Industrial Tall Column and Storage Tank Design Standards

Tall Column Design Procedure

1. Decide Column Type and Operating Conditions: Select material, design pressure, temperature, and applicable code (IS/ASME).
2. Process Height Calculation:
   • Calculate the number of trays multiplied by tray spacing or determine the packed bed height.
   • Add demister and redistributor height.
3. Add Mechanical Allowances:
   • Top vapor disengagement height.
   • Bottom liquid sump height.
   • Head heights (dished or ellipsoidal).
   • Nozzle and maintenance clearances.
4. Total Geometric Height: Final height = internal height + heads + clearances + insulation.
5. Determine Loads:
   • Dead Load: Shell, internals, platforms, insulation, and liquid hold-up.
   • Wind Load: Use code formulas to determine lateral pressure.
   • Seismic Load: Apply the equivalent static method where applicable.

Storage Tank Design Calculations

Hydrostatic and Total Pressure

Hydrostatic Pressure at Bottom:
ΔP = Density × g × Height
ΔP = 980 × 9.81 × 6 = 57,683 N/m²

Total Pressure at Bottom:
Total = ΔP + Superimposed Load
Total = 57,683 + 1,200 = 58,883 N/m²

Shell and Bottom Plate Thickness

Minimum Shell Thickness with Corrosion:
t = (P × R) / (Permissible Stress × Weld Efficiency × 0.6P)
t = (58,883 × 1.5) / (95 × 10⁶ × 0.85 × 58,883)
t = 1.09 mm + 2 mm (corrosion) = 3.09 mm

Bottom Plate Thickness:
Minimum: 6 mm + 2 mm (corrosion) = 8 mm

Plate Quantity Requirements

Number of Shell Plates:
Circumference: π × 3 = 9.42 m
Plate length: 5 m
Plates required: ⌈9.42 / 5⌉ = 2

Number of Bottom Plates:
Area: π × 1.5² = 7.07 m²
Plate area: 5 × 2 = 10 m²
Plates required: ⌈7.07 / 10⌉ = 1

Tall Column Internals and Components

• Tall columns are utilized for distillation, absorption, and stripping operations.
• Internal components are designed to promote efficient vapor–liquid contact.
• Two Main Categories:
  1. Tray (plate) columns
  2. Packed columns
• Tray Column Internals:
  1. Sieve trays (perforated plates)
  2. Valve trays (movable caps for variable load)
  3. Bubble-cap trays (stable operation for older designs)
• Tray Parts: Active area, downcomer, weir, and liquid seal.
• Operation: Vapor rises through perforations and bubbles through liquid, creating contact for mass transfer.
• Tray Spacing: Typically 450–600 mm; efficiency ranges from 60–80%.
• Packed Column Internals:
  1. Random packing: Raschig rings, Berl saddles, Pall rings.
  2. Structured packing: Geometrically arranged corrugated sheets.

Design Pressure and Temperature Parameters

Design Pressure

Design pressure is the maximum gauge pressure used to determine vessel thickness. It includes operating pressure plus allowances for surges, relief valve settings, and static head. It is usually 10–15% above the maximum operating pressure to ensure safety during abnormal conditions.

Formula: t = PD / (2SE + P)
(Where P = design pressure, D = diameter, S = allowable stress, E = weld efficiency)

Design Temperature

Design temperature is the maximum temperature considered for mechanical design. It determines material strength, allowable stress, and expansion allowances. It is usually 10–20°C above the operating temperature and affects material selection regarding creep and oxidation resistance. For vessels with cyclic temperatures, the highest value governs the design. Both design pressure and temperature must appear on the vessel nameplate.

Engineering Codes and Standards

• Standards: Established documents specifying technical requirements, dimensions, materials, and testing procedures.
• Codes: Mandatory rules providing design and safety guidelines for construction and operation.
• Purpose: To ensure safety, interchangeability, reliability, and quality in fabrication and inspection.
• Common Engineering Codes:
  1. ASME: Boiler & Pressure Vessel Code (BPVC) Section VIII.
  2. TEMA: Standards for heat exchangers.
  3. API: Standards for storage tanks and pressure-relief devices.

TEMA Heat Exchanger Classifications

TEMA Types by Application

• TEMA R: Refinery type – Heavy duty, high pressure, and severe service.
• TEMA C: Chemical process type – Used for most chemical plant exchangers.
• TEMA B: General purpose type – Light duty and low pressure.

TEMA Letter Designation

The format follows: Front Head – Shell Type – Rear Head (e.g., AES, BEU, BEM, CFU, AEP).

Front Head Types:

• A: Channel with removable cover
• B: Bonnet
• C: Channel integral with cover