Final storage tank thickness determination is required before the tank is about to be fabricated. General force balance design is not sufficient to build the tank for practical purpose. External factors to be add in the calculation to ensure the safety and life of the tank utilization. The final mechanical design of the vessel or tank provides and fulfils the necessary specification of the tank that can be built.
Factor that must be considered before finalizing the thickness of tank are
- Corrosion allowances
- Stress and strains
- Weld joints (joint efficiency)
- External and internal pressure effects and distribution
Fiber reinforced plastic has advantages than the metal in handling corrosive liquids like acids and base solutions. However, in the situation of handling vacuum and pressure metal is preferred as MOC.
Data required for storage tank thickness determination:
- Plant capacity = m kg/day
- The volume of the material = Plant capacity/ Density of the material
- Assuming tank remains half filled, volume = volume of the material x 1.5
- Assuming length = 2.5 x diameter
- 0.785 Di² L = Volume of the reactor
- Design pressure = operating pressure x 2
Storage tank thickness formula:
- Thickness, t = (P x Di )/(2σE -0.2P)
- P = design pressure
- Di = inside diameter
- σ = allowable design stress, for stainless steel=1675 kg/cm²
- E = weld joint factor = 0.85
Total thickness, t= Thickness + Corrosion allowance
The outside diameter, D0 = Di + Total thickness
Download excel based calculator for pressure vessel design to calculate internal and external pressure, cylindrical and conical shape shell and the dish of a pressure vessel.
- Calculation methods:
For cylindrical liquid storage tank, the design of its shell mostly calculated by one of the three popular methods.
- One-foot method (1FM)
- Linear analysis.
- Variable-design-point method (VDM)
These methods are in practice to meet the standards of API 650. The variables in the design required for these methods are mechanical stress factors, height and diameter.
- Temperature consideration:
When a tank is used to store hot or cold liquid. Its thickness determination should include the thermal effect in the calculation. During filling or draining the hot or cold fluids from the tank, the temperature along the surface varies from top to bottom. This cause the thermal expansion and contraction effect on storage tank thickness.
- Shape consideration:
The geometrical shape of the storage tank has great influence in the selection of storing gas or vapours.