Plastic Molded Tanks – What Kind Of Life Expectancy Do They Have?
General Rotationally Molded Polyethylene Tank Sizes:
There are many sizes available for single walled vertical tanks, ranging from 5 gallons to 15,000 gallons, as well as hundreds of sizes for horizontal, rectangular and conical bottom configurations. There are in general two styles of double walled containment systems ranging in sizes from 15 to 10,500 gallons.
Polyethylene tanks are excellent for applications storing a very wide variety of liquids up to 120 degrees F. Polyethylene tanks can handle temperatures below zero, but temperatures over 120 F will began to soften the tank walls and they will bulge. Long term exposure to elevated temperatures could result in stress cracking. Tanks with fiberglass reinforcing on the walls can handle higher temperatures.
Polyethylene Tanks are also known as – Poly Tanks | Plastic Tanks | PE Tanks
Life Expectancy of Plastic Tanks:
There is no proven way to determine the life span of a polyethylene tank. Many variables can have an effect on the tank such as:
- Are there mixed chemicals in the tank?
- Are chemicals being reacted or mixed in the tank?
- What is the temperature of the chemical?
- What is the weight of the chemical? Some chemicals are more aggressive to the tank than others, like sodium hypochlorite and some surfactants.
- How many process fittings are installed in the tank and how large are the fittings?
- What other features or accessories are attached to the tank walls and are they properly supported?
- Is there mechanical agitation in the tank?
- Is there any external loading or mechanical vibration on the tank or nozzles?
- Is the tank used temporarily at a location and then frequently moved around? The more often a tank is moved, the more physical strain it is exposed to from lifting, loading and strapping to a truck, etc. and this physical strain can also affect the tank’s integrity.
- Is the tank used indoors or outdoors?
- If the tank is outdoors, what part of the country is it located in? Tanks used outdoors in Arizona or Florida are subjected to more intense sunshine than in Minnesota for example. Even though the polyethylene resins used in molding chemical storage tanks have Ultra-Violet inhibitors in them, UV rays will have a detrimental effect on the life span of any polyethylene tank.
- Is the tank being used strictly for stationary storage or for transport? Each of these factors will have an effect on the polyethylene tank life.
Rotationally molded plastic tanks have been on the market now for nearly 40 years. We have therefore gained a good general insight into what the average life of a rotationally molded polyethylene tank will be. This is just an overall observation on average, due to many unknown factors unique to each application as outlined above. Many applications are very “tank friendly” and therefore the tanks have an unusually long service life. Other applications are especially demanding and the tanks have very short service lives as a result. We have seen tanks fail in less than two years and we have seen others well over 20 years old.
Although a guarantee cannot be made on how long a polyethylene tank will actually last, we know from experience, that the average plastic storage tank will last anywhere from 10 to 20 years, depending upon what kind of chemicals the tank is holding and the conditions of the environment they are being used in. There are exceptions to this, like 12-1/2% sodium hypochlorite, which generally causes tanks to pre-maturely age after only 5 to 7 years. This occurs on all HD Linear Polyethylene Tanks, regardless of manufacturer (it happens more quickly in Cross-linked polyethylene tanks) due to the chemical nature of sodium hypochlorite. Since each application has its own unique conditions, it is impossible to make a general statement about how long you can expect any given tank to last in a given application, unless we know all of the specifics of that particular application. Even then, the best anyone can reasonably offer is a general expectation, based upon the information available. There have been instances where a tank was purchased for a specific chemical at a specified concentration and later on, the concentration was increased or the tank was re-deployed for an entirely different chemical and service condition. In these cases, we obviously cannot predict what effect these changes may have on the service life of the tank.
General Tank Warranties:
There are standard warranties on tanks if they are used in approved applications. This type of warranty is 3 years for standard weight tanks and 5 years for heavy duty tanks. The warranty covers repair or replacement of a tank that fails due to defects in materials and workmanship. Most tanks that fail due to a manufacturing defect will do so in the first few months after the tank is placed in service. If the tank survives beyond the warranty period, it will more than likely last for 10-15 years or longer.
If A Tank Is Several Years Old, How Do You Determine If It Is Still Safe For Use?
If a plastic tank is beginning to show its age, you will be able to see fine fissures or hairline cracks referred to as crazing. Look for crazing on the dome of a vertical tank in the area where sunlight is most intense or around the nozzles on the sidewall of the tank, especially where mechanical loading from pipe and valves are connected. The top of a transport tank, which has been allowed to set outside all year, is a good place to look as well. These fine cracks will become more intense and visible as time goes on and will become stress cracks. Dropping a light inside the tank can be helpful in visual examinations, because any stress cracks will be easier to spot with the internal light source in place. The resilience of a good tank will be obvious. A tank beyond its life expectancy will sound dull when struck and could crack as a result. It is better to check regularly and early to be safe versus losing valuable chemicals and incurring cleanup expense from an un-examined tank that fails. Regular (at least annual) documented tank inspections should be a normal part of your plant safety and maintenance program. When a tank is showing signs of wear or is otherwise determined to be unsafe, it is best to replace it.