NPT thread tightening - Fluid Power engineering

Hello,

I have a question for experienced thread fitters. I have rarely used NPT threads (I am in a part of Europe where parallel fittings are used), but now I had to install some 1/2 NPT threaded male adapter to 1/2 NPT female valves.

However, after wrench tightening the adapter, I see some sort of thin metal scrap coming out of the thread. See attached picture, the thin metal scrap hanging.

Have you encountered this before? Could this be any problem or it should work fine?

The parts are made of 316 stainless steel, the adapter is made by Swagelok and the thread sealant used is Swagelok SWAK. The thread fitting was tightened about 2 turns past finger tight. It still wasn't totally locked, it can be tightened even more, but decided to stop and inquire first about that scrap.

Can you give me some advice?

Thank you for your help. I would agree with hydtools. If the debris is metal, it is likely from the female threads. Possibly machining swarf that was not cleaned out after the threads were cut. Or the un-broken tip edge of the lead-in thread that sheared off when the fitting was installed. 316 cres can be a bit gummy and thin, poorly supported edges (like thread tips) can tend to roll over from the cutter pressure rather than shear cleanly. Also it is always good practice to relieve/chamfer the first thread pitch of tapped holes.

Two turns after finger tight seems reasonable for a 1/2" NPT thread. However, since there is clearly a presence of metallic debris it would be good practice to disassemble the joint, clean the threads of both parts thoroughly, inspect the threads for any burrs or sharp edges, remove any burrs or sharp edges that are found, and then re-assemble the parts.

NPT and NPTF Tapered Pipe Threads

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NPT and NPTF pipe thread applications range from electrical conduits to high-pressure piping carrying natural gas and chemically aggressive liquids. Both thread types use a thread taper that provides solid and vibration-resistant connections. While NPT threaded plastic fittings are standard and readily available, there are distinctly specific installation and sealing recommendations for plastic versus metal installation. Both thread types also have installation differences based on gauging the threads for conformance before installation.

NPT (National Pipe Taper) is a general-purpose tapered pipe thread, and NPTF (National Pipe Taper Fuel) is a dryseal tapered pipe thread. Visually, NPT and NPTF pipe threads look identical. Both thread types rely on thread interference for tight, robust connections. This thread interference is made possible primarily by their tapered design.

The two thread standards differ in how they create leakproof seals. Both require proper installation but leakproof NPT connections require the appropriate use of thread sealants. Correctly assembling NPTF connections create leakproof seals without the use of a sealant.

NPTF threads are used instead of NPT when:

• The use of sealants could contaminate the fluids or gasses flowing through the system
• Applications are critical or the thread sealant might fail because of high heat or high pressure

There are important issues regarding the installation, mixed-use, and reuse of NPT and NPT fittings:

• NPT and NPTF fittings can be mixed but this requires using thread sealants
• NPT and sometimes NPTF fittings can be reused but only under particular conditions and cautions
• NPT and NPTF fittings both have special installation and assembly requirements for leakproof seals

What&#;s Different About NPT and NPTF Pipe Threads

Both NPT and NPTF have the same pipe taper from their center axis, which gives the internal and external threads a cone shape. This taper means the ends of external threads are narrower at the end. They mate with and screw into the larger diameter open end of internal threads where they are widest. Thread standards define the NPT and NPTF taper as a taper rate of 3/4 of an inch per foot, 1/16 inch per inch of length or about 1 degree, 47 minutes, 24 seconds (1. degrees).

Why are NPT and NPTF pipe threads tapered?

The taper of NPT and NPTF connections causes the threads to wedge together during installation. This wedging creates a strong connection that will not vibrate loose. Slight differences between NPT and NPTF threads and thread wear from reuse determine when these connections require thread sealant to get a leakproof seal.

While NPT and NPTF tapered threads are common in North America, a European tapered pipe thread standard is widely used internationally, including in North America. This pipe thread is BSPT (British Standard Pipe Taper). You can learn more about the differences between NPT and BSPT threads in our article.

