A fitting is a pipe item used for changing direction, branching or attaching in a piping system. There are many different types of fittings and they are produced in all the same sizes and weights (schedules) as the pipe. Fittings are commonly segregated into three groups; Butt-weld, Socket-weld and Screwed. Only the most common will be discussed in this article.
Materials of construction:
Like pipe, fittings are fabricated from several different types of material and usually match the material of the pipe to which they are being attached. Some fittings are Cast Iron, some are Malleable Iron, some are Forged Steel and others are even fabricated from rolled Steel Plate. The most used materials are again common carbon steel, along with chromes, stainless steel, iron, brass, copper, lead, aluminum, glass, rubber and various types of plastic and plastic lined metal materials.
Normally, fittings fall into three basic types or categories. These are In-line, On-line and Closures. The In-line fittings include elbows (Ells), Tees, Couplings and Reducers. The On-line fittings include a wide variety of "O-Let" fittings used primarily for making branch connections. The closure fittings are various types of caps and plugs used to close the end of a pipe system. We also will discuss some cases where there are alternates to these normal categories.
An Elbow is a piping fitting used for changing direction. There are five basic versions of elbows. The first and by far the most common is the 90° long radius Ell. The second is the 45° long radius Ell. The third is the 90° short radius Ell. The fourth is the long radius reducing Ell. The fifth version is the long radius 180° Return Bend. The basic Butt-Weld Ell is manufactured in 90° or 45° configurations as a standard. However for special order and extra cost, the large sizes can be made in other degrees of turn.
The standard Butt-Weld elbows (90°, 45° and 180° ) can be altered to meet any special angle needs of a piping system. Elbows like pipe can be flame cut or machine cut to the required angle. The rough end is then ground or machine beveled to the proper angle for welding. There is normally no harm to the fitting when this is done.
The terms "Long Radius" and "Short Radius" are important to understand. "Long Radius" means that the center to end dimension is one and a half times the nominal pipe size.
Nominal Line Size (and Center-tend of short radius Ell)
Center-to-end of long radius Ell
"Short Radius" means that the center to end dimension is equal to the nominal pipe size. This means that the center-to-end for a 4" short radius Ell is 4", for a 10" Ell the center-to-end is 10" and so on.
The long radius Ell is the default standard. All elbows shown in a system are assumed to be long radius 90° Ells unless noted otherwise. This means that the designer must call out any and all exceptions to this rule. If the Ell is a 90° long radius Ell then the elbow symbol is all that is required. However, if the Ell is a 45° Ell then the designer must add the notation "45° Ell" next to the elbow symbol. If the Ell is a 90° short radius Ell then the designer must place the notation S. R. next to the elbow symbol. Also if the elbow has been trimmed to any odd angle this too must be noted next to the fitting.
As stated above the 90° long radius Ell is the default standard and is the most used. The designed should use the long radius Ell at all times unless conditions exist that force another choice. The short radius 90° Ell should only be used when tight space does not allow the long radius. The 45° Ell is normally used where a simple offset is required for some purpose. The 180° Ell is used mostly by equipment manufacturers to form heating or cooling coils. Return Bends are not normally required by the piping designer unless there is a requirement to fabricate a complex configuration.
The purpose of the 90° long radius Reducing Ell is to do the job of an elbow and a reducer. (Reducers will be covered later.) As such this Ell is made with one end of one size and the other end one or two line sizes smaller. The using of the reducing Ell is not cheaper; it only takes less room. The "long radius" dimension for the 90° long radius reducing Ell is based on the size of the large end.
Because the long radius and short radius designation of the 90° Ells are based on the nominal pipe size the designer quickly learns the center-to-end dimensions. The center-to-end dimensions for the 45° Ell are normally found only on a chart. However, there is a short-cut way to "know" these dimensions. You see, these dimensions are also based on the nominal pipe size. This short-cut method works for all 45° Ells from 4" to 24" line size. You can do this in your head. You simply divide the line size in half three times. Take the answer from the first time and the third time and total them up. That will be the dimension for the 45° Ell fitting.
|Column #1 (Line size)||Column #2(½ Col. #1)||Column #3(½ Col. #2)||Column #4(½ Col. #3)|| |
Column #5 -Fitting dimension
(Total of Col. #2 & Col. #4)
|10"||5"||2 ½"||1 ¼"||6 ¼"|
|14"||7"||3 ½"||1 ¾"||8 ¾"|
The primary purpose of a Tee fitting is to make a branch from a pipe line (or run). The branch may need to be the same size as the run or it may need to be one or more sizes smaller than the run. Because of economics (the cost of special orders) the use of Tees is normally limited to size-to-size or Straight Tee, (all three connections are the same size) or Reducing Tees where the branch outlet is only one size smaller than run size. Methods for making branches of other smaller sizes will be discussed later.
The dimensions of Tees are not as simple as they are for Ells. For Tees you must look them up on a fitting chart. The dimension found there is however standardized between all manufacturers. For Straight Tees the center-to-end dimension of both ends and for the branch outlet is the same. For Reducing Tees the center-to-end of the branch outlet is different from that of the run.
