On the surface, polyethylene storage tanks might look pretty similar. It’s tough to tell a more robust tank that has been well designed and constructed from a cheap, inferior one. Over the years, we've found that many customers are unaware of the standard for polyethylene tank construction based upon resin properties and testing or they simply assume all tank manufacturers follow the ASTM D 1998 standard.
The ASTM D-1998 standard is a voluntary standard that differentiates the manufacturers that meet it vs. the ones that don't. However, as in any standard there is some subjectivity to how each manufacturer interprets the ASTM guidelines. Just meeting the standard isn't everything though; it's not an all inclusive list. The standard should be looked at as one in many items in a checklist when picking the proper tank and manufacturer for your application.
Manufacturers Meet The ASTM Standards
It’s important to verify that your storage tank has been manufactured according to ASTM D 1998 standards.
- Polyethylene producers generate the mechanical properties of their resin from samples which are compression molded.
- This process optimizes the material’s mechanical properties, however it doesn’t represent polyethylene processed using rotational molding.
- To provide the proper safety factor, ASTM has determined allowable usable ratings provided by resin producers.
This gives the end user both a margin of safety and a peace of mind knowing that your storage tank is designed and manufactured to withstand potential tank impacts, the weight of the chemical being stored, temperature fluctuations, and the pressures under storage of the chemical.
What Does the ASTM Standard Mean?
Here are a few basic components of the ASTM D 1998 guidelines:
- When calculating wall thickness using a resin with a 1200 psi hoop stress value, manufacturers are only allowed to use 600 PSI for a hoop stress rating at 100 F. even though the resin manufacturer states at a given thickness the material could withstand over 1200 PSI.
- If no standard is applied to the tank manufacturer, they in theory could select any hoop stress rating from 0-1200 PSI.
- As PSI increases, wall thickness decreases.
- This explains why two tanks designed to store up to a 1.9 specific gravity rating can vary so greatly in design strength and perform so differently. Again as stated above, the standard is part of a much larger checklist for proper tank manufacturing.
- Check to insure your tank manufaturer is using the proper design calculations for the resin used and is derating it for higher temperature applications.
The ASTM D 1998 standard determines through testing how various materials are processed together to form the finished rotational molded product.
For example, in order to determine acceptable tolerance levels, the mechanical properties of the resins used in polyethylene storage tanks are factored in so that accommodations are made for the weight of the material being stored. ASTM calculates the maximum pounds per square inch of pressure at a real-life level to determine performance numbers.
It’s important to examine and understand how stress is actually calculated, and review wall thickness data sheets from the tank manufacturer to determine if that manufacturer is meeting ASTM D 1998 testing parameters.
Tests the ASTM Guidelines Call Out
The ASTM standard calls out for several other post production tests like impact testing and gel testing. For the impact test ASTM is very clear about the thickness of the samples being used, the temperatures the samples are to be tested at, and the height of the 20 pound impact dart. For gel testing, the minimum allowable gel percentage is 60% per ASTM.
The tank manufacturer you are considering should have these technical documents, data sheets, and manufacturing quality test certificates available for your verification so you are assured that the tank is designed and manufactured to the highest safety standards.
Saving a few hundred dollars on the up-front cost of a tank could end up costing you many more thousands of dollars in the event of a catastrophic tank failure resulting from a cheap, poorly engineered and constructed tank.
Poly Processing Company has explained the ASTM standards in detail in our technical training document ASTM D-1998.