14 replies to this topic

[trungphu]

Hello,
I'm designing sieve trays with hole diameter of 5 mm and tray diameter of 900 mm. Could someone give me some advice in choosing the thickness of trays in this case?
Regards
Trungphu

Edited by trungphu, 25 October 2012 - 10:30 AM.

[trungphu]

My trays are made from carbon steel.

Edited by trungphu, 25 October 2012 - 05:12 AM.

[Bobby Strain]

So, why are you designing a sieve tray when you can buy it from a reputable vendor? Valve tray is more practical, unless this service is an extractor. 5 mm is unusually small hole.

Bobby

[Art Montemayor]

Trungphu:

What you are doing as a chemical engineering student is not a normal exercise. ChE's do not get involved in mechanical design of special equipment. If you are studying in Vietnam and that is what is expected of you, the teaching institution must have previously trained you in mechanical design. If not, I don't know how to guide you or advise you.

Additionally, teaching institutions like universities know absolutely nothing about the arcane and empirical factors involved in the fabrication of such specialized equipment such as seive trays. What may appear to be a rather simple design is much more complex than what it appears to be. No engineer, whatever the discipline, attempts to design and fabricate seive trays for a real-life, profit-oriented, industrial application. We simply do not have the resources, research, and experience to warrant the ultimate results - and that is what really matters in all cases: you must warrant (or guarantee) the results. That is why we professional engineers always resort to experienced, capable, and recognized suppliers - such as Glitsch, Koch, et.al.

If you seriously are bound to go through with this presumed assignment, good luck.

[trungphu]

Thank you so much for your advices, Mr. Montemayor and Mr.Bobby

[katmar]

Trungphu, what Bobby and Art have told you is correct from the perspective of the USA and Europe, but not necessarily of the rest of the world. There is also a difference between what you need to do as a University exercise and what a practising engineer would do. I must say that if you are able to buy trays from a reputable vendor this will probably be the more reliable and maybe even the most economical method.

I am in South Africa. There are now a few vendors here who can supply (and guarantee trays) but it wasn't always so. When I started out designing columns in the early 1980's if you didn't do it yourself it didn't get done. We got very involved in the mechanical aspects - even to the point of sitting with tool makers to design the dies for pressing bubble caps and punching holes in sieve trays. I had to learn very quickly about welding and shrinkage to be able to design trays that stayed flat. We made some mistakes, but many of those columns are still working.

As I said, things are now different here. But last year I was in China and I found chemical engineers there working in a business supplying process technology as well as fabricating the equipment - and they were designing their own trays, right down to the valves. I felt quite nostalgic and it was fun chatting about the design details again. I have also seen equipment from India that seemed to have been made in the same way i.e. with trays not supplied by a tray specialist.

Enough history and travel stories - let me answer your question. For a column of 900 diameter you would have to have a very thick tray to stop it flexing and eventually cracking. Such a thick tray would be expensive and what everyone does is use plate of about 2 mm thick for the trays, and then support them on strong beams to stop them flexing and cracking. This way the tray sections are light enough to be carried by workers into the erect column and then bolted into place. And the reverse can be done if the trays need any maintenance. A bit of Googling will find pictures of tray assemblies and the support beams. Perry probably has pictures too.

Regarding the hole diameter - in a non-fouling environment 5 mm holes are fine. I have designed several columns with that size of hole. The smaller the hole, the more efficient is the mass transfer, but the more expensive the labor to make the trays. You should be aware that a small change in the diameter of the holes (especially if they are small to start with) can make a relatively big change to the open area. A bit of corrosion on the holes can quickly change the characteristics of your tray. If the trays are only 2mm thick it might be worthwhile using stainless steel, unless you are in a totally non-corrosive environment.

[trungphu]

Thank you so much for your sharing, Mr. Katmar!

I'm sorry for replying lately. I'm very busy with my exercise here. Actually, it is difficult to find a reputable vendor buying such a tray in my country , and it's also expensive for a student like me. So I have to design it myself, and I'm glad to get your advances.

I'm a fourth year student majoring in Chemical Engineering and very interested in designing distillation column. I have read some books regarding this major like Kister's distillation design, but my knowledge is so far not enough for designing in detail. It's my pleasure to talk to you about this topic.

I have a couple of inquiries related to distillation column that I wonder if you can help me. First, should my column fabricate in a lot of piece with shell flanges? Second, should I preassemble my trays on rods into a cartridge section for loading into the column?

Regards,

[Art Montemayor]

Trungphu:

You haven’t told the Forum why, and for what purpose a 4^th year student as yourself is taking on the design and fabrication of a distillation tower. Regardless of what country you find yourself in, this is quite a rigorous and formidable assignment for any young chemical engineer – much less a student such as you. I suspect you are being forced to fabricate a column because you have to run a distillation pilot operation as part of an assignment or ChE laboratory operation and your university does not have such a facility. Is this correct?

Whatever is the case, I admire your positive outlook, aggressiveness, and enthusiasm. Like Katmar, I also was forced early in my career to confront similar taxing and challenging projects. In 1960 I was assigned to work in relatively under-developed nations as a young graduate and had to confront design and fabrication problems there - together with a very poor capital investment situation. Fortunately, I had worked summers during college as a welder so I had an advantage. The unfortunate circumstances of not having an infrastructure to support me financially, technically, and qualitatively worked to my advantage as a developing engineer – as was certainly the same case with Katmar. What was a handicap resulted in valued experience and know-how.

