4 replies to this topic

[hollerg]

An L/D of 1.5 and 3-5 min residence time is usually good. May I ask why you are replacing the steam traps?

[GKKannan]

Dear Alquimist,

Hope this is your original name.

For sizing the condensate vessel, first you have to calculate the maximum volumetric flow required. This can be done by considering the water volume for the vessel. (Full water is the condition after condesation). From this volumetric flow, based on the residence time (around 3-5 min), you can calculate the vessel volume.

As it is already advised, you have to follow certain L/D rules for the vessel design.

Hope this will be of help to you. Please elaborate how you completed this project.

Regards.

GK. Kannan.

[Art Montemayor]

Alquimista:

Bienvenido a nuestro Foro.

I can easily relate to what you have described because I introduced the same type of condensate removal from a large thermosyphon reboiler on a Furfural Stripper back in 1970. I needed efficient steam condensate removal on a continuous basis, 350 days a year for a relatively large flow of condensate. I developed my system and applied it and it worked successfully for a very long time – well over 20 years I’m told. I know of this because I received an email from a friend on the Eng-Tips Forum who lives in South Africa. He told of receiving some P&IDs and calculations on a Furfural design that he was to check over and approve for possible installation in South Africa. The documents had my name on them and they showed a thermosyphon reboiler complete with a level control on a condensate pot instead of a conventional “steam trap”. My friend deduced that perhaps this was my product and since he never had seen this application before, decided to write me to find out. He was being told to copy the design since it had worked successfully for many years on the original plant site at Cedar Rapids, Iowa and he wanted to know as much as he could about it so that he could duplicate it in South Africa.

I am attaching a brief sketch of what is involved and how it is piped. I am sure you have the same – if not, identical – arrangement, but I want to make sure and be as clear as I can. This type of system is not new – nor is it that unique – unless you have been “raised and trained” on exclusively using steam traps. I consider this system the same as a steam trap – except that it is more adaptable to large systems and can be engineered to control the available surface area of a reboiler tube bundle and thereby give you turn-down capacity control. I would point out the following important points to bear in mind when applying this type of reboiler condensate control:

1. It is very important to make sure that the condensate receiver is properly vapor-balanced with the top of the reboiler tube bundle. This is to allow free and ample condensate gravity flow. Note that I said gravity flow. You must allow proper line sizes that minimize the pressure drop and allow for venting. The vapor balance line ensures that no non-condensables will collect at the top of the condensate receiver and stop the gravity flow of condensate. You must ensure that the steam chest around the tube bundle is purged of all non-condensables – as is required in all steam-heated exchangers. I usually make this line size 1” to 1-½” in size to give generous capacity and mechanical rigidity.

2. Make sure that you design your condensate control valve properly, taking into consideration the amount of flashing that is going to take place. Don’t forget that the result downstream will be a 2-phase flow and that it can be erosive/corrosive.

3. Locate your condensate receiver well below (as much as possible) the level of your botton tube sheet (if you are using a vertical thermosyphon reboiler) – or below the lowest formation point of condensate within your reboiler. Again, remember that this is gravity flow; you have nothing else helping you move this condensate out of the tube bundle and out to the receiver, so make your condensate outlet line as generous as you can justify.

4. Your tube bundle should have no condensate accumulation around it; this is why you should try to locate your condensate receiver as low as possible to drain all condensate the moment it is formed around the tube bundle. This gives a maximum efficiency to the reboiler and greatly facilitates the efficiency of condensate removal.

5. This type of condensate receiver is nothing more than a “wide spot in the line” – but it is an important wide spot. It needs to fulfill a variety of process necessities and you should spend time thinking about them. You should decide the size of the vessel required by considering the condensate inventory you need for operability, positive control, maintenance, and space requirements. Usually the residence time for the condensate is fixed by considering the amount of time you require to react to a low or high level alarm or reading on the level control. You really normally don’t require any more condensate inventory than that which is necessary to maintain a positive liquid seal and the amount that gives you time to react to a low or high level alarm read-out. Knowing this, you may come out with a vessel size that is small enough to justify a Stainless Steel construction – which would save you a lot of worry and maintenance costs due to future corrosion being arrested by the stainless construction. Don’t forget that you have now removed concentrated potential corrosive condensate from the reboiler into the receiver. In fact, you might be able to make it out of a piece of large pipe. The residence time and the L/D ratio is strictly up to your operating and process needs, as I’ve stated above. Usually, an L/D of 2.5 – 3 is what most engineers come up with.

Before going on any further, in the event that you don’t know about the excellent information available on the proper design and operation of steam and condensate systems, I would highly recommend you go to the following sites:

http://www.spiraxsarco.com/learn/
http://www.spiraxsar...arn/modules.asp

I want to make special mention that this application is not a case for a vapor-liquid separator design. This is not the unit operation taking place. All that is involved is some very practical common sense and good engineering judgment. An example of what is involved in vapor-separation is illustrated by a copy of my workbook on the same subject – which you can download at the following thread on this same Forum:

http://www.cheresour...?showtopic=2695
Natural Gas Separator – August 24, 2006

In your case, the Brown-Souders equation does not enter into question. All you are basically doing is extending the steam chest to an external point such that it preferentially collects and handles the condensate formed and relieves the reboiler itself from doing that chore. If you have a good, reliable level controller – like a Fisher Level-Trol, model 2400 – and a properly designed condensate level control valve, you should have no flow or corrosion problems.

I hope this helps you out in your application. Do not forget to protect the reboiler and receiver with an appropriate PSV and to prepare an MOC (Management of Change) report. Buen Provecho.

Attached Files

•  ReboilerCondensate_Removal.xls   30KB   261 downloads