pipingdesigners.com Training Seminar
Section - 1
D: Flanges, Gaskets & Bolts (Just the
basics) Revision 1
By: James O. Pennock
Note: This article covers ASME B 16.5
Standard Piping Flanges up to 24" NPS. Flanges larger than 24"
fall under ASME B16.47 and while they have the same attributes they will
be covered at a later time.
A flange is defined as a plate type device, normally round, that is attached
to the end of a pipe, fitting, valve or other object to facilitate the assembly
and disassembly of a piping system. For many years the only practical method
of joining steel pipe had been by connecting threaded pipe ends with couplings.
Improvements in the welding of carbon steel reduced labor costs and provided
a completely sealed and much stronger joint. In most present day piping
systems, threaded joints are usually limited to pipe sizes 2" and smaller.
Larger pipe (3" and larger) is normally joined by butt-welding of continuous
pipe and fittings or by flanges at joints that may require dismantling.
Flanges (3" and larger) are also the default standard for connecting
to most equipment connections and valves.
Materials of construction:
Flanges are manufactured in all the different materials to match the material
of the pipe and fittings to which they are being attached. While some flanges
are made of Cast Iron. The vast majority of flanges are forged carbon steel.
Forged Flange Ratings:
Forged steel flanges are made in seven primary ratings.
These primary ratings are
o Class 150
o Class 300
o Class 400
o Class 600
o Class 900
o Class 1500
o Class 2500
The Primary Rating is on a pressure/temperature relationship.
A Class 150 Forged Flange is used for 150 PSIG
at 500º F. This same flange may also be used for 275 PSIG at 100º
F. This same flange could also be used at 100 PSIG at 750º F. Note
the inverse relationship. When the pressure goes up, the temperature goes
down and vice versa. Pressure ratings are used as a guide to safely design
piping systems and also to standardize manufactured piping components. The
same ratings hold true for screwed and socket-weld flanges.
Cast Iron Flange Ratings:
The two most common ratings for Cast Iron flanges are Class 125 and Class
250. Other flange ratings are available but are not as common. Cast Iron
flanges are generally found associated with low pressure cast iron valves
and nozzles on cast iron equipment such as some pumps and turbines. Mating
forged steel flanges to cast iron flange can pose a potential for damage
to the "weaker" cast iron. The main point to remember now is that
a Class 125 Cast Iron flange will mate to a Class 150 forged steel flange,
and a Class 250 Cast Iron flange will mate to a Class 300 forged steel flange.
The solution to the potential damage problem will be discussed later in
A flange has many dimensions. The most critical is the "length"
of the flange. This dimension will vary with each type of flange and will
be covered in the section below covering Flange Types.
All other dimensions for a flange will normally be the same across all flange
types but will vary with each flange rating.
These common dimensions include:
o Flange Outside Diameter
o Flange Thickness
o Bolt Circle
o Number of Bolts
o Bolt Hole Size
o Bolt Size
Bolt Hole Location:
The ASME B16.5 has a standard for bolt holes that are used by all (US) manufacturers
for flange sizes up through 24" For instance; the number of bolt holes
required varies with the size and rating of the flange. But the number and
size is the same no matter the type of flange. The bolt holes are evenly
spaced around the flange on a concentric bolt circle. There will always
be an even number of bolt holes, in graduations of 4 (i.e., 4, 8, 12, 16,
Unless specifically noted otherwise by the piping designer (and then only
if for good reason) all flange bolt holes shall straddle the "natural"
centerlines. This is the flange bolt hole orientation rule. This "natural"
centerline rule for flange is known, understood and followed by all responsible
equipment manufacturers and pipe fabricators.
The rule is as follows:
o For a vertical flange face (the flange face in vertical and the line is
horizontal) the bolt holes shall be oriented to straddle the vertical and
o For a horizontal flange face (the flange face is horizontal and the line
is vertical up or vertical down) the bolt holes shall be oriented to straddle
the (plant) north/south centerlines.
Care must be taken to check
all equipment vendor outlines to identify any flange orientations that do
not match this rule. When an exception is found the vendor can be requested
to change his bolt hole orientation. This is not always successful and if
not then the piping designer must insure that the piping fabrication documents
call for the correct orientation.
This rule of bolt holes straddling the natural centerlines is sometimes
referred to as "Two-Hole" the flange. This means that the two
of the holes straddle the centerline. To "One-Hole" a flange means
that the flange has been rotated so that one hole is right on the natural
centerline. I assure you that 99.999% of the time that to "One Hole"
a flange is a mistake and will add cost to the field. It also makes the
piping foreman very unhappy.
Weld Neck Flanges:
Weld Neck Flanges are distinguished from other flange types by their long
tapered hub and gentle transition of thickness in the region of the butt
weld that joins them to pipe or a fitting. A weld-neck flange is attached
to a pipe or a fitting with a single full penetration, "V" bevel
weld. The long tapered hub provides an important reinforcement of the flange
proper from the standpoint of strength and resistance to dishing. The smooth
transition from the flange thickness to the pipe wall thickness by the taper
is extremely beneficial under conditions of repeated bending caused by line
expansion or other variable forces, and produces an endurance strength of
welding neck flanged assemblies equivalent to that of a butt-welded joint.