Before you ask specific questions on how to fabricate a distillation column, you should furnish specific details, scope of work, and data. For example, whether you fabricate your column out of an existing cylindrical pipe with flanged ends depends on the availabilities and the diameters you are dealing with. I would never use a flanged joint on a column that is bigger in diameter than 12 inches. However, that is general advice. The local situation and needs have to take precedent. I also would never employ a cartridge type of tray bundle inside a column. I have participated in various projects where we had to replace this type of arrangement due to leaks and maintenance problems. But, again, the situation may call for a small pilot unit that is studying different types of trays within the same column – we don’t know because you haven’t given any specific data.

For your information I have designed, fabricated, installed, and operated such trays as bubble caps, tunnel caps, sieve trays, and slotted trays. We did these in our own plant shop or at a local sheet metal shop. So I personally know first-hand that it can be done successfully. You can also do the same. But I highly recommend that you do your internal hydraulics, vapor loadings and column sizing first.

[katmar]

Trungphu, your 2 questions to me have been well answered by Art, so all I can really do is confirm my agreement with Art's advice.

I have only once been involved with a project where the trays were inserted as a cartridge with seals against the inner wall of the column. It was a disaster and pretty much resulted in all the expenditure being wasted. This particular project involved a strong solvent and the seals around the edges of the trays could not last more than a few hours.

The question of body flanges is also related to local infrastructure. Large body flanges are expensive, and difficult to seal so they should be avoided where possible. But I have been involved in projects where there simply is no chance of getting a crane big enough to put the column in in one piece. We have used two approaches here. One is to flange the column into sections small enough for the available rigging equipment. The second is to weld the sections in situ (in the plant during erection). This does mean tying the crane up for a bit longer and a lot of careful preparation is required because it is difficult to line up the two sections well enough for welding while holding it with a crane. We have welded guide pieces to the lower section to positions the upper section while it is being welded. Welding saves the cost of a pair of body flanges, and is much less likely to leak.

If you do decide to use flanges do not try to save money by making them thin - you need a good rugged chunk of metal that can withstand serious bolting. My experience in the brandy industry, where the columns are not pressure vessels and fabricators try to get away with thin plate flanges, has been that the operators battle forever to get the flanges to seal - often resorting to injecting all sorts of epoxies and other sealants. Even if the column is not a pressure vessel use the flange dimensions given by the applicable PV code in your area.

[trungphu]

@Mr. Art

Thank you for your all your help. Yes, my university does not have a distillation column, and I am getting involved in this project simply because I am fascinated to do it.

Actually, I have calculated the internal hydraulics already, and I would like to show it to you: pressure drop per tray is 112 mmH20, maximum volumetric vapor flow-rate is 0.4655 m3/s and the internal diameter is 800 mm.
I will consider your advances carefully in my assignment.

@Mr. Katmar

Thank you kindly. Your experiences and sharing are very helpful for me who lack practical knowledge.

I can now understand the situation. However, I have some inquires remaining I wonder if both of you can answer me.

Should I joint the head to cylindrical shell by welding? In such circumstance, how do I open the column in order to fix some parts inside column in case of error in running. I think it is easier to open the column if you joint it by flange.
Should I make insulated shell for column before or after erection?
How do I assemble the stage to the cylindrical shell with diameter of 0.8 m?

I’m sorry for any inconvenient here.
Regards.

Edited by trungphu, 07 November 2012 - 01:47 AM.

[katmar]

Flanging the head to the shell is exactly like putting flanges in the shell section. I (and MANY others) have done it that way, but you can also weld the head to the shell and put a manhole (called a manway in some countries) in the head. The manhole would typically have a diameter of around 400 to 450 mm and the flanges would be cheaper. Also, if you need to get inside the column to inspect or repair trays it is a whole lot easier to lift a manhole cover than to removed the entire head - which would probably also entail removing piping. You should have a manhole in the base as well so that you can get a good circulation of air for when people go inside. As a student you may not be aware, but there are always very strict regulations about entering confined spaces like columns. This is potentially dangerous stuff, so get local advice from a safety professional.

In my experience it is better to install the insulation after erection to prevent damage during erection. Any lugs etc that are required to support the insulation can be welded on during workshop fabrication, but they can also get damaged. If it is a pressure vessel any welding onto the shell after erection will require inspection and testing.

It is difficult to give advice on how to support the column without knowing more details. Chemical engineering texts are generally very poor in giving this sort of mechanical information. If there is a mechanical engineering dept in your university rather approach them as this is something chemical engineers generally don't get involved with.

[trungphu]

Dear Mr. katmar,

Your know-how and experiences is very useful that I can not find in any book. I'm calculating the mechanical design of equipment, and it's rather difficult to me. So I will take your advance asking the mechanical engineer for detail.

Once again,thank you kindly.

Excuse me, I remain one question . In my case, if I fabricate the column of 0,9 m in diameter by welding and 2 manholes , how do I fix the error in the center of the column. I think we cannot reach any position inside the column just by 2 manholes at the top and the bottom.

Edited by trungphu, 14 November 2012 - 03:37 AM.

[trungphu]

Excuse me, I remain one question . In my case, if I fabricate the column of 0,9 m in diameter by welding and 2 manholes , how do I fix the error in the center of the column. I think we cannot reach any position inside the column just by 2 manholes at the top and the bottom

[katmar]

The tray sections are bolted to frames which are welded to the shell. Usually the center section of the tray is designed as a manway. It is a big job to get to the middle tray by unbolting a portion of every tray above it. If the trays need regular cleaning then you can install a smaller (150 to 300 mm) blank nozzle between every set of trays. Search for the Koch-Glitsch tray web page, or download their brochure, to see how trays are constructed.

[trungphu]

I really appreciate your help.