Tank inbreathing due to rapid condensation 2

[AdamEng]

I was recently put in charge of analyzing a bad situation. A client has a tank of process water, when it gets low they add more water from other locations. They have been experiencing tank buckling during some of these top offs. The major design flaws in my mind are this, too small of a vent and the much cooler makeup water enters at the top of the tank, passing through the air/vapor before joining the rest of the water.

I know I have to assume that the gas in the tank is saturated air at the temp of the water below it, and I know it will cool to near the temperature of the makeup water rather quickly.

I have looked through API 620, 520, and 2000 and also tried to find some calculation for the heat transfer from the air to the falling water, but none have those have led me any closer to an analysis.

I would appreciate any help you can give on this, thanks.

 

[Chance17]

The API 2000 Thermal In-breathing is based on 100-F per hour cooling.
In my memory banks, this was the estimated cooling during a heavy rain storm.
I know there are differences between hydrocabon and water.
But the charts should be more than adequate and apply for your situation.

As a 'war story", I am aware of a situation that once was similar to yours.
It was discovered the vent actually was not a vent - it was an induction pipe.

 

[CMA010]

API STD 2000 is based on vapour contraction, not condensation. Condensation is a whole different story and the values from API STD 2000 no longer apply, see the comments on venting during steam out.

 

[gerhardl]

A producer or supplier of air/vacuum breather valves for tanks could guide you, several reliable on the market.

Most tank inward bucklings occur typically by cleaning tanks with steam or hotwater against cool walls, causing steam pressure to collapse when steam condensates.

The same is happening in your tank: cooler air can contain less humidity, and air against cold walls causes part of the water content to condense, and pressure lowers faster than can be compensated through openings.

First step is to find max pressure difference, outside higest, the tank can bear. For thin walled SS tanks this could be 0.1 to 0,05 bar or even less in some cases, For normal SS large water mains (this is relatively solid constructions) 0,2 bars is often used. This would normally be too high for a typical tank.

A rough approximation could be to guesstimate a worstcase situation: how much volume is lowered to a given pressure (several tries) and compare this with the amount of air rushing through a given diameter at the given max pressure difference or half max pressure.

To get an idea you could also guess at a 'normal gas speed' through the valve, and indirectly say that this is what you expect and normally allow (say 20 to 60 m/s) to guess at the volume sucked in through the valve (forget for the instance the finer physical formulae, this is a very rough approximation)

This will give you an idea of necessary size, but by all means let a qualified expert supplier advice you.

 

[BenThayer]

consider filling the tank through an internal "stand-pipe" so you do not get any appreciable contact between the cold in-rush and the tank vapor space.

you can either add a "break" in the line or an anti-syphon hole if needed.