Swimming Pool Supplies & Parts
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Vertical DE Filter Grids
Q. Vertical D.E. grids look pretty simple. Is there anything specific to be alert for?
A. There are two basic designs of "curved" vertical D.E. grids - the slotted collar type used by Pac Fab, American, Hayward, and Sta-Rite, and the notched collar style used in Purex and Swimrite/Premier Atlas filters.
The good news is all four slotted grids, i.e. Pac Fab, American, Hayward, and Sta-Rite, are interchangeable (assuming full size grids are replaced with full size and small grids with smalls). Unicel offers one all-purpose replacement series.
The bad news is the two notched types are not interchangeable. Interchanging Purex and Swimrite/Premier Atlas grids in their respective tanks can cause D.E. to bleed directly into the pool. Purex grids are shorter in length than their Atlas cousins and, if put in an Atlas tank, can pop loose from the manifold during the backwash cycle. Once unseated, D.E. will pass directly into the pool when the valve is returned to the filter mode. Unicel offers two replacement series for notched collar grids.
Q. Why doesn't Unicel re-cover used D.E. grids? It certainly should be cheaper than buying a new set, and everybody wants to recycle, right?
A. That sounds good, but let's examine the facts. D.E. grids are made with a specially woven polypropylene cloth cover over a styrene plastic frame. In time, the plastic frame and grid cloth are oxidized by chlorine, weakened by acid and degraded by UV exposure. Therefore, depending on maintenance, water temperature, and the chemical environment, grids can last anywhere from 6 months to 15 years (and longer).
How long will a re-covered grid last? How old is the frame? 6 months or 15 years? How brittle is the plastic? All you really know is that the fabric has failed. That's why the grid was re-covered. Since the frame was subjected to the same conditions as the fabric, can its failure be far behind?
But the re-covered grid looks new. The exposed collar is often sanded or polished to appear new. However, plastics are porous and when the skin is broken they absorb moisture, further weakening the frame. In addition, some re-covered grids are made from two or more broken frames which have been cut and glued together. Can you be sure the reassembled grid is the correct length? One grid out of spec will result in D.E. bypass.
A recovered grid may be cheaper, but can you really afford it?
Q. What is the covering material used on D.E. filter grids and is this a special fabric?
A. The cloth used on Unicel's D.E. grids is 100% Polypropylene. This fiber, a member of the Polyolefin family, has excellent resistance to acids, alkalis, and abrasion, in addition to being both rot and mildew resistant. The use of polypropylene cloth for swimming pool filtration was originally developed by Meissner Mfg. Co. (Unicel's parent company) during the 1950s. Throughout the forty-year span that polypropylene cloth has been used in this application, this fabric has gone through several modifications. Improvements to critical properties such as yarn construction, weave pattern, and fabric finishing have allowed us to better control the fabric's "permeability" in order to optimize the D.E. cake handling characteristics. Permeability is the cloth's flow resistance, expressed in "cubic feet per minute" of airflow as measured with a Frazier permeability machine.
If D.E. grid cloth has too low a permeability, it will blind off quickly, resulting in short filter runs and frequent backwashing. On the other hand, if the material has too high permeability, D.E. can bleed back into the pool. For this reason, all polypropylene grid cloth used by Unicel (and Meissner Mfg.) is woven and finished to our exacting permeability specifications. As a result, we can assure the user the optimum in filtration efficiency, dirt holding, and cake release properties without D.E. kick back or short filter runs.