Thread Shapes and Descriptions

Thread forms are the cross-sectional shape of the thread or screw thread. Each type of thread has a different thread form that varies based on the design and parameters of that particular thread standard. The specific thread designs or forms of NPT and NPTF pipe threads allow them to work well in fluid and gas handling applications.

NPT (National Pipe Taper) is a general-purpose tapered pipe thread standard based on ANSI/ASME B1.20.1 >> 

NPTF (National Pipe Taper Fuel) is a dryseal tapered pipe thread based on ANSI/ASME B1.20.3 >> 

ANSI, the American National Standards Institute, oversees the development of voluntary consensus standards for products, services, processes, systems and personnel in the United States. It also coordinates US standards with international standards to assist in selling American products worldwide.

The American Society of Mechanical Engineers (ASME) focuses on developing solutions to mechanical engineering safety issues and problems. These efforts include working with ANSI as a standards-development organization.

Some basic screw thread terms

Here is a quick overview of the parts or sections of threads talked about in this article.

Pipe thread crests, roots and their truncation

Thread crest (see B and D above)
The thread crest is the continuous helical ridge that runs along the top of a threaded male connection or inside a nut or female port. NPT and NPTF thread crests are flat or truncated (see B above).

Thread root or base (see A and C above)
The thread root is opposite the crest at the bottom of a thread. NPT and NPTF thread crests are flat or truncated (see C above). NPTF roots have a shorter truncation than NPT thread roots, making them shallower.

Thread flank
The thread flank is the surface or straight side of the thread connecting the thread root and the thread crest.

Thread major (A) & minor (B) diameters and the thread centerline axis (C)

Major diameter
Based on the drawing above, the thread major diameter is equal to A x 2. The thread major diameter is the largest diameter between the thread crests of a male threaded connection or a thread roots of a female threaded connection. This diameter is measured in reference to the centerline or axis of the screw thread. This centerline is also the center of the flow path through a fitting or pipe. The major diameters of NPT and NPTF threads are the same, but NPTF thread crest tolerances are tighter than NPT. The result is NPTF has taller thread crests than NPT threads.

Minor diameter
Based on the drawing above, the thread minor diameter is equal to B x 2. The thread's minor diameter is the largest diameter between the thread roots of a male threaded connection or the thread crests of a female threaded connection. The minor diameter is measured in reference to the center of the flow path and the centerline of the thread axis also. The minor diameters of NPTF threads are smaller than NPT threads. The result is NPTF has shallower thread roots than NPT threads.

How NPT Threads Work

NPT pipe threads use thread taper to make a strong connection. During wrench tightening, the thread taper wedges the male and female thread flanks together. When installed correctly, this wedging creates elastic compression between the threads (thread deformation). The elastic thread deformation of the threads creates something like a spring tension that holds the threads tightly together. Thread deformation, combined with friction (interference fit) between the flank surfaces, locks the threads together in a strong, tight connection that will not vibrate loose.

Wrench tight NPT threads (left) and NPTF threads (right)

Reusing NPT threaded pipe and fittings

Thoroughly inspect NPT fitting threads for possible damage before reusing them. Reusing NPT threads is not generally recommended because of possible permanent thread deformation created by overtightening during installation. NPT threads can also be damaged from repeated assembly and disassembly, making their reuse impractical. A general guideline is to replace male NPT threaded fittings after two to three uses.

While reuse of male NPT fittings is generally not recommended, it may be practical to reuse a fitting and still get a robust and leakproof connection if:

• No more than the usual amount of thread sealant is required to make a good seal. No more than six threads from the male connection end go into the female port for a hand-tight fit.
   
• Remember that proper installation of NPT components is a significant factor determining whether they can be reused without leaking or damaging female connections. This limit on reuse is because overtightening permanently distorts tapered threads.
   
• Sealants for NPT fittings and connections
  NPT pipe thread design allows slight clearance between the thread crests and mating roots. This clearance creates a spiral leak path along the male thread crests. The spiral leak path is why NPT connections require a thread sealant to be leakproof.

