Monday, May 30, 2016

Pressure vessels: An overview spanning their construction and industrial usage

Pressure vessels
Pressure vessels are containers that are designed with the sole purpose of storing gases or liquids at pressures significantly different from the ambient pressure. One noteworthy thing here is that pressure differentials are generally very dangerous, with quite a few examples of fatal accidents having occurred as a result of the differentials. This is the reason for the design and manufacture of these vessels being regulated by laws pertaining to various parameters such as maximum safe operating temperature, safety factor etc.

Pressure vessels have been used since many centuries, with the earliest reference probably dating back to 1495 A.D. However, the kind of pressure vessels used today owe their evolution to the industrial revolution in the 19th century when boilers were used to generate steam.

As a result of the dangers involved, pressure vessel manufacturers today give a great deal of emphasis on ensuring optimal safety with zero chance of any untoward incidents while designing these vessels. This has resulted in vessel related accidents having become almost negligible in the 21st century.

Construction of pressure vessels:

Pressure vessels are typically constructed from reliable materials that can withstand high levels of stress. Selection of a sturdy and reliable material is paramount based on the all too known safety concerns during the operation of these vessels. General parameters that are weighed before selecting the material for construction are,
  • Strength of the materials- This parameter encompasses yield strength, ultimate tensile strength, creep strength and rupture strength of the materials
  • Corrosion resistance – This is the most important factor to consider, with additional thickness and alloying with various elements of the metal to be used in construction being done to increase the corrosion resistance
  • Resistance to hydrogen attack – At 300-400 degrees Fahrenheit, molecular hydrogen is formed in voids, this results in building of high pressure that can cause the vessel to crack. This tolerance to cracking under pressure of the metal is checked before being used in constructing pressure vessels
  • Fracture toughness – This simply means ability of the material to withstand conditions that can cause brittle fracture in it.

Application areas of pressure vessels:

Pressure vessels are used in a a variety of sectors such as
  •  Petroleum refining
  •  Chemical
  •  Power
  •  Pulp and paper
  •  Food
Pre-requisites for the design of pressure vessels to ensure end user(s) safety:

All pressure vessels must be designed for the most severe conditions of coincident pressure and temperature that are expected during normal service. To elaborate, normal service includes conditions during :
  •  Start up
  •  Normal operation
  •  Deviations from the normal operation that can be anticipated
  •  Shutdown