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Jump to the Various Sections of this and other Instrument Fittings and Tubing Pages; Instrument Compression Fittings | Achieving Optimal Results |Pressure Ratings| Assembly | Metric and Imperial Fittings | Thread Types |  Gauging External Taper Threads | Gauging Internal Threads | BSP (British Standard Pipe) (ISO228)  Parallel | Mixing BSP/NPT and any other Thread Standard | Making up NPT Connections | Use of Thread Tape | Thread Gauges – Use and Handling | Instrument Fittings Technical Data | Codes and Standards | Compression Fittings | Excess Flow Valves | Instrument Manifolds,Monoflanges,Valves and Accessories| Tubing Selection | Technical Information on Instrument Tubing and Fittings | Prochem Pipeline Products |

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Instrument Compression Fittings

Achieving Optimum Results

Pressure Ratings

Ensure fittings are rated to the Maximum Allowable Working Pressure (MAWP) of the tubing.


Compression fittings are supplied assembled.  The fitting, pipe thread and general appearance should be inspected visually before use.  It is recommended that fittings are not disassembled. However nuts and ferrules should be checked for correct assembly.

Metric and Imperial Fittings

Compression fittings are supplied in either Metric or Imperial tube sizes.  It is very important that types are not mixed or used on the incorrect tubing.

Thread Types

There are several common thread configurations,  in instrumentation design two types are commonally utilised.

NPT (National Pipe Tapered) threads such as Pipe Threads General Purpose (inch) conforming to ASME B1.20.1

These threads use the “interference” principle and the use of thread tape or other thread sealant is essential.  The thread tape will stop leaks via the gaps in the thread and avoid galling of the threads by giving lubrication.

Caution: In ICEWEB's opinion care should be taken on higher pressures to ensure the tolerance is better than the requirements of ASME B1.20.1 as if the male and female threads are to the outside of the tolerance then only 1.5 threads will be engaged or alternately the fittings may “bottom out”.  In both cases there are potential for blowout.  How many times have you seen this scenario?  Whilst the 1.5 threads in itself will contain most pressures without blowout it is quite possible that the Instrument Fitter will gall the threads because of insufficient thread entry.

The recommendation is NPT threads are designed to meet the dimensional requirements stated in ASME B1.20.1-1993 with the following modifications,  this will ensure that if both the fitting and the mating instrument get a 4 to 6 thread engagement. 

It is important to specify the revised tolerance on the instrument as well,  particularly on high pressure connections on equipment which are designed for noxious or hydrocarbon fluids.

The L1 thread gauging requirement in sections 8.1 and 8.2 modified to:

Gauging External Taper Threads with Ring Gauge (Male Threads)

When hand tight the male fitting must engage into the ring gauge + 1/4 to + 1 turn more than the start of the first scratch mark on the chamfer zone.

Gauging Internal Threads with Plug Gauge (Female Threads)

When hand tight the plug gauge must engage into the female fitting + 1/4 to + 1 turn below the last thread scratch on the chamfer zone.

BSP (British Standard Pipe) (ISO228)  Parallel

This type of thread utilises a washer/gasket or O-ring to achieve a seal.  It is important that any fitting also is fitted with a pin lock arrangement so that the fitting cannot vibrate loose.

Mixing BSP/NPT and any other Thread Standard on a Facility

It is highly recommended that no mixing of thread types occur on a facility.  If this culture exists it is possible for example that a Parallel thread is installed into a Tapered thread.  Thus, potential for blowout is large.

Making Up NPT Connections

Engage the threads hand tight - minimum of 4 and a maximum of 6.

An additional 0.5-1.5 turns can be expected with wrench tightness using a quality thread tape.

Hand tight engagement should be checked before applying any thread tape.

Use of Thread Tape

Thread tape should always be utilised on NPT threads,  however never on the compression fittings themselves.  Any tape should be high quality and NOT obtained from the local hardware store. A good rule is to obtain thread tape that meets the requirements of UL 340-M1979 (AGA) 3226, MIL-T-27730A or AG208.  There are thread tapes that are specifically designed for 316ss and other fittings.

There are liquid type sealants available,  these should be selected based on their specification and any anti-galling features. These sealants are particularly useful for hydraulic or clean services where small particles of tape could contaminate valves, ports etc.

Making up a Fitting when Utilising Thread Tape

Thread Gauges – Use and Handling

Primary Thread Gauges

Primary gauges are intended to be used as a master set with which regular comparative checks of secondary gauges can be made against a series of fittings.

Secondary Thread Gauges (L1 Standard)

Secondary gauges are intended for field use.  They must be regularly checked against primary gauges for wear and damage.

Testing Frequency for Thread Gauges

Frequency of testing secondary gauges depends on the frequency of use.  Testing is required after impact damage and chipping.  It is recommended that Primary gauges and secondary gauges are checked every six months by profile graph inspection.

NPT Standards Applicable to Thread Gauges

Primary master gauges and secondary working gauges should be manufactured and supplied in accordance with ASME B1.20.1-1993 specifications.

Correct Makeup Gauges

These gauges are product specific and give indication whether the make up is correct.  It is suggested that you contact your supplier to determine whether they are available for the product.  

Just click on the super ICEweb links below for more Technical Information on Instrument Tubing and Fittings.

Other Links

Fittings and Small Bore Tubing Systems - Handbook 

This superb 72 page document from the Norwegian Oil and Gas Association covers most engineering aspects of small bore tubing and fittings. 

Tube and Tube Fittings - This open source book Lessons in Industrial Instrumentation by Tony R. Kuphaldt has a useful section on instrument connections has a useful section on Instrument Tube Fittings – go to page 573. This is a fantastic resource but be prepared for the 50Mbit download!


Great technical information on other products from our sponsors PROCHEM PIPELINE PRODUCTS

INSTRUMENTATION  - Includes information on; 


HYDRAULIC- Includes information on; 

VALVES AND ACTUATORS - Includes information on;

MANUFACTURING - Includes information on; 

PIPING PRODUCTS- Includes information on; 


Welded Duplex Tube - Welded Duplex Tube to: ASTM SA789/A789 UNS S31803/S32205.
Y-Type Strainer

MORE INFORMATION? Again some great information here. This Swagelok link has considerable technical information.  It is however difficult to navigate to,  when on the main page enter as a guest or register,  go to “fittings tube” and then click on “connects tubing to tubing” and then click on any product and you will be into the technical information area.



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