8 replies to this topic

[MattyParker15]

Hi,
I am investigating the merits of steam purging vs nitrogen purging as a method of creating an inert atmosphere in a vessel. Has anyone had any experience with this practice and is it considered an acceptable method?

Cheers

Matthew

[Art Montemayor]

Matthew:

I never heard or read of steam purging a vessel - as compared with purging using nitrogen or another inert gas. Steam is a condensable vapor; nitrogen is not. While it is usual to purge or snuff out a combustion system - like a flare - with steam, it is done only because the the system is in a high temperature (+220 oF) environment and even if the steam condenses, it doesn't do any harm or create a hazard - like creating a vacuum or pools of water.

I certainly would not consider steam purging comparable to purging nitrogen. First and foremost, the vessel in question must be either designed to withstand a Full Vacuum or the venting and vacuum breaking system must be well designed and operable. Additionally, you must be prepared to drain, clean, and dry out the vessel.

Where have you obtained the idea that steam purging is comparable to nitrogen purging? - and under what considerations and circumstances?

[proinwv]

I have never heard of this, but then that doesn't make it incorrect.

What comes to mind is first, the danger of tank collapse if it is not properly vented to account for any cooling and the resulting very low pressures that can occur. (I have seen collapsed tanks!)

Second, as for any gas/vapor, I assume that the steam is completely compatible with the stored product, and any resulting liquid from it would not be detrimental.

Last, the elevated temperature must be compatible with the tank as well as other materials.
(You don't mention steam quality or pressure.)

Hopefully some of our other members will jump in and add to my simplistic comments. I too would be interested in the comments.

[MattyParker15]

The vessels we have are pressure rated and have appropriate venting to handle the pressure changes. An engineer who I used to work with recommended the practice in the absence of decent nitrogen purging system. I thought it may also have something also to do with decreasing the chances of a static discharge when filling the vessel with the flammable liquid, as the introduction of steam would have created an extremely humid atmosphere. What do you think?

[proinwv]

The static discharge is initiated by a potential difference, usually caused, in a flowing fluid. when it passes thru filters and the like. The less conductive, the more the charge buildup. At least that is my poor memory of the issue.

You say a humid atmosphere. Then the steam quality is reducing as you would not realize that condition in a sat or superheat situation. It seems that if the atmosphere is humid, it might be very conductive and enable a discharge. Or would it continually bleed off smaller charges?

Seems unstable to me. It seems to be a "convenient substitute" for a better product. If you truly inert the atmosphere, a discharge cannot ignite a O2 starved atmosphere.

I am curious to see how Art responds to this.

[Qalander (Chem)]

Dear Matthew Hello/Good Morning;
I will only add my practical experience at my previous employer that
during steam-out of a Flare KOD preparing for some emergent maintenance works;
heterogeneous fluid conditions got generated (bare vessel) and
while checking status from a bleeder valve
clear cut existence of static discharge/slight ignition of some foreign material witnessed
(Although Thanks God, nothing unfortunate took place) as alert team members immediately extinguished the very minor ignition through steam lance.
Hope this gives supportive evidence to what said by Paul
Regards
Qalander

[MattyParker15]

So, what we are saying here is that steam purging is essentially ineffective and could potentially be creating a hazard. Would the water vapour in the vessel prevent or enhance an explosion, say if a non conductive liquid such as toluene were charged and a static discharge occurred inside the reactor?

[KZLow]

I've heard of steam purging for palm oil refinery plant at lower temperature. (if i'm not mistaken, they used it while the oil is still under 100 deg Celcius, and use nitrogen purging for oil above that.)

I believed it will be cheaper to use steam than nitrogen as an advantage.

KZ LOW
Project Engineer

[eman3001]

Steam purging is sometimes used as a means of "vacuum" purging since you can condense the steam( by cooling) and produce a vacuum with the vacuum pump. The vacuum give less resistence to diffusion than the inert nitrogen and hence may take less time.