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What is Flash Steam?

“Flash steam” is simply steam. Which is produced when high pressure condensate is discharge to a lower pressure.

In a process, this typically occurs at the hot well or downstream of steam traps, level control valves, by-phase control valves, etc. If not properly accounted for in the design of a steam system, flash steam can lead to operational issues and damage to equipment.

Flash steam formed- Steam at any given pressure contains both sensible and latent heat energy. The latent heat is available while the steam is in its gas phase, upon condensing this latent heat is released and what remains in the condensate is the sensible heat.  As an explanation let us look at steam at two different pressure points. The below values are for saturated steam and are taken from a steam table:

At 9 bar steam contains 2,014 kJ/kg of latent heat and 763 kJ/kg of sensible heat.

At 0 bar steam contains 2,256 kJ/kg of latent heat and 419 kJ/kg of sensible heat.

Now you will note that the difference in sensible heat between 9 bar and 0 bar is 344 kJ/kg, it is this excess energy that will suddenly change a small amount of condensate into steam, should the 9 bar condensate be reduced in pressure to 0 bar Upon flashing there will be a corresponding loss of condensate mass. This only applies to hot condensate, you cannot flash cold condensate, as it has already given up its excess energy either to the process or as a heat loss.

The below example calculations demonstrate exactly how much flash steam would be produced from 5,000 kg of condensate at 9 bar and what this might be worth fiscally. You can use this calculation to determine how much flash steam you are generating within your own process, after which you can start looking into recovering some of this energy.

Example flash steam loss to atmosphere calculation

[ (hf – 419) x Q ] / 2256 = flash steam loss kg/hr

Where; hf = sensible heat of condensate at P1 (from steam tables) and Q = mass of condensate

Example; at 9 bar, hf = 763 kJ/kg, Q = 5,000 kg/hr ;

                [ (763 – 419) x 5,000 ] / 2,256 = 762 kg/hr flash steam loss*

*The initial 5,000 kg of condensate is now reduced by 762 kg.

Calculating quantity of flash steam formed

(Hf1-Hf2)/Hfg2

Where,

Hf1= Sensible heat present in the condensate at higher pressure.

Hf2= Sensible heat present in the condensate at the lower pressure.

Hfg2= Latent heat of steam formation at lower pressure.