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 requirement that a tube fitting accomplish these two functions has been the driving factor in tube fitting design for decades. A multitude of factors influence the design of a tube fitting to meet a desired grip and seal performance criteria, but basic to any tube fitting design will be:
1.0 The characteristics of the tubing that the fitting must work with, including the material, outside diameter and wall thickness; and
2.0 The tube grip and seal performance level required of the tube fitting for its intended applications.
Tube fittings that are intended for use with stainless steel tubing, for example, are particularly challenging to design in order to achieve the desired tube grip and seal functions. This arises from the nature of stainless steel which, in terms of typical commercially available tubing material, is a very hard material, usually on the order of up to 200 Vickers. Stainless steel tubing is also used for high pressure applications in which the tubing wall thickness is substantial (referred to in this paper as "heavy walled" tubing). Heavy wall tubing is difficult to grip because it is not only hard but it is also not particularly ductile. Low ductility makes it more difficult to deform the tubing plastically so as to achieve a desired tube grip.
A tube fitting has to meet the following requirements:
- Offer reliable installation over a range of field conditions, since improper make-up and tightening remain the leading causes for leakage.
- Deliver a predictable, consistent “feel” to installers, who sometimes judge installation quality by effort (torque) rather than the recommended installation practice. Fittings that require high installation torque or that vary widely in the “feel” and effort required to achieve complete pull-up may cause installers to improperly tighten components and severely degrade tube fitting performance.
- Fittings should be of a compatible material with the tubing or pipe material on which they are used to avoid electrolysis and to provide acceptable weld joints.
- Tube fittings should be used at pressure-temperature ratings not exceeding the recommendation of the tube fitting manufacturer and to meet the environmental and process system requirements.
- Tube fittings should be installed in accordance with manufacturer's recommendations.
- In the absence of any existing standards, the designer should determine that the type of fitting selected is qualified for design conditions (including vibration, pressure, and thermal shock and applicable environmental conditions) or should demonstrate this by testing the fitting's ability to perform its intended function. The fittings selected should not degrade the inherent strength of the tubing specified.
- Screwed joints in which pipe threads provide the only seal may be used, as long as they are in compliance with the appropriate code and system temperature and pressure requirements.
- Thread sealant should be suitable for the required service conditions considering the process media, radiation environment, and compatibility with the materials of construction.
- It should withstand the temperature and pressure cycling as per appropriate standard (PB-E-146).
- Pull Out Capability: Tube fitting should provide sufficiently robust grip on the tube such that when a tensile load (e.g. during hydro test or during operating conditions) is applied on it the tube does not pull out of the grip.
- Generally the acceptable pull out tension load is more than four times the hydrostatic test pressure load.
- It is recommended that compression type tube fitting should not be used for welded tubes. As in such type of tubes the hardness differs at the point of welding. This difference makes the gripping action of the ferrule unreliable. (This may be noted that NAPS onwards welded tubes have replaced by Seamless annealed tubing in all NPCIL plants/projects)
Tube fittings for stainless steel tubing typically include an assembly of:
- a tube gripping device, often in the form of a ferrule or ferrules, or a gripping ring-like structure, and
- a pull-up mechanism for causing the tube gripping device to be installed on a tube end so as to grip the tube end and provide a seal against leakage.
The term "pull-up" simply refers to the operation of tightening the tube fitting assembly so as to complete the assembly of the fitting onto the tube end with the desired tube grip and seal.
