36 replies to this topic

[curious_cat]

 

Go back and re-read what Zauberberg said.  It is not the filling and emptying of tanks that causes problems.  It is the non-routine things like collapsed steam due to a rain storm (that was never planned) that destroys tanks.

 

What's a good way to size the flow for the steaming / rain storm scenario? What's the right way to estimate the flowrate needed.

[Zauberberg]

Estimating how fast the tank cools and its content (in this case - steam) condenses. Depending on size of the tank and intensity/temperature of rain, this can happen real fast - down to a few minutes.

 

Condensation of steam means reduction of pressure because volume is constant. The worst theoretical case would be to assume direct contact of rain with steam (maximum rainfall data is available in environmental datasheets) and there you are. Over-conservative but you sleep quiet at night. Who cares if the vent pipe is 4" or 8".

[fallah]

Katmar:

 

Why should the problems in tanks be limited to, say, collapsing due to steam condensation after rain storm during steam out operation?

 

If the vent line being undersized and not to be sized for maximum possible filling/emptying rates, once the filling/emptying pump due to,say, discharge failure is going to be operated at end of curve conditions; the tank roof will be thrown away or the tank itself will be collapsed...

[katmar]

I was not trying to put any limit on the design.  I was just trying to give an example of something that might not be immediately obvious that it should be included in the design considerations. I'm sure that you could list many more possibilities. In this particular design the pumping rates are 200 m3/h, but if the tank has little liquid in it and is full of steam a bad operation (eg spraying cold water into the tank) or just bad luck (the rain storm) could result in an air volume requirement of 1000 m3 in a minute.  This is vastly worse than the pumping situation.

 

A large part of safe design is being able to see problems before they happen.  Hope for the best, but plan for the worst!

[curious_cat]

 

 

I don't think Katmar was limiting himself to rainstorm scenarios. 

 

Finally it is a question of what is / will be / can be the governing scenario for sizing. I think Katmar's point (& Zaurberg's) is that very often for large storage tanks these other scenarios will turn out to be governing cases & not the simple filling / unloading

 

Please correct me Katmar if I am wrong or mischaracterising you position. 

[curious_cat]

 

@Zaurberg

 

Indeed. I did a back of the envelope estimate & I get on the order of 5 minutes for one case I assumed. I'm posting my assumptions.

 

Does this seem a reasonable estimate? Assumptions on the rainwater Temperature & HTC seem critical

 

[fallah]

Curious_cat:

 

 

I don't think Katmar was limiting himself to rainstorm scenarios. 

 

Finally it is a question of what is / will be / can be the governing scenario for sizing.

 

When someone intended to specify governing scenario of vent sizing for a storage tank, all credible scenarios shall be specified and known...

 

If you go back and read the OP query, you will see the only scenario based on which he requested to size the vent is tank emptying but even though the steam out operation can be a credible scenario to create partial vacuum in the tank, it wouldn't applied for all storage tanks and if is applied shall be specified in relevant data sheet by tank owner, the case never mentioned by OP and the discussions naturally  continued in tank emptying that, contrary to the steam out case, is a credible scenario in all storage tank for vent sizing...
 

[curious_cat]

@fallah:

 

Makes sense. You are right. 

[katmar]

Fallah, as will be clear from our previous interactions on various threads I have the greatest respect for your experience and advice but in this case I must differ from your recommendation and conclusions.  As you have pointed out, the OP spoke only of in and out breathing of air to compensate for liquid transfers.  And in my first post I advised that the pressure in the tank was the critical parameter and that the vent sizing should be based on that, together with the given transfer rates.

 

But then followed a clarifying post saying something along the lines of "similar tanks nearby have very much larger vents and I don't understand why".  That was when I started raising the questions of what other causes there could be for the need to vent air in or out.  We do not know the level of experience of the person who asked the original question and I do not believe it is wrong to ask "have you considered such-and-such?"  It was not my intention to say he MUST allow for steam collapse in his design.  Some tanks do not need that level of protection.  My intention was to give a gentle prod and ask "could there be other reasons for the larger vents you find on the other tanks?".

 

You and I have been around the block enough times to start recognizing the scenery, and I believe we can help others by getting them to ask the right questions.  It was long ago that I discovered that asking the right question in the right way was more than 50% of the journey to getting the right answer.  The perfect answer to the wrong question achieves nothing.

[fallah]

Katmar:

 

I did respect to your experience too and didn't insist on your agreement to my recommendation and conclusions in this case at all. OP never said that "similar tank has larger vent..." but he mentioned another tank has 16,000 m³ (i.e. with capacity of 32 times of the first tank) but with the same output rate as the first one.  For that tank with very much higher capacity mostly the higher filling rate causes having larger vent size than the first one; because those have adequate experiences in storage tanks design and operation know that a water tank rarely need to be subject to steam out operation, then this case automatically would remove from credible scenarios and shouldn't be referred in analysis of redible over pressure/vacuum scenarios...

 

Indeed, it's obvious that for such huge tank with 16,000 m³ capacity, even if steam out was a credible scenario a 8" vent size (per OP info) coudn't handle the subsequent vacuum due to a probable sudden raining...

[Shahzad Ahmad]

Katmar,

 

Thank you for your guidenese,my experience as a process engineer is not too much. Sorry for having asked and will be asking in future too much stupid/immature questions. But the point I want to clear is that, the purpose is to clear myself only not anything else..... 

 

The Continuous discussion of you people unearth more and more dimensions and concepts of the same topic. 

 

 

Regards and Respect,

[katmar]

SAK, there is no need to apologise for asking a question. We are all ignorant of certain aspects of engineering.  Many questions are asked here for which I have no idea of the answer. Indeed, that is the reason that I participate in these forums - I learn a lot more than I teach.

 

Sometimes an engineer has to be brave and expose his lack of knowledge by asking questions. But the wise engineer knows that it is better to momentarily appear ignorant than to remain ignorant for ever.  Good luck with your career - keep learning, but also be willing to help others where you have relevant knowledge and experience.