There are many reasons we choose to use poly tanks, including: availability, cost effectiveness, ease of handling, corrosion resistance, and visibility of liquid in the tank. However, using poly tanks leave some concerns, and now is a great time to evaluate your tanks before they are filled with liquids this spring.
One of the biggest causes of failures to poly tanks is their degradation due to the exposure to ultraviolet (UV) light. When exposed to sunlight, UV radiation changes the material from tough and resilient to a hard, brittle material. This process will make the tanks more prone to leakage and rupturing. Some agricultural producers that I've come across seem to relate tank color to strength. Color really has a very small difference in reducing UV degradation; however some manufacturers will use colors to separate their product lines (different specific gravities of tanks). One of the first considerations you must make when purchasing a tank should be the specific gravity (density) of the tank. Take note to the specific gravities and their ratings:
- 1.0 specific gravity should be used for water only (8.3 lbs./gal)
- 1.5 specific gravity should be used for liquids weighing no more than 12.5 lbs./gal
- 1.9 specific gravity should be used for liquids weighing no more than 15.8 lbs./gal
- In general, the weight of liquid fertilizers ranges from 10-12 lbs./gal, so a minimum tank density of 1.5 should be used.
Once you have figured out your tank density, you should be checking your poly tanks for signs of wear. Here are some tips for checking the condition of your tanks:
- If you are transporting material, first insure that you are using a tank that is designed for transportation instead of storage.
- Make sure that the base underneath your tank remains solid. Insure that the tank is sitting on a level surface.
- Inspect your threaded poly fittings for cracks and leaks.
- When looking at the tank itself, be on the lookout for cracking and crazing.
- Crazing (figure 1) is the appearance of very fine cracks (spider-webbing) within the tank wall. These cracks usually cannot be felt with a fingernail. In this case, the tank will still hold liquids, but its structural integrity is reduces. This is a sign of serious deterioration, which can lead to cracks and failures.
- Cracking (figure 2) reveals very abrupt lines that may run parallel or cross at right angles. It has a dry rot or alligator skin appearance. Your fingernail may catch if you run your fingers across cracking.
Cracking (figure 2)
You can inspect you tanks for cracking or crazing in several different ways:
- Mark your tank with a water soluble marker. Color several 6X6" sections on the sides of the tank that are exposed to the sun, on its top, and around fittings. Then, quickly rub off the ink with a dry cloth or paper towel. Looking at the results can reveal some things not seen by the naked eye. If your tank reveals crazing, consider using it for water only. If you see cracking, consider replacing the tank.
- Candle the tank with a bright, cool light. Placing a light inside the poly tank, while conducting an inspection from the outside, can reveal defects and cracks. You can also shine the light from the outside and have someone look through the fill neck. Never enter the tank!
- Hit your empty tank with a baseball bat. If your tank is sound, it will have the ability to flex as it is filled and emptied. It is impossible to crack a good tank using this method because of the strength of the polymer. Hit the tank on the top and the sides (where it receives the most sunlight). If you can crack a tank using this method, it should have been placed out of service anyway. Sounds kind of crazy, but having a tank break from a swing of the bat is much better than losing 2,000 gallons of fertilizer or pesticide from a failure. Little League season is just around the corner; get your kids some early practice swinging the bat!
A PDF copy of an 82-page publication is available from Purdue University, which contains more great information and color photos.
Also, a fact sheet with much of the same information can be found here from the Ontario Ministry of Agriculture, Food and Rural Affairs.