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Heights of Air Pressure!

On a summer afternoon in 1988, Aloha Airlines flight 243 was cruising under clear blue skies making the most of the fair weather. The Boeing 737 with 89 passengers on board was flying from Hilo to Honolulu in Hawaii. Barely 10 minutes into the flight as the plane levelled out at 24,000 feet, the pilots heard a loud ‘clap’ followed by a sharp wind noise behind them. They turned back to find the cockpit entry door missing. A flight attendant was swept out of the cabin through the hole in the fuselage. The plane began descending rapidly and suffered extensive damage due to the explosive decompression of the fuselage. Miraculously, the crew managed to land safely at Kahului Airport in Maui.

But what does an airline disaster have to do with pressure vessels? A lot actually, since airplane cabins are pressure vessels. They are pressure containing structures. These are essentially closed containers designed to hold air, gases or liquids at a pressure substantially different from that of the ambient pressure. Due to this difference, especially when the vessel is at a height, it can be a safety hazard that may cause a rupture or an explosion. However, parameters such as shape, thickness and strength of its material, physical properties, maximum safe operating pressure and temperature also play a part.

Pressure vessels have a variety of applications that date back to the industrial revolution, following the invention of boilers and steam engines. Today, industrial application of pressure vessels spans a wide range – from chemical industry to cosmetics, food and beverage to oil and fuel, paper industry to pharmaceutical, plastics and power generation to energy processing. Pressure vessels can be classified according to their dimensions or end construction.

When a vessel is pressurized, the pressure is exerted against its walls. Pressure is always normal at the surface, regardless of shape. Theoretically, a pressure vessel can be designed to have almost any shape but from a safety point of view, spherical and cylindrical ones are most common. Pressure vessels are able to hold up against internal pressure due to tensile forces within their walls. The normal tensile stress in the walls of the vessel is proportional to the pressure and radius of the vessel and inversely proportional to its thickness.

Compressed air is used at various points in the manufacturing process of pressure vessels. Deep drawing presses utilise compressed air for certain pneumatic components. Hydrostatic and pressure testing of vessels also employ compressed air. A number of Indian manufacturers produce domestic LPG cylinders, industrial gas cylinders and auto LPG tanks among other pressure vessels, catering to both Indian and foreign market. ELGi compressors have been meeting the compressed air needs of this industrial segment for several years.