| Instrumentation :: Tube Fittings :: Features& Installation |
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| DESIGN FEATURES |
| PMT tube fittings are Composed of four precision engineered components (as shown in following fig.), all manufactured to very stringent tolerances under rigid quality control procedures. The consistency and quality of these matched components has permitted the use in many critical applications and severe services. |
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Although the fittings consist of four components: Nut, back ferrule, front ferrule and the body, it becomes a four piece
connection when affixed to tubing. The two ferrule design and sequential action of the fittings overcome in-variation of tube wall thickness and hardness to assure safe, reliable, leak-free connections PMT tube fittings are easily installed without
any special tools. The front ferrule swages onto the tube as it moves down the cone of the body creating a pressure/vacuum tight seal on both tube and body by interface pressure and surface finish of matching components. The
back ferrule, deforms into the tube creating a strong mechanical and anti-vibration hold on the tube. The internal diameter
of body and nut are closely controlled which constrain the tube within a close tolerance of its axis ensuring accurate
alignment within the assembled fitting. In case of S.S. tube fittings high hardness of ferrules
of ferrules is assured / maintained (more than Rd 90 ) For reliable joints the positive tolerances on inside diameter of ferrules and body is kept to minimum. The nut
threads are silver coated to ensure no galling
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Thus the design features on the tube end side can be summarised as below.
1) The back ferrule dampens the tube circuit vibration through a spring like action. Further more, it prevents a carry over
of tightening torque from the fitting nut to front ferrule
2) The front ferrule circumferentially seals the surface between the tube and fitting. The inner taper of front ferrule is
pressed on the tube so that a pressure tight impression is formed.
3) Silvering of nut threads prevents seizing/galling. This allows repeated use of fittings.
4) A deep body bore on tube side and taper, guarantees accurate positioning and centering of tube.
The twin ferrule connection does not in any way reduce the strength of the tube and therefore tube having thin wall PMT twin ferrule connection does not in any way reduce the strength of the tube and therefore tube having thin wall
can be used without affecting the safety of installation. |
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On the pipe end connection side of fittings, most widely used "NPT" threads are machined and rolled as per ANSI
B2.1/ANSI B1.20.1. For leak proof connection suitable sealing material (Teflon tape/paste) should be used. On request tube fitting with dry seal threads can be supplied. The straight
threads are fine threads machined as per ANSI B1.1 or equivalent standards.
Some of the important features/characteristics of PMT
Twin ferrule tube fittings are :
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- Withstands high pressure and vacuum.
- Leak tight assembly from vacuum to high pressure is assured.
- Excellent make and re-make life
- No Twisting of tubes while assembly.
- Assembling with self aligning front and back ferrule.
- No transmission of torque on tubing while assembly.
- Controlled tightening, no distortion of tube, no rupture of tube fibres and minimum reduction of tube bore size.
- Low tightening torque (the nut being of non binding design)
- Fittings can be used with thin and thick walled tubing. (As per guidlines of Table 1, 2 & 3)
- Silver plated nut to avoid galling and give low assembly torque. No pre-lubrication needed.
- Fittings can be used (ensuring leak tightness) in low to high temperature range.
- Anti-vibration hold on the tube (Vibration resistant).
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| IDENTIFICATION OF METRIC TUBE FITTINGS |
| Metric tube fittings are identified by a stepped shoulder on both the body and the threaded end of the nut as illustrated in the following fig |
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| ASSEMBLY INSTRUCTIONS OF PMT TUBE FITTINGS |
Following instruction should be followed while doing the first assembly. |
- Ensure that the end of the tube is cut square and any burrs removed without causing causing undue chamfering to the tube end.
- Simply insert yhe tubing into PMT tube fitting. Make sure that the tube is firmly butted on the shoulder of fitting and the nut is finger tight.
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| 3. Before lighting the PMT nut, scribe the nut at the 6.00 ‘o’ clock position as illustrated in the Following figure. |
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| 4. Now while holding the fitting body firmly with a backup wrench, tighten the nut one quarter Turns, watching the scribe mark. |
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| NOTE : |
Due to variation in tube diameters for high pressure and safety systems, a common starting point is desirable. To achieve this ( a wrench snug up the nut until the tube cannot be rotated by hand. From this point the nut should be rotated
one and a quarter turns.)
For sizes upto 1/8" in imperial tube fittings and 4 mm in metric tube fittings only 3/4 turn from finger tight position is necessary |
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