15 replies to this topic

[Sridhar P]

Dear Sir,

What is the preferred L/D ratio for atmospheric tanks.

Say for atmospheric water storage tanks- Length greater than Diameter (or) Diameter greater than Length is preferred?

Does L/D ratio also varies with increase in tank capacities?

AWWA D100 or API-650 gives option for both ratios. Which ratio is better.

Thanks and Regards,
P.Sridhar
Sr. Design Engineer

[ankur2061]

Dear Sir,

What is the preferred L/D ratio for atmospheric tanks.

Say for atmospheric water storage tanks- Length greater than Diameter (or) Diameter greater than Length is preferred?

Does L/D ratio also varies with increase in tank capacities?

AWWA D100 or API-650 gives option for both ratios. Which ratio is better.

Thanks and Regards,
P.Sridhar
Sr. Design Engineer

Sridhar,

Where I did not have to face layout constraints and where easy accessibility of tank appurtenances were required for very large volume atmospheric storage tanks (>5000 m3), I have used a L/D ratio of even 0.6 for storage of water or even petroleum products. I think you need to evaluate all these aspects such as layout, maintenance accessibility & the economics of a large site fabricated atmospheric storage tank to arrive at the correct L/D ratio. In my practical experience I haven't seen large atmospheric storage tanks with L/D ratios greater than 1.2-1.3. Many engineering companies generally go by a rule of thumb of L/D of 0.8-1.0 for large atmospheric storage tanks. But I would like to emphasize that, do your engineering and economic evaluation before deciding on the L/D ratio.

Hope, this helps.

Regards,
Ankur.

[djack77494]

I don't believe that L/D ratio is a valid design parameter for atmospheric storage tanks. While I do consider L/D and I do have preferred ranges for pressure vessels (drums), they do not have a role in AST's. Practical considerations will lead the tank fabricator to want to fully utilize standard (8' wide) steel plates for most AST's. Other factors will limit the height to 40 or perhaps 48 feet. Diameter is: as needed up to practical limits. Role of L/D ratio: None. Just my humble opinion.

[ankur2061]

I don't believe that L/D ratio is a valid design parameter for atmospheric storage tanks. While I do consider L/D and I do have preferred ranges for pressure vessels (drums), they do not have a role in AST's. Practical considerations will lead the tank fabricator to want to fully utilize standard (8' wide) steel plates for most AST's. Other factors will limit the height to 40 or perhaps 48 feet. Diameter is: as needed up to practical limits. Role of L/D ratio: None. Just my humble opinion.

Doug,

As a process engineer when I need to fill the tank process data sheet, I need to specify the dimensions of the tank. As far as I know, hardly any process engineer knows or wants to know what plate width a tank fabricator is going to use. All the process engineer wants to do is fill the height or length & diameter of the tank in the process data sheet besides the other important data such as operating/design pressure/temperature, stored fluid properties (sp. gravity, vapor pressure etc.) and nozzle sizes and locations. Essentially it means that he or she has to have a fair idea of L/D ratios even for atmospheric storage tanks. That is where my reply to the original post was trying to help by giving some idea about L/D ratios. I am sure you will agree that for a large atmospheric storage tank an L/D ratio of 2 or greater doesn't seem right whereas it may be perfectly applicable to a pressure vessel.

I would love to know any further thoughts on this by you.

Regards,
Ankur.

[Rama]

Dear Ankur, Doug,

I think both of you are right from your respective points of view.

To repeat, the following points do figure in the reckoning when designing a tank.
1. Location and space constraints. 2. Availability of plates and their sizes. 3. Cost. (Apart from safety and other considerations.)

It is easy to check by a small excel calculation or by a simple differential equation that the best economy in the case of a tank where the plates are of the same thickness (both the cylindrical portion as well as the circular top & bottoms) is when the area of plate required is minimum; this happens when the L/D ratio is 1. If the top is conical the L/D increases slightly to 1.1 or 1.2.
If we factor in the change in the thickness of the tank when the diameter increases, or when the height increases (at least the bottom courses), there will be a further change in the most economical L/D ratio.
So for an initial estimate L/D can be taken as anything between 1.0 & 1.3, whereas if the tank is large it might be prudent to check a few combinations of L/D in the above range to get the best economy of design, if space is not a constraint.

Edited by Rama, 28 September 2009 - 01:32 PM.

[Sridhar P]

Is that tanks with L/D ratio less than 1 (say 0.8) is stronger than tanks with L/D > 1 (Say 1.2) for atmospheric tanks with same volume and service conditions.

Regards,
P.Sridhar

[fallah]

Is that tanks with L/D ratio less than 1 (say 0.8) is stronger than tanks with L/D > 1 (Say 1.2) for atmospheric tanks with same volume and service conditions.

