(a) Type and service pressure. A DOT 8 cylinder is a seamless cylinder with a service pressure of 250 psig. The following steel is authorized:
(1) A longitudinal seam if forge lap welded;
(2) Attachment of heads by welding or by brazing by dipping process; or
(3) A welded circumferential body seam if the cylinder has no longitudinal seam.
(b) Steel. Open-hearth, electric or basic oxygen process steel of uniform quality must be used. Content percent may not exceed the following: Carbon, 0.25; phosphorus, 0.045; sulphur, 0.050.
(c) Identification of steel. Materials must be identified by any suitable method except that plates and billets for hot-drawn cylinders must be marked with the heat number.
(d) Manufacture. Cylinders must be manufactured using equipment and processes adequate to ensure that each cylinder produced conforms to the requirements of this subpart. No defect is acceptable that is likely to weaken the finished cylinder appreciably. A reasonably smooth and uniform surface finish is required. Welding procedures and operators must be qualified in accordance with CGA Pamphlet C-3 (IBR, see § 171.7 of this subchapter).
(e) Exposed bottom welds. Exposed bottom welds on cylinders over 18 inches long must be protected by footrings.
(f) Heat treatment. Body and heads formed by drawing or pressing must be uniformly and properly heat treated prior to tests.
(g) Openings. Openings in the cylinders must comply with the following:
(1) Standard taper pipe threads are required;
(2) Length may not be less than as specified for American Standard pipe threads; tapped to gauge; clean cut, even, and without checks.
(h) Pressure testing. Each cylinder must successfully withstand a pressure test as follows:
(1) Lot testing.
(i) At least one (1) cylinder selected at random out of each lot of 200 or fewer must be tested by water-jacket or direct expansion method as prescribed in CGA C-1 (IBR; see § 171.7 of this subchapter). The testing equipment must be calibrated as prescribed in CGA C-1. All testing equipment and pressure indicating devices must be accurate within the parameters defined in CGA C-1.
(ii) The selected cylinder must be tested to a minimum of 750 psig.
(iii) The minimum test pressure must be maintained at least 30 seconds and sufficiently longer to ensure complete expansion. Any internal pressure applied after heat-treatment and prior to the official test may not exceed 90 percent of the test pressure. If, due to failure of the test apparatus or operator error, the test pressure cannot be maintained, the test may be repeated in accordance with CGA C-1, section 5.7.2.
(iv) Permanent volumetric expansion may not exceed 10 percent of the total volumetric expansion at test pressure.
(v) If the selected cylinder passes the volumetric expansion test, each remaining cylinder in the lot must be pressure tested in accordance with paragraph (h)(2) of this section. If the selected cylinder fails, each cylinder in the lot must be tested by water-jacket or direct expansion method as prescribed in CGA C-1 at 750 psig. Each cylinder with a permanent expansion that does not exceed 10% is acceptable.
(2) Pressure testing.
(i) If the selected cylinder passes the water-jacket or direct expansion test, the remaining cylinders in each lot must be pressure tested by the proof pressure, water-jacket or direct expansion test method as prescribed in CGA C-1. The minimum test pressure must be maintained for the specific timeframe and the testing equipment must be calibrated as prescribed in CGA C-1. Further, all testing equipment and pressure indicating devices must be accurate within the parameters defined in CGA C-1.
(ii) Each cylinder must be tested between 500 and 600 psig and show no defect. If, due to failure of the test apparatus or operator error, the test pressure cannot be maintained, the test may be repeated in accordance with CGA C-1 section 5.7.2 or 7.1.2, as appropriate. Determination of expansion properties is not required.
(i) Leakage test. Cylinders with bottoms closed in by spinning must be subjected to a leakage test by setting the interior air or gas pressure to not less than the service pressure. Cylinders which leak must be rejected.
(j) Physical test. A physical test must be conducted as follows:
(1) The test is required on 2 specimens cut longitudinally from 1 cylinder or part thereof taken at random out of each lot of 200 or less, after heat treatment.
(2) Specimens must conform to a gauge length of 8 inches with a width not over 11⁄2 inches, a gauge length of 2 inches with width not over 11⁄2, or a gauge length at least 24 times thickness with a width not over 6 times thickness is authorized when a cylinder wall is not over 3⁄16 inch thick.
(3) The yield strength in tension must be the stress corresponding to a permanent strain of 0.2 percent of the gauge length. The following conditions apply:
(i) The yield strength must be determined by either the “offset” method or the “extension under load” method as prescribed in ASTM E 8 (IBR, see § 171.7 of this subchapter).
(ii) In using the “extension under load” method, the total strain (or “extension under load”) corresponding to the stress at which the 0.2 percent permanent strain occurs may be determined with sufficient accuracy by calculating the elastic extension of the gauge length under appropriate load and adding thereto 0.2 percent of the gauge length. Elastic extension calculations must be based on an elastic modulus of 30,000,000. In the event of controversy, the entire stress-strain diagram must be plotted and the yield strength determined from the 0.2 offset.