How NPTF Threads Work

Crests of the threads are crushed against the mating thread roots when assembling NPTF threaded components. This crushing is in addition to the interference fit provided by the thread taper wedging the thread flanks together. Like NPT connections, tapered threads hold NPTF connections together. And these connections are solid and tight. Crushing the thread crests fills the potential spiral leak path in NPT connections. Filling this leak path is why NPTF connections do not generally need thread sealants and why NPTF is considered a dryseal connection.

Reusing NPTF threaded pipe and fittings
The NPTF pipe thread design uses truncated thread roots. Truncated thread roots provide an interference fit so that the thread crests crush against the opposing thread roots. Because of this, you should generally not reuse NPTF threaded fittings. Despite this best practice recommendation, reuse of NPTF threaded fittings sometimes happens. NPTF reuse requires careful inspection for thread distortion and proper thread engagement, plus the use of a thread sealant for a leakproof seal. Never reuse NPTF fittings for flammable and combustible gases or liquids service or systems carrying hazardous and aggressive gases or liquids.

Thread sealants and NPTF

There are some conditions where NPTF threaded connections need a thread sealant:

• When reusing NPTF fittings, which is not advised
• When NPTF and NPT threaded parts are mixed or used together

Thread Sealants

Sealants are Teflon (PTFE) tape or thread sealing compounds, also called pipe dope. When assembling connections, they fill any voids between the mating thread crests and roots that leave a spiral leak path. Sealants also provide lubrication that makes metal thread installation more manageable, and they help prevent galling.

Teflon tape has the advantage of being cheap, easy to carry and less messy than sealant pastes or compounds. Technically, Teflon tape is not a thread sealant. Instead, it seals NPT connections by clogging the spiral path around the thread crests. There are different types and colors of Teflon tape. Variations in coloring, thickness and material composition provide options that accommodate the piping size and what the piping carries. For instance, white single-density Teflon tape is common in water plumbing.

Pipe dopes or pastes are solvent-based, are usually applied with a brush, and are compatible with metals and plastics. There are also anaerobic resin compounds used for thread sealants. Anaerobic sealants are not solvent-based and provide longer-lasting seals, but their chemistry must be correctly matched for compatibility when used with plastic fittings.

Plant Engineering magazine has a helpful overview article regarding guidelines for choosing a pipe thread sealant.

FMA series female by male NPT threaded plastic elbow >> 

If you are looking for more details, kindly visit NPT Pipe Fitting.

Additional reading:
TOP 5 BEST ADVANTAGES OF METAL POLISHING
Key Factors for Choosing 4 Inch Wood Cutting Wheels

In general, the suitable sealant for threaded plastic connections should be non-hardening, compatible with the type of plastic, and not contain a lubricant that makes overtightening easier. You can check out ISM's Plastic Tapered Thread Fittings Installation Guide for more information about this.  

Metal Pipe Thread Galling

What is galling?
Thread galling is a type of metal wear called adhesive wear. When two metal surfaces slide against each other under enough load, the pressure and friction cause metal to tear or shear away from one surface and weld itself (cold welding) to the other surface. The result is scoring that causes thread damage, leaking and seizing.

Galling can happen to threaded stainless steel, steel, aluminum, or brass connections during assembly and disassembly. The tendency for galling is especially true for tapered thread connections. While brass and bronze are resistant to galling and fully hardened steel is very resistant to galling, thread galling is common in stainless steel.

The lingering effects of galling

• Galling stresses and tears threads which distorts threaded connections
• Galling leaves behind tiny metal particles and debris that become potential contaminants
• Internal port thread galling causes thread damage that is expensive and difficult to repair

How to prevent galling
Use specialty seal tapes or thread sealants to prevent metal thread galling, especially with stainless steel tapered thread connections like NPT and NPTF. Sealants and tape for metal threads, especially those made for stainless steel, are formulated to prevent galling, seizing and corrosion.

series Camozzi NPTF threaded stainless steel bushing >> > 

Heat is a significant factor in metal thread galling. Slowing the wrench tightening of threaded metal connections generates less heat from friction and allows generated heat to dissipate more efficiently.

