ASTM D1998-21

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D1998 − 15 D1998 − 21
Standard Specification for
Polyethylene Upright Storage Tanks
This standard is issued under the fixed designation D1998; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This specification covers flat-bottom, upright, cylindrical tanks molded in one-piece seamless construction by rotational
molding. The tanks are molded from polyethylene for above ground, vertical installation and are capable of containing aggressive
chemicals at atmospheric pressure. Included are requirements for materials, properties, design, construction, dimensions,
tolerances, workmanship and appearance. Tank capacities are from 1900 L (500 gal) up.
1.2 This specification covers the design of stationery vessels for use at atmospheric pressure intended for use with liquids heated
below their flash points and continuous service temperatures below 66°C (150°F) for Type I tanks and below 60°C (140°F) for
Type II tanks.
1.2.1 NFPA Standards 30 and NFPA 31 shall be consulted for installations that are subject to the requirements of these standards.
1.3 For service requirements beyond the scope of this specification (1.2), such as externally imposed mechanical forces, internal
pressure or vacuum, higher temperature service, etc., other relevant sources of standards, for example, local and state building
codes, NFPA, ASME, ARM, etc., shall be consulted.
1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
NOTE 1—ISO 13341:2005+A1:2011 and ISO 13575:2012 are similar, but not equivalent to this standard.
1.5 The following precautionary caveat pertains only to the test methods portion, Section 11, of this specification: This standard
does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this
standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the applicability of
regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D618 Practice for Conditioning Plastics for Testing
This specification is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.19 on Film, Sheeting, and Molded
Products (Section D20.15.01).
Current edition approved Sept. 15, 2015July 15, 2021. Published October 2015August 2021. Originally approved in 1991. Last previous edition approved in 20132015
as D1998 - 13.D1998 - 15. DOI: 10.1520/D1998-15.10.1520/D1998-21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1998 − 21
D883 Terminology Relating to Plastics
D1693 Test Method for Environmental Stress-Cracking of Ethylene Plastics
D2837 Test Method for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials or Pressure Design Basis for
Thermoplastic Pipe Products
D3892 Practice for Packaging/Packing of Plastics
D4703 Practice for Compression Molding Thermoplastic Materials into Test Specimens, Plaques, or Sheets
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
F412 Terminology Relating to Plastic Piping Systems
2.2 OSHA Standard:
29 CFR 1910.106 Occupational Safety and Health Administration, Flammable and Combustible Liquids
2.3 ANSI Standard:
B-16.5 Pipe Flanges and Flanged Fittings
2.4 NFPA Standards:
30 Flammable and Combustible Liquid Code
31 Installation of Oil Burning Equipment
2.5 ISO Standards:
ISO 13341:2005+A1:2011 Static Thermoplastic Tanks for the Above Ground Storage of Chemicals – Blow Moulded or
Rotationally Moulded Polyethylene Tanks – Requirements and Test Methods
ISO 13575:2012 Static Thermoplastic Tanks for Above Ground Storage of Domestic Heating Oils, Kerosene and Diesel Fuels
– Blow Moulded and Rotationally Moulded Polyethylene Tanks and Rotationally Moulded Tanks Made of Anionically
Polymerized Polyamide 6 – Requirements and Test Methods
3. Terminology
3.1 Definitions—Definitions are in accordance with For definitions of terms used in this specification and associated with plastics
issues, refer to the terminology contained in Terminologies D883 and F412 and the Association of Rotational Molders (ARM)
Glossary of Terms, unless otherwise indicated.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 bottom knuckle radius, n—the outside corner radius.
3.2.2 impact failure, n—any crack in the test specimen resulting from the impact and visible in normal room lighting to a person
with normal eyesight.
3.2.3 rotational molding, n—a three-stage commercial process consisting of loading the mold with powdered resin, fusing the resin
by heating while rotating the mold about more than one axis, and cooling and removing the molded article.