Usually a stainless steel tube fitting is first assembled in a "finger tight" condition and then a wrench or other suitable tool is used to tighten or "pull up" the fitting to its final initial and complete assembled condition. In some cases, especially for larger tube sizes, a swaging tool is used to pre-install a ferrule onto the tubing. The pull up mechanism most commonly used is a threaded connection of a female threaded nut component and a male threaded body component, with the tube gripping device being acted upon by these two components as they are threaded and tightened together. The body includes a tube end receiving bore with an angled camming surface at the outer portion of that bore. The most commonly used camming surfaces are frusto-conical such that the term "camming angle" refers to the cone angle of the camming surface relative to the tube end longitudinal axis or outer surface. The tube end is axially inser ted into the body bore and extends past the frusto-conical camming surface. The gripping device is slipped onto the tube end and the nut is partially threaded onto the body to the finger tight position such that the tube gripping device captured axially between the camming surface and the nut. The nut typically includes an inward shoulder that drives the tube gripping device into engagement with the angled camming surface on the body as the nut and body components are threadably tightened together. The angled camming surface imparts a radial compression to the tube gripping device, forcing the tube gripping device into a gripping engagement with the tube end.
The tube gripping device typically is to form a seal against the outer surface of the tubing and also against the angled camming surface.
Generally following types of fittings are available:
(a) Flareless Compression Type (Single Ferrule)
(b) Flareless Compression Type (Double Ferrule)
(c) Bite Type
(d) Flared Fitting
A flareless tube fitting generally refers to a type of tube fitting in which the tube end remains substantially tubular, in contrast to a flared tube fitting in which the tube end is outwardly flared over a fitting component. Flared tube ends are commonly encountered in use with plastic tubing and plastic tube fittings.
The present note is not directed to plastic tubing or tube fittings because such fittings have significantly different challenges and material properties that affect the ability of the fitting to both grip the tube and provide an adequate seal.
Operating pressures and temperatures are also typically substantially lower in the plastics tubing systems. In other words, with respect to tube grip and seal, whatever works in a plastic tube fitting provides little or no guidance for a non-plastic tube fitting.
Among the above, the recommended fitting is flareless compression type twin ferrule tube fitting. Because of its ease of installation and higher reliability this type of fitting is most commonly used.
This is made up of a nut, sleeve and body with a flare or coned end. In some instances, the sleeve is used as a self-flaring option, usually on thinner wall or softer tubing materials. Compared to the original compression fitting, the flare fitting can handle higher pressures and wider system parameters. It is also available in a larger variety of materials and has a larger seal area, which provides remake capabilities in maintenance applications. However, special flaring tools are required to prepare the tubing for installation. Additionally, flaring of the tubing can cause stress risers at the base of the flare or cause axial cracks on thin or brittle tubing. Uneven tube cuts will create an uneven sealing surface.
As compared to ordinary compression joints, the ferrule holds the pipe in its place to give a proper seal when the nut is screwed on to the body. When it is fully tightened, the case hardened ferrule is pushed slightly in the middle where it acts as a spring. This maintains a continuous friction between the body and nut and which help prevent the nut from loosening under stress and repeated vibration.
Bite-type fittings are typically single ferrule in design. This requires the nose of the ferrule to perform two functions: to bite into the tube to hold it, and to provide a sealing element for the coupling body, an action that can easily compromise one or both functions. A two-ferrule separation of functions (the first to seal, the second to hold the tube) would solve this problem, as the separation would permit each of the elements to be designed specifically for the task.
Over the years there have been numerous tube fitting designs that do not rely on a "bite" type action, but rather merely radially compress the tube gripping device against the tubing outer surface, some with the effect of indenting into the tubing without creating a bite.
The most common commercially available stainless steel tube fittings especially for high pressure applications have historically been of two radically distinct designs of the tube gripping device--single ferrule tube fittings and two ferrule tube fittings.
A single ferrule tube fitting, as the name implies, uses a single ferrule to accomplish both the tube grip and seal functions.
For single ferrule tube fittings, the tube gripping action is usually associated with the single ferrule being designed to bow in a radially outward direction from the tube wall in the central region or mid-portion of the single ferrule body between the front and back ends thereof.
