Vacuum design of a open end steam piping

[mat211743]

Hello Friends, Does it make sense to design a blowdown steam piping that is open to atmosphere for full vacuum? it is in SAGD operation where a 24", ANSI 150 class piping is releasing steam and water at 100 psi and 400F during steam gen blow down events to the pond. Rest of the time it is sitting idle open to atmosphere. This line is being designed for full vacuum conditions. As it is open to atmosphere i am not able to make sense of FV requirement. Am I missing anything here?? Thanks for the help.

 

[EmmanuelTop]

It is hard to say without seeing the actual layout of the system. Generally, full vacuum design in steam systems ensures that no damage will occur during steam-out conditions, i.e. when steam inside the piping or vessels starts to condense due to atmospheric conditions, once when the source of steam is closed. During ambient cooldown, steam follows its pressure-temperature chart and as the temperature is reduced, the pressure inside the system gets reduced as well. These systems almost always encounter a vacuum, unless the inflow of air from the vent is sufficient to maintain the pressure, or other means are used (i.e. N2 purge following steam blowdown etc.) to prevent vacuum. Design for full vacuum is usually prescribed of the anticipated pressure falls below certain value (Norsok: 0.35 bara).

The lines subject of steam out conditions typically require stress analysis (e.g. Norsok standard specifies stress analysis for all lines with NPS 3" and higher, if they will be exposed to steam out conditions during lifetime).

http://www.standard.no/pagefiles/1212/l-002.pdf

http://www.standard.no/pagefiles/1130/p-001r3.pdf

Dejan IVANOVIC
Process Engineer, MSChE

 

[BigInch]

If the outlet of the pipe were to be below the lake's water level, when the lake is high, could a vacuum be developed as the system cools?

 

[KevinNZ]

Very easy for steam lines to pull negative pressure. You only need a low point with the drain closed. Then you also need to worry about Condensate Induced Water Hammer. This far worst than just designing for vacuum.

https://www.youtube.com/watch?v=ToOS6lGnU8U