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Process Vessel--Values Don't Make Sense

process vessel sizing

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#1 Bal

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Posted 10 November 2015 - 12:09 AM

Hello,

 

I have calculated the size of a vessel. The diameter is 5.6 m and the height is 21 m and that's a horizontal vessel.  

The numbers are disturbing. what do you think and what should I do? 

 

I have sized the vessel based on the procedure given by Wankat in his book (Separation Process Engineering)

 

 

Is there any heuristics for the maximum height of a vessel?  or the maximum diameter? I found heuristic on the height to diameter ratio.

 

 

Waiting for you prompt response :)



#2 fallah

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Posted 10 November 2015 - 01:33 AM

 

I have calculated the size of a vessel. The diameter is 5.6 m and the height is 21 m and that's a horizontal vessel.  

The numbers are disturbing. what do you think and what should I do? 

 

I have sized the vessel based on the procedure given by Wankat in his book (Separation Process Engineering)

 

 

Is there any heuristics for the maximum height of a vessel?  or the maximum diameter? I found heuristic on the height to diameter ratio.

 

 

Bal,

 

For horizontal vessel diameter and length are to be specified and height makes no sense with such vessels...

 

L/D equal to around four is reasonable for horizontal vessels; then that of yours can be acceptable...



#3 Art Montemayor

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Posted 10 November 2015 - 04:13 PM

Bal:

 

You are the only one that knows what the vessel is designed to accomplish.  Fallah - nor the rest of our members - know that, so we can't explain why you are concerned by the dimensions you have calculated.  You haven't submitted your calculations, so we can't even begin to guess if you made a mistake or error in your calculations.

 

As Fallah states, the height should not play a role in the design of the vessel itself.  However, the application can require an elevation (or "height") of the vessel's installation - but we don't know that.  What is the vessel supposed to accomplish, how, and where?

 

Give us the total details of this query: scope, basic data (liquids, gases, pressure, temperature) and the unit operation or unit process this vessel is designed for, together with the detailed calculations for its size.  I am sure our members will give you their best comments and ideas on the topic.



#4 MTumack

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Posted 10 November 2015 - 05:33 PM

I concur with Fallah, the number itself doesn't seem out to lunch. Depends what your process conditions and requirements are.

 

Depending on the specific application though, you might be better off with a smaller diameter and a longer can; I say this because your thickness will increase with respect to inner diameter: At 1440 psig design you would have an estimated shell thickness of 8.375 inches @ a 1/16" Corrosion Allowance (assuming carbon steel)! Welding at these sizes is either

1) Ridiculous with standard ASME Section IX 30 degree weld tapers (you're talking like 32 square inches of Weld! That is like 21 sub-arc passes!) or

 

 2) fancy / proprietary and therefor very very expensive due to Technology monopolies in that market. 

 

At the same specs w/ a 120" ID shell in lieu of proposed 220" ID gives you an estimated thickness of only 4 inches, which is only about a third of the welding required for the 8 inch. Your amount of actual steel is likely not that different either in this scenario (ie your Heuristics are not accurate at these sizes), maybe double but your circ seam welding is likely much much easier and more manageable, even if you are quadrupling your amount of circ seams. God forbid your fabricator has to do a weld repair on a circ seam like that, what a huge nightmare... you know your price will include a few weld repairs if the shop is reputable because welders and even sub arcs make mistakes. It happens. At 4 inches, you have half the passes required to weld, at a third of the mean weld area to be filled. You are welding 6 times less per Circ seam! That means you are welding 4/6=75% as much as with the other option. And at this larger size you would likely be looking at introducing 2 long seams, as many plates wouldn't be able to get that circumference from standard sizes... Now you have 12 times the long seam weld per unit length of shell, with maybe 1/4 of the amount of total shell length, again 3x more welding on the large size. That labour will kill you.

 

Very few people can build vessels 6 meters in Diameter; you require billions of dollars worth of equipment to properly roll, maneuver, weld, heat treat, paint... etc etc etc. You bet you'll be paying for the maintenance on this specialty equipment. On the other hand, most medium to large well known fabricators could easily fabricate a 120" ID unit.

 

So, I suppose there are a few things to consider. If it was me, I would give the vessel to all vendors you are interested in having fabricate it, and have them size it. They will provide you will the cheapest option based on their equipment and your design specifications. It could be the 4:1 ratio, but I'd guess at this size it isn't, to be honest with you. I suppose your site constraints might govern as well.

 

All of these sizes are guesstimated.

 

EDIT: I see you are a student, my apologies. Anyway, a few logistical things to consider, from a Mechie that Builds these things for a living. But you are right, that L/D Ratio is pretty common and a good place to start at, anyway. There are a hundred things that could swing the economics a bunch of different directions.

 

Take a gander at my quick sketch for what I mean re Weld sizes on these seams.

Attached Files


Edited by MTumack, 10 November 2015 - 05:53 PM.


#5 cylai

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Posted 10 November 2015 - 08:53 PM

Hi Bal,

 

It is very important for you to tell us what the vessel is for. Separation? (should be since you referred to Separation Process Engineering document), storage? If it's for separation, what are you separating? Oil and water? traces of oil?

 

More importantly, the water, oil and gas flow and the operating conditions. At high pressure, gas volume can be compressed and the vessel size can go alot smaller. At low operating temperature, viscosity increases, and separation becomes harder. If there are minimal oil, bucket separators can be used. If minimal water, perhaps a double barrel and so on. Point is, there are some minimal information that we need to look at, instead of leading you the unoptimized way.

 

If you could furnish more information, we can give you a better explanation and technical reasons. Too large vessels are not transportable (by road, and the fabrication yard will need to be right next to the port etc), difficult to fabricate, not economical, tough during inspection and so on...

 

Hope to hear from you soon.

 

Lai



#6 breizh

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Posted 10 November 2015 - 11:20 PM

Bal,

Consider the document attached ( end ) : Heuristics in CHE .

 

As others said , not enough info from you to get  valuable answers . 

 

Good luck ,

 

Breizh

Attached Files






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