in OD vessel that you need to keep at vacuum? I don't have enough experience with large PV's to comment on this. It seems to me that it might be a bit insubstantial for its size. It should support the vacuum, but don't you dare bang into it with anything.

6. Sep 27, 2011

### buytree

Thanks all for your response.
@Travis King I felt the same way, the thickness is small. Even after adding the 20% more the t is 3.59 mm. I was thinking I have to add the effect of the external pressure on the duct (ASME B31.3 Sec 304.1.3), I am working on it.

7. Sep 30, 2011

### buytree

I referred to the steps followed in ASME VIII Div 1. UG 27 Thickness of Shells under Internal Pressure used the formula in (1). which give .4954 mm + 3 mm. Am I using the right code or not? My question is, my duct is maintained under vacuum pressure to let the natural flow to the duct, from which the steam/water is distributed through the stubs/pipes. In my case am I using the right ASME code?? Any help will be appreciated.

8. Oct 1, 2011

### Q_Goest

Hi buytree, ASME B31.3 para. 304.1.1 and 304.1.2 are for internal pressure only. They don't apply to external pressure. Para. 304.1.3 is applicable for external pressure and it basically tells you to go to the boiler and pressure vessel (BPV) code and use para. UG-28 through UG-30 to determine wall thickness for externally pressurized pipe. Externally pressurized cylindrical sections are prone to buckling so they typically are much thicker than internally pressurized pipe for the same pressure differential.
Note that there is no additive affect of external pressure on the duct. If the duct is under external pressure only, then 304.1.3 applies. If it is subject to internal pressure 304.1.2 applies. If it is sometimes under internal pressure and sometimes under external pressure then each case can be considered separately. You simply need to ensure the duct meets the applicable paragraph for each case. There is no need to make the wall thicker than is required by either of the individual cases. Hope that's clear.

Take a look at the BPV code for external pressure and if you have questions on it's application feel free to ask.

Last edited: Oct 1, 2011
9. Oct 3, 2011

### buytree

Thank you Q_Goest for taking time to reply. What you wrote makes perfect sense.

I have few more questions,
1. Problem statement, I am trying to select a thickness for a 1100 mm OD, 16000 mm long duct. It is maintained at vacuum pressure to suck the steam which is at higher pressure at the other end of the duct. And also to find the stiffener sizing for the duct.

2. I used the ASME B31.3 para 304.1.1 & 304.1.2 to find the minimum thickness due to internal pressure. I used the pressure value as -14.69 Psig ( which I am not sure is the right value for vacuum pressure). I am getting a minimum thickness of 2.43 mm (which includes corrossion allowance of 3 mm). My doubt, Is 2.43 mm thickness is sufficient for my duct with vacuum inside and (atmospheric pressure exerted on the outside, will there be an effect on the duct or not?). The pressure inside the duct is vacuum so I think the duct will try to stress towards inside and also the external pressure will try to stress the duct?

3. I used the ASME Sec VIII Div 1 mentioned in ASME B 31.3 Sec 304.1.3 to find the thickness due to external pressure. I calculated the factor A and B, but I have a lot of confusion when finding factor B. My duct working temperature is 120 deg c and carbon steel A 515 grade B material. Which chart/table should I use to find factor B? Either based on the yield strength or temperature? For checking I selected CS-2 chart, after finding the factor B using it to find pressure gives me pressure which is pa = 87 MPa when using factor A, pa = 9112 MPa when using factor B.

4. According to step 8 in Sec VIII Div 1, the pressure obtained is much higher than the actual pressure I used as internal pressure. I think I am safe. Comment pl?

5. When comparing the minimum thickness formula for internal pressure in ASME B31.3 and the ASME Sec VIII. Div 1 there is a slight variation in formula right?

6. How can I determine the stiffener required for the particulaer duct?

10. Oct 3, 2011

### Q_Goest

hi buytree. I see you have a parallel thread in engtips.
http://www.eng-tips.com/viewthread.cfm?qid=307182&page=1

That's a good place for advice. As mentioned:
- You can't simply use the internal pressure calculations to determine external collapse pressure.
- The ASME BPV code is fairly complex for externally pressurized structures. You'll need to dig into UG-28 through UG-30 yourself to do this analysis.

