Floating roof storage tanks design consultants-wisdom engineering

Floating roof storage tanks basics & design 

• --As the name suggests, a floating roof tank is armed with a floating roof.

• Floating roof tank is the most economical and efficient solution for reducing emission and product loss.

• Provides flexible use of storage capacity and reduces evaporation loss.​

• Floating roof tanks reduce the risk of explosion and fire in above ground storage tanks for extremely volatile organic liquids

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This floating roof floating up and down in shell, by adjusts its height by depending on the product level.

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The factors influencing the product evaporation are:
a) temperature of the liquid;
b) the presence and the dimension of the vapor space above the liquid;
c) the possibility for ventilating of the vapor space;

d) the presence of the free surface of the product

The safety from fire is increased which is a result of the decreased evaporation of the volatile stuff hence the lower risk from compound between the air and evaporating product which can be easily detonated.

Floating roof tanks keeps the stored oil pollution-free against rain, wind, sand, snow, or dust.

Conclusion

Floating roof tanks are used for storing a variety of different products across the globe. Most of them are extremely flammable products with low flashpoints.

Floating roof tanks are nearly ignition-proof. It is the rim of a tank that is most likely to catch fire. It is advisable to keep rim seals well maintained and check rim seals for deficiencies to help prevent ignition and fires.

External floating roofs.
The floating roofs projected and used for open on the top tanks.
They are made from steel and are welded.

Loadings
a) weather conditions (rain, snow, wind);
- 250 mm rain water for the period of 24 hours.
With tank owner agreement the real values for geographical area can be accepted for calculating the rain loading;
- the normal snow loading for consider is  st = 2,0 kN/m2. Depends on location of country.

b) live loading on the roof;
- 2,2 kN movable concentrated loading of two persons upon 0,1 m2, who can stay on each one point on the roof, which is floating or is in the lower position upon its supports;
c) loading  from rolling ladder and supporting truss;
d) loading from additional manholes, nozzles, roof drain, and seal;
e) loading of the product upon the roof;
f) single deck roof – membrane and two neighboring compartments are punctured.
  double deck roof  - two neighboring compartments are punctured.

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Internal floating roof tanks
They appear because of the increased requirements of environment safety and to decrease the loss of oil product. These floating roofs are fit inside under the fixed roof itself. They can be fit even in the tanks with supported cone roof.
They can be made from steel,.

Loadings:
a) 2,2 kN live concentrated load from two persons upon the 0,1m2, who can stay upon any point on the roof, when it is floating or is in its lower position supported by its supports;
b) if there are not other conditions, all project calculation must be done when the product density is 0,7t/m3;

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Single deck, with outsider ring of pontoons on the periphery and a membrane in the middle.
a) in the regions with small wind pressure where it is not possible to tear the membrane from the product;
b) in the regions with small sun shining where the lower temperature of the membrane helps the decrease of evaporation of the product;

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Double deck floating roof, with two membranes covering all surface of the roof and structure between them.
a) in regions with increased wind pressure, where it is possible to tear the membrane from the product and it must be prevented;
b) in regions with increased sun shining when it is necessary to decrease temperature of the membrane which contacts with the product;
c) more often when the tank has diameter D > 50 m.

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 Criteria for determining the type of the floating roof
The purpose of the tank, kind and the characteristics of the stored product, the conditions of the site must be taken into account when the type of floating roof is being chosen. The correct determination of its kind and the facilities connected with it can assures easer maintenance and quick return of the investments

a) Stability of the floating roof:
It is one of the most important conditions for the correct work. Moving loads which are not balanced upon the roof can cause inclination of the roof and floating with slope. There is the higher risk for the roof to sink there. It is necessary to assure the work of the roof in the projected position by means of correct.

b) Product
- aggressiveness of the stored product. It is necessary to take care of chemical characteristic of the liquid and the possibility to damage the roof equipment which is not metal. Some chemical products can destroy the seals of the roof in several weeks of exploitation;
- density of the product. According to the API Std. 650 volume weight of the product which are used when the calculations for the buoyancy are made is ρf  = 0,7 t/m3. It is not likely to store product with smaller density but it is possible to happen. The information for the minimal values of the ρf allows the designer

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CASE STUDY OF PROJECT:

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• Client ; NUMALIGARH REFINERY  LIMITED (INDIA)

• Consultant : TECHNIP INDIA LIMITED

• Scope of Work : Design, Analysis and Detail Engineering for External floating roof tank.

• Tank Design Code : API 650  ‐ 13th Ed.2020, Annex C, E, P 

• SERVICE : LIGHT SLOPE TANK

• Type   : EXTERNAL FLOATING ROOF TANK

• FLOATING ROOF TYPE : SINGLE DECK ANNULAR PONTOON

• OPERATING TEMPERATURE  : 40-45 0 C

• SPECIFIC GRAVITY OF LIQUID  : 0.700‐0.995 (AT 40°C )

• Material of construction : A36.

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Wisd provided design & Engineering services to NUMALIGARH REFINERY  LIMITED (INDIA) ,Calculations,  GA, Detail drawing services for External floating roof tank,

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tank diameter 23.5 m and tank height is 15 m. tank instillation location Haryana ,INDIA. The major challenging for this design services is in this location Seismic effect (Earthquake} is very high compare to the rest of India.  So client provided 15 m height of tank, We checked tank height extra 1.5 m height is   require for free board for seismic effect. So final height of tank 16.5 m provided.

And have provided major items like total weight of tank, weight of floating roof, all thickness, sizing of normal vent, bleeder vent, and flexible pipe for rain fall,  clean out door design, stability checking for floating roof, supporting legs design etc--

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Wisdom provide  Design & Engineering of this  storage tank , based on requirements for material, design, fabrication, erection, and testing according to Design code API 650.

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Considered Vertical shell joints are , single –V butt joint or double –V butt joint , butt joints with complete penetration and complete fusion . Vertical joints in adjacent shell courses shall not be aligned, but shall be offset from each other a minimum distance of 5t, where t is the plate thickness of the thicker course at the point of offset. Horizontal weld joints are considered single bevel butt joint  with complete penetration and double bevel butt joints with full penetration.. Bottom plate joints are lap welded and annular plates are but-welded. Shell to bottom plate or annular plate joints are fillet weld. Wind girder ring section are but-weld , continuous weld on top side and seal weld on  bottom side .

Nozzles design

Where nozzle necks are fabricated from pipe, the pipe shell be seamless. Where nozzles are 10” nominal bore and larger may be fabricated from rolled steel plate and butt welded using full penetration welds.

Man-ways

For storage tank, man-ways with DN600 (24”) size shall be used.

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GA drawing

GA drawing covering all details shown in data sheets like list of appurtenances, lists of standards and specifications, material of construction, tank design data, General notes including details of test to be conducted on tank,

Details of spiral stairway and extension stairway, roiling ladder to still well platform/LT platform, landing and handrails with bill of materials. Details of wind girders with bill of materials..