What is ASTM A688 U Bend Stainless Steel Tube for Feedwater Heater?

What is ASTM A688/A688M?
ASTM A688/A688M is a standard specification for U-bend stainless steel tubes used in feedwater heaters, covering both seamless and welded austenitic stainless steel tubes with specific size ranges and requiring corrosion resistance testing. These tubes are used in various industrial applications, including power plants and chemical plants, due to stainless steel's high-temperature resistance and durability.

 

Key Aspects of ASTM A688 U-Bend Tubes:
Standard Specification: ASTM A688/A688M covers seamless and welded austenitic stainless steel feedwater heater tubes.
Application: The primary application is in tubular feedwater heaters, where the U-bend form facilitates easy withdrawal and cleaning of tube bundles.
Tube Types: Both seamless and welded tubes are included, with high-quality TIG welding often used for welded tubes to ensure integrity.
Sizes: The specification covers outside diameters from 5/8 to 1 inch (15.9 to 25.4 mm) and wall thicknesses of 0.028 inches (0.7 mm) or more.
Properties: The tubing is selected for its high-temperature resistance, corrosion resistance, and durability, making it suitable for high-temperature, corrosive environments.
Testing: Nondestructive testing, such as electric tests, and checks for corrosion resistance are performed to ensure quality.
Finishing: Tubes can be supplied in various finishes, including annealing, pickling, or bright annealing, depending on the requirements.

 

ASTM A688 U Bend Stainless Steel Tube Specification

shall be applied to the U-bend area plus approximately 6 in. [150 mm] of each leg beyond the tangent point of the U-bend after bending, if specified.

: max 30000 mm

: 12000 mm, points of tangency of the bend to the leg end

: max 1500 mm

equal to 2 x leg length + 3.1416 x R

 

ASTM A688 Stainless Steel Chemical Composition

Elem-Ent		TP304	TP304L	TP304LN	TP316	TP316L	TP316LN	TP XM-29	TP 304N	TP 316N
%		S30400	S30403	S30453	S31600	S31603	S31653	S24000	S30451	S31651
		0.08	0.035	0.035	0.08	0.035	0.035	0.060	0.08	0.08
		2.00	2.00	2.00	2.00	2.00	2.00	11.50-14.50	2.00	2.00
		0.040	0.040	0.040	0.040	0.040	0.040	0.060	0.040	0.040
		0.030	0.030	0.030	0.030	0.030	0.030	0.030	0.030	0.030
		0.75	0.75	0.75	0.75	0.75	0.75	1.00	0.75	0.75
		8.00-11.00	8.00-13.00	8.00-13.00	10.00-14.00	10.00-15.00	10.00-15.00	2.25-3.75	8.00-1.00	10.00-14.00
		18.00-20.00	18.00-20.00	18.00-20.00	16.00-18.00	16.00-18.00	16.00-18.00	17.00-19.00	18.00-20.00	16.00-18.00
		-	-	-	2.00-3.00	2.00-3.00	2.00-3.00	-	-	2.00-3.00
N		-	-	0.10-0.16	-	-	0.10-0.16	0.20-0.40	0.10-0.16	0.10-0.16

 

 

ASTM A688 U Bend Stainless Steel Tube Dimensional Tolerance

ASTM A688 U bend tube tolerance of outside should be accordance with specification A1016/M, the 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).

<= 20 (6)	1/8 (3.2)
>20(6), <=30(9)	5/32 (4.0)
>30 (9), <=40 (12.2)	3/16 (4.8)

<=5.8 (15.9)	0.010 (0.25)
>5.8 (15.9), <=1 (25.4)	0.016 (0.4)

Compared to ASTM A249/A268 specification, ASTM A688 requires more and specific tests to ensure high-quality tube, and more tighter tolerances.

 

All tubing shall be packaged and blocked in such a manner as to prevent damage in ordinary handling and transportation. The boxes shall be constructed in such a manner that no nails, staples, screws, or similar fasteners are required to close and secure the box after the tubes have been placed in the box. The box shall be lined with plastic sheet or vapor barrier materials so as to prevent chloride contamination of the tube during handling, transportation, and storage.

Package in Waterproof Paper bundles and put into Plywood Cases. ASME SA688 SA213 TP316L Stainless Steel U Bend Tube Pictures as below: