Instrumentation :: Tube Fittings :: Features& Installation
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.

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
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.
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 :
  • 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).
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

Following instruction should be followed while doing the first assembly.

  1. Ensure that the end of the tube is cut square and any burrs removed without causing causing undue chamfering to the tube end.
  2. 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.
          3.   Before lighting the PMT nut, scribe the nut at the 6.00 ‘o’ clock position as illustrated in the Following figure.
          4.   Now while holding the fitting body firmly with a backup wrench, tighten the nut one quarter Turns, watching the scribe mark.
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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