Wet steam (sometimes called ‘supersaturated steam’) is a mixture of steam and liquid water. That’s why it is said to be a two-phase mix : steam contains droplets of water that hasn’t changed phase.
In wet steam, water and steam are at saturation temperature. If additional heat is added to wet steam at a set pressure, temperature will remain the same until all the liquid is evaporated. Only then will the temperature rise above saturation, allowing the formation of superheated steam.
We sometimes speak of the ‘dryness fraction’ to describe the dryness level of steam. If the steam’s water content is 5% of its total mass, we would say that the dryness fraction is 95%.
It is generally impossible to produce 100% dry saturated steam and keep it perfectly dry in pipelines. Water droplets escape from the surface in the boiler. When steam bubbles break the surface of liquid water, they carry water droplets. This is called ‘entrainment’. Entrainment can happen at the boiler outlet or at the outlet of a revaporization system.
To entrainment, we must add a certain amount of condensate that unavoidably forms in pipelines as a result of heat loss. This condensate is normally not problematic since it gets eliminated by end-of-line steam traps. Problems begin when entrainment is so important that end-of-line steam traps cannot eliminate everything.
Problems associated with wet steam
Wet steam can cause important problems in a steam system :
- hydraulic water hammer
- deposits of water treatment chemicals
- Destruction of turbines
Hydraulic water hammer
Hydraulic water hammer in a steam system is caused by by an accumulation of condensate or humidity in pipelines or appliances, which is carried by rapidly moving steam.
The resistance of water being superior to that of steam, excessive presence of water in steam lines promotes erosion, especially in elbows.
Deposits of water treatment chemicals
Water droplets in wet steam are in fact treated water from the boiler. Water treatment chemicals in wet steam can degrade heat transfers by forming deposits in heat exchangers.
In processes using steam injection, traces of treated water in the steam can contaminate the product.
Destruction of turbines
Introduction of wet steam in a turbine will have catastrophic consequences. The water droplets will literally destroy the vanes of a turbine.