Pipe design for external pressure 1

[swapanburo]

I am checking a 42" steam turbine LP extraction piping for operation at the vacuum condition , i.e. subject to external pressure. Clause 104.1.3 of B31.1 refers to the procedure in UG-28, UG-29 and UG-30 of Section VIII, Div 1 for this purpose. Section-VIII, Div-1, UG-28 provides a methodology to check this.

There is a term "L" used in the methodology suggested in Section-VIII, Div 1, UG28. I am, however, not sure what value should be considered for this "L" in a piping system. Will it be the support span in the pipe, OR the maximum length between two bends in the piping system ? It is not clear from the figures UG-28 or UG-28.1

I will appreciate if anyone can suggest me something on the above issue from his experience of working on this subject.

 

[racookpe1978]

I'm not sure extraction steam is always at a vacuum: What are you startup and mid-load conditions both INSIDE the pipe and outside (in the condensor and the traverse to the heaters and in the heaters)?

 

[C2it]

The usual interpretation of 'L' in Piping terms is the maximum length between buckle arresting components, such as a flange, a stiffening ring or a support with a circumferential attachment. Personally I would not rely upon an elbow in this respect.

 

[jte]

On the other hand, sometimes it doesn't matter. Work the problem backwards and see if you can eliminate needing a precise answer.

From Chart CS-2, if A is to the left of the curves then I use the formula in Step 7 of UG-28. Hold that thought for a minute.

If I assume an infinite L, then L/Do maxes out at 50. Fine. Looking at CS-2, I see that if I use an A of 0.00018 or so I am to the left of the curves. [I'll suggest that you read U-2(I) before anybody gets on me for the "or so" lack of precision.] Now moving to Figure G, I see that with an A set to 0.00018 or so and reading the L/Do = 50 line at the top my maximum Do/t that I can have is about 80.

This is key: So if my Do/t exceeds 80 I use the formula in Step 7. Given the 42" Do, the maximum t would be ~1/2", not unreasonable for a low pressure steam line. Now, applying the formula in Step 7 using E = 25 E6 psi, I get.. Well, let's just say well over 15 psi or Full Vacuum.

So if the line is not over 1/2" thick, then it's fine. If the line exceeds 1/2", then use the charts and I'm guessing the value will be higher than what you'd get with Step 7.

Either way, allowable pressure not precisely calculated, but enough information developed to allow for a decision.

 

[swapanburo]

Thanks for all the suggestions . These were really helpful. In fact I used XS pipe , i.e. about 1/2 " thick and considered L/D as 50, and got A factor on the left side of curve CS-2. Then using Step-7 , with E about 26.5 E6 psi at 600 F , got about 30 psi allowable external pressure, which is fine.
One last question. How that max value of L/D of 50 has been arrived at in UG-28 and Figure-G ? What is the idea behind this? Any thought ?