7.0 Methods of Connections of Instrumentation Tubes

Different types of connections/joints can be used in instrument sensing lines as discussed below:

7.1 WELDED JOINTS
a) Butt-welded joints should be made wherever possible.

b) Socket-welded joints are permitted but limited to tube/pipe size of 50 mm and less. Socket welded joints should conform to requirements of ANSI-B-16.11. While performing socket-welding, approximately 1.6 mm should be provided between the end of pipe/tube and bottom of socket, before welding.

c) Socket welds should not be used where the existence of crevices could accelerate corrosion.

7.2 FLARED, FLARELESS & COMPRESSION JOINTS
Flareless and compression type tube fittings may be used in instrument sensing lines for the tube sizes not exceeding 25mm (1”) O.D. provided following conditions are met.

a) Fittings and their joints should be of compatible material with the tubing or pipe material with which they are used and should conform to the range of wall thickness and method of assembly recommended by the manufacturers.

b) Tube fittings should be used at pressure-temperature ratings, not exceeding the recommendations of the manufacturer.

c) Wherever compression type flareless tube fittings are used, they should be made of design in which the gripping members or sleeve shall grip or bite into the outer surface of the tube hold the tube against pressure (to prevent blow out) but, without seriously deforming the inside diameter. The gripping member or sleeve should form a pressure seal against a fitting body.

d) Tube fittings should be installed in accordance with manufacturer’s recommended procedures.

e) The fitting parts of the original flareless tube fittings should be designed and manufactured by same company. Replacement parts from different manufacturers should be qualified to meet and seal properly at the design service conditions, or the complete fittings should be replaced when needed.

f) The fittings selected should not degrade the inherent strength of the tubing specified.

g) Service conditions such as vibration and thermal cycling should also be considered in the application.

h) Metal tubing material should be softer than fitting material. For example, stainless steel tubing should not be used with brass fittings.

i) When tubing and fittings are made of the same material, tubing must be fully annealed and ferrule of fittings should have more hardness than tubing material.

7.3 THREADED JOINTS
Threaded joints in which the threads provide the only seal may be avoided as far as possible. Threaded joints should not be used between the process taps and accessible isolating valves. However, threaded joints between accessible isolating valve and instrument may be used within the limitations specified below:

a) Thread size should not exceed ¾” (N.B.)

b) Pipe threads should be taper pipe threads in accordance with applicable standard. Suitable sealant should be used on pipe threads depending upon the type of threads.

c) Threaded joints should not be used when severe erosion, crevice corrosion, shock or vibrations are expected to occur.

d) In case of steam or hot water if service temperature is more than 100oC, threaded connections should be used only if process pressure is less than 100 kg/cm2.

e) Threaded connections should not be used at temperature above 495oC.

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