There is a variety of stainless steel tube fitting designs and installation procedures available today. Tube fittings have proven to be a fast, cost efficient, and reliable method for constructing analytical and process instrumentation systems. Tube fittings are also heavily used in process control systems, CNG vehicles, and utility systems such as pneumatics, hydraulics, and steam.
Tube Fitting Advantages
Designers like tube fittings and tubing systems because:
Tube fittings are fast and easy to install without special equipment or certified personnel
In most cases, the added costs of stainless steel tubing and tube fitting components are more than offset by the labor savings when comparing stainless steel tubing systems to welded or threaded pipe systems.
Tubing can be bent reducing the number of potential leak points versus threaded pipe and weld system
Tube fittings are remakeable there by simplifying component changes and reducing maintenance time and costs.
Stainless steel tubing is considerably less expensive than hose for comparable service pressure, temperature and flow rates.
Stainless Steel Tubing and Tube Fitting Requirements
While each tube fitting design has its advantages and disadvantages, every design is dependent on the quality of the tubing on which the tube fittings are installed. When selecting tubing, purchasers must consider tubing that meets the following requirements.
ASTM A269 section 12.2 allows some thin-walled stainless steel tubing to be produced with ovality of the tube twice the OD tolerance of ±.005”. This degree of out-of-roundness would prevent the tubing from even inserting into the tube fitting properly. Even when tubing can be inserted into the tube fitting restricting tubing ovality tolerance to better control ovality can improve tube fitting installation and performance.
ASTM specifications are ambiguous on the subject of finishes. Instrumentation tubing should be free from nicks, gouges, and deep scratches which can interfere with proper sealing, particularly in gas service. Checking suppliers packaging and preservation standards can ensure that tubing arrives in proper condition.
All instrumentation tube fittings require that the tubing be softer than the fitting material. While most metal tubing is restricted to a maximum hardness measured on the Rockwell hardness scale, a more restricted maximum hardness can reduce installed cost by allowing the tubing to be more easily bent and installed.
Material and Method of Manufacture
Select Dual Grade stainless steel tubing where possible. Dual Grade means alloy with identical physical characteristics and only slightly different chemical characteristics are specification-controlled favoring the grade with the more refined chemical characteristics.
Improving Stainless Steel Tubing - Tube Fitting Installation
Other considerations are features that support proper tubing storage and selection as well as proper tube fitting installation.
Plastic protective caps are installed onto the end of each stainless steel tube to preserve internal cleanliness from the factory as well as to protect the end of the tube for maximum yield. Improperly protected tube ends must be trimmed prior to using the tube. Tube caps prevent wasted time and material associated with preparing the tube end for installation and removing damaged tubing.
MAWP on the Tubing Lay Line
Improve system safety and increase installer and operator confidence by printing Maximum Allowable Working Pressure [MAWP] in psig and bar right onto the tubing lay line. This feature provides an additional measure of safety against using an inadequately rated tube in service. MAWP are calculated using widely accepted ANSI /ASME B31.3 Process Piping specifications and code calculations.
Stainless Steel Tubing Inspection and Certification
Stainless steel tubing should be fully heat and lot traceable. Chemical composition, yield, tensile strength, and elongation testing is performed. SSP Tubes pass functional tests including flattening, flaring, dimensional, and leak testing via Eddy current to ASTM A-450, as required. Because mills are so far from customers accountability for tubing quality usually rest with the reseller. Therefore, tubing should be certified by suppliers using Energy-dispersive X-Ray Fluorescence (EDXRF) technology or other non-destructive testing.
Inspection certificates and summarized test results are issued with each tube of different size or lot. Certification is always issued in compliance with EN 10 204 3.1 and ISO 10474.
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