3.2.4 service factor, n—a number less than 1.0 (that takes into consideration all the variables and degrees of safety involved in
a polyethylene storage tank installation) that is multiplied by the hydrostatic design basis to give the design hoop stress.
4. Classification
4.1 Tanks meeting this specification are classified according to type as follows, and it is the responsibility of the purchaser to
specify Type I or Type II:
4.1.1 Type I—Tanks molded from cross-linkable polyethylene.
4.1.2 Type II—Tanks molded from non-cross-linkable polyethylene.
5. Materials
5.1 This specification is based upon the use of 100 % virgin polyethylene intended for the rotational molding process. Any use
Available from Occupational Safety and Health Administration (OSHA), 200 Constitution Ave., NW, Washington, DC 20210.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036.10036, http://www.ansi.org.
Available from National Fire Protection Association (NFPA), 1 Batterymarch Park, Quincy, MA 02269-9101.02169-7471, http://www.nfpa.org.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Available from Association of Rotational Molders, 800 Roosevelt Road, C-312, Glen Ellyn, IL 60137, tel: 630-942-6589.
D1998 − 21
of regrind, recycled or reprocessed materials, or combinations of such materials, shall not rely upon the performance data of their
original constituents, but must meet the requirements of this specification in its own right.
5.1.1 The polyethylene shall have a stress-cracking resistance of 500 h minimum F50 in accordance with Test Method D1693,
Condition A, full-strength stress-cracking agent. The test specimens shall be compression molded or rotational molded. If
compression molded, Procedure C of Annex A1 of Practice D4703 shall be followed for both types of polyethylene with a
minimum platen temperature of 177°C (350°F) for Type II materials. The temperature for Type I (cross-linkable) polyethylene shall
be 197°C (390°F) and the platen shall be kept closed under full pressure for five minutes at the specified temperature in order to
bring about the crosslinking reaction. If the test specimens are rotational molded, the conditions for rotational molding shall be
similar to the conditions used for molding a tank from this polyethylene.
NOTE 2—The stress-cracking test is not used as an indicator of general chemical resistance of a polyethylene. Refer to the polyethylene supplier’s or
molder’s chemical-resistance chart for information on the resistance of the polyethylene to specific chemicals or products, or test specific products or
chemicals prior to use.
5.2 All tanks used for outdoor installation shall contain an ultraviolet stabilizer at a level adequate to give protection for the
intended service life of the tanks. This stabilizer shall be compounded into the polyethylene.
5.3 Any pigments added must be compatible with the polyethylene and shall not exceed 0.5 % dry blended, and 2 % compounded
in, of the total weight.
NOTE 3—The use of dry-blended pigments maycan have an effect on physical properties, that is, impact strength.
5.4 Each resin used in designing tanks covered by this specification shall have hydrostatic-hoop-stress data available.
6. Design Requirements for Both Type I and Type II Tanks
6.1 Cylinder Shell (Unsupported Portion of Tanks)—The minimum required wall thickness of the cylindrical shell at any fluid
level (F) shall be determined by the following equations, but shall not be less than 4.7 mm (0.187 in.) thick. The tolerance indicated
in 9.1.2 applies to these dimensions.
HL 5 HM 2 F (1)
where:
HL = head at level F, m (ft),
HM = maximum fluid head, m (ft), and
F = any fluid level, m (ft).
For illustration, see Fig. 1.
T 5 P 3OD/2 SD (2)
where:
T = wall thickness, mm (in.),
P = pressure, MPa (0.0098 MPa/m-H O × SG × HL (m)), or psi (0.433 psi/ft-H O × SG × HL (ft)),
2 2
SG = specific gravity of fluid,
HL = head at level F, m (ft),
OD = outside diameter of tank, mm (in.), and
SD = hydrostatic design stress, MPa (psi).
