3
bolt holes that allow for the connection of multiple parts,
forming a sealed mechanical connection. There are two
main types of FRP flanges commonly used in the industry,
Full Face (FF) and Lap Joint (LJ) (see Figure 1)
A full-face flange is a type of pipe flange that is charac-
terized by its solid, circular design, which covers the entire
face of the pipe end.
A LJ flange is a type of flange used to connect two
piping components and allows for easy alignment, disas-
sembly, and maintenance. The LJ flange contact face only
covers the area inside the bolts and not the entire face of the
flange like FF flanges. It consists of two main components:
(i) Stub End: This is a short, cylindrical pipe section with a
larger OD than the pipe at the end. It is typically attached
to one of the pipes being joined. (ii) Backing Flange: The
backing flange is a separate, flat plate with bolt holes. It is
usually made of a material such as carbon steel, stainless
steel, or FRP. The backing flange is not laminated to the
pipe and is free to rotate and move.Table 1 summarizes the
difference between FF flanges and LJ flanges.
GASKETS
When using FRP flanges, it’s crucial to select an appropri-
ate gasket to ensure a reliable and leak-free connection. The
choice of gasket material should consider the specific appli-
cation, the chemicals or fluids being transported, tempera-
ture, and pressure.
One of the important factors to consider when select-
ing a gasket for FRP flanges is seating stress. This is the
stress required for the gasket to conform to the flanges and
provide a seal at the operating pressure. The most common
type of gaskets being used with FRP flanges are soft gaskets
(60–70 durometer) such as EPDM (Ethylene Propylene
Diene Monomer), Viton, and neoprene. These rubber gas-
kets have the lowest seating stress of all types hence less
demanding to achieve a seal.
For more chemically aggressive services, expanded
PTFE (Polytetrafluoroethylene) can be used for its excep-
tional chemical resistance. However, virgin expanded PTFE
typically has a high seating stress which makes it unsuitable
for FRP flanges. As a workaround, the gasket manufactur-
ers have created different low seating stress expanded PTFE
gaskets using a variety of techniques such as re-structur-
ing the gasket or reducing the area. This low seating stress
makes them suitable for FRP flanges.
FRP FLANGE DESIGN
Designing flanges poses a significant challenge among
standard piping components due to the intricate nature
of flange design theory and the reliance on trial-and-error
methods. The process typically involves assuming a flange
configuration, assessing its stress levels, and if deemed
unacceptable, iteratively adjusting the configuration and
reevaluating the stresses. Flange design is inherently com-
plex for several reasons:
• Flanges consist of multiple components that act
together, such as the flange ring, flange hub, bolts,
and gasket.
• The individual components of the flange are typically
composed of materials with significantly different
properties. For Instance, the bolts are metallic and
hence very stiff the gasket is typically elastomeric
and hence very flexible.
As a result, most flange design approaches are either “sim-
plified” elastic analyses or empirical in nature. The primary
design methods used for contact molded, full-face FRP pipe
flanges, closely align with standards like ASME NM.29,
ASME RTP-110, and ASME BPV Code Section X11.
These methods are adapted from metallic flange design
principles outlined in ASME BPV Code Section VIII12 for
metallic pressure vessels. Notably, Section VIII assumes a
Table 1. LJ flanges vs FF flanges
Performance
Aspect FF LJ Comments
Design Pressure For the common pressure ratings for FRP flanges, there is no difference between FF and LJ and
both can be designed to take the pressure
Design
Temperature
There is no difference between FF and LJ regarding the design temperature
Installation With a loose backing ring, it is easier to align and install LJ flanges
Backing ring Since the backing ring is separate from the flange, a different material can be selected for the
backing ring to take advantage of higher strength material when there is no contact with service
fluid
Sealing Since the contact area for LJ flanges is smaller than FF flanges, a certain amount of torque will
lead to higher seating stress in the gasket providing an easier seal.
bolt holes that allow for the connection of multiple parts,
forming a sealed mechanical connection. There are two
main types of FRP flanges commonly used in the industry,
Full Face (FF) and Lap Joint (LJ) (see Figure 1)
A full-face flange is a type of pipe flange that is charac-
terized by its solid, circular design, which covers the entire
face of the pipe end.
