We offer ASTM A688 seamless and welded U bend stainless steel tube for feedwater heaters, Austenitic stainless steel and Ferritic stainless steels, surface condition in annealing and pickling, or bright annealed finishing, used in heat exchanger, boiler, condenser, heat transfer system.
ASTM A688 / A688M covers seamless and welded Austenitic stainless steel feedwater heater tubes including U bending tube for application in tubular feed-water heaters, 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 Stainless Steel Tube Specifications
Straight length
max 30000 mm
Leg length
12000 mm, points of tangency of the bend to the leg end
Bending radius
max 1500 mm
Manufacturing method
TIG welding, Cold drawn
Finishes: Annealing Pickling (AP), Bright Annealed (BA)
Straight length equal to 2 x leg length + 3.1416 x R
Heat treatment 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.
ASTM A688 Stainless Steel Chemical Composition
| Elem-Ent | Grade | TP304 | TP304L | TP304LN | TP316 | TP316L | TP316LN | TP XM-29 | TP 304N | TP 316N |
|---|---|---|---|---|---|---|---|---|---|---|
| % | UNS | S30400 | S30403 | S30453 | S31600 | S31603 | S31653 | S24000 | S30451 | S31651 |
| C, Max | 0.08 | 0.035 | 0.035 | 0.08 | 0.035 | 0.035 | 0.060 | 0.08 | 0.08 | |
| Mn, Max | 2.00 | 2.00 | 2.00 | 2.00 | 2.00 | 2.00 | 11.50-14.50 | 2.00 | 2.00 | |
| Ph, Max | 0.040 | 0.040 | 0.040 | 0.040 | 0.040 | 0.040 | 0.060 | 0.040 | 0.040 | |
| S, Max | 0.030 | 0.030 | 0.030 | 0.030 | 0.030 | 0.030 | 0.030 | 0.030 | 0.030 | |
| Si,,Max | 0.75 | 0.75 | 0.75 | 0.75 | 0.75 | 0.75 | 1.00 | 0.75 | 0.75 | |
| Ni | 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 | |
| Cr | 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 | |
| Mo | — | — | — | 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 TP304, ASTM A688 TP304L, ASTM A688 TP304N, ASTM A688 TP316, ASTM A688 TP316L, A688 TP316N.
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).
Tube Leg Length Tolerance
| Leg Length, Ft (M) | Plus Tolerance, In (Mm) |
|---|---|
| <= 20 (6) | 1/8 (3.2) |
| >20(6), <=30(9) | 5/32 (4.0) |
| >30 (9), <=40 (12.2) | 3/16 (4.8) |
Squareness Of Ends Tolerance
| Tube OD, In, (Mm) | Tolerance, In, (Mm) |
|---|---|
| <=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.
Testing & Inspection
- Tensile Strength
- Reverse bending test
- Flattening test
- Flare test
- Flange test
- Dimensional
- Hydrostatic Test
- Pneumatic Test
- Hardness
- Eddy Current test
- Intergranular Corrosion (option)
ASTM A688/ASME SA688 Summary of Tolerances
| Acceptable Description | Tolerances |
|---|---|
| Deviation from plane of bend | ≤ 1.5 mm |
| Leg lengths different | ≤ 3.2 mm |
| Leg lengths tolerance L L ≤ 6 m 6 > L ≤ 9 m L > 9 m | +3.2 mm – 0 +4 mm – 0 +4.8 mm – 0 |
| Radius R | ± 1.5 mm |
| Leg spacing (measured at tangency point) | ± 1.5 mm |
| Ovality in the bent portion (also called flattening) | ± 10 % This can be described as follow. (Dmax – D) / D < 10% (D – Dmin) / D < 10% “neither the major nor minor diameter of the tube shall deviate from the nominal diameter prior to bending by more than 10%” |
| Wall thickness in bent portion | shall not be less than the value determined by thebelow equation: tf = (4R x T) / (4R + D) where : tf = wall thickness after bending T = minimum wall thickness before bending R = centerline bend radius D = nominal outside tube diameter |
| Squareness of ends D ≤ 15.9 mm 15.9 > D ≤ 25.4 D ≥ 25.4 ≤ 38.1 | 0.25 mm 0.4 mm 0.4 mm |
ASTM A688, ASME SA688 Specification
This specification covers standard requirements for welded austenitic stainless steel feedforward heater tubes including those bent, if specified, into the form of U-tubes for application in tubular feed-water heaters.
All finished or straight tubing ready for U-bending shall be furnished in the solution-annealed condition. The steel shall conform to the required chemical composition for carbon, phosphorus, chromium, molybdenum, nitrogen, and copper. The material shall also conform to tensile properties such as tensile strength, yield strength, and elongation. The steel shall undergo mechanical tests such as tension test, hardness test, reverse bend test, flattening test, flange test, pressure test, hydrostatic test, and air underwater test. Nondestructive test (electric test) shall be performed and corrosion-resisting properties shall be determined for each sample tube.
Austenitic Stainless Steel – Feedwater Heater Tubes – Heat-Transfer Apparatus – Seamless Pipes And Tubes – Seamless Stainless Steel Tubes – Stainless Steel Tubes – Tubular Products – Welded Pipes And Tubes – Welded Stainless Steel Tubes.
Feed Water Heater is a device to pre-heat the water using extraction steam in a steam and water heat exchanger system, usually used in power plants, this component reduces heat loss during the heat transfer process resulting from cooling water, therefore improving transfer efficiency. The performance of the feed water heater impacts the whole system’s economical efficiency and safety, tubes play an important role in feeding the water heater, resisting high temperature and corrosion, and failed tubes cause leakage problems, which will increase maintenance costs by repairing.
ASTM A688, ASTM A803 Feed Water Heater Tube
There are two specifications specified for feed water heater tubes, ASTM A688 and ASTM A803, compared to ASTM A213, A249, A268, and A269 standard, these two standards are special for heater tubes, requiring tight tolerances and more testing to ensure tubes to suitable in critical environment.
Common Stainless Steel Alloys
TP304. TP304L, TP316L, 439, and 446 stainless steel are common grades to produce tubes, and derivatives, also duplex steels are available, like 2205, selecting which grade depends on applications, the user should know single stainless steel grade has special properties. 304 is widely used 18 Chromium 8 Nickel stainless steel, its properties make it used in most applications.
The “L” grade has low carbon which provides significant extra resistance to corrosion, like TP304L, and TP416L, which are used in environments requiring tubes to enable to resist corrosion.
304, 304L, 316, 316L, 316H, 316Ti, 317L, 321, 347/347H, 904L, 254SMO/S31254, 309/309S, 310/310S, 409, 430, 439, 2205, S32750, S32760
U bend tubes are designed commonly for feed water heater tubes, both welded and seamless tubes are for opting, max length to 30 meters, bending portion heat treated, maximum bend radius reach 1.5 meters, tolerances are controlled as ASTM A688.
Testing and Inspection
Non-destructive testing and pressure tests should be conducted, in practice, eddy current testing and air underwater testing are common methods to ensure tubes are in superior condition, optional tests can be made for feed water heater tubes.