(iii) For the purpose of strain measurement, the initial strain must be set while the specimen is under a stress of 12,000 psi and the strain indicator reading being set at the calculated corresponding strain.
(iv) Cross-head speed of the testing machine may not exceed 1⁄8 inch per minute during yield strength determination.
(4) Yield strength may not exceed 73 percent of tensile strength. Elongation must be at least 40 percent in 2 inch or 20 percent in other cases.
(k) Rejected cylinders. Reheat treatment of rejected cylinder is authorized. Subsequent thereto, cylinders must pass all prescribed tests to be acceptable. Repair by welding is authorized.
(l) Porous filling.
(1) Cylinders must be filled with a porous material in accordance with the following:
(i) The porous material may not disintegrate or sag when wet with solvent or when subjected to normal service;
(ii) The porous filling material must be uniform in quality and free of voids, except that a well drilled into the filling material beneath the valve is authorized if the well is filled with a material of such type that the functions of the filling material are not impaired;
(iii) Overall shrinkage of the filling material is authorized if the total clearance between the cylinder shell and filling material, after solvent has been added, does not exceed 1⁄2 of 1 percent of the respective diameter or length, but not to exceed 1⁄8 inch, measured diametrically and longitudinally;
(iv) The clearance may not impair the functions of the filling material;
(v) The installed filling material must meet the requirements of CGA C-12 (IBR, see § 171.7 of this subchapter); and
(vi) Porosity of filling material may not exceed 80 percent except that filling material with a porosity of up to 92 percent may be used when tested with satisfactory results in accordance with CGA Pamphlet C-12.
(2) When the porosity of each cylinder is not known, a cylinder taken at random from a lot of 200 or less must be tested for porosity. If the test cylinder fails, each cylinder in the lot may be tested individually and those cylinders that pass the test are acceptable.
(3) For filling that is molded and dried before insertion in cylinders, porosity test may be made on a sample block taken at random from material to be used.
(4) The porosity of the filling material must be determined. The amount of solvent at 70 °F for a cylinder:
(i) Having shell volumetric capacity above 20 pounds water capacity (nominal) may not exceed the following:
| Percent porosity of filler | Maximum acetone solvent percent shell capacity by volume |
|---|---|
| 90 to 92 | 43.4 |
| 87 to 90 | 42.0 |
| 83 to 87 | 40.0 |
| 80 to 83 | 38.6 |
| 75 to 80 | 36.2 |
| 70 to 75 | 33.8 |
| 65 to 70 | 31.4 |
(ii) Having volumetric capacity of 20 pounds or less water capacity (nominal), may not exceed the following:
| Percent porosity of filler | Maximum acetone solvent percent shell capacity by volume |
|---|---|
| 90 to 92 | 41.8 |
| 83 to 90 | 38.5 |
| 80 to 83 | 37.1 |
| 75 to 80 | 34.8 |
| 70 to 75 | 32.5 |
| 65 to 70 | 30.2 |
(m) Tare weight. The tare weight is the combined weight of the cylinder proper, porous filling, valve, and solvent, without removable cap.
(n) Duties of inspector. In addition to the requirements of § 178.35, the inspector is required to -
(1) Certify chemical analyses of steel used, signed by manufacturer thereof; also verify by, check analyses of samples taken from each heat or from 1 out of each lot of 200 or less, plates, shells, or tubes used.
(2) Verify compliance of cylinder shells with all shell requirements; inspect inside before closing in both ends; verify heat treatment as proper; obtain all samples for all tests and for check analyses; witness all tests; verify threads by gauge; report volumetric capacity and minimum thickness of wall noted.
(3) Prepare report on manufacture of steel shells in form prescribed in § 178.35. Furnish one copy to manufacturer and three copies to the company that is to complete the cylinders.
(4) Determine porosity of filling and tare weights; verify compliance of marking with prescribed requirements; obtain necessary copies of steel shell reports; and furnish complete reports required by this specification to the person who has completed the manufacture of the cylinders and, upon request, to the purchaser. The test reports must be retained by the inspector for fifteen years from the original test date of the cylinder.
(o) Marking.
(1) Marking on each cylinder must be stamped plainly and permanently on or near the shoulder, top head, neck or valve protection collar which is permanently attached to the cylinder and forming integral part thereof.
(2) Tare weight of cylinder, in pounds and ounces, must be marked on the cylinder.
(3) Cylinders, not completed, when delivered must each be marked for identification of each lot of 200 or less.
[Amdt. 178-114, 61 FR 25942, May 23, 1996, as amended at 66 FR 45386, Aug. 28, 2001; 67 FR 61016, Sept. 27, 2002; 67 FR 51654, Aug. 8, 2002; 68 FR 75748, 75749, Dec. 31, 2003; 85 FR 85427, Dec. 28, 2020]