A LJ flange is a type of flange used to connect two
piping components and allows for easy alignment, disas-
sembly, and maintenance. The LJ flange contact face only
covers the area inside the bolts and not the entire face of the
flange like FF flanges. It consists of two main components:
(i) Stub End: This is a short, cylindrical pipe section with a
larger OD than the pipe at the end. It is typically attached
to one of the pipes being joined. (ii) Backing Flange: The
backing flange is a separate, flat plate with bolt holes. It is
usually made of a material such as carbon steel, stainless
steel, or FRP. The backing flange is not laminated to the
pipe and is free to rotate and move.Table 1 summarizes the
difference between FF flanges and LJ flanges.
GASKETS
When using FRP flanges, it’s crucial to select an appropri-
ate gasket to ensure a reliable and leak-free connection. The
choice of gasket material should consider the specific appli-
cation, the chemicals or fluids being transported, tempera-
ture, and pressure.
One of the important factors to consider when select-
ing a gasket for FRP flanges is seating stress. This is the
stress required for the gasket to conform to the flanges and
provide a seal at the operating pressure. The most common
type of gaskets being used with FRP flanges are soft gaskets
(60–70 durometer) such as EPDM (Ethylene Propylene
Diene Monomer), Viton, and neoprene. These rubber gas-
kets have the lowest seating stress of all types hence less
demanding to achieve a seal.
For more chemically aggressive services, expanded
PTFE (Polytetrafluoroethylene) can be used for its excep-
tional chemical resistance. However, virgin expanded PTFE
typically has a high seating stress which makes it unsuitable
for FRP flanges. As a workaround, the gasket manufactur-
ers have created different low seating stress expanded PTFE
gaskets using a variety of techniques such as re-structur-
ing the gasket or reducing the area. This low seating stress
makes them suitable for FRP flanges.
FRP FLANGE DESIGN
Designing flanges poses a significant challenge among
standard piping components due to the intricate nature
of flange design theory and the reliance on trial-and-error
methods. The process typically involves assuming a flange
configuration, assessing its stress levels, and if deemed
unacceptable, iteratively adjusting the configuration and
reevaluating the stresses. Flange design is inherently com-
plex for several reasons:
• Flanges consist of multiple components that act
together, such as the flange ring, flange hub, bolts,
and gasket.
• The individual components of the flange are typically
composed of materials with significantly different
properties. For Instance, the bolts are metallic and
hence very stiff the gasket is typically elastomeric
and hence very flexible.
As a result, most flange design approaches are either “sim-
plified” elastic analyses or empirical in nature. The primary
design methods used for contact molded, full-face FRP pipe
flanges, closely align with standards like ASME NM.29,
ASME RTP-110, and ASME BPV Code Section X11.
These methods are adapted from metallic flange design
principles outlined in ASME BPV Code Section VIII12 for
metallic pressure vessels. Notably, Section VIII assumes a
Table 1. LJ flanges vs FF flanges
Performance
Aspect FF LJ Comments
Design Pressure For the common pressure ratings for FRP flanges, there is no difference between FF and LJ and
both can be designed to take the pressure
Design
Temperature
There is no difference between FF and LJ regarding the design temperature
Installation With a loose backing ring, it is easier to align and install LJ flanges
Backing ring Since the backing ring is separate from the flange, a different material can be selected for the
backing ring to take advantage of higher strength material when there is no contact with service
fluid
Sealing Since the contact area for LJ flanges is smaller than FF flanges, a certain amount of torque will
lead to higher seating stress in the gasket providing an easier seal.