5 replies to this topic

[ELEMAN]

Hello everybody:

In order to obtain some sedimentation and/or settling of solids and water contained in heavy fuel oil (Bunker C or distillate #6), we are planning to heat this fuel oil in their above ground uninsulated storage tanks. The heating fluid available is saturated steam that would be feeded to an immersed steam coil in the tanks.

We plan to fill up these tanks and then, supply the heat.

The tanks are uninsulated, with fixed and vented roof.

Tanks diameter: 16,8 m
Tanks height: 9,1 m
Maximum level of fuel in the tanks: 8 m
Average tank plate thickness: 0,0079 m
Ambient temperature: 35º C
The wind in the tank farm area is still
Initial temperature of fuel in the tanks: 40º C
Final temperature of fuel in the tanks: 65º C
Initial volume of fuel in the tank: 130 m³ at T = 65º C
Final volume of fuel in the tank: 1800 m³ at T ~ 40º C
Mass flow rate of filling up: 210 m³/h at T = 40º C
Time for filling up: ~ 8 hours
Density of the fuel oil: 998 kg/m³ @ 15º C
Kinematics viscosity of the fuel: 635 cSt @ 50º C
Specific heat of the fuel: 2,1 kJ/kg ºK
Heating time: 24 hours
Steam pressure before regulating valve: 6,5 barg @ 168º C
Steam pressure after regulating valve: 4,35 bara @ 147º C

After the filling up is achieved, the fuel remains at rest (no fuel in and no fuel out of the tank) for 50 hours. At the end of this settling time, the tank starts to be emptied.

From my calculations I have this:

Heat required: 1.723 kW
Latent heat of vaporization: 2126 kJ/kg
Steam mass flow rate: 2918 kg/h
Heat transfer area: 261 m²
Steam coil diameter: 162 mm (more than 6")
Steam coil length: 495 m

For a heating time of 36 hours, the coil diameter is 128 mm and length of 371 m.

The maximum settling time of 50 hours obliges to heat the fuel oil to 65º C in a very short time.

I am surprised with those dimensions of the steam coils.

Please submit your comments regarding to this subject. Thanks in advance.

[Art Montemayor]

Eleman:

From what you have described, I believe that you are proposing to design a batch heat-up operation within a limited amount of time - and do this within a large, un-insulated storage tank.

I believe the benefits you are looking for are to be obtained by what you are trying to do. However, you need to heat up the fuel oil and then give it as much residence time withing the tank that you can possible give it - while it is hot and in a calm and un-disturbed state. This will yield the maximum amount of separation by precipitation of the heavy impurities in your fuel oil. This is an excellent fuel oil pre-treatment that will surely yield profitable results downstream at your burners and fuel oil handling facilities.

However, I would not recommend a batch heat-up system. It will require an inordinate (large) amount of heat transfer area - especially if you have no positive and effective agitator within the tank to create the necessary convection currents that are needed to yield a decent heat transfer coefficient. I am afraid that you will be at the mercy of whatever natural currents you can create - and they will be very inefficient. Additionally, you will be having an internal coil mounted probably on the floor of the storage tank. This necessity to mount the heavy coil will mean that it will be subject to becoming fouled with the heavy sediments precipitating within the oil. This further compounds your heat transfer capabilities.

I would take another path to heat up the oil and stimulate separation of heavies and impurities. I would heat up the fuel oil under steady-state conditions as it enters the storage tank for its residence time of 50 hours. This does several things:

1. The oil is easily heated up faster and with much less of a heat transfer area being required. The necessary heat transfer can take place in a TEMA type shell & tube exchanger that can be locally built without any problems since it is relatively low pressure and has fixed tube sheets (an AEA unit would be ideal). The oil is circulated in the multi-pass tube side (4 or more passes) and the steam is condensed in the shell side. Please refer to the attached heat exchanger size estimation that I’ve calculated for you to compare with the size of your coil.
2. This maximizes the residence time of the heated oil and maximizes the amount of precipitation and phase separation taking place in the tank.
3. This avoids any fouling of a heat transfer coil within the tank and a need to have human entry into a hazardous confined space such as the tank in order to clean it out. The heat exchanger with A type of front and rear heads can easily be rodded out - if need be - in order to remove fouling in the tube side. Additionally, tubeside fouling can be minimized if the correct, high oil velocity is designed into the tube passes. Higher tube velocity yields less (or no) fouling and much better heat transfer coefficients.
4. You have to ensure that your filling oil pump has enough head to allow for a decent pressure drop through the exchanger. I suspect you may already have this ability in your present transfer pump and motor.

This is my recommendation and I offer my workbook for your scrutiny and examination. I would prefer to never have personnel enter a fuel oil tank to do internal maintenance. Like any other confined space, it is an accident waiting to happen. On top of that, the coil heat transfer is very inefficient and its installation will, in my opinion, be just as expensive – or more so – than the heat exchanger that I have estimated.

I hope this response is of help to you in your application.

Saludos
 Fuel_Oil_Tank_Heat_Up.xls   574.5KB   735 downloads

[ELEMAN]

Hello everybody:

Art:

thank you very much for your valuable help. I will study your workbook and will compare with my calculations basis in order to take the best decision. I will be in contact with the Forum.

[ELEMAN]

Hello everybody:

Art, I am in the point to ask for a quotation of a TEMA type shell and tube exchanger but, I have been unable to find a specific site on the web.

I wonder if you know a manufacturer and could advise me in this matter?

Thanks and saludos.

[Art Montemayor]

Eleman:

The TEMA type I would use on this Fuel Oil heater would be a AEA type. This would allow you to open both channel covers and have easy clean-out or rodding of the tubes. The tube sheets would be fixed because I presume your steam is clean and you never need to go into the shell side. This is a very simple and easy to fabricate heat exchanger type; I would not have any doubts that it can be fabricated in your country by local fabrication shops.

Be sure to fill in an appropriate Specification Sheet for your exchanger quote request. That's why I included a sample Spec sheet in the workbook I prepared for you. Some possible Shell & Tube heat exchanger designers and fabricators you can request a quote from are:

Southern Heat Exchanger
http://www.souheat.com/quickship2.html

Perry Products
http://www.perryproducts.com/

Ward Tank & Heat Exchanger Corp
http://www.wardtank.com/heatexchangers.htm

http://www.heattransfersys.com/index.html

http://www.ittstandard.com/Tools/Portfolio...amp;strMetaTag=

I hope this helps you out.

[ELEMAN]

Hello everybody

Art, thanks for your quick response to my request for information. Sure, I think this information handed over for you will help to us to find a solution to our project.

As always, I will be in contact with the Forum in order to share with all our experiences (good or bad) in this matter.

Once again, thanks a lot.