A boiler of 1.6 m diameter is made of a 20-mm thick plate. What will be the permissible steam pressure in the boiler in the maximum tensile stress in the plates must NOT exceed 80 MPa?

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MPPGCL JE ME 19 March 2019 Official Paper
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  1. MPa
  2. GPa
  3. 20 MPa
  4. 20 GPa

Answer (Detailed Solution Below)

Option 1 : MPa
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Detailed Solution

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Concept:

For thin cylinder: 

Hoop stress

\({σ _h} = \frac{{Pd}}{{2t}} = {σ _1}\)

Hoop strain

\({\epsilon_h} = \frac{{Pd}}{{4tE}}\left( {2 - \mu } \right)\)

Longitudinal stress

\({σ _L} = \frac{{Pd}}{{4t}} = {σ _2} = \frac{{{σ _1}}}{2}\)

Longitudinal strain

\({\epsilon_L} = \frac{{Pd}}{{4tE}}\left( {1 - 2\mu } \right) = \frac{{{σ _2}}}{E}\left( {1 - 2\mu } \right)\)

Calculation:

Given:
d = 1.6 m, t = 20 mm, σmax = 80 MPa = σl (Longitudinal stress in case of thin cylinder)  

Maximum working stress

\({σ _1} = {σ _h} = \frac{{Pd}}{{2t}} \Rightarrow {\rm{P}} = \frac{{{\rm{2t \sigma_h }}}}{{{{\rm{d }}_{\rm{}}}}} = \frac{{2 \times 20 \times 80}}{{ 1.6 \times 1000}} = 2\ MPa\)

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