Understanding Flash Steam

Effective flash steam recovery can be a valuable source of heat.


Flash steam is low-pressure steam created when hot water is released from a high pressure to a lower pressure within a steam system. For example, it can be released through a steam trap or from boiler blowdown.

Flash steam is sometimes seen as a waste product with little value compared to live steam straight from the boiler. This can be an expensive mistake. Steam, however generated, can be an efficient heat carrier. In addition to being wasteful, the venting of flash steam is unsightly and can be both dangerous to people and damaging to a building’s fabric. Far from being waste, flash steam is a valuable source of heat.

Steam tables show that when steam condenses, around 25% of its heat remains in the condensate. With the right flash steam recovery system, around half of this heat can be recovered as flash steam.

What’s the solution?

A flash steam recovery system releases flash steam into a vessel, rather than to atmosphere. This means it can be reintegrated into the steam system or used to feed low-pressure applications, such as space heating. Condensate, which is already flashing as it leaves steam traps on high-pressure units, enters the flash vessel roughly half way up the steam and condensate separate to the top and bottom of the vessel respectively.

There are three basic requirements for a site to be a viable candidate for effective flash steam recovery:

1. There must be sufficient high pressure condensate to release enough flash steam to make recovery economically effective.

2. There must be a suitable low pressure application for the recovered flash steam. Clearly, demand for flash steam must be in step with its availability.

3. The application for the flash steam should be reasonably close to the high pressure condensate source. Piping for low pressure steam is relatively large and can be costly to install in long runs.

- Content provided by Spirax Sarco, originally published in Steam News Magazine.

- Edited by Jessica DuBois-Maahs, Associate Content Manager, CFE Media, Plant Engineering, Control Engineering

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