pressure vessel having steam out condition=0.5 kg/cm2g @ 200 deg C, in which conditions this vessel can be designed without any external pressure considering this given steam out condition ?

Hi ketan daraji

The pressure you quoted seems to be a standard used by refineries. The problem of partial vacuum occurs with the steam condensing thus reducing the partial pressure of the condensate. Obviously, the ideal would be for the vessel to be vented during the process, especially when the steam out ceases.

In the case of vessels have a large D/t ratio, extreme care has to be exercised to ensure the vessel will not collapse as the steam out process comes to an end.

I would say that a vacuum of 50 kPa should be allowed for under all conditions. It is better to be safe than sorry.