Test the Mettle of FRP Tanks

Commissioning and operating such vessels demands special care.

By Dirk Willard, Contributing Editor

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It exploded! An Indian worker almost drowned when a fiber-reinforced plastic (FRP) tank unzipped during a fill test. Eliminating the final band around the circumferences saved the manufacturer money but almost flushed a guy into the Indian Ocean.

The truth is FRP fabrication is still an art.

FRP tank mishaps occur in developed as well as developing countries. The truth is that FRP fabrication is still an art. Manufacturers can't easily correct mistakes or deficiencies like they can with steel; this tempts some vendors to ship a marginally acceptable tank in the hope it will pass on-site inspection.

So, what should you look for when you commission an FRP tank? Inspect the piping and foundation as well as the tank itself.

Piping should be isolated by expansion joints from the tank, and must be freestanding — don't support it from the tank. Check nozzle connections with a torque wrench.

Inspect concrete prior to tank installation. Ensure the concrete under where the tank will be is smooth with a rubber mat, not sand, filling the inevitable gaps. Don't rest an FRP tank on bare ground for long-term service.

For the tank itself, start with a visual inspection: there should be no swelling, blisters, discoloration, softening, cracks, visible fibers or delamination. Aged resin ingredients can cause bubbles, discoloration, softening and blisters. Poor mixing and winding can lead to visible fibers, spots, delamination and cracks. Another item on the checklist is flatness: ¼-in. maximum deviation per ft for the bottom, 1/16 in. from a right angle for nozzles — flanges should be mandatory.

Keep anchors loose until the tank has reached operating temperature and then tighten them gently. Restrict temperature to below 50°C for long-term use (>15 years); operating above 50°C shortens the life of FRP, even for tanks made with special high-temperature resins. Temperature can go up to 90°C for special service, but excursions can't exceed 98°C. Confirm the placement of lugs because improper design could have damaged the tank. You can use rope slings but never steer or lift a tank by a nozzle.

Now, let's consider testing. Assess pre-mix samples of resins and resin/fiber samples to check ingredient quality control. FRP is an anisotropic material — meaning the stress is unique in each of three directions — so destructive testing results would vary between samples taken at nozzles and those taken from the shell or head. Vessels are woven and wound to form the shape of the final form. Also perform nondestructive testing on the tank. In the field, unless you want to hire an expert, you're limited to infra-red analysis of the vessel while it's in service; obviously, a hot spot could indicate a weakness such as thinning of the resin, but I doubt it because the differences are so slight between the insulation from a good wall and a thinned wall. If you want to check out an FRP expert ask about spark testing — the person should advise against it. The expert may suggest acoustic emission testing, which shows promise. Conduct annual inspections after the first year; twice a year during new service or after ten years.

High or low pH service, operating at vacuum or in an aerated environment shorten vessel life more than they do for a steel tank. Cooling and heating an FRP tank causes rings to form in the shell. Aeration, such as from circulating water inside a scrubber, forms a whitened zone at the water/air interface. Such visual signs indicate wear. Typical service is 10–15 years for hydrochloric acid regeneration plants where, at the worst case, vacuum runs down to 650 torr absolute with normal temperatures as high as 90°C and pH lower than 1.0. Avoid freezing temperatures because an FRP vessel, unlike a steel tank, is prone to failure from expansion.

Don't use FRP tanks for some chemicals: hydrofluoric acid and chromates, many acetates and carbonates, acetone, and bleaching agents like sodium hypochlorite. Vessels can tolerate up to 25% sulfuric acid and phosphoric acid, and 32% hydrochloric acid (but for safe operation, keep HCl concentration below 10%). Avoid hydroxides above 5%. These recommendations are at ambient temperatures; for high temperatures consult the tank manufacturer.

So, what should you do if you find a crack or leak? For swelling or weeping, call an expert; the bond between the resin and the glass fibers has failed, indicating a general decline in vessel integrity. It's often possible to repair a crack, provided it hasn't penetrated more than 30% into a web. Cracks usually appear in groups, e.g., around vessel anchors. Allowing these to grow more than a few inches in length will compromise tank integrity.

As with all things fiberglass, consult an expert before attempting repairs yourself.

DIRK WILLARD is a Chemical Processing contributing editor. You can e-mail him at dwillard@putman.net.

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