Preventing tapered thread galling:

• Do not overtighten metal NPT and NPTF connections, especially stainless steel ones. Proper sealant
• Use appropriate metal anti-seize lubricants for NPTF and lubricating thread sealants for NPT connections.

Interference fit for both NPT and NPTF

Proper installation of NPT and NPTF connections creates a friction type of interference fit. The threaded taper wedges the flanks or sides of the threads together. This force of this wedging places constant elastic stress (elastic deformation) on the thread flanks. This elastic stress and friction between the mating surfaces make NPT and NPTF connections tight and robust.

NPT and NPTF threads will screw together, but they do differ slightly in the diameter of their thread crests (major diameter) and thread roots (minor diameter), as well as their allowed tolerances. While NPTF thread crests have the same shape and major diameter as NPT, their tolerance range is narrower and more carefully controlled. NPTF minor diameters are slightly smaller than NPT, making their thread roots truncated or shallower than NPT threads.

While NPTF has the same taper as NPT threads, they have truncated or shortened thread roots that interfere with the crests of the mating thread. This interference fit crushes the thread crests against the truncated thread roots. The result is a mechanical leakproof seal that does not need a thread sealant.

NPT and NPTF are Torque Sensitive Connections

NPT and NPTF use thread taper to create interference between male and female threads. This interference causes elastic deformation as the connections are tightened and hold the mated threads tightly together. NPTF uses additional thread interference to crush the thread crests against the thread roots. Crushing the thread crests creates a mechanical leakproof seal. Both types of interference fits require carefully tightening to get the desired effect. Under tightening can lead to poor seals and weak connections, but Overtightening can cause more complex problems to fix. Torque sensitive is the term used to refer to this need for carefully controlled NPT and NPTF wrench-tight installation.

Torque sensitive essentially means that NPT and NPTF are sensitive to overtightening because they can permanently distort the threads. The main reason for this is that NPT and NPTF are tapered threads. While the taper creates a functional friction interference fit between the thread flanks, this taper can also act as a wedge. It is relatively easy to overtighten and permanently distort threads or even split or crack a female connection. Instead of torque measurement, it is hand tight plus the proper wrench tight distance (wrench tight engagement) that produces a good connection.

Note: Do not use torque as a reference for installing NPT and NPTF threaded components because there will always be variations in pipe joints:

• Possible mixed materials for male and female threads,
• Different types of sealant slipperiness or lubricity
• Variations in component wall thicknesses
• Differences in thread surface roughness
• Variances in threads and their taper
• And other factors

The installation required for strong, leak-free connections

Properly installing NPT and NPTF starts with a finger or hand tight install plus some wrench tight turns. This wrench tightening is called wrench tight engagement. A simple rule of thumb for installing tapered pipe threads, both metal and plastic, is finger tight plus one to two turns with a wrench. Be cautious of overtightening NPT and NPTF connections.

FTHT (Fit to Hand Tight) is the threaded assembly screwed together hand-tight but without any sealant applied to the threads.

TFFT (Turns From Finger Tight) or TPFT (Turns Past Finger Tight) is the number of complete rotations of the fitting recommended after making the connection finger tight.

FFWR (Flats From Wrench Resistance), also called the flats method, is the number of flats or hexes on a fitting that should pass the dry fit position once you feel wrench resistance. The dry-fit position is where you tighten the fitting to hand-tight without any thread sealant or lubricant on the threads.

Hand tight thread engagement (A) & wrench tight thread engagement (B)

Installing NPT threaded components
These are the steps for installing NPT and NPTF threaded components commonly used to assemble these connections to get leakproof seals:

1. Check that the male and female threads are clean and free from burs and nicks
2. Correctly apply an appropriate and compatible sealant to the male threads
3. Tighten the fitting by hand until it is finger tight in the female threaded port
4. Use a wrench to tighten the threaded joint up to the wrench-tight thread engagement point*
5. For shaped fittings, wrench tighten to the minimum recommended turns past finger tight and then tighten as needed to get the desired orientation

