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What is Flash Steam?

What is Flash Steam?

“Flash steam” is simply steam. The “flash” prefix is used to inform the reader/listener that this steam has been generated from hot pressurized condensate/water that has suddenly been reduced in pressure.

In a process, this typically occurs at the hotwell 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.

Benefits of flash steam recovery

  1. Process improvement by recovering valuable heat energy from within the condensate, improving overall energy efficiency and line profitability
  2. An increase in boiler capacity by reducing steam lost as flash steam, that steam is then available to other users which is beneficial for sites where steam generation capacity is an operating concern
  3. Carbon (CO2) emissions reduction

How is 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 barg steam contains 2,014 kJ/kg of latent heat and 763 kJ/kg of sensible heat.

At 0 barg 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 barg and 0 barg is 344 kJ/kg, it is this excess energy that will suddenly change a small amount of condensate into steam, should the 9 barg condensate be reduced in pressure to 0 barg. 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 barg 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.

Example fiscal loss calculation

Qloss x Hrs x SteamCost = Fiscal loss £ per year

Where; Qloss = mass of lost steam, Hrs = annual operation, SteamCost = overall steam generation cost including boiler house chemicals, etc… typically £20 to £25/tn

Example; For a process losing 762kg/hr as flash steam, that operates 8,000 hr per year, with a steam generation cost of £25/ton ;-

0.762 x 8000 x 25 = £152,400 loss per annum.

PLEASE NOTE: 
The steam costs in the examples are for conditions in the United Kingdom. The costs in other countries may be different.

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