Resources
ASTM D1998 Standard for Polyethylene Vertical Tanks
ASTM D1998-21 sets wall thickness requirements for polyethylene vertical storage tanks of 500 gallons and larger. Bailiff-molded HDPE vertical tanks of 500 gallons and up meet or exceed it. Smaller tanks, and tanks we sell from other manufacturers, are not built to this standard. Below is our plain-language summary of what the standard requires.
Overview
What ASTM D1998-21 Covers
ASTM D1998 is ASTM International’s specification for polyethylene upright storage tanks. The current edition, D1998-21, was approved in July 2021 and replaced D1998-15; the standard was first adopted in 1991. This page is our summary of the main requirements. It is not the standard itself; the full text is copyrighted and available from ASTM (link below).
Tanks covered
Flat-bottom, upright, cylindrical tanks rotationally molded in one seamless piece, installed above ground and holding liquids, including aggressive chemicals, at atmospheric pressure. Capacities of 500 gallons (1,900 L) and up.
Temperature limits
Continuous service below 150°F for Type I (crosslinked) tanks and below 140°F for Type II tanks, with liquids kept below their flash points. Allowable wall stress is reduced for service above 73.4°F.
Not covered
Pressure or vacuum service, portable tanks, and outside forces such as wind, seismic loads or agitation. For those, the standard points to local building codes, NFPA 30 and 31, ASME and similar codes.
Classification
Type I and Type II Tanks
The buyer specifies which type is wanted.
Type I: crosslinked polyethylene
Molded from cross-linkable polyethylene (XLPE). These tanks must also pass a gel test showing at least 60% crosslinking in the inner 1/8 in. of the wall.
Type II: non-crosslinked polyethylene
Molded from polyethylene that is not crosslinked, such as the HDPE Bailiff molds. Type II tanks meet the same design, impact and water-test requirements. See how HDPE compares with crosslinked polyethylene.
Materials
Resin Requirements
- The specification is based on 100% virgin polyethylene made for rotational molding. Tanks with regrind or recycled resin must meet every requirement on their own, without relying on the original resin’s test data.
- The resin must resist environmental stress cracking for at least 500 hours in ASTM D1693 testing. That test is not a measure of chemical resistance; check our chemical compatibility database or send us your SDS for that.
- Tanks for outdoor use must have a UV stabilizer compounded into the resin, enough to protect the tank for its intended service life.
- Pigment is limited to 0.5% of total weight when dry blended and 2% when compounded in.
- Every resin used must have hydrostatic hoop-stress data, which the wall-thickness design is based on.
Design
Wall Thickness and Construction
The heart of the standard is the minimum wall thickness, which is why the SG (specific gravity) rating of a tank matters.
Calculated at every level
The required wall is figured at each liquid level from the liquid head above that point, the specific gravity of the liquid and the tank diameter, divided by the resin’s design stress. Deeper, heavier liquid and larger diameters need a thicker wall, so a 1.9 SG tank has a thicker wall than a 1.5 SG tank of the same size.
Built-in safety margin
The resin’s long-term strength is multiplied by a service factor of no more than 0.5 (0.475 for walls 3/8 in. and thicker) to set the design stress. No wall may be thinner than 0.187 in.
Top, bottom and corners
The top and bottom heads are molded in one piece with the wall. The bottom corner radius must be at least 1 in. on tanks under 6 ft in diameter and 1.5 in. on larger tanks, and that corner must be as thick as the thickest part of the wall. Open-top tanks need a reinforced rim.
| Tolerance | Requirement |
|---|---|
| Outside diameter | Within ±3%, including out-of-roundness |
| Wall and head thickness | Average no more than 10% under design (no limit over); individual readings no more than 20% under; thin areas no more than 10% of the surface |
| Fitting locations | Within 1/2 in. in elevation and 2° around the tank |
Fittings
Fittings, Vents and Tie-Downs
- Nozzles, gaskets and bolts must be compatible with the chemical stored.
- Holes cut for fittings must not have sharp corners.
- Flanged connections are drilled to ANSI/ASME B16.5 Class 150, with bolt holes straddling the tank’s centerline.
- Vents must meet OSHA 29 CFR 1910.106 normal venting, or another accepted standard, or be at least as large as the largest fill or withdrawal connection, and never smaller than 1 in.
- Outdoor tanks must have provisions for hold-down devices. See our seismic and wind restraints.
- Manway and fitting sizes and locations are agreed between buyer and manufacturer. See our fittings and options.
Quality
Testing and Inspection
Cold impact test
Samples cut from the tank, or from a test mold run with it, are chilled to -20°F and struck with a dart. A properly molded part bends instead of cracking. Some cold-climate service calls for testing at -40°F.
Water test
Every tank is filled completely with water by the manufacturer before release. The standard also calls for the user to fill it with water at installation, which checks the fittings too.
Visual inspection
Walls are checked for inclusions, bubbles, pinholes, cracks, crazing and delamination that would affect service.
| Wall thickness | Minimum impact energy at -20°F |
|---|---|
| 0.187 in. to 0.25 in. | 90 ft-lb |
| 0.26 in. to 0.50 in. | 100 ft-lb |
| 0.51 in. to 0.75 in. | 150 ft-lb |
| Over 0.75 in. | 200 ft-lb |
Marking and shipping
Labels, Shipping and Delivery
Permanent marking
Each tank is marked with the manufacturer, month and year made, capacity, maximum design specific gravity, serial number, and Type I or Type II, plus the OSHA warning signs that apply.
Shipping
Fittings and flange faces are covered for protection, loose parts are packaged separately, and the tank stays vented in transit.
On arrival
The buyer inspects the tank for shipping damage and files any claim with the carrier. Tell us about any damage before the tank goes into service.
Bailiff tanks
What This Means for Your Tank
Bailiff-molded HDPE vertical tanks of 500 gallons and larger meet or exceed the ASTM D1998-21 wall thickness requirements. They are Type II tanks, and every one is water-tested before it leaves our Willis, TX plant. Smaller tanks, cone bottom and horizontal tanks, and tanks we sell from other manufacturers are outside this standard. Browse HDPE single wall vertical tanks or HDPE double wall vertical tanks, or see storage tanks by chemical.
FAQ
ASTM D1998 Questions
Does ASTM D1998 apply to tanks under 500 gallons?
No. ASTM D1998-21 covers upright, flat-bottom polyethylene tanks of 500 gallons and larger.
What is the difference between Type I and Type II tanks?
Type I tanks are molded from crosslinked polyethylene (XLPE) and must pass a gel test for crosslinking. Type II tanks are molded from polyethylene that is not crosslinked, such as HDPE. Both must meet the same wall thickness, impact and water-test requirements.
How does ASTM D1998 set wall thickness?
The wall is calculated at every liquid level from the liquid head, the liquid’s specific gravity and the tank diameter, using a design stress with a safety factor of at least 2 built in. No wall may be thinner than 0.187 in. That’s why a tank’s SG rating should be at or above the specific gravity of your liquid.
What temperatures does the standard allow?
Continuous service below 150°F for Type I tanks and below 140°F for Type II tanks. Allowable wall stress is reduced for service above 73.4°F, so tell us your operating temperature when you request a quote.
Do I need to water-test a new tank?
The standard calls for the manufacturer to fill every tank with water before release, and for the user to fill it with water again at installation. The second test also checks that the fittings were installed properly.
Ready to Talk About Your Tank?
Tell us the liquid, volume and fittings. We’ll come back with a price and lead time.