Instrumentation tube fittings

Tube fittings are used to join or connect a tube end to another member, whether that other member be another tube end such as through T-fittings and elbow fittings, for example, or a device that needs to be in fluid communication with the tube end, such as for example, a valve.

Any tube fitting must accomplish two important functions within the pressure, temperature and vibration criteria that the tube fitting is designed to meet. First, the tube fitting must grip the tube end so as to prevent loss of seal or tube blow out. Secondly, the tube fitting must maintain a primary seal against leakage

The different types of tubing are

a) Flareless Compression Type (Single Ferrule)

(b) Flareless Compression Type (Double Ferrule)

(c) Bite Type

(d) Flared Fitting


The important parts of fitting are


Before that male and female threads are distinguished as follows


The different types of fittings based on type of threads is as below


The fitting can be of brass, carbon steel, SS 316/316L,Duplex steel, super duplex steel, Super 6Mo, Monel 400, Alloy 825,Alloy 625, Alloy Carbon 276, Titanium grade 2 and 5 and PVC

The fittings can be classified as

  1. 1.NIPPLE (both side NPT/BSP)

    2.UNION (both side Metric or fractional OD with end caps)

    3.CONNECTOR (One side Metric/Fractional OD compulsory and other side may be NPT/BSP)

    4.CONVERTER (Fraction to metric conversion, i.e. one side end tube of inch and other side OD metric conversion with end caps )

    5.ADAPTER (one side tube end with NPT/BSP/METRIC/FRACTIONAL and other side with machine threaded NPT/BSP/METRIC/FRACTIONAL/SPECIAL THREADED)

    6.REDUCER (one side end tube with inch/metric and other side threaded with inch/metric, but of different sizes. Here it should be noted that if one side is metric other side will also be metric, only its size changes)

    7.PLUG (one side NPT/BSP/METRIC OD and the other side blinded)

    8.ELBOW (in shape of our elbow)

    9.T (in the shape of T)

    10.CROSS (in the shape of cross)

    11.BARBED (simply push the fitting inside the tube)

    12.INSERT (inserting fitting inside connecting tubes with different ends)

Let me explain in detail

1.NIPPLE {both side NPT or both side BSP}:

Here are some situations in which two female ends in a tube system need to be connected. The fitting for this job is a nipple. A nipple is a fitting with two male-threaded ends. It will be having either both side NPT or BSP.

Nipple is used to connect two other fittings as shown below



A union allows the convenient future disconnection of tubes for maintenance or fixture replacement. A union allows easy connection and disconnection, multiple times if needed. This type of fitting has either metric of fractional OD with end caps on both sides



This will be having one side compulsorily Metric fractional OD and another side NPT or BSP




A converter have port one side as tube end (inch) and port 2 as OD metric conversion (with end caps) which means if port 1 is metric then port 2 will be inch and is used for inch metric conversion



 These fittings are designed to change the end type of a tube, allowing it to connect to fittings and tubes of many sizes. They can have threaded or slip socket ends to connect to an endless variety of tubes and fittings. Adapters can be either male or female-threaded, as well as socket or spigot.

 An adapter have port one side as turned end tube (with NPT /BSP/METRIC/FRACTIONAL measurements) and port two side as machined end tube or threaded  NPT /BSP/METRIC/FRACTIONAL/special threads like barbed thread but of different sizes. Thus in an adapter both sides will be of different types threading.



A reducer allows for a change in tube size to meet hydraulic flow requirements of the system or adapt to existing tube of a different size.

Reducer have port one side as (tube end) and port two as machine threaded but the remains same type (either inch or metric) and obviously of different sizes in order to reduce. This means if port one is metric then port 2 will also be metric but size changes.



A PLUG HAVE having one side as NPT/BSP/METRIC/FRACTIONAL and the other end as blinded.Plugs are like caps, but instead of stopping the flow in a tube, they stop the flow in a fitting.

They are put on the end of a pipeline that does not need to be connected to another pipe. They can stop a tube line that you plan on expanding later or give you easy access to a system when needed.


8.ELBOW in the shape of elbow

An elbow is installed between two lengths of pipe (or tubing) to allow a change of direction, usually a 90° or 45° angle; 22.5° elbows are also available. The ends may be machined for butt welding,threaded(usually female). When the ends differ in size, it is known as a reducing (or reducer) elbow.


9.T (in the shape of T)

A tee, the most common pipe fitting, is used to combine (or divide) fluid flow. It is available with female thread sockets, solvent-weld sockets or opposed solvent-weld sockets and a female-threaded side outlet. Tees can connect tubes of different diameters or change the direction of a tube run, or both


Ts are of many types, some of them are

This will combine or reduce a flow.

Types of T are

a.Union T


b.Drop size T


c.Run T here a portion of flow from main flow will be diverted into a side branch


d.Branch T


10.CROSS in the shape of cross

Crosses can add a great deal of structural integrity to a framework for multiple flow direction in a tube


11.BARBED (Because they’re insert fittings, they’re sized by the inside diameter (ID) of tubing. Fortunately, most tubing has the ID printed right on it, making it easy to find the right size. Installing barbed fittings is simple, as well: simply push the fitting inside the tube)



This is used for inserting fitting inside connecting tubes with different ends.


Most popular make of fittings are Parker, Swagelok, Midland, HOKE, MANNESMANN, FESTO, HY-LOK, FESTO etc


To know more about Instrumentation and Control purchase your copy now,


Basic Instrumentation and Calibration

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