4 replies to this topic

[deltaChe]

I found out our steam header design pressure/temperautre = 25/FV barg and 250 C in
the utiltiy condtion table.

Why does FV(full vacuum) use as minium design pressure for
steam header pressure?
Can any master in process field hep me out?

Thank you very much.

[ankur2061]

Any system handling steam can see vacuum because of the simple fact that when steam condenses (for example during a shutdown or process upset), it causes vacuum in the closed confines of the system it occupies whether it be a pipe or vessel.

However, it is a difficult task to exactly arrive at the vacuum (sub-atmospheric pressure) value. This prompts a conservative design specifying full vacuum for the system (pipe or vessel). If you are certain of the maximum vacuum that you can achieve in a system of given volume for the given steam system then you can specify the same. However, by specifying F.V. (full vacuum) you are making the design safe. It is also possible that there may hardly be any thickness variation of the metal used for construction of the pipe or vessel by specifying F.V. in comparison to specifying a lower vacuum value.

To conclude, specifying F.V. as a design condition has been used for ages as a good engineering practice for chemical process equipment (pipe, vessels, tanks) which can see a vacuum condition during any phase of the operation of the equipment such as normal operation, start-up, shut-down & process upset.

Hope this helps.

Regards,
Ankur.

Edited by ankur2061, 23 May 2010 - 12:40 AM.

[fallah]

Why does FV(full vacuum) use as minium design pressure for
steam header pressure?

Possibility of sudden condensation!

[ankur2061]

You may also look at the following post for further details in determining design vacuum conditions. The post is specifically for vessels but is applicable to piping as well.

http://www.cheresour...__1

Regards,
Ankur.

[sheiko]

I found out our steam header design pressure/temperautre = 25/FV barg and 250 C in
the utiltiy condtion table.

Why does FV(full vacuum) use as minium design pressure for
steam header pressure?
Can any master in process field hep me out?

Thank you very much.

If you fill your pipeline with steam and isolate it and let it cool down the pressure will be determined by the vapour pressure of water at the temperature reached. The temperature reached will depend on the time and the external weather conditions, but if you left it long enough in freezing conditions the pressure in the pipe would get down to below 1 kPa absolute. Effectively full vacuum.