How to Calculate Minimum Wall Thickness of Pipe


        The determination of pipe wall thickness is influenced by various elements, including the pipe material, the pressure inside the pipe, the temperature of the fluid running through the pipe, and the pipe material's permitted stress.

The general steps for estimating pipe wall thickness are as follows:

1. Define the design conditions:   The pressure and temperature of the fluid that will flow through the pipe must be determined. Determine the pipe's material as well.

2. Determine the nominal size of the pipe:   The nominal size of the pipe is the size of the pipe utilised in design calculations. This is not always the actual size of the pipe.

3. Determine the design pressure:   The design pressure is the maximum pressure that the pipe will withstand. It is determined using the fluid pressure, the safety factor, and other design considerations.

4. Calculate the acceptable stress of the pipe:   The allowable stress is the maximum stress that the pipe material can sustain without failing. This value is determined by the pipe material, fluid temperature, and other design variables.

 Minimum Pipe Wall Thickness Calculation Formula as Per Requirement

            The International Organization for Standardization (ISO) has created a number of pipe design and construction standards. The pipe thickness formula is determined by the type of pipe and the design circumstances. Here are a couple such examples:

1.  ISO 14692 :  The general formula for calculating the minimum required wall thickness of a GRP pipe for internal pressure is:

t = (P x D)/(2 x S x F)

where:

t = minimum required wall thickness of the GRP pipe (mm or inches)

P = internal pressure (bar or psi)

D = outside diameter of the pipe (mm or inches)

S = long-term modulus of elasticity of the GRP pipe (MPa or psi)

F = design factor (dimensionless),

The design factor F takes into account various design conditions such as the maximum temperature, type of fluid being transported. The ISO 14692 standard provides guidelines for selecting an appropriate design factor based on these conditions.

 

2.  Metallic pipework according to ISO 10631:2006: The minimum wall thickness for carbon steel and stainless steel pipes subject to internal pressure is calculated as:

t = (PD)/(2SE - 0.2P)

where:

t = minimum wall thickness (mm)

P = internal pressure (MPa)

D = outside diameter of pipe (mm)

 S = allowable stress for the material at design temperature (MPa)

E = weld joint efficiency (dimensionless)

 

3.  Polyethylene (PE) pipes: ISO 4427-2:2019  :    The formula for calculating the minimum wall thickness of PE pipes for water supply systems is:

e = (M - 1) x (D/2)

where:

e = minimum wall thickness (mm)

M = minimum required strength ratio (dimensionless)

D = outside diameter of pipe (mm)

 

4.  Glass-reinforced thermosetting plastic (GRP) pipe systems: ISO 14692:   The minimum wall thickness for GRP pipes subject to internal pressure is calculated as:

t = (P x D)/(2 x (S x E + P))

where: t = minimum wall thickness (mm) P = internal pressure (MPa) D = outside diameter of pipe (mm) S = hoop stress allowable (MPa) E = joint efficiency factor (dimensionless)

5.   ISO 11296-4 for non-pressure subterranean drainage and sewerage PVC-U and PVC-HD pipes:   The formula for calculating the minimum wall thickness of PVC pipes is:

t = (K x (D + 2s) x S)/(2 x (C + 1))

where:

t = minimum wall thickness (mm)

K = dimensionless coefficient depending on the pipe stiffness, modulus of elasticity and internal pressure (refer to the standard for values)

D = outside diameter of pipe (mm)

s = socket depth (mm)

 S = minimum required pipe stiffness (kN/m²)

C = safety factor (dimensionless)

7.  ASME B31.1 Power Piping:   This standard applies to power pipe systems that transport under pressure steam, water, and other liquids and gases. This standard's pipe thickness formula is as follows:

t = (P * D)/(2*(SEW+PY))

where:

t = minimum pipe wall thickness (inches)

P = maximum allowable working pressure (psi)

D = outside diameter of the pipe (inches)

S = allowable stress for the material of the pipe (psi)

E = joint efficiency factor (dimensionless)

W = welding joint strength reduction factor (dimensionless)

Y = coefficient of wall thickness variation (dimensionless)

 

8.  ASME B31.3 Process Piping:   This standard covers process pipe systems that convey fluids like chemicals, gases, and petroleum products. This standard's pipe thickness formula is identical to ASME B31.1, but it adds a corrosion allowance:

t = (P * D)/(2*(SEW+PY)) + C

where

C is the corrosion allowance (inches or mm)

9.   Piping Inspection Code API 570:   This standard applies to petroleum and chemical industry pipe systems. This standard's pipe thickness formula is identical to ASME B31.3, but it adds a manufacturing tolerance:

t = (P * D)/(2*(SEW+PY)) + C + F

where

F is the fabrication tolerance (inches or mm).

These standards provide guidelines for designing, fabricating, and inspecting piping systems and ensure that the piping system is safe and reliable.

 

10.  General Formula from ASME B31.3, B31.1 or B31.2 :   The formula you provided is a general formula for calculating the minimum required wall thickness of a pipe under internal pressure according to the Barlow's formula, where:

t   =   P x D / (2 x F x S x E)

 t              :               Calculated Wall thickness (mm)

P              :               Design pressure for the pipeline (N/mm²)

D             :                Outside diameter of pipe (mm)

F              :               Design factor

S              :               Specified Minimum Yield Strength (MPa)

E              :               Longitudinal   joint   factor

 

Hence

Calculated wall thickness (t,  in mm)

Assume That :

t              =             ?

P             =             25 N/mm²

D            =             Ø50 mm

F             =             0.75

S             =             350 N/mm²

E             =             1

t             =            10x25/(2x0.75x350x1)

t             =            2.41 mm

It is customary to include a corrosion/erosion allowance, especially in the case of carbon steel. This is typically estimated using the projected corrosion/erosion rate. If the pipe ends are threaded, a mechanical allowance based on the thread or groove depth must be included. These additional allowances need to be taken into account when establishing the actual required minimum thickness of the pipe protective measure value = 0.25 mm

Nominal wall thickness = 2.41+2.25 = 4.66 mm

So, any available thickness greater than 4.4 mm can be used as a selected thickness.

It is customary to include a corrosion/erosion allowance, especially in the case of carbon steel. This is typically estimated using the projected corrosion/erosion rate. If the pipe ends are threaded, a mechanical allowance based on the thread or groove depth must be included. Temperature is significant because as it rises, the material's strength decreases; hence, the allowed stress varies with temperature. Since the pressure is not constant over the temperature range in which the pipe will run, the minimum needed wall thickness must be calculated at a number of different pressures and temperatures. The most significant value for pipe wall thickness will be used in the design.

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