The front end of the ferrule is driven against the angled camming surface of the body by the nut pushing against the back end of the ferrule. The bowing action helps direct the front end of the single ferrule into the tube end. The bowing action is also used to cause the back end of the ferrule to likewise engage and grip the tube end. This is accomplished usually by providing an angled drive surface on the nut shoulder that engages the back end of the single ferrule so as to radially compress the back end of the ferrule into a gripping action on the tube end. In some single ferrule designs, the back end of the ferrule apparently is intended to bite into the tube end.
This back end tube grip is sometimes used with the single ferrule in order to attempt to improve the tube fitting's performance under vibration because the back end grip attempts to isolate down-tube vibration from affecting the front end tube bite.
The use of a back end tube grip actually works against the effort to grip the tube end at the front end of the single ferrule. Ideally, the single ferrule should be completely in three dimensional compression between the nut and the camming surface of the body. Providing a back end grip actually places a counter acting tension to the single ferrule that works against the front end compression being used to provide the tube grip. Additionally, the outward bowing action tends to work against the effort to grip the tube at the front end of the single ferrule because, in order to enable the outward bowing action, the single ferrule requires a lessened mass that is adjacent the tube gripping "bite". The outward bowing
action radially displaces ferrule mass from central of the ferrule body to away from the tube end. Consequently, an outwardly bowed single ferrule fitting could be more susceptible to ferrule collapse, loss of seal and possibly tube blow out at higher pressures.
In order to achieve an adequate tube grip on stainless steel tubing, single ferrule stainless steel tube fittings have historically used a rather shallow camming angle of between 10o and 20o. This range of angles is referred to herein as "shallow" only as a term of convenience in that the angle is rather small. The shallow camming angle has been used in single ferrule fittings to obtain a mechanical advantage because the shallow angle provides an axially elongated camming surface against which to slide and radially compress the single ferrule front end to bite into the tube end outer surface. Hard stainless steel tubing material necessitated this elongated sliding camming action in order to be able to get the single ferrule to create an adequate bite for tube grip. Over the years, the single ferrule has been ‘through hardened’ or ‘case hardened’ so as to be significantly harder than the stainless steel tubing, however, the shallow camming angle is still used today in such single ferrule fittings to obtain a mechanical advantage from the ferrule sliding along the camming surface to produce the "bite" so as to assure an
adequate tube grip. An example of a commercially available single ferrule tube fitting that uses a case hardened ferrule and a shallow camming angle of about twenty degrees is the CPI fitting line available from Parker-Hannifin Corporation. Another example is the EO fitting line available from Ermeto GmbH that uses a through hardened single ferrule and a twelve degree camming angle.
In some single ferrule designs, a non-conical camming surface has been tried whereby an attempt is made to simply press the ferrule against the outer surface of the tube end, thereby not creating a bite. The result in such cases however is a low grip or low-pressure-only fitting that are not well suited to stainless steel fittings.
It is becoming increasingly recognized that the two primary functions of a tube fitting viz. tube gripping and sealing are at odds with each other when designing a tube fitting that can meet a desired tube grip and seal performance criteria. This is because the design criteria needed to assure that the tube fitting achieves an adequate tube grip usually works against the ability of the single ferrule to also provide an effective seal. Consequently, although single ferrule fittings can achieve adequate tube grip in some cases, this tube grip performance comes at the expense of having a less effective seal. The shallow camming angle and elongated camming surface and axial movement needed to achieve an adequate tube grip with a single ferrule fitting, however, compromises the ability of the single ferrule to achieve the seal function, especially in extreme environments and for sealing gas. This is because the front end of the single ferrule attempts to make the seal against the axially elongated camming surface. The radially outward bowing action causes a larger portion of the outer surface of the front end of the single ferrule to come into contact with the camming surface against which it is being driven. The result necessarily is a larger seal surface area between the outer surface of the single ferrule and the camming surface. This enlarged seal area causes an unwanted distribution of the sealing force between the single ferrule and the camming surface, and also creates a larger area for surface imperfections to allow leaks to occur. This is particularly a metal to metal seal issue (as contrasted to non-metal to non-metal seals: for example, in a plastic fitting it is usually desirable to provide an enlarged seal contact area because the more highly ductile plastic material can better form a seal between the two surfaces.)
