How to find the minimum thickness of duct with vacuum pressure? 3

[buytree]

How to find the minimum thickness of a duct or pipe carrying fluid which is maintained at vacuum or negative pressure inside at 120 deg c? Thanks.

 

[TGS4]

It all depends - what Code of Construction? What Material? How negative is the pressure? How big is the duct/pipe? It is round/obround/rectangular?

 

[buytree]

Thanks for your response.
I am following ASME.
The duct is a circular carbon steel duct of dia 1100 mm which carries water/steam at 120 deg c. It is maintained in vacuum pressure so the flow is natural.

What other codes are available?

 

[TGS4]

ASME Section I
ASME Section III - multiple Divisions
ASME Section IV
ASME Section V
ASME Section VIII - multiple Divisions
ASME B31.1
ASME B31.3
ASME B31.4
ASME B31.8

There's also AWWA, some various API standards, etc.

FYI - ASME refers to the organization that publishes the standards/codes (American Society of Mechanical Engineers). They publish way more than what I have listed above, but you get the idea.

You wouldn't happen to know what specification and/or grade of "carbon steel" that you would be using? That, I should mention, makes a big difference, too.

This water/steam that is being carried, does it have a specific gravity? Is it possible that there could be water part-way up the pipe, with external pressure? How have you calculated and evaluated the loads due to thermal expansion? Again - how negative is the pressure? What is the distance between supports or vacuum stiffening rings or similar?

 

[pipesnpumps]

ASME piping codes defer to ASME BPV code for calculating external pressures, at least the last time I checked about 5 years ago they did. I recall using sched 40 piping with up to something like 450 psig external and it was fine. When using smaller wall thickness or higher pressures/temps you should most definitely check.

For ductwork you can use SMACNA tables, but if the pressure is very low the tables won't apply, and the duct manufacturer will have to input it into their software to tell you. It doesn't really matter because you only need to the specify the design conditions and materials, they can choose which combination of wall thickness and stiffeners is best. It really depends on size, material availability, etc. and that is an economic decision that should be made in the shop.



Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.

 

[buytree]

Thanks for your reply.

I was using 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). For now I did not consider the thermal expansion. But there will be half water and steam inside the duct will it impact on the thickness? Do I have to consider external pressure and do a calculation? Thanks again.

 

[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.

 

[gr2vessels]

buytree,
Vacuum pressure is interpreted in ASME Code as external pressure, not internal pressure. Look in the ASME Section VIII, Div 1 for design of shells under external pressure.

 

[brubakerj]

I've never liked the ASME rules for external pressure. Are you allowed to do FEA under Section VIII-I to find buckling pressure?

 

[MikeHalloran]

I don't know if you can call it ductwork; I'm getting a minimum wall of 6.6-ish mm for a perfect tube, 7.0-ish mm if it's not more than 10mm out of round.

In the US, that's rolled plate, not sheet.






Mike Halloran
Pembroke Pines, FL, USA

 

[TGS4]

brubakerj - no. If there are rules in Div 1, then you must follow them. Only if there are no rules, then you can apply U-2(g).

Note that there is a new Code Case, that will allow the Design-By-Rules portion of Division2 to be used in lieu of the rules in Div. 1. That might provide the additional rules that you might like to use.

What is it, specifically, about Div 1 rules for external pressure that you don't like?

 

[buytree]

I have some 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?

 

[TGS4]

OK. I think that you have now reached the end of what you are going to find for free. You are asking specific question that are best answered by a qualified and experienced mechanical engineer. Please hire one and follow their advice. Or, alternatively, find a course on pressure vessel and/or piping design, attend, and ask questions of the instructor.

Use of a negative pressure in the internal pressure calculations is incorrect. NO NOT DO THAT! gr2vessels, on October 2nd, already told you that.

There are so many factors that go into designing for external pressure - as described in UG-28 and UG-29.

Rather than blindly following rules and procedures that you do not understand, maybe I will ask this question: do you understand why external pressure is treated differently than internal pressure?

 

[pipesnpumps]

If you can't read ASME requirements and apply them properly, get someone to help you design it. This is a very basic subject and you need only read the code to find the answer.

Question 5.) The design for internal pressure is ONLY when there IS an INTERNAL pressure.. If you used -14.69 psig you are completely misapplying the formula; it is not meant to be used with a negative internal pressure. In your design case (full vacuum), there is only a net EXTERNAL pressure and therefore you need to follow those requirements. These are two completely different requirements.. failure to understand that is quite scary and you should not be doing designs such a low level of knowledge.

Question 6.) If you can properly calculate the wall thickness per ASME for external pressure, based on the full length of the duct, then no stiffener is required.

Don't forget about accounting for thermal stresses too, not just external pressure.






Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.

 

[salmon2]

Please reference API 5C3, which is a short document and a pretty good start to get around this collapse pressure thing. You will see there are elastic collapse, plastic collapse, etc. Of course you have to use the minimum one from all kinds of scenarios.

Also collapse pressure is much much more senstive to dimentional tolerance or imperfections compared to burst (ID) pressure. Such as OD tolerance, aka ovality, and wall thickness tolerance, aka concentricity.

Temperature effect can be incorporated by degraded yield strength, use the yield strength at your specific temperature. Residual stress is also a factor. I think you can find a empirical equation in 5C3 for residual stress, ovalty and concentricity. If you have a vacuum pressure inside, just add that pressure to the external pressure shall be fine.

 

[buytree]

Thank you everyone for your valuable information.

 

[buytree]

at TGS4, I never used ASME codes before. This is my first time I used it, I read the ASME B31.3 there it was mentioned about the Section VIII Div 1. Then I started reading the codes and did a sample calculation. The numbers obtained don't look obvious to me, so I posted in here. Literally I am trying to learn.

I agree I don't know the exact reason why external pressure is treated different than Internal pressure. All I assumed is vacuum is inside the duct that might create an internal pressure and because of the low pressure inside the external atmospheric pressure may impact on the duct. But sadly I understand my assumption was wrong.

at pipesnpumps, Now using the UG-28 for external pressure, find the allowable pressure Pa that should be greater than vacuum pressure of -14.69 Psig. Is that right? I worked out the numbers, they are of course greater than -14.69 Psig but my doubt is they are way too much than the actual pressure. Any comments?

And it makes sense that if I select a proper thickness then there is no need of stiffener. But is it not true that adding stiffeners be an additional strength for the duct?

Thanks again.

 

[buytree]

at salmon2,

I haven't started using API codes, but sure will definitely try using them. Thanks for your comments.

 

[TGS4]

buytree - glad to have you trying and learning. This is probably not the problem that you want to be cutting your teeth on, though.

Do you have any other engineers at your work with experience in this type of thing? What's your mentoring situation? Are there any related courses in your immediate vicinity that you could take to assist you?

Good luck. (I'll give you a hint about why external pressure is treated differently from internal pressure - buckling. Now go and look through some college course notes - MIT's open courseware is a good place if you don't have your college notes.)

 

[buytree]

Anyways I just want to thank everyone who helped me. I successfully did the calculation and arrived at the perfect result. Thanks a lot.