This type of flange is preferred for severe service conditions, whether
loading conditions are substantially constant or fluctuate between wide
The weld neck flange is used
in each of the seven flange ratings and has the advantage of requiring only
one weld to attach it to the adjacent pipe or fitting.
The key dimension for a weld neck flange is the length through the hub from
the beveled end to the contact face of the flange. This "length"
includes the bevel, the tapered hub, and the thickness of the plate part
of the flange and the raised face. To obtain the correct dimension you must
look at a correctly constructed flange dimension chart (see the "Tools"
button on this website) or a flange manufacturers catalog. Electronic piping
design software will normally already have the correct dimension built-in.
It is important to understand
and remember that the (1/16") raised face on the Class 150 raised face
and on the Class 300 raised face flanges is normally included in the length
dimension. However, the ¼" raised face is not included in the
chart or catalog length dimension for the Class 400 and higher pressure
rated flanges. The raised face dimension for Class 400 flanges (and up)
normally must be added to the chart or catalog length to arrive at the true
total length of these higher-pressure flanges.
Slip-On (SO) Flanges are preferred by some contractors, over the Weld-neck,
because of the lower initial cost. However, this may be offset by the added
cost of the two fillet welds required for proper installation. The strength
of the slip-on flange is ample for it's rating, but its life under fatigue
conditions is considered to be only one-third that of the weld-neck flange.
The slip-on flange may be
attached to the end of a piece of pipe or to one or more ends of a pipefitting.
The slip-on flange is positioned so the inserted end of the pipe or fitting
is set back or short of the flange face by the thickness of the pipe wall
plus 1/8 of an inch. This allows for a fillet weld inside the SO flange
equal to the thickness of the pipe wall without doing any damage to the
flange face. The back or outside of the flange is also welded with a fillet
A variation of the Slip-On
flange also exists. This is the Slip-On Reducing Flange. This is simply
a larger (say a 14") Slip-On flange blank that, instead of the Center
(pipe) hole being cut out (or drilled out) for 14" pipe it is cut out
for a 6" (or some other size) pipe. The SO Reducing flange is basically
used for reducing the line size where space limitations will not allow the
length of a weld neck flange and reducer combination. The use of the Slip-On
Reducing Flange should only be used where the flow direction is from the
smaller size into the larger size.
Lap Joint Flanges:
A Lap Joint Flange is a two piece device that is much like a weld-neck flange
but also like a loose slip-on flange. One piece is a sleeve called a 'Stub-end"
and is shaped like a short piece of pipe with a weld bevel on one end and
a narrow shoulder on the other end called the hub. The hub is the same outside
diameter as the raised face (gasket contact surface) of a weld neck flange.
The thickness of the hub is normally about ¼" to 3/8".
The back face of the hub has a rounded transition (or inside fillet) that
joins the hub to the sleeve.
The other piece of a Lap Joint
Flange is the backing flange. This flange has all the same common dimensions
(O.D., bolt circle, bolt hole size, etc.) as any other flange however it
does not have a raised face. One side, the backside, has a slight shoulder
that is square cut at the center or pipe hole. The front side has flat face
and at the center hole an outside fillet to match the fillet of the "Stub-end"
piece. The flange part of the Lap-joint flange assembly is slipped on to
the stub-end prior to the sleeve being welded to the adjoining pipe or fitting.
The flange itself is not welded or fixed in any way. It is free to spin
for proper alignment with what ever it is joining to.
The "Stub-end" can
normally be purchased in two lengths. There is a short version, about 3"
long and a long version of about 6" long. It is prudent for the piping
designer to know which version is in the piping specification.
Because of it's two piece
configuration, the Lap Joint Flange offers a way to cut cost or simplify
work. The cost saving comes when the piping system requires a high cost
alloy for all "wetted" parts to reduce corrosion. The sleeve or
Stub-end can be the required higher cost alloy but the flange can be the
lower cost forged carbon steel.
The work simplification comes
into the picture where there are cases that require frequent and rapid disassembly
and assembly during the operation of a plant. The ability to spin that backing
flange compensates for misalignment of the bolt holes during reassembly.
Screwed (or Threaded) Flanges:
Screwed flanges look very much like a Slip-On flange in some ways. The main
difference is the Screwed flange was bored out initially to match a specific
pipe inside diameter. The backside of this center opening is then threaded
with the proper sized tapered pipe thread. This flange is primarily used
to make flanged joints where required in small sizes in threaded pipe specs
Socket Weld Flanges:
Socket Weld flanges also look very much like a Slip-On flange. Here the
main difference is the Socket Weld flange was also bored out initially to
match a specific pipe inside diameter. Here however, the backside of this
center opening is then counter bored to form the proper size socket to take
the pipe O.D. This flange is primarily used to make flanged joints where
required in small sizes in socket welded pipe specs
Blind flanges are a round plate with all the proper bolt holes but no center
hole. This flange is used to provide positive closer at the ends of pipes,
valves or equipment nozzles.
