Ask
Dan! Column*
 |
Welcome
to Ask Dan!, a monthly column by our resident water treatment
guru, Dan Capano of DTS, Inc. Dan addresses the issues
that bug you the most. And Dan knows!! Working
in the design and construction on both the public and private
sector water-treatment projects. He has published articles
on the subject and has a book, Network Cabling for Contractors,
published by McGraw-Hill. |
Specifying
Rotationally-Molded PE Storage Tanks
Guest Article by Darrell Oltman & Arthur Blaker
Each
of the commonly used materials of construction for PE bulk storage
and processing tanks has advantages and disadvantages in particular
use conditions. For some applications, several materials of construction
may be acceptable. The key is recognizing each materials' strengths
and weaknesses and applying them to the application requirements.
Applications
Polyethylene
(PE) tanks can be used successfully in a variety of liquid and
dry material applications.
 Rotationally
molded PE tanks are also acceptable for storing many liquids at
atmospheric pressure and can be designed for use with corrosive
liquids, commonly used in the water and wastewater treatment industry,
such as sulfuric acid, hydrochloric acid, sodium hypochlorite,
hydrofluoric acid, and sodium hydroxide.
PE
tanks are highly impact resistant, have homogeneous wall construction,
good weatherability, broad chemical resistance, excellent adaptability
for numerous nozzle (outlet) configurations. These attributes
make PE tanks an effective alternative to costlier FRP and metallic
fabricated vessel assemblies. A result of stress-free construction,
PE resists cracking, does not dent, and is rotationally molded
seamless.
PE
tanks are not suitable for high heat (over 150°F), pressurized
applications, and exposure to aromatic hydrocarbons, halogenated
solvents, some aliphatics, and chemical reactions or processes
that release free oxygen molecules. In applications that do not
involve these conditions, rotationally molded polyethylene tanks
should be considered.
Some
engineers are not aware of what rotational molding is capable
of producing. HDLPE tanks are molded for a wide variety of applications
in vertical, horizontal, and conical bottom configurations.
Sizes
range from five gallons to 16,500 gallon vertical! Wall thickness
vary from 0.125 inches to over 2.75 inches. PE tanks are extremely
durable and may be constructed from both crosslinkable and linear
PE resins. The two resin types have similarities, but also have
some very basic differences.
Linear
vs. Crosslinked Polyethylene
Linear
polyethylene is a long-chain polymer consisting primarily of ethylene
groups. On the molecular scale, it may be described as a long
chain of repeating groups, each having one carbon atom connected
to two hydrogen atoms, one on either side. At each end, the carbon
of one group is connected to another group.
Individual
molecules are very long, generally containing between 10,000 and
100,000 atoms in the carbon backbone of each molecule. The solid
polymer contains millions of those long molecular chains entangled
together. The strength of the molded polyethylene part lies in
the complexity of that entanglement, which mechanically restricts
the polymer from changing shape.
Crosslinkable
PE resins contain a small amount of crosslinking agent dispersed
throughout a linear polyethylene material. The crosslinking agent
is designed to decompose at a selected temperature during the
rotational molding process.
During
the decomposition sequence, crosslinking agents break down into
two reactive products, each of which strips a single hydrogen
atom from the carbon backbone of the nearest PE molecules. Stripped
of their hydrogen, the remaining carbon atoms are reactive and
combine with each other, thus "crosslinking" the carbon
chains of two molecules.
Strength
of the crosslinked PE depends solely on the links, usually no
more than one or two for every 1000 carbon atoms. The strength
of crosslinked PE no longer depends solely on the mechanical entanglement
of molecules, but is reinforced by a network of chemical bonds.
These
basic differences between the resins cause variation in properties.
Linear PE is capable of being recycled and welded (repaired).
Has good cold impact resistance, excellent elongation characteristics,
meets the requirements of the U.S. Food and Drug Administration
(FDA) Regulation 177.1520, has good environmental stress-crack
resistance and can develop high tensile strength. Crosslinked
PE is a tough material that cannot be recycled or welded (repaired).
Has excellent cold impact resistance, good elongation characteristics,
excellent environmental stress-crack resistance and can develop
a high tensile strength if processed correctly.
Crosslinked
PE resins are not FDA approved because of the by-products of the
crosslinking agent decomposition during the rotational molding
process. A crosslinked resin, even with a linear liner, should
not be used for food processing or potable water applications.
Discuss
Each Application
Different
materials and processing methods can also cause variations in
other physical characteristics of the material. Discuss each application
with the rotational molders' representative.
 Type
of chemical, concentration, specific gravity, temperature range,
dimensions, mechanical loading, indoor or out-of doors exposure,
and physical location of tanks are just a few of the considerations
that must be analyzed during the specification phase of PE storage
tanks.
Compare
competitive quotations to ensure they are for similar conditions.
Here are some points to consider:
- Have
the tank described to a consistent spec such as ASTM D-1998-97.
- Check
the accessories quoted to ensure they are of comparable construction
and quality.
- Specify
that the manufacturer will use only virgin materials (no regrind
material).
- Insist
on a "chemical specific" warranty not "workmanship
and materials."
- Larger
tanks are heavy, insist on lifting lugs. The best lifting/tiedown
lugs are rotationally molded as part of the vessel; no drilling,
no expensive alloy hardware.
- Insist
on ample flat areas on the dome of the vessel for fittings and
accessories.
- Full
drainage molded-in FNPT inserts available in hastelloy, titanium,
or stainless steel make tank draining and cleaning chores "routine."
- When
working with double wall tank assemblies, insist on a UFO (Unified
Fitting Outlet) design that allows side wall fittings to be
installed and safely sealed through both walls, primary and
secondary.
*
The Ask Dan Column can be found at Water and Waste Water Dot Com.
|