Regards,
P.Sridhar

Yes,provided that adequate area would be available.

[Rama]

Dear Sridhar,

The formula for stress shows that the stress is directly proportional to the radius and the pressure – in this case the hydraulic head. But it also means that as the diameter increases, the head decreases inversely proportional to the square of the radius. (For the same volume, as the radius doubles, the height would become a quarter) So apparently there is an advantage in reducing the head. At the same time, as I said earlier, the area of the plates required and hence the cost will also go up, may be marginally. So is the trade off in terms of thickness worth it? That depends on individual cases. If we have a smaller tank (in Height) would we not go for smaller thickness, thus negating the advantage of “stronger tanks”?

In any case 'stronger' has no meaning as any properly designed and maintained tank lasts its full life time. The additional 'strength' is not going to give it any 'special' advantage!

[djack77494]

I would love to know any further thoughts on this by you.
Regards,
Ankur.

Ankur,
Thank you for your astute comments. While I find many of your comments to be useful, I cannot totally agree with your comment that you must fill out the data sheet. That is if, by that, you mean all information on the datasheet. Most types of datasheets contain fields that should be completed by the process engineer and other fields that are not within the realm of process engineering and are best left for experts in other fields. Concerning a storage tank, the process engineer is familiar with the process conditions and properties of the fluid. He/she would also be aware of nozzle requirements for inflows, outflows, measurements, etc., though often the specialist - for example an instrument engineer - would be needed to confirm the final sizing required.

Now, tell me why a process engineer would be particularly interested in or knowledgable about the best combination of height and diameter to use. The realm of process engineering include specifying the capacity of the tank, but the manner in which that capacity is achieved is best left to the specialist. There are various "rules of thumb" that can be good guidelines in restricted circumstances, but those generally serve only as an initial estimate. I am aware of large AST's with diameters nearly 300 feet or 100 meters. Their height is still 40 to 48 feet or 12 to 15 meters resulting in very small L/D ratios. Does anyone care? Of course not. The dimensions chosen made sense since tall tanks would need very thick walls more to support "a structure" than to contain the pressure. On the other hand, talk about an alloy vessel designed for very high pressure and the circumstances are totally different - an L/D of 5 might be appropriate. I tend to specify capacity and not dimensions for storage tanks because that is what I am interested in. I urge others not to wander into areas outside their region of expertise, but if you feel compelled to do so (a common malady), then indicate the entries are estimates, not requirements.

I hope I have satisfied your yearnings.
Doug

Edited by djack77494, 30 September 2009 - 02:02 PM.

[ankur2061]

I urge others not to wander into areas outside their region of expertise, but if you feel compelled to do so (a common malady), then indicate the entries are estimates, not requirements.

I hope I have satisfied your yearnings.
Doug

Doug,

I 100% agree with you that an engineer should work within the bounds of his expertise and knowledge. But unfortunately in the part of the world where I live, you can only be counted if you are a 'know-it-all' even if that is half-baked. People in this part of the world consider the phrase "I don't know" as heresy. This is very unfortunate but this is what the fact is. I have worked with a lot of counterparts from the west & I have many a times heard them say that they don't know the answer and I should better ask an expert or specialist. While initially it sounded brutally rude & intimidating (because of my eastern background) on deeper contemplation it was very honest & that is how it should be in real life.

Well on the lighter side, I would prefer to be honest like that, but only when in my part of the world there will be social security payouts.

Regards,
Ankur.

[katmar]

I suppose that we are wandering away from the original topic, but I find these last two posts very interesting. While I agree with Doug that we should not step over other engineers' areas of expertise, it is a fact that our designs are iterative and the further we can push each iteration towards the final solution the fewer iterations we need to go through. For example, the L/D ratio of the tank can affect the static head available, or to be overcome, and this is sometimes an input into the process engineer's work. If we have to do a rough hydraulic calculation, then wait for the mechanical specialist to tell us the actual height of the tank, and then come back and redo the hydraulics it all takes time and costs money. I know that I am guilty of poking my nose into mechanical and instrumentation areas where it does not belong, but when working in small teams it is sometimes necessary. I think the important thing to remember is that we should not specify things we are not knowledgeable about and experienced with.

Wow Ankur - thanks for posting this. I have worked quite a bit with people from the sub-continent and was aware that they never said "I don't know". But I was not aware that they regarded me as rude when I admitted that "I don't know" (which happens all too often!!). I don't know (there I go again!) how we get around this communication wrinkle. Any suggestions?

