Is the Pressure Vessel Design for a Railway Cistern Appropriate?

AI Thread Summary
The discussion focuses on the design of a railway cistern intended to hold 120,000 kg of liquid, with specific dimensions and material properties. Key considerations include evaluating the shell thickness of 32mm against various forces: internal pressure, weight of the liquid, and dynamic forces from braking and incline. The necessity of pressurization for a cistern, typically a non-pressurized container, is questioned, highlighting that liquids exert hydrostatic pressure differently than solids. The calculation of forces, including friction and net force on an incline, is also discussed, emphasizing the need to determine the coefficient of friction experimentally or through reference. Overall, the design approach appears sound, but clarification on the need for pressurization remains a point of inquiry.
lak91
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Design a railway cistern for 120,000 kg of Liquid. It has to be a cylindrical shell on two supports.

The cistern is
18 m long
3 m in diameter (inner).
It is made of steel with the yield strength of SY= 240MPa.
Safety factor of FY= 1.9;
Total corrosion allowance, c= 3.0 mm;
Welded joint efficiency = 0.85
Shell thickness, t - 32mm

Check whether this thickness is sufficient to withstand combined action of the internal pressure, t
he weight of liquid, and the action of longitudinal external forces (tensile or compressive) .
Assume the mass of the train as m= 12 x 10^6kg, deceleration during braking of a = 3m/sec
, and during climbing uphill assume the inclination angle of 10 degrees


I assume there will be 3 design cases for this problem:
Case 1 Internal Pressure+ Weight of Liquid
Case 2Internal Pressure+ Weight of Liquid + Inertia during breaking of the railway cistern
Case 3 Forces acting on the pressure vessel: Internal Pressure + Weight of liquid+ inertia of breaking + inclination angle (locomotive applied to the first cistern in the train)

Wondering if I am going about this correctly?
 
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I don't see any problem with your method of reasoning, but my question is why a railway cistern needs to be pressurized. AFAIK, a cistern just a big container to hold water or some other liquid, isn't it?
 
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So for each case I work out the total longitudinal stress?

For case three I am unsure if I am going about it correctly:

Determine mgsinx and Ff(friction).
mgsinx pushes block down the incline and Ff(friction) opposes this.

Write the equation
Fnet = mgsinx - FfSince Ff=uFn and we know Fn=mgcosx and we also know Fnet=ma, we write

ma = mgsinx- u(mgcosx)

Divide by m throughout, so we don't need mass a=gsinx - ugcosx

How would I work out u ?
 
I think in your equations, "u" is really mu, the coefficient of friction. You should just look it up or experimentally determine it.
 
timthereaper said:
I don't see any problem with your method of reasoning, but my question is why a railway cistern needs to be pressurized. AFAIK, a cistern just a big container to hold water or some other liquid, isn't it?

The liquid exerts a hydrostatic pressure on ALL the surface of the container. That is different from a truck containing a solid. For example a liquid would exert a force on the ends of the cylinder when the train was horizontal, but a solid would not.
 
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