One result of this situation is that some single ferrule tube fittings have been designed with additional components and techniques to achieve an adequate seal. Less than optimum seal performance is particularly noted in single ferrule fittings that attempt to seal against gas, and especially high pressure gas. Single ferrule tube fittings thus are usually more suited to lower pressure liquid applications such as hydraulics, however, even in such lower pressure applications single ferrule seal performance remains less than desired.
The double ferrule fitting has the ability to lock onto the tube with a 'double bite' feature. Each ring bites in to the tube giving two separate sealing areas. This style of fitting does so without transmitting torque or twisting the tube ensuring that the tube does not become 'stressed'. Therefore, the mechanical properties of the tube are maintained. A further sealing point occurs at the bottom of the tube abutment. The abutment has an angle which the tube is forced into when the rings bite and drive the tube forward.
13.8.1 Ferrule and its purpose
highly engineered and, to function properly, requires considerable design, metallurgy, and production expertise. Not all products on the market meet these stringent requirements.
For instance, the ferrule must precisely deform elastically and plastically during fitting assembly to properly grip and seal the tubing. Its front edge must be harder than the tubing to grip and seal through surface scratches and defects, but if the entire ferrule is too hard, it may not deform properly. Therefore, only the gripping edge of the ferrule is hardened while the rest has different, tightly controlled mechanical properties. Also, the hardening process must not compromise stainless steel's corrosion resistance. And finally, production processes must consistently turn out defect-free ferrules that hold tight tolerances and maintain metallurgical specifications.
Ferrules provide a reliable, leak-proof connection in instrumentation and process tubing systems. These tube fittings consist of four precision-machined components: body, front ferrules, back ferrules, and nut. Ferrules make up for the variation in the tubing material, hardness and thickness of the tube wall in order to provide leak-proof connections in a large number of applications. They also reduce the number of potential leak paths in the connection, boosting safety, reliability and integrity. They also simplify assembly and maintenance.
Ferrules can generally handle pressures up to 15,000 PSI / 1,034bar. They eliminate the time-consuming 'coning and threading' that usually needs to be performed when applying traditional high-pressure flared fittings, allowing fittings to be installed in seconds by simply tightening a nut.
Back and front ferrules are designed to provide leak resistant, secure and tight connections for operations at high pressures. These fittings provide a tight pressure seal and have a long thread area for improved resistance to pressure and load on ferrules. Long support area of back ferrules improves resistance to vibration and line loads.
Swaging is a process that is used to reduce or increase the diameter of tubes. A swaged piece is created by placing the tube inside a die that applies compressive force by hammering radially.
Swaging can be further expanded by placing a mandrel inside the tube and applying radial compressive forces on the outer diameter. Thus, through the swage process, the inner tube diameter can be a different shape, for example a hexagon, and the outer is still circular. Flared piece of pipes are sometimes known as "swage nipples," "pipe swages," "swedge nipples," or "reducing nipples".
13.8.3 Operation of a twin ferrule tube fitting
Function of Front ferrule
In the two ferrule fitting, the tube grip and seal functions also are separately achieved by the use of two ferrules. The forward or front ferrule provides an excellent seal even against gas, and the back or rear ferrule provides an excellent tube grip.
The front ferrule achieves an excellent seal by camming against a shallow camming surface angle such as twenty degrees. This is because the front ferrule does not need to slide excessively on the camming surface in order to achieve a tube grip function. Likewise, the front ferrule is not case hardened because the primary purpose of the front ferrule is to seal and is not to bite into the tube end. Thus the relatively "softer" front ferrule achieves an excellent seal, particularly against gas, even though the body conical camming surface presents a camming angle of about twenty degrees.
