5 replies to this topic

[Dev 009]

Dear All, We have designed a LPG storage & transport system for one of our client, at present I am struck with requirement of spray cooling system for LPG sphere.

The spheres are not insulated, & hence i assume that during afternoon the temp. of vapor space may go as high as 60°C due to heat absorption from atm. while the same time the bulk liquid may remain same at normal temp. What we understand that we can cool the vapor space by spraying the LPG from transfer pumps at lower flow rates ( flow load is decided based on heat load in vaporised state).

I am furnishing the detailed design data could you please help me out if iam not wrong, & suggest me .

1. LPG Operating pressure of the Sphere : 11.4 barg
2. Initial temperature of vapor inside the Sphere : 50­°C
3. Volume of the Sphere : 3140 m³
4. Initial vapor space available : 15% of Total volume i.e 471 m³
5. Avg. Molecular weight of LPG : 47.91 kg/kmole (HYSYS)
6. Liquid desntiy at 50°C : 500 kg/m³ ( HYSYS)
7. Avg. Specific heat of LPG Vapor at 50°C : 143.5 kJ/kgmole°C
8. Avg. Specific heat of LPG Vapor at 60°C : 92.35 kJ/kgmole°C
9. Vapor pressure at 50°C : 10.87 barg
10 Vapor pressure at 60°C : 13.56 barg

Assuming a steady state I have calculated vapor load in sphere for a temp rise of 10°C ( from 50 to 60) & total amount of heat in the vapors at 60°C.

Amount of vapor formed by the increase in temp. comes around 2921.12 kg to cool the same I have used bulk LPG from same sphere assuming it to be at 30°C.

I have estimated a cooling flow rate requirement of around 25 m³ to sprayed in one hour.

We have detailed out some spray arrangement also attached here for your reference, I need to confirm about the spray arrangement & requirement for LPG cooling .

Whether cooling requirement may arise due to other reasons also apart from heat absorption please let me know.

Attached Files

•  SPHERE_1_Model__1_.pdf   211.33KB   246 downloads

[Qalander (Chem)]

Dear Just a wild thought;

Do you have abundant water source,or if possible sunshade covering arrangement [i][u]should be useful!

Regards
Qalander

[Art Montemayor]

Rohit:

What you have furnished is an outline of the tank’s fabrication. There are no heat transfer calculations included.

Your query raises some questions:

1) If you have designed your sphere for your location, why can’t the sphere withstand the normal, local ambient temperature’s vapor pressure? - even if it reaches 60 oC (which is an incredible 140 oF!). I don't believe human life is possible under this temperature.
2) You are only addressing the temperature of the vapor space – where the film coefficient is the worse value. It is the liquid LPG that generates the vapor pressure inside the tank – not the heatup of the vapor space. That is why I request your total calculations before I can make a detailed comment.
3) Have you or your company ever designed an LPG spraying system? If not, then I would recommend you refer to an experienced consultant or company for that design.
4) Have you considered a simple vapor re-compression/refrigeration system for this application? Why did you select the spray system instead?
5) If your local ambient temperature is really going to reach 60 oC, why didn’t you apply insulation to your tank? Did you make an optimization study on the tank regarding the effect of applying insulation?

Spray cooling is normally used in cooling down the vapors in a transfer operation – not usually for long-term storage. The reason for that is that the liquid spray operation is considerably more expensive to operate due to the inefficient energy consumption as opposed to vapor recompression and refrigeration.

I hope we can receive your detailed engineering calculations on this application.

[Dev 009]

First of sorry for delayed response as i got ill & was not there in office

Dear Art Thanks a lot for your reply, my reply to your queries are as mentioned below

1. The design pressure is based on vapour pressure at 60°C + static head (0.75
bar) + 10% or 2 bar whichever is more. This works out to 17.75 bar rounded off to 18
bar. The sphere is internally designed for 18 barg well above the vapor pressure of LPG ( 11.4 barg @50°C), the dry bulb temp at site is given as 50°C. What i feel is that during direct exposer to sun light the metal temp may becomes hotter than the actual ambient temp at that time. Hence selected a temp of 60°C for surface, though i know its very rare.

2. I have chosen only the vapor phase expansion due to heat up by exposed surface area, keeping liquid temp constant. You are correct that its liquid LPG that generates the vap pressure inside tank. Knowingly i did this which can give me a conservative figure.

