(Glass-Lined / Enamel-Coated Electric Water Heater)

Electric water heater tanks are designed as thin cylindrical pressure vessels.
The wall thickness must safely withstand the internal water pressure while considering material strength, weld efficiency, and enamel coating requirements.

1️ - Hoop Stress in Cylindrical Tank

Internal pressure creates circumferential (hoop) stress, which is the dominant stress in the tank wall.

σh=P×D / 2t​

Where:

σh   :Hoop stress (MPa)

P     :Internal pressure (MPa)

D     :Internal tank diameter (mm)

t       : Tank wall thickness (mm)

2️ - Allowable Stress Condition

To ensure safe operation: σh ≤ S×E

Where:

S  :  Allowable stress of steel (MPa)

E  : Weld joint efficiency

Typical values for water heater tanks:

Allowable stress S ≈ 120 MPa

Weld efficiency E ≈ 0.80 – 0.90

3️ -  Wall Thickness Formula

Final Formula  : t = ( P×D ) / ( 2×S×E−P )

Where:

t   : Required wall thickness (mm)

P  : Design pressure (MPa)

D  : Internal diameter (mm)

S  : Allowable stress (MPa)

E  : Weld efficiency

4️ -  Example Calculation (Typical Water Heater)

Assume:

P = 0.8 MPa

D = 400 mm

S = 120 MPa

E = 0.85

t =  ( 0.8×400 )  / ( 2×120×0.85−0.8 ) ≈1.58 mm

5️ - Effect of Enamel Coating

Glass-lined tanks require additional thickness for several reasons:

Enamel firing process

During enamel coating the tank is heated to:

820 – 860 °C

The steel must maintain dimensional stability and resist thermal distortion.

Corrosion protection

Although the enamel protects the steel, micro-cracks may occur, so manufacturers include additional corrosion allowance.

Typical allowance: 0.3 – 0.5 mm

6️ - Practical Manufacturing Thickness

For enamel-coated water heaters the final thickness becomes:

50 L : 1.8 – 2.0 mm

80 L :  2.0 – 2.2 mm

100 L :  2.2 – 2.5 mm

150 L :  2.5 – 3.0 mm

7️ - Final Design Rule (Glass-Lined Tanks)

Final thickness should be:  corrosion allowance  +  enamel process margin

Example: 

Calculated thickness = 1.58 mm

Corrosion allowance = 0.40 mm

Final thickness: t ≈ 2.0 mm steel

8️ - Standards Reference

Common standards used in water heater tank design:

• EN 60335-2-21 – Safety of electric water heaters

• EN 12897 – Indirect storage water heaters

• ASME Boiler & Pressure Vessel Code (Section VIII)

• UL 174 / UL 1453
  
  

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