Function of a Back Ferrule
The back ferrule achieves the tube grip function in the two ferrule tube fitting. The back ferrule is case hardened to be substantially harder than the tube end. Tube fittings depend on a balance of factors to ensure proper installation and performance. In a two-ferrule tube fitting design, the back ferrule moves the front ferrule forward to spring load the fitting assembly, burnish and seal with the fitting body, and create the primary tubing seal. The front end of the back ferrule cams against a frusto-conical camming surface formed in the back end of the front ferrule. The ostensible angle of this camming surface is forty-five degrees, but due to the sliding movement of the front ferrule, the effective camming angle is
actually a shallow angle of about fifteen to twenty degrees. Although the effective camming angle for the back ferrule is shallow, the back ferrule is not required to provide a primary seal (although it can form secondary or backup seals). The back ferrule also does not exhibit the undesired bowing action but rather grips the tube end as a function of a radially inward hinging action. As used herein, the term "hinging" refers to a controlled deformation of the ferrule such that a central region or mid-portion of the ferrule body undergoes an inwardly radial compression, as distinctly contrasted to a bowing or radially outward displacement. Thus, the effective shallow camming angle not only does not compromise the fitting seal capability, it actually substantially enhances the overall performance of the tube fitting especially for stainless steel tubing.
By using separate ferrules for each to achieve primarily only one of the key tube fitting functions, the two ferrule tube fitting achieves tremendous tube grip and seal functions.
The back ferrule also swages the tube to provide the grip needed to keep the fitting and tubing firmly in place. To swage and grip the tube properly, the back ferrule’s leading edge must be sufficiently harder than the tube. Two methods of producing this differential hardness may be employed—
1. Complete surface hardening of the back ferrule:
The use of complete surface hardening on a conventional back ferrule can have several drawbacks.
First, it typically increases installation torque because a surface-hardened, conventional back ferrule is unable to flex or “hinge” downward to improve swaging action on the tube. Instead, it must be wedged into position using installer torque, and as a result, more torque typically is required. Second, because it is not engineered to hinge and absorb installer torque on remakes, a conventional surface-hardened back ferrule can tend to overdrive the front ferrule when remade. This condition can potentially damage the tubing and fitting body and compromise the front ferrule action required for consistent gas-tight remakes.
2. Selectively hardened back ferrule: Use of a selectively hardened back ferrule, Swagelok reduced installation torque while providing the swaging and gripping action needed to perform in combination with a wide variation of commercial grade tubing. In manufacturing back ferrules selectively hardening the nose of the back ferrule is done, yet the center section and rear flange are left softer. During make-up, this softer center section acts as a hinge point when force is applied to the flange. This hinging mechanism helps limit the amount of torque required by the installer, yet delivers the right amount of swaging action through the nose of the back ferrule.
The improved engineered hinging action of the back ferrule (Figure 4, next page) provides several benefits:
- It advances and seals the front ferrule predictably and accurately.
- It flexes to maintain installation torque at a predictable and manageable level, even on hard materials.
- It smoothly and efficiently delivers more swaging energy earlier in the pull-up process. As a result, it reduces the potential for improper installation and leakage in cases where the fitting is less than properly tightened.
- Its proprietary metallurgy and hinging action can absorb excess torque inputs to help prevent overdriving of the front ferrule, thus ensuring more predictable gas-tight sealing during remakes.
An important aspect of the choice of material s is that the ferrule preferably should be case or through hardened to a ratio of at least about 3.3 and preferably 4 or more times harder than the hardest tubing material that the fitting will be used with. Therefore, the ferrule need not be made of the same material as the tubing itself. For example, the ferrule may be s elected from the stainless steel material s or other suitable materials that can be case hardened, such as magnesium, titanium and aluminum, to name some additional examples.
