7 replies to this topic

[nefertiti]

The questions I have below arised after reading an article posted by Mr Art Montemayor titled Design and Operations of Natural Gas Handling Facilities (uploaded on 13 Aug 2013, authored by Majid A Abdi).

The article suggested that for refrigeration systems operating under vacuum: flanges should be eliminated where possible and welded valves should be used.

 

Aside from preventing cryogenic leakage, are there any other reasons why such design is recommended?

 

Must we select top-entry/side-entry type of cryogenic valves for maintenance purposes since the valves are welded to the pipeline?

 

If the refrigeration system is not under vacuum, would such recommendations be valid?

 

If the refrigerant used are not hydrocarbon type (e.g. N2), should we still follow these recommended design considerations?

Additionally, I have an enquiry on LNG regasification process: I had came across a couple of proposed LNG vaporizers of shell & tube design using direct seawater as the heating medium (LNG on the tube side and SW on the shell side, with no intermediate fluid) which have tilted/slanted arrangement (refer to the attachment for illustration). Why is such arrangement employed? My guess is, this is done to aid vapor-liquid separation, but I am not too certain about it. 

Looking forward to hearing your thoughts and explanations. Thanks!

Attached Files

•  Doc3.pdf   49.37KB   73 downloads

[Art Montemayor]

Hida:

 

The author carefully explains that for mechanical refrigeration systems where a hydrocarbon is the refrigerant employed, care should be taken when subjecting the vaporizer and the suction of the compressor to a VACUUM condition in order to achieve the low temperature desired in the vaporizer.  This is explained in detail, stating that such a condition could allow for the seepage of atmospheric air into the vacuum system and cause a mixture of hydrocarbons and oxygen within the refrigerant system – which would be hazardous.  THAT is why the author gives the recommendations and not for preventing cryogenic leakage.

 

You don’t have to select top-entry type of cryogenic valves in an LNG facility, but it would be a wise move in my opinion.  The price difference in the valves when one considers the total investment of an LNG facility (in the Billions of $) is miniscule.

 

If the refrigeration vaporizer is under positive pressure and not under vacuum, the concerns shifts to one of leaks and spillage.  I would still opt for a totally welded system.  You will find that a cryogenic process is subjected to a lot stresses and forces during startup and shutdowns/warmups/defrosting operations.  Welded construction removes all those concerns.  Besides, how can you – as a knowledgeable engineer knowing full well that the fluid in question is SUPER clean and pure (due to the need of liquefaction) – justify the need for flanges or “break-out” points in the piping??  You certainly have no reasonable need to inspect the piping and equipment for fouling or plugging.  By the time the natural gas reaches the cold box it is probably one of the most cleaned-up and pure fluids in any process.  Otherwise, it would not be allowed to enter a multi-billion dollar process to be liquefied.  The majority of design principles employed in an LNG process are those of practical horse sense: the magnitude of the capital investment and the amount of continuous, steady operation required of the process makes it imperative that all the fluid entering the liquefaction (refrigeration) section be of the utmost cleanliness, purity, and steady process conditions.  LNG facilities are not designed with routine monthly (or yearly, sometimes) maintenance or “turn-arounds”.  By nature, all LNG projects are almost always undertaken under the concept of PROJECT FINANCING.  This means that all the billions of monies borrowed to build the installation have to be paid by the generation of profits from the operation – and this, of course, means that any stoppage (for whatever reason) cannot be tolerated.  All design is scrutinized to make sure that no stoppages, interruptions, or breakdowns occur during the life of an LNG facitility.

 

I have never seen LNG seawater vaporizers.  Where did you see these?  Or is the drawing just someone’s idea of a design?  I can see the usual problems of freeze ups and corrosion when using seawater as the heat sink, and perhaps gravity drainage is why the vaporizers are shown tilted or slanted.

 

[curious_cat]

I have never seen LNG seawater vaporizers.  Where did you see these?  Or is the drawing just someone’s idea of a design?  I can see the usual problems of freeze ups and corrosion when using seawater as the heat sink, and perhaps gravity drainage is why the vaporizers are shown tilted or slanted. 

 

I have no idea but on some googling I came across a few seawater units. Not sure if this is established tech. or  just a fancy idea.  @Art probably knows best. For sure, they look a unique design and not a S&T type. 

 

e.g. 

 

What is an ORV?
A vaporizer is a device used to re-gasify liquefied natural gas (LNG), which is cooled at super low temperatures, by heat exchange. The open rack type LNG vaporizer (ORV) uses seawater as its heat source. By running seawater over the aluminum tubes, the LNG inside the tubes converts from a liquid into a gas. Because this method is very cost-efficient to operate and consists of a simplistic structure, it is easy to operate and maintain. It is the safest and most reliable vaporizer. ORVs are normally used for a base load due to these features.

 

http://www.tokyo-gas...jp/lngtech/orv/

 

 

 

Edited by curious_cat, 15 November 2013 - 09:30 AM.

[Art Montemayor]

Curious:

 

What I stated is true.  I have never seen a seawater LNG vaporizer.  However, that doesn't mean that they don't exist - and possibly used a lot.  I do know that when we evaluated using them in the mid-70's we faced serious corrosion, fouling, and freezing problems and so opted for heated types - such as submerged combustion.  I think that some large seawater-based units are used in Japan, but don't know where - and how.

[curious_cat]

Thanks @Art. Interesting technology indeed. 

 

Are there any practical options to integrate this large capacity  source of very cold LNG and make it do any useful cooling? Or is it not worth the heat integration expense, safety hazards, complexity, capex etc..

 

Are there applications where the vaporizer is integrated into any sort of other application at the terminals?

Edited by curious_cat, 15 November 2013 - 12:12 PM.

[Art Montemayor]

Curious:

 

Yes.  When I was a Project Manager at El Paso LNG in 1977, we had engineering studies (by Fluor) looking into using the inherent cold sink for air separation plant applications and also as coolant for separating the heavier compounds from the methane, since they were worth much more in the raw materials market rather than as fuel.  Whether any of these "add-ons" have been implemented in world-scale LNG reception terminals - such as those in Japan, Europe, and elsewhere is something I have not kept up with.

[curious_cat]

Whether any of these "add-ons" have been implemented in world-scale LNG reception terminals - such as those in Japan, Europe, and elsewhere is something I have not kept up with.

 

Thanks again @Art. 

 

I will investigate. Not out of a project need but pure professional curiosity. 

 

If the idea was even mildly feasible in the 70's it ought to be far more attractive now with increased energy costs, larger scale,etc. 

[thorium90]

I happen to know a plant that uses seawater... There are quite many others too now.

 

http://www.slng.com....ct-updates.html

 

http://www.google.co...6,d.bmk&cad=rja

Edited by thorium90, 16 November 2013 - 12:29 AM.