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Pressure vessels problem

by edz2012
Tags: pressure, vessels
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edz2012
#1
Nov26-12, 09:56 AM
P: 3
1. The problem statement, all variables and given/known data
20 m3 of gas at a pressure of 25 bar is to be stored in a cylindrical
pressure vessel 2 m long. Given the following information :

The yield strength of the vessel material is 14,000 psi

If a factor of safety of 5 is to be used, determine:

Whether the vessel should be treated as a thin or thick cylinder.


2. Relevant equations
iv been given the feed back as follows:
For this question you need to apply the thin cylinder theory to determine the thickness t, then depending on the answer for r/t, determine whether the cylinder should be treated as a thick cylinder. If it is a thick cylinder, then the thick cylinder theory must be applied to determine the thickness of vessel required.

The attempt at a solution

3.1 bar = 100,000 Pa
factor 5 means that maximumstrength
yield strength
=1 /5
1 psi=6894.7N /m2
a) We have PV = RT = PSL (L= 2 m long.)
So the strength of our vessel should be 25⋅105 Pa≈362.6 psi
From the factor of safety we can find the the maximum strength should be
14/5⋅103 psi=2.8⋅103 psi
So, the vessel should be treated as a thick.


im told that my attempt is incorrect but i dont know how else to solve it useing the feed back im given.
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CFDFEAGURU
#2
Nov26-12, 10:42 AM
CFDFEAGURU's Avatar
P: 723
The length is fixed at 2 m. So, calculate what the diameter needs to be to hold 20 m^3 of gas. From the diameter, calculate what the thickness needs to be and then check the r/t value to determine what set of equations should be used.

Thanks
Matt
SteamKing
#3
Nov26-12, 10:43 AM
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Thanks
PF Gold
P: 6,534
Did you work out the radius of the cylinder required to hold the compressed gas?

The decision on whether to apply thick or thin cylinder theory depends on the ratio r/t, not on what the ratio of the wall stress to yield might be.

edz2012
#4
Nov26-12, 10:44 AM
P: 3
Pressure vessels problem

can you explain to me how to do this, what equations do i need to use.
CFDFEAGURU
#5
Nov26-12, 10:46 AM
CFDFEAGURU's Avatar
P: 723
The equation to figure out the volume is straight forward. Just look it up for a cylinder. A basic thickness equation can easily be derived (or looked up) for the stress in the hoop direction. The longitudinal stress is always 1/2 of the hoop stress. So, the hoop stress governs.

Thanks
Matt


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