6.1.1 The hydrostatic design stress that is used to determine the minimum wall thickness at any fluid level must be based on hoop
stress data for the resin. The hoop stress data, obtained in accordance with the procedures of Test Method D2837, provide a
hydrostatic-design-basis for the resin. The hydrostatic-design-basis must be reduced by a service factor to determine the actual
hydrostatic design stress. The maximum service factor shall be 0.5 for wall thicknesses less than 9.5 mm (0.375 in.). For
thicknesses equal to or greater than 9.5 mm (0.375 in.), the maximum service factor shall be 0.475. For example, if the
hydrostatic-design-basis for the resin is 8.7 MPa (1260 psi), the hydrostatic design stress for a tank with wall thickness greater than
9.5 mm (0.375 in.) is 0.475 × 8.7 = 4.1 MPa (or 0.475 × 1260 = 600 psi).
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FIG. 1 Illustration of Cylinder Shell
6.1.2 Tank hoop stresses shall be derated for service above 23°C (73.4°F).
6.2 Cylinder Shell (Externally Supported Tanks)—The minimum required wall thickness for the cylinder straight shell must be
sufficient to support its own weight in an upright position without any external support, but shall not be less than 4.7 mm (0.187
in.) thick. The tolerance indicated in 9.1.2 applies to these dimensions.
6.3 Top Head—Must be integrally molded with the cylinder shell. The minimum thickness of the top head shall be equal to the
top of the straight wall.
6.4 Bottom Head—Must be integrally molded with the cylinder shell. The minimum thickness for a full-supported flat-bottom
head shall be 4.7 mm (0.187 in.). The radius of the bottom knuckle of a flat-bottom tank shall not be less than 25.4 mm (1 in.)
for tanks with a diameter less than 1.8 m (6 ft) and 38.1 mm (1.5 in.) for a diameter greater than 1.8 m (6 ft). The minimum
thickness of the radius shall not be less than the maximum thickness of the cylinder wall.
6.5 Bottom Knuckle Radius—The minimum thickness of the outer radius shall not be less than the maximum thickness of the
cylinder wall.
NOTE 4—Since it is difficult to control the absolute radius dimension and, in the absence of any FEA or scientific analysis, it is recommended that the
outside radius of the bottom knuckle of a flat-bottom tank not be less than 25.4 mm (1 in.) for tanks with a diameter less than 1.8 m (6 ft) and 38.1 mm
(1.5 in.) for a diameter greater than 1.8 (6 ft).
6.6 Open-Top Tanks—The top edge of open tanks shall be reinforced by design to maintain its shape after installation.
7. Fittings
7.1 Fabricated nozzles, gaskets, and other fitting accessories must be chemically compatible with the materials to be handled in
the tanks.
7.2 Openings that are cut in tanks to install fittings must not have sharp corners. Holes shall have minimum clearance to insure
best performance of fittings.
7.3 The size, location, and specification, and so forth, for manways and fittings shall be agreed upon between the purchaser and
the manufacturer.
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7.4 The vents must comply with OSHA 1910.106 (F) (iii) (2) (IV) (9) normal venting for atmospheric tanks, or other accepted
standard, or shall be at least as large as the filling or withdrawal connection, whichever is larger but in no case less than 25.4 mm
(1 in.) nominal inside diameter.
7.5 Fittings installed in tanks shall be of appropriate strength to meet manufacturer and purchaser specifications.
7.6 Bolts securing mechanical fittings must be manufactured of materials compatible with tank contents.
7.7 Provisions shall be made to attach hold-down devices to the tanks for outdoor service.
7.8 For all flanged connectors, the flange drilling and bolting shall be in accordance with ANSI/ASME B-16.5 for 150 psi (1 MPa)
pressure class straddling the principal centerline of the vessel.