Figure 13-4: 316 SS Advanced Swagelok Tube Fitting Prior to Make –up
During make-up, the front ferrule (center) is driven into the body of the fitting (right) and the tube (bottom) to create primary seals (tube and body), while the back ferrule (left) hinges inward to create a strong grip on the tube. The back ferrule geometry allows for an improved engineering hinging action that translates axial (forward) motion into radial swaging action on the tube, yet operates with a low input force (torque) requirement. The improved radial colleting action of the back ferrule (the area to the left of the swage point) isolates and protects the swaged area of the tube, preventing the exposed vibration stress riser that is typical of bitetype fittings.
A distinct advantage of the contoured back ferrule is that pull up forces between the nut drive surface and the contoured face of the Back ferrule are more uniformly distributed across the surface of the back ferrule, thus reducing and substantially eliminating force concentrations. This further reduction of force concentrations on the drive nut reduces pull up torque and reduces galling, thus facilitating re-make of the fitting.
13.8.4 Effect of Tube thickness on Swaging
The strength of the fitting is such that the tube contained will burst before the fitting shows any sign of a leak or movement. This is subject to certain constraints on the wall thickness of the tube. Tube thickness is decided by following factors
a. Pressure rating
b. Corrosion/Threading allowance
c. Swaging considerations
For swaging over thickness may lead to unreliable joint and in very thin tube it may lead to distortion of tube leading to leakage. Thus considering all the above factors, optimal thickness should be selected when use of compression type of tube fittings is envisaged.
A heavy wall tube resists ferrule action more than a thin wall tube, allowing the ferrules to coin out minor surface imperfections. If the wall is too heavy the rings will not bite.
A thin wall tube offers less resistance to ferrule action during installation, reducing the chance of coining out surface defects, such as scratches. When the tube wall is too thin, the tube will collapse rather than allow the rings to bite fully. Within the applicable suggested allowable working pressure table, select a tube wall thickness whose working pressure is outside of the shaded areas. Reference to the manufacturers' product information should be made in all instances. The tube should generally have a hardness of no more than 80 on the Rockwell 'B' scale.
Advanced Swagelok Fitting
The advanced Swagelok two- ferrule tube fitting offers predictable, leak free performance up to the burst pressure of ANSI 316 and 304 stainless steel tubing. A summary of its benefits include:
Wider Target for Proper Installation: The engineered hinging action of the back ferrule delivers energy to not only seal the front ferrule, but also to deliver greater swaging action throughout the pull –up process . As a result, this fitting reduces the potential for improper installation and system leakage, even in cases where the fitting was less than properly tightened.
Enhanced Gas Seal : The back ferrule hinge delivers steady force to seal the front ferrule consistently on a wide range of tubing. Because the advanced back ferrule can hinge and absorb more energy than a conventional hardened back ferrule, this design reduces the potential for overdriving the front ferrule, thereby ensuring reliable operation and gas seal for repeated remakes .
Vibration Fatigue Resistance: The engineered back ferrule hinging action delivers a more consistent radial colleting action to give improved support to the tube behind the point of grip. This colleting protects the swaged area of the tube more effectively from system vibration and fatigue .
Greater Margin of Performance on Commercial Tubing: Textbook calculations , such as Lame’ s formula for determining minimum rupture pressure of a tube , use the minimum allowable ultimate tensile strength, minimum allowable wall thickness, and maximum allowable outer diameter for tube burst calculations —as they should. However, these calculations offer a conservative estimate of the tube’ s pressure-containing ability . In reality, stainless steel tubing manufacturers do not always run their processes for the minimum required material strength values cited by ASTM and other standards for determining the rupture pressure of a tube. The result is stronger, harder tubing with burst pressures of ten significantly higher than what occurs under least case conditions . The advanced Swagelok tube fitting is robust enough to grip and exceed the burs t pressure of these stronger, available tubing materials . In addition, the uniform surface -hardened design of the back ferrule offers high corrosion resistance.
Compatibility with Original Design Swagelok Tube Fittings: The advanced Swagelok fitting pulls up using the same one-and-one-quarter - turn procedure as the original design Swagelok tube fitting.