This version of the steps for installing NPT and NPTF threaded components is a more detailed and careful way to assemble these connections getting the best assurance of a leakproof seal:

1. Check that the male and female threads are clean and free from burs and nicks
2. Confirm that both the female and male threads are cut properly by gaging them per the appropriate thread standard
3. Screw in and tighten the fitting until it is hand-tight
4. Count the remaining exposed threads and also mark both components in this dry-fit position, which is the hand-tight or finger-tight thread engagement point
5. Disassemble the threaded parts and correctly apply an appropriate and compatible sealant or lubricant to the male threads
6. Tighten the parts again by hand until they are at the dry fit position
7. Use a wrench to tighten the threaded joint up to the wrench-tight thread engagement point
8. For shaped fittings, wrench tighten to the minimum recommended turns past finger tight and then tighten as needed to get the desired orientation

*The wrench tight thread engagement point for NPT and NPTF fittings smaller than 1 inch is typically about 1-1/2 to 3 turns. Wrench tight for fittings from 1 inch to 2 inches is about 1 to 2-1/2 turns. Consult "Machinery's Handbook" for more detailed wrench-tight thread engagement information.

"Machinery's Handbook" is a highly recommended reference book that provides the exact wrench-tight distance (thread engagement) for the NPT and NPTF thread installations. Keep in mind that there are thread variances between manufacturers and allowed tolerances.

In general, especially for shaped fittings (tees, elbows and so on), never back off an installed fitting to get the correct alignment. Loosening installed pipe fittings disrupts the sealant, which causes leaking.

How installing NPTF differs from NPT
In general, no thread sealant is necessary for NPTF threads. Also, gaging NPTF threads to confirm they meet NPTF thread standards is more involved than the relatively uncomplicated use of "Go" / "No Go" gaging for NPT connections. NPTF gaging involves using two thread plug gages for internal threads and two other ring gages for external threads. When no gaging is done, follow the normal NPT installation process.

NPTF thread inspection requirements

NPTF and thread class
NPTF dryseal pipe threads have two thread classifications called Class 1 and Class 2. These thread classes differ only in the inspections required by the NPTF thread standard. Class 2 threads are identical to Class 1 threads except for additional thread inspections.

Class 1 NPTF threads
Class 1 NPTF gaging checks the thread taper and the pitch diameter plus the hand tight and wrench tight thread engagement lengths. Class 1 crest and root truncations are not gauged because the threads are generally considered to be produced by well-controlled tooling. Some users add thread sealant to Class 1 NPTF threads when critical applications require it.

Class 2 NPTF threads
Class 2 NPTF gaging adds required checks for the thread and crest truncation. Class 2 gaging provides the greatest assurance of getting a leakproof seal from crushing the thread crests during installation. There are some applications where the media flowing through the system is too corrosive for sealants, but leakproof seals are critical.

Additional Resources for NPT and NPTF Gaging

A good resource for better understanding NPT and NPTF thread gages is the NPT and NPTF Gage Basic User Guide from Vermont Gage. This PDF download has excellent illustrations and a summary of the fundamental issues but little background information or instruction on using the gages.  

Vermont Gage pipe thread plug gage.

Glastonbury Southern Gage (GSG) has an excellent technical seminar available as a PDF download through their distributor, Willrich Precision Instrument (WPI). Screw Threads, An Intermediate Level Technical Seminar Covering Design, Nomenclature, Acceptability, and Gaging is probably one of the best and most accessible resources on the subject. It covers a lot of material from many different threads in its well-illustrated 52 pages, but you can quickly jump to the NPT and NPTF sections.

Vermont Gage pipe thread ring gages

NPT and NPTF Pipe Thread Takeaways

While NPT and NPTF thread forms are almost identical, NPT is a general-purpose thread that requires thread sealant for a leakproof seal, and you need to choose the sealant thoughtfully. NPTF creates a strong, leakproof mechanical seal without using a sealant. Mixing NPT and NPTF threaded fittings requires thread sealants for leakproof connections. NPT fittings can be reused but require careful inspection for thread damage from overtightening. In general, do not reuse NPTF fittings.

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