Flanges, Gaskets & Bolts (Just the basics) Revision 1

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 as follows: 

  • Class 150
  • Class 300
  • Class 400
  • Class 600
  • Class 900
  • Class 1500
  • 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 flange facings.

Flange Dimensions:

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: 

  • Flange Outside Diameter
  • Flange Thickness
  • Bolt Circle
  • Number of Bolts
  • Bolt Hole Size
  • 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, etc.). 
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:

  • 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 horizontal centerlines.
  • 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.

Flange Types:

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 limits. 

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 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 weld. 

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:

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.

Flange Faces:

Face Types:

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: 

  • 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. 
  • 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.
  • 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. 
  • 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.
  • 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.

Stud Bolts:

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.

Cap Screws:

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.

Machine Bolts:

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 fittings see the "Codes / Standards" section of this website. 

About the Author


{cb:James O. Pennock has more than forty-five years in the process plant design profession. He has been involved in both home office and job site assignments on refinery, chemical, petrochemical, power and other projects. His experience ranges from entry level designer to engineering manager. Much of this was with Fluor. He is also the author of the book "Piping Engineering Leadership for Process Plant Projects." He is now retired, living in Florida, USA and does only occasional consulting work.

Mr. Pennock can be contacted via E-Mail at This email address is being protected from spambots. You need JavaScript enabled to view it..}

Log in to comment

Rendering Error in layout Topic/Item: Call to a member function getIcon() on null. Please enable debug mode for more information.