In addition, the advanced Swagelok fitting uses the same installation inspection gauges as before. However, what every ins taller will notice is a more consistent feel, from a more consistent range of torque on every pull-up to an even more consistent, leak free connection.
Applicability to New Alloys: The advanced Swagelok fitting demonstrates it is practical to develop an easy - to install, high-performance tube fitting that can be built using advanced alloys, such as super duplex steel, despite their increased strength and advanced mechanical properties .
13.8.5 Safety precautions for tube fitting installation
Following safety precautions should be taken while installing the tube fitting1
- Do not bleed the system by loosening the fitting nut or fitting plug.
- Do not make up and tighten fittings when the system is pressurized.
- Make sure that the tubing rests firmly on the shoulder of the tube fitting body before tightening the nut.
- Use the gap inspection gauge to ensure sufficient pull-up upon initial installation.
- Never allow problems to go unreported.
- Always use proper thread sealants on tapered pipe threads. In NPCIL a Nickel compound based sealant is used (Never Siege compound) to avoid galling
- Do not mix materials or fitting components from various manufacturers— tubing, ferrules, nuts, and fitting bodies.
- Never turn the fitting body. Instead, hold the fitting body and turn the nut.
- Avoid unnecessary disassembly of unused fittings.
- Stainless-steel parts that rub together under high pressure have a strong tendency to cold weld and seize. And to form high-integrity, leak-free tubing connections, ferrules must only slide forward during assembly and not rotate with the nut. To prevent seizing and ensure only linear ferrule movement, surface conditions and lubrication at the nut/ferrule and nut/body interfaces should be precisely controlled..
- All mating surfaces must be smooth and free of defects, which exacerbate seizing. A bonded molybdenum-disulfide coating is the recommended lubricant for many compression fittings.
- Solid molybdenum disulfide readily adheres to surfaces, is noted for its lubrication and anti-seizing properties, and the solid does not squeeze out like liquid or soft, waxy lubricants under extreme pressure. The result is low assembly torque and consistent performance, even with repeated remakes.
- Additional tubing considerations:
- Always use an insert with extremely soft or pliable plastic tubing.
- Wall thickness should always be checked against the fitting manufacturer’s suggested minimum and maximum wall thickness limitations.
- Surface finish is very important to proper sealing. Tubing with any kind of depression, scratch, raised portion, or other surface defect will be difficult to seal, particularly in gas service.
- Tubing that is oval and will not easily fit through fitting nuts, ferrules, and bodies should never be forced into the fitting.
- When installing fittings near tube bends, there must be a sufficient straight length of tubing to allow the tube to be Bottomed in the fitting (see figure-13-7). The following table indicates the minimum straight length required.
Special precautions for Gas Service
Gases (air, hydrogen, helium, nitrogen, etc.) have very small molecules that can escape through even the most minute leak path. Some surface defects on the tubing can provide such a leak path. As tube outside diameter (OD) increases, so does the likelihood of a scratch or other surface defect interfering with proper sealing.
The most successful connection for gas service will occur if all installation instructions are carefully followed and the heavier wall thicknesses of tubing on the tables-1-3 to 1-10 are selected.
Repeated assembly and disassembly of the tube fitting causes the reduction in the distance between the two ferrules. As the distance between the two ferrules reduces over a period of time the back ferrule’s spring action diminishes and a time comes when both
the ferrules touch each other and the leak tightness provided by this assembly is no longer assured. Therefore it is necessary to keep this gap under check and whenever this gap is found to be very little the new tube fitting and swaging should be used. The figure:13-8 shows this type of action.
Figure-13-8: Tube fitting in assembled condition
1. Pipe threads should be NPT threads as per ANSI-B-1.20.1
2. Needs the above seismic test is required on 1% of each type of fitting. This test should be performed on limited samples taken from the lot.