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Moisture in Steam

Last updated: September 24, 2019

What Does Moisture in Steam Mean?

Moisture in steam is the amount of water particles carried by the steam, expressed as a percentage by weight. The mixture of steam and water droplets, often referred to as wet steam, is a major challenge in boiler and steam systems. The moisture can lead to process and maintenance problems such as reduced productivity, poor-quality products, erosion and corrosion.


Corrosionpedia Explains Moisture in Steam

Moisture in steam is caused by:

  • Water molecules that do not vaporize during the steam generation process, carried by the steam as droplets
  • Water molecules in steam losing their energy due to turbulence and splashing, resulting in condensation into water droplets
  • Heat loss in the pipes, leading to condensation of steam
  • When steam enters the processing equipment, it gives up some energy, which is used to warm up the equipment while some is transferred to the processes.

It is almost impossible to produce and maintain a 100 percent steam throughout the entire piping system. Dry steam is the actual steam without water droplets and is usually between 95 and 98 percent, while the moisture accounts for the remaining percentage. The water droplets can, however, be removed from the steam using methods such as:

  • Using a steam separator (moisture separator) to remove suspended droplets from the steam
  • Using steam traps for trapping and draining the condensed water
  • Insulating the pipes to reduce heat loss

The negative effects of the moisture in steam are:

  • Reduced steam quality and thermal efficiency, causing reduced plant productivity and increased energy costs
  • Equipment malfunction and damage to parts, causing expensive repairs and downtime
  • Water accumulation in the piston engine cylinders, causing hydraulic lock, which can damage the engine
  • Water droplets in high-velocity steam can impinge on and erode turbine internals such as the turbine blades

Erosion of carbon steel piping interior surfaces causes thinning of the pipes and removal of protective coating, which can lead to corrosion of the inner walls of the steam pipe. Holes may form in the piping, causing steam leaks that could be a safety hazard.


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