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.
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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