Section 7 Boiler tubes subject to internal pressure
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Naval Ships, January 2023 - Volume 2 Machinery and Engineering Systems - Part 8 Pressure Plant - Chapter 1 Steam Raising Plant and Associated Pressure Vessels - Section 7 Boiler tubes subject to internal pressure

Section 7 Boiler tubes subject to internal pressure

Parent topic: Chapter 1 Steam Raising Plant and Associated Pressure Vessels

7.1 Minimum thickness

7.1.1 The minimum wall thickness of straight tubes subject to internal pressure is to be determined by the following formula:

where t, p, D o and σ are as defined in Vol 2, Pt 8, Ch 1, 1.2 Definition of symbols

Note 1. Provision must be made for minus tolerances where necessary and also in cases where abnormal corrosion or erosion is expected in service. For bending allowances, see Vol 2, Pt 8, Ch 1, 7.2 Tube bending

Note 2. Thickness is in no case to be less than the minimum shown in Table 1.7.1 Minimum thickness of tubes

Table 1.7.1 Minimum thickness of tubes

Nominal outside
diameter of tube
in mm 		Minimum
thickness
in mm
≤ 38 1,75
> 38 ≤ 50 2,16
> 50 ≤ 70 2,40
> 70 ≤ 75 2,67
> 75 ≤ 95 3,05
> 95 ≤ 100 3,28
> 100 ≤ 125 35,0

7.1.2 The minimum thickness of boiler, superheater, reheater and economiser tubes is to be determined by using the design stress appropriate to the mean wall temperature, which will be considered to be the metal temperature. Unless it is otherwise agreed between the manufacturer and LR, the metal temperature used to decide the value of σ for these tubes is to be determined as follows:

  1. The calculation temperature for boiler tubes is to be taken as not less than the saturated steam temperature, plus 25°C for tubes mainly subject to convection heat, or plus 50°C for tubes mainly subject to radiant heat.

  2. The calculation temperature for superheater and reheater tubes is to be generally taken as not less than the steam temperature expected in the part being considered, plus 35°C for tubes mainly subject to convection heat. For tubes mainly subject to radiant heat, the calculation temperature is generally to be taken as not less than the steam temperature expected in the part being considered, plus 50°C, but the actual metal temperature expected is to be stated when submitting plans.

  3. The calculation temperature for economiser tubes is to be taken as not less than 35°C in excess of the maximum temperature of the internal fluid.

7.1.3 The minimum thickness of downcomer tubes and pipes which form an integral part of the boiler and which are not exposed to combustion gases is to comply with the requirements for steam pipes.

7.2 Tube bending

7.2.1 Where boiler, superheater, reheater and economiser tubes are bent, the resulting thickness of the tubes at the thinnest part is to be not less than that required for straight tubes, unless it can be demonstrated that the method of forming the bend results in no decrease in strength at the bend. The manufacturer is to demonstrate in connection with any new method of tube bending that this condition is satisfied.

7.2.2 Tube bending and subsequent heat treatment, where necessary, is to be carried out as to ensure that residual stresses do not adversely affect the strength of the tube for the design purpose intended.

7.3 CROSS-REFERENCES

7.3.1 For details of manholes, sight holes and doors, see Vol 2, Pt 8, Ch 1, 14.1 Access arrangements. For details of tube holes and fitting of tubes, see Vol 2, Pt 8, Ch 1, 14.6 Fitting of tubes in water tube boilers.

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