Steel pipe thickness for 70m oil head with safety factor 1.5

[DylanW]

1. Homework Statement
A 1.2m diamater steel pipe carries oil of relative density 0.822 under a head of 70m of oil. What minimum thickness of 120MPa (yield stress) steel would be required for a safety factor of 1.5?


2. Homework Equations
Head = pressure/rho.g
thickness = (safety factor.Pressure.radius.)/Tensile Strength

3. The Attempt at a Solution
Okay so I worked out pressure by Pressure = head x density x gravity = 563892 Pa
Then I worked out the thickness by (1.5 x 562492 x 0.6)/120,000,000 = 0.0042 m = 4.2mm

Seems a bit too small to me, could someone please tell me if this is plausible or if not where I have gone wrong?

 

[DylanW]

Hey just a brainwave am I maybe supposed to use the diamater or circumference? Gives a more plausible seeming result

 

[SteamKing]

What formula are you using to calculate the stress in the pipe wall?

 

[DylanW]

I didn't think that I needed to from the research I've done. The only formula I could find for minimum thickness is the one I posted above. Am I missing something?

 

[SteamKing]

If the formula says radius, you use the radius.