Importance of Tank Design Standards.
1. Why are the ASTM specifications so important? Simple language describing hoop stress in relation to specific
gravity.Why is the UL rating irrelevant?
The ASTM D 1998-97 standard utilizes the same hoop stress formula that has been used by Sii design engineers for
the past twenty years. Due to the superior Sii process control methods that have been developed, a stratified wall
thickness is produced in large bulk tanks. Sii calculates the hoop stress and required tank wall thickness in one
foot vertical intervals or less, and designs the part tooling to achieve these required thickness' in each of the
calculated intervals. In the end, the only point that really matters is that Snyder Industries engineered and provided
a tank best suited for the specific application and will stand behind our products.
UL Approvals - Do Snyder tanks meet any UL approvals? Snyder has not formally petitioned UL for any approvals,
however Snyder tanks DO meet the same UL Class 3B standards that one of our XLPE tank competitor's tanks meet.
This standard simply states that materials with a flash point higher than 200 F can be safely stored. The equivalent
NFPA standard for above ground storage tanks is NFPA 30.
2. Relationship of wall thickness and tare weight to overall performance. When the stratified wall is better than
a heavier tank?
In short, Sii puts the strength where it is needed. Some of Sii's competitors use a uniform tank wall thickness
and claim this is required by the ASTM standard. Anyone who claims this should read section 6.1 carefully. The
stratified wall optimizes the placement of the material where it is more needed. Sii has well established tank
wall thickness requirements that are published for each tank and are consistent with the ASTM hoop stress formula
and Sii warranty. Sii will continue to provide wall thickness information on request.
The wall thickness of a tank is not the only factor relevant to determining the strength of a tank; in fact it
can be a very small factor. Proper material cure is more important than wall thickness in determining tank strength.
The cure of the material is a determining factor for the long-term hydrostatic strength, flexural modulus, and
the ESCR (Environmental Stress Crack Resistance) rating of the material. Long-term hydrostatic strength test values
could vary from as high as 1600 psi for a properly cured material to as low as 160 psi for a poorly cured material
according to tests conducted by an independent testing lab for Exxon Chemical. With these values, it would take
a poorly cured tank with a 2.5 inch wall thickness to have the same hydrostatic strength as a properly cured tank
with a 0.25 inch wall thickness. This clearly demonstrates why tank weight and/or wall thickness may not be a very
good indication of tank physical strength. Therefore, it is very important to have a manufacturing process that
can produce a properly cured product with a high degree of reliability without degrading the material.
Material cure is highly dependent upon the time of exposure to heat conducting through the tooling and the plastic
material. Excessive exposure to high temperature will degrade the plastic material and cause a weak part. The ideal
cure is to reach the proper curing temperature as fast as possible, maintain that temperature for the correct period
of time, and to cool the part and the tooling at the proper rate of part temperature decrease that does not induce
stresses into the tank wall. The superior Sii manufacturing process and oven capacity makes it possible to reach
the proper cure temperature quickly and avoid the material degradation caused by slow oven temperature rise. Therefore,
it is improper manufacturing technique to compensate for poor material cure with excessively thick tank walls.
With the continuing improvements of true high density linear PE Snyder has taken tank design to a new level. We
have participated in a test program with the University of Nebraska to determine the best resin choice for the
application. The test program is based upon current resins and state of the art molding technology along with specific
chemicals, concentrations and temperature of application.
Since Snyder has the ability and technology to supply either resin, we are able to make unbiased recommendations
for tank design.
The crosslinked polyethylene and the high density linear offered by Snyder are both virgin number one grade resins.
Snyder does not utilize any regrind or blend resins in the molding of industrial tank systems. Both resins have
UV inhibitors incorporated into the raw resin. Along with the economics of a high density linear PE tank, a linear
tank is weldable/repairable in the event of damage and in theory the tank could be recycled in the future once
its use has been exhausted.
Recycling: crosslinks are thermosets that means they cannot be re-melted. Thus, many applications that required
molding, though they can be granulated into road fillers, are excluded. Linears can be re-melted and thus the ease
of recycling and possible end-uses are greatly enhanced.
Download the Sierra Sales Specification for Polyethylene Upright Storage Tanks
here.
or view the Sierra Sales Specification here.
Download the Sierra Sales Tank Use and Installation
Guide (pdf format).
View the Sierra Sales Tank Use and Installation
Guide (html format).
Capacities and Dimensions of
| Vertical Tanks
| Cone Bottom Tanks
| Horizontal Tanks | Double Wall Tanks
|
