Vessel Steamout Condition 1

[vsldsgnr]

I am a newbie with vessel design and I would like to be enlightened on the topic of "steamout".

A process data sheet for a vessel specifies a steamout condition of 1.5 barg/FV @ 150 deg. C. Is 1.5 barg the design internal pressure and full vacuum the design external pressure for steamout condition?

Or does this mean that 1.5 barg of steam is introduced into the vessel during steamout and that the vessel should be designed for full vacuum in case of an upset (i.e. steam condenses)?

What is happening inside the vessel during steamout? Is steam confined in the vessel until it condenses? Or does steam just flows in and out of the vessel?

Thanks,
Kris

 

[rzrbk]

During steam-out, steam will be introduced to the vessel and allowed to vent out the top, while condensate drains from the bottom.

Generally I have seen conditions for steam out given with saturation temperature of the class of steam being used. Since the vessel is steamed with vent(s) open, the pressure is usually either ignored, or a minimum pressure given to allow for some pressure-up but not affect vessel wall, as full steam pressure might. Main concern in my mind is for thermal growth, coatings, gasket selection, etc, that might be affected by the higher than normal temp.

Full vacuum is usually specified for steam-out vessels as you surmised, for the vacuum caused by condensing steam in the case that there are insufficient vents open or the vents are closed prematurely.

 

[reena1957]

Dear vslgsdnr,
Steam-out is a steam purging process to remove either hydrocarbons, prior to vessel opening for maintenance and inspection OR to remove air in the vessel prior to start-up. Steam free of condensate is introduced into the column/vessel at the bottom and allowed to flow through to the top vent into the atmosphere, sweeping out the air /hydrocarbons to atmosphere. Gradually the vessel gets heated up and the resulting condensate is drained at the bottom. Only after the vessel is pressurized (slightly above atmospheric pressure), the vent is opened as it may suck air into the vessel otherwise, which may lead to an explosive mixture in case the vessel holds hydrocarbons. Again at the end of steaming ( checked by hydrocarbons OR Oxygen in vented Steam), the vent is closed first in case of start-up and the vessel filled with an inert gas like Nitrogen to take the place of steam. In case of shut-down, the vent is left open and air is allowed to enter the vessel prior to opening of manholes. Also sometimes, the vent is closed for leak testing with pressurized steam at its saturation temperature.
Best wishes

 

[vsldsgnr]

Thanks to reena1957 & rzrbk for the replies.

 

[CJKruger]

I think reena1957 & rzrbk has covered it well.

The real difficulty for steam out is to decide whether the relief valve must be sized for blocked in conditions during steam out.

Different companies do it differently - some say this is not a case as it is cover by operating procedure. While others will calculate a steam flow (and maybe add a RO) and size the relief valve for it.

 

[reena1957]

Dear Kruger,
I have come across normally cases where the vessel design pressure and Vacuum conditions refer to the Max steam pressure and Full vacuum conditions. With this design conditions, I feel, one need not check the safety valve for blocked conditions. However, the Steam line should have a safety valve to limit the maximum steam pressure in the line as well as that admiited into the vessel.
Best wishes

 

[TD2K]

I would take your design conditions to mean the vessel can be operated up to 1.5 barg internal pressure at 150C OR with 0 internal pressure absolute (equivalent to a full vacuum) at 150C.

 

[CHEMANG]

All said and done as eloberated already, i have my doubts in the practice of venting the steamed out vapors into the atmosphere.

Don't you think it is unsafe to vent the mixture of hydrocarbons and steam to the atmosphere wherever you feel like.

I feel venting shall be to a safe location in the atmosphere.

 

[sshep]

CHEMANG,

If you are steaming a column and the condenser is adequate for steam out, then it is usually a simple matter to condense and collect the condensate without steaming to the air. In other cases it may be be possible to clear other pieces of equipment (feed heaters and lines, etc) into the process with steam.

Unfortunately to design a system to capture steam out vapors is not always practical (or safe). For enviromentally sensitive applications you can hire companies to use oxidizing chemicals or other methods. Each application requires its own consideration, but steaming out to the atmosphere will likely remain a workhorse for clearing equipment for most of your career.

best wishes,
sshep