Flash steam is formed when the condensate at high pressure is expanded. Once the condensate is at a lower pressure, part of the condensate will vaporise again and form flash steam. Flash steam contains both the purified water and a large part of the available energy, which is still present in the condensate.
Energy recovery can be achieved through heat exchange with make-up water. If the blowdown water is brought to a lower pressure in a flash tank beforehand, then steam will be formed at a lower pressure. This flash steam can be moved directly to the degasser and can thus be mixed with the fresh make-up water. The flash steam does not contain any dissolved salts and the steam represents a large portion of the energy in the blowdown.
Flash steam does, how ever, occupy a much larger volume than condensate. T he return pipes must be able to deal with this without pressure increases. Otherwise, the resulting backpressure may hamper the proper functioning of steam traps and other components upstream.
In the boilerhouse, the flash steam, like the condensate, can be used to heat the fresh feed-water in the degasser. Other possibilities include the use of the flash steam for air heating.
Outside the boilerhouse, flash steam can be used to heat components to under 100 °C. In practice, there are steam uses at the pressure of 1 barg. Flash steam can thus be injected into these pipes. Flash steam can also be used to preheat air, etc.
Low pressure process steam requirements are usually met by throttling high pressure steam, but a portion of the process requirements can be achieved at low cost by flashing high pressure condensate. Flashing is particularly attractive when it is not economically feasible to return the high pressure condensate to the boiler.
Where flash steam is produced from pressurised condensate, the temperature ( and energy content) of the condensate returning to the boiler is lowered. Where an economiser is fitted, this has the potential advantage that the economiser can then recover more energy from the exhaust stack into the return/feed-water stream, and the boiler efficiency will improve. This is the most energy efficient combination. However, there must be a use for the low pressure ( LP) steam from flashing, taking into account that LP steam (from all sources) can only be moved limited distances. In many cases (such as in refineries and chemical plants) there is a surplus of LP steam, and there is often no use for the steam from flashing. In such cases, the best option is to return the condensate to the deaerator, as flashing steam to the atmosphere is a waste of energy. To avoid condensate problems, condensate can be collected locally in a specific unit or activity and pumped back to the deaerator.
The installation of either option depends on the cost-benefit of installing the necessary pipework and other equipment (see Section 1.1.6).
This technique applies when the site has a steam network with pressures lower than the pressure at which steam is generated. Then, re-using flash steam can be exergetically more favourable than just exchanging the heat in the blowdown via a heat exchanger.
In theory, any energy use at a lower temperature can be a possible use for flash steam instead of fresh steam and there will be a range of opportunities on investigation, although implementation is not always easy. It is widely applicable in the petrochemical industry.
The recovery of flash steam saves on fresh top-up water and its treatment, although the main cost savings are in energy. The recovery of flash steam leads to much greater energy savings than with the simple collection of liquid condensate.
Energy Efficiency (2009) 3.2.14