Drop-in tank linings the solution to corrosion woes
Every plant maintenance professional dreads the realization that an open process tank somewhere in the building is leaking. Visions of OSHA and EPA problems float about. Questions of what to do with the liquid in the tank raise logistics problems. Should the tank be replaced? If so, how do you get the old one out and the new one into place? Getting operations to shut down a process for more than a day or two almost requires an Act of Congress. Let’s face it, leaking tanks anywhere on the property are a headache.
In most cases, replacing a tank is out of the question because it represents an unplanned, long-lead capital expenditure that most plants can ill afford. But how does one get a tank back in service quickly?
The advantages of drop-in linings
These inexpensive liners can be installed in less than 48 hours after a leak is discovered. The repair process requires no sandblasting or removal of the old lining. A drainage layer installed between the new liner and the tank provides early leak detection. The flexible liner offers resistance to crack-through on concrete, and is available as thick as 3/16 inches (187 mils).
High-temperature polyvinylchloride liners are used for service temperatures to 180 F and fluoropolymers for liquids as hot as 300 F. Drop-in liners can be used to contain most chemicals found in industrial tanks.
The versatility of drop-in liners makes them suitable for lining a wide variety of tanks, including:
- Steel (welded, bolted or riveted)
- Alloys
- Concrete
- Brick
- Tile
- Wood
- Rubber-lined
- Fiberglass
- Glass-lined
- Polymer-coated
The technology works on open- or closed-top, horizontal or vertical tanks. Drop-in liners are used in simple, open top, 2 by 2 by 2 foot square tanks, as well as for oil storage tanks that are more than 200 feet in diameter. The liners are fabricated to order to fit the tank; therefore, outlets, baffles, overflows or unusual shapes are easily incorporated into the design. Special formulations are available, such as high-temperature or oil-resistant polyvinylchloride.
Drop-in linings (also called bag liners or bladders) are made from flexible plastics such as polyvinylchloride, polyethylene or fluoropolymers. These liners have been used successfully for more than 40 years. Polyvinylchloride or fluoropolymer liners are so flexible that they are folded and boxed for shipment, and can be installed through a 24-inch manway. Liners for open, rectangular plating tanks or a 12 foot diameter by 12 foot high water or chemical tank can be shipped within 24 hours by air and installed in a few hours. This reduces tank downtime to two days or less.
Steel tanks
Drop-in liners provide corrosion protection for the tank, contain the liquid and offer the possibility for leak detection and secondary containment. Repairing the lining in a steel tank in the traditional manner, whether welded, bolted or riveted, requires a white-metal sandblast for liquid coatings and bonded rubber. Rubber also requires precision. Drop-in liners, on the other hand, do not require vulcanization.
It is nearly impossible to line bolted steel tanks using a liquid coating. The area where sections of tank wall overlap is difficult to properly cover with a continuous liquid coating. Both the shape of the tank and its tendency to move consistently compromise the integrity of any liquid coating. Drop-in liners are easily installed and perform well in bolted steel tanks. A layer of geotextile installed between the liner and the tank protects the liner from abrasion when the tank flexes.
Riveted steel tanks frequently develop leaks as they age. Various approaches, such as caulking the tanks from the outside, have been used to try to stop the leaks, but with marginal success. Drop-in liners, however, offer a long lasting solution to stop the leaks and contain the liquid.
Concrete tanks
Concrete tanks crack with age. The cracks “mirror,” or “telegraph” through the concrete and this cracks liquid or bonded linings.
Mechanically attached at the top of the tank, drop-in liners are impervious to normal cracking if the concrete develops a crack. Geotextile is frequently used behind the liner to pad it against the surface roughness of concrete tanks, eliminating the time-consuming requirement of grinding or resurfacing the concrete.
Concrete also expands as its temperature rises. Liquid coatings frequently develop pinholes as gases in the concrete expand and are forced through the coating. Concrete is properly coated only when the temperature is falling, usually at night. Drop-in linings can be installed anytime, day or night, whether the temperature is rising or falling.
Concrete absorbs large amounts of chemicals, which must be removed if a liquid or bonded coating is to adhere properly. Removing the chemicals requires careful and time-consuming washing, neutralizing and, sometimes, steam cleaning. Much less substrate preparation is needed when installing a drop-in lining.
Other aspects
Wooden tanks are particularly prone to developing leaks as they age. Most conventional tank linings are not suitable for wood tanks. Drop-in linings can dramatically extend the life of wooden tanks. The material is also available for water tanks, as it complies with NSF 61 and FDA requirements. Drop-in linings have the added advantage of offering a tank liner with the addition of secondary containment (leak monitoring) at little additional cost. The geotextile padding in steel or concrete tanks also allows the option of installing a leak monitor. Unlike other lining methods, the combination of a drop-in liner with geotextile padding offers continuous detection of leaks. This method is sometimes used to satisfy secondary containment requirements.
Environmental issues
The process of applying a liquid coating, or any other material that requires a sandblasted finish, generates a large amount of spent sand and old coating. Many older coatings contain material, such as lead or chrome pigments, which qualify as hazardous waste and require expensive disposal methods. Producing a proper white metal sandblast for a rubber lining may result in 10 to 20 pounds of spent material per square foot of tank area. Since the dust from sandblasting is thought to cause pulmonary problems similar to Black Lung Disease (commonly associated with coal mining), workers must use special equipment to protect themselves from the dust. The fugitive dust that floats through a plant when blasting with a sand of hardness 9 on the Mohs scale quickly damages bearings, pumps and instrumentation far outside the area where work is being done.
RFQ requirements
A supplier of tank liners needs the following information to prepare a quotation for a drop-in lining:
- Chemical(s) and concentration(s)
- Temperature range
- Type of tank, i.e., open- or closed-top, concrete or steel, etc.
- Number and size of inlets, outlets, manways, etc.
- Tank dimensions
For a quotation, also provide an up-to-date engineering drawing or marked-up shop drawing of the tank. Sooner or later, you may find a need to promptly repair a problem tank. Installing a drop-in tank liner allows every owner, engineer and maintenance manager to win against the forces of corrosion and leaks.