ASTM A688 / A688M covers seamless and welded Austenitic stainless steel feedwater heater tubes including stainless steel U bending tube for application in tubular feed-water heaters, surface condition in annealing and pickling, or bright annealing finishing, used in heat exchanger, boiler, condenser, heat transfer system.the tubing sizes covered shall be 5⁄8 in to 1 in. [15.9 to 25.4 mm] inclusive outside diameter, and average or minimum wall thicknesses of 0.028 in. [0.7 mm] and heavier.
ASTM A688 U bend tube tolerance of outside should be accordance with specification A1016/M, bent portion of U tubes for R =2 X D or greater, tolerance shall deviate from the nominal diameter prior to bending by more than 10 %, if less than 2 X D is speciﬁed, tolerances could be greater, minimum wall thickness shall not exceed +20 % − 0, average wall thickness are 10 % of the nominal wall thickness, bent area wall tolerance shall not be less than value determined by the equation: 4RT/(4R+D).
1.Heat treatment: Stress relive annealing of U-tubes after bending
2.Dimensional tolerances U-tubes acc. to TEMA R.C.B.
3.Length of straight part -0/+5 mm
4.Flattening (also called “ovality”) at the bend shall not exceed 10% of the nominal tube outside diameter.
5.Wall thickness in bending part acc. to TEMA RCB 2.31
6.Minimum tube wall thickness in the bend part (T min) T(min) ≥ (SW × (2×R + D))/( 2× (R+D)
where: SW=smallest wall thickness, D=Nominal outside diameter,R=radius
7.Radius tolerance 1) for R 100 mm +/- 3 mm, 2) for R ≥ 100 mm +/- 5 mm
8.Straightness tolerance max. 1,5 mm per 1 m
9.U-Tube ends: plain, vertically cut to the tube axis
10.Repairs by welding: prohibited
11. Surface condition Finished U-tubes shall be free of scale, without scratches after bending,
12. Delivery Condition: Passivation and oiled, Black paint or varnish, Bright Annealed
13. Packing:Plastic plugs in both ends, bundles of max. 3,000kg with several steel strips, Two tags on each bundle, Wrapped in waterproof paper packed in wooden case
Main Testing Items and Procedure:
3. Chemical Composition Analysis Test With 100% PMI and One tube from each heat by Direct Reading Spectrometer
4. Visual inspection for waviness and cracks by Dye Penetrant Test. and Endoscope Test for Surface Quality Test:
5. 100% Hydrostatic Test: Minimum:10 Mpa-25Mpa and 100% Eddy Current Test before U bend Formed
6. Ultrasonic Test before U bend Formed subject to the MPS (Material Purchase Specification)
7. Mechanical Tests includes Tension Test, Flattening Test, FlaringTest, Hardness Test
8. Impact Test subject to Standard request
9. Grain Size Test and Intergranular Corrosion Test
10. Ultrasonic measuring of Wall Thickness
11. Stress Relieve Annealing on U bend Parts after bending
12.Air Under Water Test After Bending
13. Cotton Ball Passing Test for ID cleanness