A Reducer is a fitting used to change the line size one or more sizes smaller (or larger). There are two versions of Reducers. There is Concentric Reducers- where the centerline of the inlet and the outlet are the same. There is Eccentric Reducers- where the centerline of the inlet is different than the centerline of the outlet. With the Eccentric Reducer, one side is flat. Depending on how it is installed you may have bottom flat (BF) or top flat TF). You may also have a need to have (*) side flat (*= north, south, east or west). It is about a toss-up as to which is used more. Concentric Reducers are used mostly in situations where the reducer is in a vertical run of pipe. Eccentric Reducers are used in horizontal runs of pipe such as pipeways or in pump suctions.
The dimensions for reducers must be looked up but are normally standardized among the manufacturers for a given size. The length of a reducer is the same for a range of sizes (Example: The end-to-end dimension for 10" x 4", 10" x 6" and 10" x 8" reducers is 7"). As you can see the length of a Reducer is very short in relation to the diameter.
The weld Cap is a fitting used to close the end of a pipe. The closed end of the Cap is semi-elliptical in shape. The dimension of a weld cap is a look-up item. Weld caps are most often found at the bottom of a piping configuration called a "Boot." A boot is a short length of pipe with a pipe Cap that is attached to the bottom of steam line and provides for the collection of condensate.
Here are a few alternates to the normal methods of doing business discussed above.
We talked about elbows as a way to change direction. You can change direction without using elbows. You might do this with a Miter Ell (or Mitre, both spellings are correct). A Miter Ell is where no fitting is used. Miters are normally used in large size/low pressure piping.
You fabricate the Miter or change in direction from pipe segments (or pieces) that are cut at specific angles depending on the number of pieces and welds required. This is really effective when really odd angles are required. Two of the pieces are the incoming pipe and the out-going pipe. There may be no middle piece or there may be one (or more) other short middle pieces depending on the angle of the turn. A simple turn of 45° might be made with a two-piece/one weld miter. Other changes in direction might be three piece/two weld miters, three piece/two weld miters and so on. The number of welds is always one less than the number of pieces.
Depending on the size and schedule of the pipe a Miter might be cheaper than buying fittings. In small diameter piping the miter is more expensive (labor costs) and there is more pressure drop through a small miter than a small fitting. Miters are also not recommended for high temperature lines because miters are more susceptible to overstressing.
We talked about using Straight Tees and Reducing Tees as a way to make branches from a line. For low pressure (or reasonably low pressure) there is another way to make branches from a line. This method uses only pipe. It is normally used only for low pressure/low temperature applications where the branch is reducing. The ASME B31.3 (and other piping B31 Code sections) recognize two basic versions of the pipe to pipe branch.
One method is where the run pipe has a hole cut the outside diameter of the branch pipe. This opening is then beveled for a "full penetration weld" The branch pipe is saddle cut (with no bevel) to match the I. D. of the run pipe. They are then fitted together and welded.
The second method is where the diameter of the hole in the run pipe is the same I. D. as the I. D. of the branch pipe. This hole does not get a bevel. The end of the branch pipe is saddle cut to fit the run pipe and is then beveled for a full penetration weld.
With the first method, the branch pipe is inserted in the run pipe. With the second method, the branch pipe is set on the run pipe. Both are still commonly referred to as "Stub-ins"
Both of these can come non-reinforced (as described above) or reinforced. The reinforced version is normally only required for higher stress situations. The reinforcement is a "ring" plate cut from some scrape run pipe or the same material as the run pipe. At the center is a hole the same size as the branch pipe. If cut from flat plate it is then shaped to fit around the run pipe. The width of the ring is normally one half the diameter of the branch pipe. The ring is intended to replace the material that was removed when the hole was cut in the run pipe. A small diameter hole (1/4" NPT) is normally drilled (and tapped) in the ring to act as a vent during the welding process and to allow for Hydrotesting of the welds. The ring is then welded to the branch pipe and the run pipe with full penetration welds. The small hole is fitted with a plug after work is completed.
Another way to make branch connections on pipe and vessels is by using an "O-Let" fitting. An "O-Let fitting is designed for use on 3" and larger welded pipe. The main feature of the typical O-Let fitting is the built-up base design which eliminates the need of any other form of branch reinforcement.
The O-let fitting is manufactured in a number of styles.
Weld-O-Let - (common) - This fitting is best described as an odd shaped "donut." It's purpose is to make self-reinforced branch outlets on a larger (one size or more) run of pipe. The base of the common weld-o-let has a saddle shape to fit the run pipe. The outlet end of the weld-o-let has a beveled-end allowing for butt welding a pipe or fitting. Weld-O-Lets come in a wide range of sizes and materials. The size call-out is normally the run (header) size by the branch size (Example: 24" x 4" WOL). It may be of some interest to know that most O-Let fittings are made with the base that covers a range of header sizes. This means that the 24" x 4" WOL will also fit on all pipe sizes from 24" pipe to 36" pipe.
