Instrumentation :: Tube Fittings :: Features& Installation
 
RE-ASSEMBLY INSTRUCTIONS OF   TUBE FITTINGS
With PMT tube fittings the connection can be disconnected and retightened many times and the same reliable, safe and leak proof connection can be obtained. For re-assembly the following steps should be followed
 
          1.   Insert tube with pre-swaged ferrules into the tube fitting until front ferrule seats in fittings.
 
 
          2.   Holding the body with spanner, the nut is tightened to the original position and then given a slight extra effort to retain positive sealing
 
 
PIPE THREAD CONNECTION
Most widely used NPT threads have been machined for connecting the threaded fittings. Leak tighteness is not achieved by  the  thread  alone  without  sealant.  Commercially  available  Teflon  tape/paste  is  to  be  used  to  provide  sealing  and lubricating agent. Wrap the tape 4 to 5 layers around the male threads in a clockwise direction. Ensure that the tape/paste does not over hang the first thread.
 
SELECTION OF TUBING
The PMT twin ferrule tube fitting is precision engineered to exacting standards. Careful selection of high quality tubing together with appropriate Super-lok fitting will ensure the installation of safe, leak free systems.

PMT tube fittings are designed to work on line material. Stainless steel fittings should be used with S.S. tubing. The practice of mixing material is strongly discouraged. The only exception is brass fittings with copper tubing.

Disimilar materials in contact may be susceptible to galvanic corrosion. Further different materials have different levels of hardness, and can adversely affect the fittings ability to seal on the tubing.

The tubing wall thickness is decided based on maximum pressure rating. Thick walled tubing helps to provide resistance Very low wall thickness of tubing may result in unreliable joints. For PMT S.S. tube fittings seamless S.S. 316/316L/304L fully annealed tubing of high commercial quality as per ASTM-A-.269/ASTM-A-213 is recommended

PMT tube fittings are normally not recommended for tube wall thickness outside the ranges shown in Table I, II & III.

The key to proper tube hardness for use with PMT tube fitting is that the tubing should be softer than the fitting material Based on this, preferred hardness of tubing is Rb 80 or less. If  harder tube is used the maximum hardness should not exceed Rb 90.
 
TUBING FOR GAS SERVICE
Special care must be taken while selecting tubing for gas service. In gasses even a small surface defect may provide a leak path.  In  order  to  achieve  a  gas  tight  seal,  ferrules in  instrument fittings  must  seal  any  surface  imperfections.  This  is accomplished by the ferrules penetrating the surface of the tubing.

Penetration can only be achieved if the tubing provides radial resistance and if the tubing material is softer than the ferrules Though maximum allowable hardness of S.S. tubing as per ASTM 269/213 is Rb 90, preferred hardness of tubing should be Rb 80 or less for reliable and leak proof joints mainly in gas application.

The suggested minimum wall thickness of S.S. tubing for gas service for different sizes of tubing is given in following table

SUGGESTED MINIMUM WALL THICKNESS OF S.S. TUBING FOR GAS SERVICE
 
TUBE O.D
(Inch)
SUGGESTED MINIMUM
WALL THICKNESS
(Inch)
1/8 0.028
3/16 0.028
1/4 0.028
5/6 0.035
3/8 0.035
1/2 0.041
5/8 0.052
3/4 0.062
7/8 0.073
1 0.083
 
RECOMMENDED MAXIMUM WORKING PRESSURE FOR DIFFERENT TUBING
The maximum allowable working pressure for different tubings (as given in following tables) have been calculated using the  maximum  allowable  stress  levels  in  accordance  with  ANSI  B  31.3.  The  calculations  are  based  on  nominal  wall thickness and at ambient temp. At higher temperature the maximum working pressure will be less as the allowable stress will be reduced. The maximum working pressure in the following tables have been given only for recommended minimum to maximum wall thickness for different sizes of tubing

TABLE-1
RECOMMENDED  MAXIMUM  WORKING  PRESSURE  FOR  SS  304/316  TUBING  (SEAMLESS)  IN  POUNDS  PER SQUARE INCH.
 
