Why are tankers that haul liquid cylindrical? Can't it be, say, rectangular?

greenrun (66) in stemng • 7 years ago (edited)

You must be familiar with truck tankers that ferry all types of liquid/fluid through the highway and our neighbourhood. The liquid product may either be liquefied natural gas (LNG) such as methane or ethane, crude oil, petroleum products such as tar, petrol, kerosene, etc. and industrial chemicals. Some are pressurised and others non-pressurised. But they all have one thing in common; they are cylindrical or elliptical.

^{[Image Credit]Pxhere: A Truck Tanker}

Why is the size not rectangular seeing that manufacture of these cylindrical tankers is not as easy as it would have been to produce the rectangular tanker?

First on the reason why cylinder shaped tanker is the choice of all tankers is the strength. Cylindrical tankers have no weak spots that may require reinforcement. The corners of a rectangular tanker are its most vulnerable point concerning its ability to withstand pressure. Comparatively, the rectangular tank would fail faster under pressure than its cylindrical counterpart.

^{^{Pexels Image}}

That is the same reason airliner's windows are oval.

In the early days of aviation, the first commercial airliner de Havilland DH 106 Comet have large square windows.

^{^{Wikipedia CC0: DH Jetliner with Square Windows}}

Due to the pressuring and the depressuring of aircraft, the fuselage is under pressure, and a square window corner experiences thrice the pressure compared to the rest of the aeroplane. This continuous high pressure leads to the aeroplane body experiencing fatigue.

In 1954, the Comets jet experienced two fatal aircraft involving flight 781 from Rome that disintegrated after 20 minutes after takeoff and Flight 201 from London to South Africa that ended up crashing into the Meditarian Sea near Naples.

The cause of the crashes investigations revealed to be from the square windows.

You can take a look at this short animation analogy from the Federal Aviation Administration (FAA) here that showed how the square window builds up pressure on the plane's body.

The rectangular tanker has the welded joints in the four corners as its weak spots that will fail on continuous pressure. Therefore the cylindrical tanker is more immune to failure from corners since it has none.

Sloshing: Sloshing is the movement of liquid contents on a tanker that is not full. The action is as a result of inertia, the tendency of a body to continue in its present state of motion. A cylindrical container reduces the effects of these "surges" in liquid. Though in some tankers they install baffles which are a liquid-restricting solid divider (with holes in them) that are placed in multiple places inside the tanker to reduce the sloshing effect.

Experienced tanker drives do not come to an abrupt stop and tend to maintain the same level of speed while accelerating or slowing down to reduce sloshing effect.

The big liquid movement could create a high impact pressure on the tank which may lead to structural damage.

Stability: This is a critical requirement of the tanker. The centre of gravity of a cylindrical tanker is lower than that of a rectangular tank; that makes the former to be more stable than the latter.

^{^{Wikipedia CC-BY-SA-4.0 by Sinotrailer: A cement tanker with sloped bottom to enable ease of cement evacuation}}

Ease of liquid evacuation: The cylindrical shape tankers makes extraction very easy. The liquid can easily be channelled down to the bottom of the tank especially if the tanker is designed like on this tanker on the left which is usually employed in cement haulage.

Economics/ Ease of Maintenance: A cylindrical tanker will contain the maximum volume of any liquid product with the minimum amount of material. The cylinder has the least circumference to area compared to the rectangle.

^{^{Wikipedia CC-BY-SA-3.0 by Cmglee:Graphs of surface area, A against volume, V of the Platonic solids and a sphere. The surface area decreases for rounder shapes, and the surface-area-to-volume ratio decreases with increasing volume.}}

So to create the large containers of equal volume, there would be less material for a cylinder than a rectangle.

Due to the corners inherent in the rectangular tanker, cleaning and maintaining it is difficult as liquids and fluid can get stuck at the corners. But the cylindrical truck is easy to cleanup having no edges.

^REFERENCES

^{Sloshing characteristics in half-full horizontal elliptical tanks with vertical baffles}
^{Scientific Reason for Round Windows on Planes}
^{The tragedies that changed the shape of plane windows forever}
^{Why Do Liquid-Carrying Trucks Have Cylindrical Shaped Tankers?}

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