Equipment Engineering and Design
Background
A 6 ton pressure vessel - see dimensioned drawing - was being lowered onto a "stillage" or temporary supporting structure for testing outside a factory in the open yard.. The lower lugs were attached to the stillage and as the slings were being loosened by the crane operator, the vessel tilted slightly.
At this point one of the legs of the stillage collapsed. Luckily the slings had not been removed, so the vessel did not fall - but a serious accident was only narrowly averted.
It was subsequently noticed that the ground on which the vessel and stillage were being placed sloped at 5 degrees.
Problem
1. Why did the leg collapse? What effects or details did the designer ignore?
2. Redesign the stillage to avoid similar incidents in future, without increasing its footprint (width, length) since space is needed around it for testing.
3. Make recommendations on safe lifting and mechanical handling of the vessel.
Tips: work out the weight and the centre of gravity of the vessel. Check the load on each leg. Estimate the collapse load using a suitable formula and relevant assumptions.
Vessel details
Wall thickness 12mm. Hemispherical ends. Carbon steel.
Overall length 7.2m, diameter 2.4m
3 Nozzles (all on vessel centre line), pipe o/d 500mm, i/d 420mm, 500mm long.
Nozzle flanges, o/d 600mm, i/d 420mm, thickness 22mm
Stillage details
Section: 50 x 50 x 5 mm rolled steel angle. Welded construction. S275 steel (i.e. yield stress of 275 MPa).
Section Properties:
overall width 2m, length 4.4m, height 2.4m