TUBE O.D
(Inch)
TUBE WALL THICKNESS (inch)
0.028 0.035 0.049 0.065 0.083 0.095 0.109 0.120
1/8 8600 11200 - - - - - -
3/16 5500 7000 10400 - - - - -
1/4 4000 5100 7500 10500 - - - -
5/6 - 4100 5900 8100 - - - -
3/8 - 3300 4800 6600 - - - -
1/2 - 2500 3500 4800 6300 - - -
5/8 - - 3000 4000 5200 6100 - -
3/4 - - 2400 3300 4300 5000 5800 -
7/8 - - 2100 2800 3600 4200 4900 -
1 - - - 2400 3200 3700 4200 4700
 
TABLE-2
RECOMMENDED MAXIMUM WORKING PRESSURE FOR STEEL (SEAMLESS! IN POUNDS PER SQUARE INCH)
 
TUBE O.D
(Inch)
TUBE WALL THICKNESS (inch)
0.028 0.035 0.049 0.065 0.083 0.095 0.109 0.120
1/8 8200 10900 - - - - - -
3/16 5200 6700 10100 - - - - -
1/4 3800 4900 7200 10100 - - - -
5/6 - 3800 5500 7700 - - - -
3/8 - 3100 4500 6200 - - - -
1/2 - 2300 3300 4500 6000 - - -
5/8 - 1800 2600 3500 4600 5400 - -
3/4 - - 2200 2900 3800 4400 5100 -
7/8 - - 1800 2500 3200 3700 4300 -
1 - - 1600 2100 2800 3200 3700 4100
 
TABLE-3
RECOMMENDED  MAXIMUM  WORKING  PRESSURE  FOR  COPPER  (SEAMLESS)  TUBING  IN  POUNDS  PER
 
TUBE O.D
(Inch)
TUBE WALL THICKNESS (inch)
0.028 0.035 0.049 0.065 0.083 0.095 0.109 0.120
1/8 2800 3800 - - - - - -
3/16 1600 2300 3600 - - - - -
1/4 - 1700 2600 3600 - - - -
5/6 - 1700 2000 2800 - - - -
3/8 - 1100 1600 2300 - - - -
1/2 - 800 1200 1600 2100 - - -
5/8 - - 900 1300 1700 2000 - -
3/4 - - 800 1000 1400 1600 1900 -
7/8 - - 600 900 1100 1300 1600 -
1 - - 550 800 1000 1200 1400 -
 
QUALITY ASSURANCE PLAN
PMT Twin ferrule tube fittings are manufactured to conform with the applicable sections of the American standard code for pressure piping (ASME codes). Manufacturing process is precisely controlled by a well established and approved quality assurance programme.

Manufacturing is started only  after verifying the chemical composition and hardness of raw material.  Unless otherwise asked for special material, PMT twin ferrule tube fittings are manufactured from S.S. 316 material. Ail bar stock items, i.e nut  ferrules  and  straight  connectors,  unions,  etc.  are  manufactured  from  S.S.  316  meeting  ASTM-A-479/276 requirements. All shaped fittings i.e. tees, elbows, etc. are manufactured from forgings meeting requirementsofASTM-A- 182.

Batch production of fittings is allowed only after qualification of sample fitting parts. Internal bores, seat angle, tube bore depth, ferrule and body inner dimensions and pipe threads are checked on 100% basis.

To check the mechanical conformance of the fittings/parts the following test are performed :
A) Material Test
B) Hydro Test
C) Vibration Test
D) Leak Test
E) Dimensional Conformance Test
F) Hardness test

Hardness test on ferrules, pressure test, make remake test, vibration test, pull out tests, pressure & temp cycling tests are done on limited samples selected randomly from the lot and in case of any failure, full lot is tested for above tests.

In house Extensive test facility is being developed for performing different tests on tube fittings.
 
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