[ankur2061]

Wow Ankur - thanks for posting this. I have worked quite a bit with people from the sub-continent and was aware that they never said "I don't know". But I was not aware that they regarded me as rude when I admitted that "I don't know" (which happens all too often!!). I don't know (there I go again!) how we get around this communication wrinkle. Any suggestions?

Harvey,

As a child in my house I was taught not to say no & totally forbidden to say "I cannot help you". It was considered as being rude and callous. So, when I was a newcomer to the industry as a young man in his early twenties and when I had my first opportunity to deal with people from the west who were frank enought to say 'I don't know' & 'I cannot help you', it was a big culture shock for me. However, over the period of time I could understand the underlying sentiment of basic honesty and the fact that you need not be a know-it-all not only to survive but also thrive.

It has been a long time since those initial days and today I have no hesitation in admitting that it is absolutely OK not to know something but it is totally unscruplous on anybody's part to pretend to know something and misguide somebody who is asking for help. All the more so if you are an engineer and where safety and livelihood of people are at stake.

I suppose at a very young age you are too presumptious about life & people.

Regards,
Ankur.

Edited by ankur2061, 01 October 2009 - 04:19 AM.

[djack77494]

Well I wasn't expecting to revisit this thread, but now feel compelled to do so. I must admit that if you're not in a large engineering office filled with people who really know the answers to many of the messy details in the engineering and design business, then you will be expected to fill in for "the experts". I've been in that situation often enough and know that "I don't know" is often not an acceptable answer. Since I'm not a proponent of professional suicide, and especially if I don't foresee any serious consequences to providing information that I'm not totally confident of, then I'd cross over the boundary lines I've just advocated avoiding.

So now if we're talking about the dimensions of an atmospheric storage tank and we have only a requirement to provide x hours/days/weeks of storage, we can go ahead and calculate a needed capacity. Let's be honest about this - we really don't care about the physical dimensions of the tank. But someone will want us to provide estimated dimensions for the tank, and also we'll probably need to do some pump calcs and other typical process work that will require some assumptions about the tank. Under these conditions, I do propose that we estimate tank dimensions as required to advance the project. I'd contend that we can be reasonably secure in the knowledge that many other engineers, designers, and fabricators (i.e. experts) will review my estimated dimensions at some point before the tank is fabricated. If I've done something less than smart, I would hope that one of these reviewers would be able to say that the dimensions should be modified (for good reason). At that point, I'd readily concede their point and agree to the more reasonable dimensions. Please understand that I'm not being cagey about this issue - after all, what's the worse that could happen if I choose less than ideal dimensions for my tank. Just don't feel obligated to defend those dimensions in the face of good reasons to back down.

[MI_Sidd]

Hi
this thread has been a real help. thanks everybody.


This is my first post & here is the question

what is the maximum tank height which can be considered?
I have read a few papers where it says it is to be limited to 40 feet or 15 meters.
what is the reason behind this?
Are there any cases where we cross this limit & what are the implications for that

Regards,

[ankur2061]

Hi
this thread has been a real help. thanks everybody.


This is my first post & here is the question

what is the maximum tank height which can be considered?
I have read a few papers where it says it is to be limited to 40 feet or 15 meters.
what is the reason behind this?
Are there any cases where we cross this limit & what are the implications for that

Regards,

Sidd,

I have an excel file now with a table pasted from a magazine article (don't know the source) for vertical cylindrical tanks which does not agrre with your premise of restricting the height to 15 meters.

It is attached. Have a look at it.

The only reason I can think of restricting the height is unavailbility or difficult availability of steel plates of bigger dimensions.

Regards,
Ankur.

Regards,
Ankur

Attached Files

•  Std_Vertical_tank_dmns_vs_volume.xls   94KB   1148 downloads

[MI_Sidd]

Sidd,

I have an excel file now with a table pasted from a magazine article (don't know the source) for vertical cylindrical tanks which does not agrre with your premise of restricting the height to 15 meters.

It is attached. Have a look at it.

The only reason I can think of restricting the height is unavailbility or difficult availability of steel plates of bigger dimensions.

Regards,
Ankur.

Regards,
Ankur

Ankur,

This is an extract from paper by siddharth mukherjee "Understanding Atmospheric Storage Tanks" in chemical engineering april 2006

"As the tank height increases, wall thickness plays a more important role. Higher tanks also put a greater load on the soil. If the pressure becomes more than the soil-allowable bearing pressure, pile-supported foundations become necessary and are expensive. This concern is particularly applicable for poor soils. In general, tanks that are higher than 15 m are not commonly used in industry."

Also , one of the recent bid projects which i was working on. I observed that the tank height was limited to 48 ft. and also the height was a factor of 8 ft as already pointed out by djack77494