Whether a tank is a few years old or 20 years old, the only way to be sure it is structurally sound is to perform inspections before filling and again at the end of the storage period. The inspection, prior to filling, provides reassurance that the tank can safely store or transport the fertilizer or pesticide required. Inspect the tank after use to determine if a new tank is required before the next filling (if the tank is defective or unserviceable, there is time to consider a replacement). Make sure that the base under a vertical tank remains solid. Animals can burrow underneath, causing the base to become uneven. Keep a written record of each inspection for liability or warranty concerns.
Threaded poly fittings sometimes develop leaks when left dry for extended periods of time, so be sure to check them before use. Some of the older poly tanks are plumbed with black iron fittings versus poly fittings, and after years of use the iron starts to deteriorate from the inside out. Inspect these fittings and consider replacing them with newer poly fittings.
Before conducting an inspection of the poly tank, it is important to know the difference between crazing within the tank wall (cracks that extend through the tank wall) and surface scratches.
Crazing is the appearance of very fine cracks within the tank wall, usually appearing as a network of fine lines that cannot be felt with a fingernail. The tank will still hold liquids, but its structural integrity is significantly reduced. Crazing occurs in both high-density and cross-linked poly tanks; it is a sign of serious deterioration within the plastic, which leads to cracks and fractures.
Tank showing evidence of crazing (in blue) and cracking (black) when coloured with a water-soluble marker.
Cracking causes no displaced material; very abrupt lines may run parallel or cross at right angles to each other. UV cracking has a dry rot or alligator-skin look in advanced stages; a fingernail may catch on the cracks. It is common for the poly material around the crack to appear whiter than the surrounding polymer.
Most scratches displace minute amounts of polymer but remain superficial. Scratches are open to the surface. Displaced material is evident on the tank’s surface. A fingernail would catch on the scratch.
It is difficult to visually determine a good tank from a bad tank. Three inspection techniques are used for inspecting poly tank integrity:
writing on the tank with water-soluble ink
candling the tank with light
hitting the tank with a baseball bat
These techniques can pinpoint weakened walls and stressed areas around the fittings.
Marking the Tank with a Water Soluble Marker
Crazing may signal UV damage. UV crazing, which is very difficult to see, forms in areas where the tank gets maximum sunlight exposure; the lines become more visible when the tank is “coloured” with a water-soluble marker. Rub the marker over several 6-in.-x-6-in. sections on the sides of the tank exposed to sun, on its top and around fittings. Quickly rub off the ink with a dry cloth or paper towel. The ink left behind will penetrate the surface of the tank and reveal any crazing present .
Poly tank showing evidence of crazing after coloured with a water-soluble marker.
Crazing is one of the first signs of deterioration, so check tanks with crazing often. Consider using crazed tanks for water only. If the ink reveals cracking or spider webbing, where the lines go in all directions, classic UV radiation damage is indicated. Advanced deterioration to the plastic presents a checkered or “dry rot” appearance, indicating loss of elasticity. Replace a tank showing these symptoms.
The appearance of parallel lines signals early UV damage and the need for continual inspections. Replace tanks with parallel lines in the plastic around fittings immediately or use the tanks to store water only.
A bright light is used to candle the poly tank from the inside of the tank.
Candling: Visual Inspection With a Light
Candling consists of placing a bright, cool light source inside a poly tank while conducting a visual inspection from the outside (do not use a hot lamp, as it could melt the tank).
Repeat this procedure with the light on the outside of the tank and someone looking through the fill neck or man way. Do not enter the tank. A camera, camcorder or other optical device may be helpful in detecting defects.
A bright light is used to candle the poly tank from the outside of the tank. Defects and cracks in the tank will show up as areas or lines of different light intensity .
Cracking in poly tank at junction point.
Cracks exposed in poly tank thru candling from the inside of the tank.
Candling reveals crazing of this tank.
Tank showing signs of crazing and cracking when tested with a water soluble marker.
Hitting an Empty Tank with a Baseball Bat
An empty tank showing UV cracking can be further evaluated by striking it with a baseball bat. Some people are afraid to hit their empty tank with a bat, fearing that the bat might break the tank. Do not hesitate to perform this test, because if the tank ruptures in this situation, it should not be in service. Cracking an empty tank with a bat is a better option than risking the rupture of a tank filled with fertilizer or pesticide.
A good tank has the flexibility to bend outward as it is filled and inward as it is emptied. Tanks that are brittle (i.e., that show excessive or advanced cracking) have lost the ability to flex under pressure and to rebound when impacted. Test the brittleness of an empty tank with a solid swing of a baseball bat where the signs of cracking were discovered during the water-soluble ink inspection. Hit the tank along the sides and top where they receive the most sunlight, then check the tank for signs of breakage. It is impossible to crack a good tank using this method, because the polymer is strong and resilient; if the tank cracks or breaks open when hit by a bat, a potential spill has been averted.
Upon being struck with a baseball bat to determine the tank integrity, this tank failed due to the brittleness of the plastic.