Filtrex Incorporated

P.O. Box 2273  •  Wayne, New Jersey 07474-2273
Phone: 973-595-0400  •  Fax: 973-595-6506

IS5899A Why Stainless Steel?

PER Corporation (Filtrex's predecessor) and Filtrex, manufactured Inc. carbon steel tanks for several years, up to 1976 (Filtrex's last carbon steel tank). It was certainly an attractive option as it lowers material costs by 30-40% depending on filter model. The main problem with a carbon steel tank is field longevity. Our stated goal in manufacture is that our products should give an effective service life equal to that of the facility itself. This in our opinion cannot be achieved with carbon steel construction regardless of surface treatments, coatings, linings, and processes.

In general, unprotected carbon steel fares poorly in swimming pool filter and recirculation systems. It suffers from both oxidative and chloride mediated corrosion processes. Coatings and surface treatments may be a partial answer in sand filters, balancing tanks and the like. However, the factors involved in a modern regenerative filter render these solutions temporary at best.

The two main factors which tend to thwart the use of surface treatments and coatings are; flow velocity and precoat recycle surface ablation. Flow velocities, especially in the lower portion of a regenerative filter (below the inlet distributor) are quite high (8-10 times higher than found in a typical sand filter), moreover the flows are highly turbulent with some locally concentrated vectors producing flow "hot spots." In addition during precoat recycle the liquid in the closed loop may contain up to 10,000 PPM silica (D.E.) particles. This in effect "sand blasts" areas in the filter leading to ablation and eventual coating failure. Although this effect is transitory in nature, it tends to roughen the coating surface leading to further erosion.

We have, over the years tried vulcanized rubbers, vinyls, urethanes, and a variety of epoxies (filled and non-filled) in our testing lab. Our testing protocols simulate 1 year of operation (8750 hrs) under normal conditions. Coatings are evaluated with a thickness tester and optically examined with a stereo microscope. We have observed ablation rates in the materials mentioned above, between 8-36 MPY (mils per year). The poorest being the urethanes and the best being with the filled epoxies.

The above cited problems combined with normal application problems; surface bonding, pinholes, voids, cracks (cracking being particular to epoxy treatments) and uniform coating of the complex geometries of the inlet distributors, in our opinion do not lead to adequate protection against corrosion. In fact, they can, under some circumstance exacerbate corrosion problems.

By contrast premium grades of stainless stainless steel used in conjunction with proper welding alloy/practices, shielding gas, and post fabrication passivation will produce a filter tank with a lifespan wich can and will (if maintained properly) outlast the facility that it is installed in.