Thread-O-Let - The Thread-O-Let is made much the same as the Weld-O-Let except that the outlet is threaded to match the normal tapered pipe threads. The threaded outlet sizes are normally limited to the smaller (2" and under) pipe sizes.
Sock-O-Let - The Sock-O-Let is also made much the same as the Weld-O-Let except that the outlet has a socket to match the socket welded piping fittings and pipe. The socket outlet sizes are normally limited to the smaller (2" and under) pipe sizes.
Latrolet - A Latrolet is a weld on branch fitting that is attached to the run pipe at a 45° angle. The angle attachment is sometimes required on high pressure relief systems. A Latrolet may be ordered with; a Butt-weld outlet end, a threaded outlet end or a socket weld end.
Elbowlet - The Elbowlet is made to be fitted on the back side of a long radius 90° elbow. An Elbowlet may also be ordered with; a Butt-weld outlet end, a threaded outlet end or a socket weld outlet end.
Nip-O-Let - A Nip-O-Let is a fitting that has the reinforced base for attaching to the run pipe and then has a short pipe extension with a threaded or plain outlet end. The Nip-O-Let does come in a range of sizes, however they are limited to the smaller sizes. This fitting is normally used for vent, drain and pressure gage connections.
Flange-O-Let - This fitting is much like the Nip-O-Let but has a flanged outlet end. The purpose is the same as for the Nip-O-Let.
Couplings: (as a branch outlet fitting)
The common pipe Coupling (to be discussed later) can also be used in the making of small size branches from a larger header or run pipe. One end of the (Threaded or Socket Weld) Coupling is shaped to match the O. D. of the larger pipe. This shaped end is then ground to form a beveled end which allows for a full penetration weld.
Screwed and Socket-Welded Fittings
These fittings perform the same function as the Butt-Weld fittings. There function is the same but the method of joining and the dimensioning is different. Normally these fittings are used in sizes 1-1/2" (or 2") and smaller. Welded fittings are specified the same as the pipe, by weight, schedule or wall thickness. Screwed and Socket-Weld fittings are specified per the pressure class.
Thread engagements as well as the depths of the sockets for different pipe sizes are different and must be looked-up on an approved dimension table.
Threaded fitting pressure classes:
- 125# Cast Iron
- 250# Cast Iron
- 150# Malleable Iron
- 300# Malleable Iron
- 2000# Forged Steel *
- 3000# Forged Steel *
- 6000# Forged Steel
* Most common
The Cast Iron and Malleable Iron fittings are basically used for air and water services at a low temperature and pressure. Forged fittings are normally used for higher pressures and temperatures as well as for the more complex commodities.
The majority of the screwed fittings will have female (internal) threads per NPT (National Pipe Thread). The exception will be the swages and the plugs - they will have male (external) threads.
Socket-Weld fittings are manufactured in two classes.
- 3000# Forged Steel
- 6000# Forged Steel
Socket-Weld fittings have a deep socket into which the pipe slips and aligns itself. The weld is then made on the outer surface of the pipe and fitting. This eliminate the need for or use of special clamps or tack welding for alignment prior to the final fit-up welding. At the bottom of the socket a 1/16" gap is left to compensate for expansion when the weld is made. This gap is called a root-gap. The swage does not have an internal socket; it will fit into the socket of a fitting or be butt-welded to a pipe.
The dimensions for screwed and socket-weld fittings must be looked up on a standard fitting dimension chart. There are no dimension short-cuts for these fittings.
Common Screwed an Socket-Weld fittings:
Elbows (Ells): Here again we have a fitting whose purpose is to change direction. There are only two versions. There is the 90° Ell and the 45° Ell. With the Screwed and Socket-Weld Ells there is no long radius or short radius. They are just as they are and they cannot be "trimmed" to allow for odd angles..
Tees: The Screwed and Socket-Weld Tee fittings are used for making branches. They do come in straight and some reducing sizes.
Swages: The Screwed and Socket-Weld Swage comes in both the concentric and the eccentric shapes. Swages do have an important feature that every designer needs to know and accept. Where a Butt-Weld reducer is short relative to the diameter, the swage is very long relative the diameter. Screwed and Socket-Weld swages are made by the same people and in some cases by the same machine. Some are then threaded and some are left with a plain end or beveled for welding. The extra length on the Screwed Swage allows space for the pipe wrench.
Caps and Plugs: Caps and Plugs are intended to provide for the closer of the end of a pipe or fitting.
Nipples: A Nipple is a name given to a short length of pipe. It is not really a fitting in the same context as an elbow or a Tee. Nipples are cut from pipe and can be purchased in 4", 6" and 12" standard lengths. Pipe Nipples can also be made by the piping crew in the field.
Unions: The Union is basically used as a dismantling fitting, and in many cases it is necessary for assembly. The field crew may install extra Unions at their own discretion to expedite and facilitate the construction of socket-weld and screwed piping.
About the Author
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.