8. Performance Requirements
8.1 The following performance requirements shall be met by Type I and Type II tanks:
8.1.1 Low-Temperature Impact—Low-temperature impact shall be determined using the test method described in 11.3. The
requirements for Type I and Type II tanks are as follows:
Impact energy,
Wall thickness, mm (in.)
min. J (ft-lb)
4.7 mm (0.187 in.) to and including 6.4 mm (0.25 in.) 122.0 (90)
6.6 mm (0.26 in.) to and including 12.9 mm (0.50 in.) 135.5 (100)
12.9 mm (0.51 in.) to and including 19.3 mm (0.75 in.) 203.2 (150)
19.3 mm (0.76 in.) to and including 25.4 mm (1.00 in.) 271.0 (200)
Greater than 25.4 mm (1.00 in.) 271.0 (200)
8.1.2 Percent Gel, for Type I Tanks Only—The percent gel level shall be determined using the test method described in 11.4. The
percent gel level for Type I tanks on the inside 3.2 mm (0.125 in.) of the wall shall be a minimum of 60 %.
9. Dimensions and Tolerances
9.1 General—All dimensions will be taken with the tank in the vertical position, unfilled. Tank dimensions will represent the
exterior measurements.
9.1.1 Outside Diameter—The tolerance for the outside diameter, including out of roundness, shall be 63 %.
9.1.2 Shell Wall and Head Thickness—The tolerance for average thickness at each elevation shall be − 10 % of the design
thickness on the low side and shall be unlimited on the high side. The tolerance for individual audit readings shall be limited to
− 20 % of the design thickness. The total amount of surface area on the low side of the tolerance shall not exceed 10 % of the
total surface area.
9.1.3 Placement of Fittings—The tolerance for fitting placements shall be 12.7 mm (0.5 in.) in elevation and 2° radial at ambient
temperature.
10. Workmanship
10.1 Type I finished tank walls shall be free, as commercially practicable, of visual defects such as foreign inclusions, air bubbles,
pinholes, pimples, crazing, cracking and delaminations that will impair the serviceability of the vessel. Fine bubbles are acceptable
with Type II tanks to the degree to which they do not interfere with proper fusion of the resin melt.
10.2 The acceptable finish shall be predetermined by agreement between the molder and the buyer.
D1998 − 21
11. Test Methods
11.1 Test Specimens—Test specimens shall be taken from an area that is representative of the bottom side wall. If no representative
sample cut-out area in the tank is available, test specimens shall be molded in a test mold. In either case, prior testing shall verify
that the tank wall and the test specimen have equal impact resistance.
11.1.1 The test mold shall be constructed of the same type material and have the same wall thickness as the tank mold. The
thickness of the specimen from a test mold shall be the same as the thickness of the bottom sidewall within the tolerances as defined
in 9.1.2. The test mold shall be molded with each tank.
11.2 Conditioning—If requested, test specimens shall be conditioned at 23 6 2°C (73.4 6 3.6°F) and 50 6 10 % relative humidity
for not less than 40 h prior to testing in accordance with Procedure A of Practice D618.
11.3 Low-Temperature Impact Test:
11.3.1 Scope—This test method is for the determination of the impact property of rotational-molded polyethylene tanks at low
temperature. The test method is used on tanks molded from both crosslinked and non-crosslinked polyethylenes.
11.3.2 Summary of Test Method—Test specimens are cut from available areas on the tank and conditioned at − 29°C −29°C
(−20°F) for a specified time. A suitable type of test apparatus is shown in Fig. 2 and Fig. 3. The specimens are placed,
inside-surface down, in the sample holder and immediately impacted from a prescribed height with a dart of specified weight and
tip radius. The specimen is observed for failure on both surfaces. The test prescribes a minimum impact value that the specimen
must pass.
11.3.3 Significance and Use:
11.3.3.1 The dart impact test at −29°C (−20°F) produces a value that is used as an indication of the quality of the tank. If the
molding conditions were inadequate and a homogenous melt was not obtained, the impact will likely be low. Higher impact values
are obtained with ideal molding conditions indicating that a quality part with good impact resistance has been molded.
FIG. 2 Dart Drop Impact Test Apparatus
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FIG. 3 Dart Drop Impact Test Apparat
...