Good question, which figure to use for your material? Use CS-2 since if you check ASME B31.3 in the materials section you’ll find A 515 Grade 60 (presume you mean 60, not grade B) has a yield of 32 ksi.

Correct. If you go through the steps in UG-28 and get to step 8 and find your actual pressure is less than the allowable pressure (Pa) then you’re pipe wall thickness is sufficient.

Yes, but don’t worry about ASME Sec VIII. Div 1 for internal pressure. Use B31.3 for internal pressure.

UG-29 covers stiffeners.

11. Oct 4, 2011

### buytree

Thanks for your reply.
After reading all the comments it makes more sense to me.

12. Oct 7, 2011

### buytree

I did a sample calculation, I read in the ASME Sec VIII UG 28 that the allowable pressure should be higher than the applied pressure. If I choose a thickness that brings the allowable pressure higher than the applies pressure then my thickness is almost 1 inch which sounds ridiculous to me. If anyone can point out where I am going wrong that would be great. If you can suggest me a alternative way to find it would also be helpful. Thanks everyone.

L = 1181.10 inch
Do = 43.31 inch
t = .31 inch (assumed)
P = 14.69 Psi
Y = 165 MPa
Material A 515 Grade 60/65
S = 128 Mpa = 18564.83 Psi
E = 195 x 10^3 MPa = 28282351.50 Psi
Joint Efficiency E = 1

Do/t = 137.5

L/Do = 27.27

Due to external pressure ASME Section VIII Div 1 UG 28

Factor A = 6.02 x 10^-5

Factor B = 940 (Very much doubt it)

Max Allowable pressure Pa = (4*940)/(3*137.5) = 9.12 In

Minimum thickness due to internal pressure ASME Section VIII Div 1 UG 27

(1) Circumferential stress
t = (7.35 *21.33)/(( 18564.83 * 1) –( 0.6 * 7.35)) + .158 inch Corrosion Allowance
t = 0.0085 + .158 inch = 0.1665 inch

(2) Longitudinal stress
t = (7.35 *21.33)/(( 2 * 18564.83 * 1) +( 0.4 * 7.35)) + .158 inch Corrosion Allowance
t = 0.0043 + .158 inch = 0.1623 inch

13. Oct 7, 2011

### Q_Goest

If your pipe is under external pressure, UG-27 does not apply and you can ignore it.

It looks like you're doing the external pressure analysis correctly but I'm not sure why you think the final thickness needs to be around 1". You went through the calculations correctly for a thickness of 0.31 with the exception that you shouldn't try to extrapolate factor B from Figure CS-2. The minimum value of A that can be used for Figure CS-2 (assuming 300 F) is around 0.00018. When that happens, then per UG-28, step 4, it says:
If you go to step 7, it provides an equation for the allowable pressure. Putting the values in with A = 0.00006 gives a pressure of around 8.6 psi which is clearly not sufficient for what you need. You need a value of 14.69.

So now you need to iterate. Increase thickness a bit. Note that this step by step method results in a highly nonlinear result for Pa. Try for example, a thickness of 1/2". In that case, you get:
L/Do = 27
Do/t = 87

Now go back to Figure G and determine Factor A. To me, it looks like A should be around .00015. Looking at Figure CS-2, we find that factor A is still to the left of the lines on the graph so again we go to Step 7 and use that equation. Now you should find Pa = 33 psi which is greater than the value you need (14.69) so 1/2" thick plate is sufficient and it could even be made thinner.

Try iterating a few more times and see what value of thickness results in a value of Pa = 14.7 psi.

14. Oct 10, 2011

### buytree

@ Q_Goest Thank you very much. I was assuming the Pa values are linear and came to a conclusion. It is so nice and kind of you to provide answers to all the questions. God bless you. Thanks a bunch again.

15. Oct 18, 2011

### buytree

Thanks a lot for your help. I have successfully completed both the Minimum thickness and the stiffener calculation. Thanks a lot for your motivation.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook