4
raised-face configuration where there is no contact between
flanges beyond the bolt circle, leading to high longitudinal
hub stresses. Such stress levels are challenging for materi-
als like FRP to handle, which is why FRP flanges are fre-
quently designed with full flange face contact to reduce
these stresses. This configuration, however, necessitates
higher initial bolt loads to effectively seat gaskets, limiting
its use to relatively low pressures, such as 150 psi.
The ASME standards mentioned here, do not yet offer
a design method for FRP lap joint flanges. RPS Composites
has an internal design method developed for the lap joint
flanges that closely follows the approach used for full-face
flanges but with contact confined within the bolt circle,
making the ASME Section VIII design method more appli-
cable. We have qualified our design method for lap joint
flanges by meeting the performance requirements specified
in ASTM D542113. These requirements include a sealing
test, bolt torque test, and burst test, ensuring that the flange
can withstand pressure, torque, and burst conditions with-
out failure.
While the discussed design methods are suitable for
contact-molded flanges, it’s worth noting that some FRP
flanges are manufactured through methods other than con-
tact molding, such as filament winding. In this manufac-
turing method, the flange mold is rotated about the flange
axis, and continuous strands of glass are applied in the hoop
direction. These flanges exhibit superior hoop strength and
stiffness, enabling them to withstand higher bolt-tight-
ening and operational loads. Due to their non-standard
geometry and orthotropic material properties, standard-
ized design methods have yet to be established. Therefore,
filament wound flanges are typically designed empirically,
with proven configurations modified as needed to meet
specific design conditions. Verification of filament wound
flange performance can be conducted using the criteria out-
lined in ASTM D402414 which includes a sealing test, bolt
torque test, and burst test.
The main challenge inherent in the design of a flange
stems from the limited space available for the flange hub.
Typical applications necessitate compliance with standard-
ized bolting patterns as per ASME B16.515 and ASME
B16.4716. These standards specify the dimensions of the
flanges including the Bolt Circle Diameter (BCD) and
the flange Outside Diameter (OD). On the other hand,
the inner pipe diameter (ID) imposes limitations on the
inside dimension. These two constraints invariably restrict
the thickness of the flange hub, regardless of the necessary
thickness to withstand applied mechanical stresses. This
issue is further exacerbated when designing LJ flanges or
machine-backed FF flanges, where all hub material on the
back of the flange up to the ID of the backing ring must be
removed to accommodate the backing ring. In the case of
spot-faced FF flanges, only minimal material is machined
to fit a narrow washer, retaining some material between the
bolt holes that contributes to the flange’s structural integ-
rity. Having a thick sacrificial corrosion barrier (CB) or
dual laminate products (an FRP pipe lined with thermo-
plastic corrosion liner) will further limit the available space
for structural layers of the hub compounding the issue.
Unfortunately, there is no one-size-fits-all solution
to this problem. Multiple approaches can render a flange
design viable, but each solution comes with undesirable
side effects. The choice of the best solution depends on
several critical factors, including customer-specific require-
ments and constraints, cost considerations, and prevailing
design conditions. In this discussion, we elaborate on these
potential solutions:
1. Increasing the Flange Thickness
The limitation imposed on hub thickness, as explained ear-
lier, results in elevated stresses within the hub. One solution
to mitigate these elevated stresses is to increase the flange
thickness. Unlike the hub, there is typically no inherent
limitation on increasing the flange thickness. However, this
approach presents certain drawbacks:
• A substantial increase in flange thickness is often
required to reduce hub stresses to acceptable levels.
• It increases the overall cost of flange, especially for
larger sizes
• Longer bolts may be required to accommodate the
increased flange thickness
• An increase in the weight of the flange is another
consequence of this strategy.
2. Reducing the Pipe ID
As explained in the preceding section, hub thickness is deter-
mined by the dimensions of the pipe ID and BCD. One
solution to create more space for the hub is to reduce the
pipe ID. However, this approach has certain disadvantages:
• A reduced ID could cause flow-related issues and
result in increased pressure losses.
• This may lead to higher fabrication costs, as a custom
mold may be required to accommodate the altered
pipe ID.
3. Changing Flange Drilling Patterns or Sizing:
Relief from the constraints on hub thickness can be achieved
by expanding the BCD. This can be done by altering the
drilling pattern to a higher pressure class, such as using
Class 300 drilling pattern for a 150 psi flange. A similar
raised-face configuration where there is no contact between
flanges beyond the bolt circle, leading to high longitudinal
hub stresses. Such stress levels are challenging for materi-
als like FRP to handle, which is why FRP flanges are fre-
quently designed with full flange face contact to reduce
these stresses. This configuration, however, necessitates
higher initial bolt loads to effectively seat gaskets, limiting
its use to relatively low pressures, such as 150 psi.
The ASME standards mentioned here, do not yet offer
a design method for FRP lap joint flanges. RPS Composites
has an internal design method developed for the lap joint
flanges that closely follows the approach used for full-face
flanges but with contact confined within the bolt circle,
making the ASME Section VIII design method more appli-
cable. We have qualified our design method for lap joint
flanges by meeting the performance requirements specified
in ASTM D542113. These requirements include a sealing
test, bolt torque test, and burst test, ensuring that the flange
can withstand pressure, torque, and burst conditions with-
out failure.
While the discussed design methods are suitable for
contact-molded flanges, it’s worth noting that some FRP
flanges are manufactured through methods other than con-
tact molding, such as filament winding. In this manufac-
turing method, the flange mold is rotated about the flange
axis, and continuous strands of glass are applied in the hoop
direction. These flanges exhibit superior hoop strength and
stiffness, enabling them to withstand higher bolt-tight-
ening and operational loads. Due to their non-standard
geometry and orthotropic material properties, standard-
ized design methods have yet to be established. Therefore,
filament wound flanges are typically designed empirically,
with proven configurations modified as needed to meet
specific design conditions. Verification of filament wound
flange performance can be conducted using the criteria out-
lined in ASTM D402414 which includes a sealing test, bolt
torque test, and burst test.
The main challenge inherent in the design of a flange
stems from the limited space available for the flange hub.
Typical applications necessitate compliance with standard-
ized bolting patterns as per ASME B16.515 and ASME
B16.4716. These standards specify the dimensions of the
flanges including the Bolt Circle Diameter (BCD) and
the flange Outside Diameter (OD). On the other hand,
the inner pipe diameter (ID) imposes limitations on the
inside dimension. These two constraints invariably restrict
the thickness of the flange hub, regardless of the necessary
thickness to withstand applied mechanical stresses. This
issue is further exacerbated when designing LJ flanges or
machine-backed FF flanges, where all hub material on the
back of the flange up to the ID of the backing ring must be
removed to accommodate the backing ring. In the case of
spot-faced FF flanges, only minimal material is machined
to fit a narrow washer, retaining some material between the
bolt holes that contributes to the flange’s structural integ-
rity. Having a thick sacrificial corrosion barrier (CB) or
dual laminate products (an FRP pipe lined with thermo-
plastic corrosion liner) will further limit the available space
for structural layers of the hub compounding the issue.
Unfortunately, there is no one-size-fits-all solution
to this problem. Multiple approaches can render a flange
design viable, but each solution comes with undesirable
side effects. The choice of the best solution depends on
several critical factors, including customer-specific require-
ments and constraints, cost considerations, and prevailing
design conditions. In this discussion, we elaborate on these
potential solutions:
1. Increasing the Flange Thickness
The limitation imposed on hub thickness, as explained ear-
lier, results in elevated stresses within the hub. One solution
to mitigate these elevated stresses is to increase the flange
thickness. Unlike the hub, there is typically no inherent
limitation on increasing the flange thickness. However, this
approach presents certain drawbacks:
• A substantial increase in flange thickness is often
required to reduce hub stresses to acceptable levels.
• It increases the overall cost of flange, especially for
larger sizes
• Longer bolts may be required to accommodate the
increased flange thickness
• An increase in the weight of the flange is another
consequence of this strategy.
2. Reducing the Pipe ID
As explained in the preceding section, hub thickness is deter-
mined by the dimensions of the pipe ID and BCD. One
solution to create more space for the hub is to reduce the
pipe ID. However, this approach has certain disadvantages:
• A reduced ID could cause flow-related issues and
result in increased pressure losses.
• This may lead to higher fabrication costs, as a custom
mold may be required to accommodate the altered
pipe ID.
3. Changing Flange Drilling Patterns or Sizing:
Relief from the constraints on hub thickness can be achieved
by expanding the BCD. This can be done by altering the
drilling pattern to a higher pressure class, such as using
Class 300 drilling pattern for a 150 psi flange. A similar