# NCEES problem 525



## Master slacker (Apr 12, 2009)

Really, where does that equation in the solutions come from?

t = r_i * root { [ ( sigma_t + p ) / ( sigma_t - p ) ] - 1 }

By plugging in the numbers from the solution itself, the thickness doesn't end up being what was calculated. The book has 5.8 inches whereas when I use the solution's numbers, I get 13.1 inches. What gives?

:huh:


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## bph (Apr 13, 2009)

Master slacker said:


> Really, where does that equation in the solutions come from?
> t = r_i * root { [ ( sigma_t + p ) / ( sigma_t - p ) ] - 1 }
> 
> By plugging in the numbers from the solution itself, the thickness doesn't end up being what was calculated. The book has 5.8 inches whereas when I use the solution's numbers, I get 13.1 inches. What gives?
> ...


That equation does not seem correct, did you copy correctly?

I seem to remember that question, and I got it right, 5.8 inches, you use the pressure vessel eq, if I remember right (from MERM) but I don't have the exam with me, so can't recall the details of the question or answer. I will answer this question when I get home tonight and see the exam, but again, did you type solution in your post correctly? Maybe I'm thinking of something else, will post solution later when I get home.

BPH


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## Master slacker (Apr 13, 2009)

Yeah, that equation is written exactly as it is shown in the solutions. I've seen several errors on this test. Too bad no erratta has been issued yet.


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## bph (Apr 13, 2009)

Master slacker said:


> Yeah, that equation is written exactly as it is shown in the solutions. I've seen several errors on this test. Too bad no erratta has been issued yet.


Ok, here it is. I got the answer right on the practice test, so I didn't bother to look at the solution. I just used the standard equation for a pressure vessel from the MERM 12ed, page 53-9, equation 53.3 and it gives the same answer as the exam solution, but they have a different equation.

the Equation I used (Eq. 53.3) is

t = ri * P / (SE - 0.6*P)

Where ri is the inner radius, P is the pressure, SE is the allowable stress.

So;

t = 24 /2 * 7500 / (20,000 - 0.6 * 7500) = 5.8 inches

So, the answer looks correct, but their equation looks wrong?

BPH


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## MikeR (Apr 13, 2009)

bph said:


> Ok, here it is. I got the answer right on the practice test, so I didn't bother to look at the solution. I just used the standard equation for a pressure vessel from the MERM 12ed, page 53-9, equation 53.3 and it gives the same answer as the exam solution, but they have a different equation.the Equation I used (Eq. 53.3) is
> 
> t = ri * P / (SE - 0.6*P)
> 
> ...


Yep. I also used eq. 53.3 and got the correct answer.


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## Master slacker (Apr 14, 2009)

Found the errata: http://www.ncees.org/exams/study_materials...ical_errata.pdf


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