This excess energy in the water (condensate) causes part of the water (condensate) to flash.
Flashing is the name given to formation of steam from the hot-condensate when the same is released at reduced pressure.
Whether Flash Steam is a Different Steam ?
Flash steam is similar to any other normal steam, but its name is given on the basis of how it is produced.
How the Generation of Flash Steam is Different from Normal Steam?
Flash steam is produced/generated, when a high pressure condensate before the steam trap is exposed to a large pressure drop during its exit.
Flash steam generation is an automatic phenomenon depending upon the condensate parameters (system parameters). Do not depend upon the primary or secondary source of fuel.
Where as normal steam is produced mainly from Boiler or waste heat recovery steam generator (HRSG). Normal steam generation requires the firing of primary or secondary fuel in the Boiler or HRSG.
Flashing of Steam
It is the Excess energy or enthalpy available (due to fall in pressure) will flashes/evaporate some portion of the water/condensate when pressure falls.
Amount of Flash steam produced is given by the formula:
hf at 6 bar = 697.22 kJ/Kg
hf at 0 bar = 417.5 kJ/Kg
hfg at 0 bar = 2258 kJ/Kg
Flash steam generation for the condensate for values as given in Figure 1 = 0.124 Kg of steam/per Kg of water or 12.4 %.
Principal of energy of conservation associated with the generation of Flash steam is described in the given below example:
Let the condensate (water) is at a pressure of 6 Bar(g) as shown in the Figure 1 above.
1 kg of condensate at 6 Bar(g) and at 165oC produces 0.124 kg of flash steam at atmospheric pressure. As the summation of mass of flash steam and condensate is equal to 1, therefore amount of condensate is (1- 0.124) = 0.876 kg.
|Pressure in Bar(g)||Saturation Temperature in oC||Specific Enthalpies kJ/Kg|
|Inlet Condition or condition before the steam trap||6||165||697.1||2066.3||2763.5|
|Outlet Condition or condition after the steam trap||0||100||417.46||2258.0||2675.5|
Therefore from the above table:
|Total enthalpy at atmospheric pressure of saturated water is hf||417.46 kJ/kg|
|Total enthalpy in atmospheric pressure of saturated water is hg||2675.5 kJ/kg|
|Therefore at exit condition or lower pressure of 0 bar(g)|
|Condensate (water) total enthalpy (from fig:2)||0.876 × 417 = 365.3||A|
|Steam total enthalpy (from fig:2)||0.124 × 2675.5 = 331.76||B|
Thus at the exit point or lower pressure, total enthalpy in water ( condensate) and steam is A + B = 697.1
Thus when law of conservation of energy principle applies to Flash steam it is proved that the energy associated with the condensate at the inlet of the steam trap is equal to the energy associated with the water at the outlet of the steam trap.
Misconception when Visualising Flash Steam
First thing that strikes while visualising flash steam clouds, as if leaking of live steam. But usually the flash steam clouds are on account of hot condensate being passed to atmosphere through a steam trap.
Volume of Flash steam generated
Water is denser than steam, so the specific volume of steam is far more than that of water. Thus a small increase in the generation of flash steam results in many fold increase in volume of steam generated.
Larger or greater is the differential pressure between the trap inlet and out parameters larger is the amount of flash steam generated and larger its volume.
Flash Steam Effects
Following are the areas where Flash steam effect should be given due consideration other it may lead to water hammer:
- Condensate-receiver tank vent sizing
- Steam-traps discharge piping.
- Condensate return line sizing.