3. Its true this is first time I am trying myself to do this terrific exercise.

4. Its clients requirement, already they told us to design at 18barg.

5. The temp will go to 50°C only during summers. Client doesn't want to apply insulation which he feels associated with various problems down the line, like corrosion at bottom due to moisture etc.

6. Its absolutely correct that LPG spray is basically designed for LPG transfer operation.

The pressure in the receiving sphere would be 11.4 bar.This is based on the assumption that liquid flow rate in to sphere is slow enough to allow the vapor space to be fully saturated and no pressure build-up takes place. The LPG flow from B.L. is 18m³/hr. Hence I feel cooling of vapor space is not required through spray.

The withdrawal rate is high 500 m³/hr to ship filling. This is I feel an adiabatic operation which will cool down the vapor space & also reduce the system pressure, know to push out remaining product we would have to increase the vapor pressure of the system which will be done by spraying the liquid LPG from the pump line.

Art what problem I am facing here is that I am not able to predict the temp drop in the system since this is a dynamic situation which will change by time as the sphere will emptying from initial 85% to 15% level. I have tried through ASPEN HYSIS Dynamics but still I am not on any conclusive answer.

What I wanted is to predict the flow of liquid LPG required to increase the vapor pressure of system to push the same.

I am sorry that at present stage I am not in a position to share the detailed calculation. They are still in crude stage. Spray ring details are attached here in line with DEP 30.06.10.12 Gen Nov 2003, which says that 8 to10 mm dia holes should be used for liquid spray distribution. I need to confirm whether 25m³/hr spray volume is large enough or I may need to cut down the flow rate as one of my colleague advised from site.


Regards
Rohit

Attached Files

•  IOT_BY_07004_44_110_REV.pdf   232.64KB   135 downloads

[Art Montemayor]

Rohit:

You haven't responded to all my queries. If your client demands a liquid spray system to cool down the tank, so be it. I recommend an experienced design be incorporated for such a large tank. If you or your company have never done this successfully before, then you are taking a large risk – especially since you have no detailed calculations to back up your design.

Your basis for heat transfer into the tank and the effect this has on the tanks vapor pressure is basically wrong. Even if the skin temperature of the tank is kept at 60 oC (which I don't believe), the heat transfer RATE through the vapor space is going to be very poor because of the terrible gas film coefficient (thanks to God). It is the liquid temperature at its surface that causes the internal vapor pressure and this liquid has to be heated. No one in his right mind is going to allow the tank wall to reach 60 oC AND MAINTAIN IT ALL DAY without first taking the precaution to paint the external wall with a REFLECTIVE PAINT. Everyone does this in Saudi Arabia, Qatar, Abu Dhabi, etc. Why don't you do the same? It is the AVERAGE wall temperature on the tank that will conduct heat into the liquid – and this is at an AVERAGE RATE and for a LIMITED time (mid-day probably being the time for the highest rate) – and the tank's heat pick up for the day should be based on these detailed, special, unsteady-state heat transfer calculations. These are not easy calculations and almost always are based on experienced, empirical data – not 100% theoretical. They depend on the location, the type and size of tank, the external surface, the climatic average conditions, etc., etc.. You have not addressed any of these factors.

Now you mention "ship filling" and wanting to predict the flow of liquid LPG required to increase the vapor pressure of the system – all of which I don't understand what you mean since we have no detailed scope of work, calculations, PFDs, P&IDs, or any description of the "system".

All we know is you want to cool an LPG sphere by liquid spray – which can be done – and you can't (or won't) show your engineering calculations for the heat transfer, pressure buildup, and fluid flow. Without any detailed engineering information I'm not in a position to comment on how a special liquid spray refrigeration system should be fabricated or installed. Sorry.

[djack77494]

Best suggestion is by Art to use a highly reflective paint. This is nearly ALWAYS done for bare vessels holding liquified gases. I do not subscribe to the approach of using a supermaximum ambient temperature for your maximum operating pressure calculation. As always, look at what others have done for built and operating facilities and use their approach. The sphere's contents will probably be much closer to the average daily temperature than to the maximum daily temperature. Also, I saw you mentioned that you included static head. What static head? There is none since your design pressure applies to the very top of your vessel. Seek help from someone who has done this before, preferably at your location or one similar.