Flanges faces come in different forms. Some forms are more common and others
are old and out of date forms. These old forms may be ordered but possibly
only to match an existing piece of old equipment.
Flange face forms are:
o Flat Face (FF) - The Flat Face is primarily used on Cast Iron flanges.
With this face the whole contact face of the flange is machined flat.
o Raised Face (RF) - The Raised Face is most common of all flange faces.
The flange has a raised area machined on the flange face equal to the contact
area of a gasket.
o Ring-type Joint (RTJ) - This is a form of flange face that is becoming
obsolete. This type has a higher raised portion on the face into which a
ring groove is then machined.
o Tongue and Groove (T&G) - This is also a form of flange face that
in becoming obsolete. With this type the flanges must be matched. One flange
face has a raised ring (Tongue) machined onto the flange face while the
mating flange has a matching depression (Groove) machined into it's face.
o Male-and -Female (M&F) - This is another form of flange face that
is obsolete. With this type the flanges must also be matched. One flange
face has an area that extends beyond the normal flange face (Male). The
companion flange or mating flange has a matching depression (Female) machined
into it's face.
Dissimilar flange faces such
as the RTJ, T&G and the F&M shall never be bolted together. The
primary reason for this is that the contact surfaces do not match and there
is no gasket that has one type on one side and another type on the other
side. Don't even think about it!
Flat face flanges are never
to be bolted to a raised face flange. If you need to bolt a Forged steel
flange to cast iron then you must call for the forged steel flange to be
machined off to a flat face. For more information on this see this
link to Goulds pumps
Flange Face Finish:
The part of a flange where the gasket touches is called the contact surface.
This area is the most critical area to the prevention of leaks. This area
of a flange must be protected from the time it is machined clear through
all the various shipping, storage, fabrication and installation periods.
Flange faces are machined with standard finishes. No doubt your piping material
engineer could request another special finish but that would only add extra
cost. The most common finish for the contact face of a flange is a concentric
(or phonographic) groove. This pattern is machined into the flange face
and provides the grip for the gasket.
You can have Class 600 stainless steel flanges and have the bolts fully
tight and if you do not have a gasket (or the proper gasket) you will have
a lot of leaks. Having the gasket and the right gasket is very important.
Gaskets provide the tight seal that retains the pressure and keeps the gas
or liquid in the pipe. In a vacuum system it keeps the outside air from
getting in. Gaskets are designed and later chosen considering all the same
issues as were used to select the pipe. These include pressure, temperature,
and corrosiveness of the commodity, among others. Gaskets are made of a
wide range of materials. These include rubber, elastomers and graphite.
The Spiral Wound gasket has a graphite or Teflon material wound with a metal
strip which is then held in shape by a flat metal ring. This metal retainer
ring also acts as a centering tool to insure that the casket is not misaligned
or blocks the product flow.
Gaskets for Ring Type Joint
flanges are simply a solid metal ring. There are two basic cross-sectional
shapes for the RTJ gasket. These are "Oval" and "Hexagonal."
Bolting is the final element of a complete flange joint assembly. Here again
we have some variations. The most common is the Stud Bolt. Next is normally
the Cap Screw. And finally we have the Machine Bolt.
The Stud Bolt is a long threaded rod (with no head on either end) and two
nuts. The Stud Bolt is used in all locations where you have two normal flanges
with access to the backside of both flanges and both ends of the stud.
The Cap Screw is a fully threaded rod with a head on one end. No nut is
used with the Cap Screw. The Cap Screw is normally used in all locations
where a flange is being attached to a piece of equipment where there are
only tapped holes (i.e.: no access to the backside). Cap Screws are also
used to attach threaded-lug type wafer valves (Butterfly Valves) between
a pair of flanges. For this application the length of the Cap Screw selected
is critical. Two Cap Screws are used at each lug position, one from one
side and one from the other side. The Cap Screw must be long enough to go
through the flange, the raised face and half of the threaded lug minus 1/16
of an inch. This leaves a 1/8 inch total gap between the ends of the two
cap screws when the screws are tight.
A Machine Bolt is a rod with a hexagon head on one end and threads on some
of the length. Machine Bolts are normally made of a lower strength material
than Stud Bolts and are therefore considered only where low strength bolting
is required. These applications most often include Cast Iron flanges.
For additional information about flanges, gaskets
and bolts see the "Standards"
tab on the pipingdesigners.com website.
piping designers . com | Training - Flanges, Gaskets, Bolts
James O. Pennock is a former Piper with more than 45 years experience
covering process plant engineering, design, training, pipe fabrication and
construction. He is now retired and lives in Florida, USA.