The importance or significance of pressure drop is that it determines the hydraulic life (capacity before becoming plugged with sediment) of filters. One end-point of mechanical filtration is the appearance of either particles of the filter media or the filtered solids in the effluent. The force of the flowing water carries particles toward the effluent, where they eventually break through if they can. If the pressure and flow are very regular, this can occur “on schedule,” and the hydraulic life is somewhat predictable. Usually, however, flow surges or water hammers cause the media to shift or flex, so that particles previously removed from the flow (filtered out) are dislodged and stream toward the outlet suddenly. This is known as channeling and dumping, and once a filter has had a channeling and dumping event, it is usually finished as an effective mechanical filter. Some media may be able to “repair” themselves momentarily and thus last a long time with repeating cycles of filtering and dumping, until the medium is either backwashed and rebedded or replaced. Channeling and dumping is both dangerous and disgusting, and it should not be permitted. It is the job of filter designers to assure that filtration will not break down (allow channeling and dumping) until the ∆P exceeds a certain level, usually 40 psi (276 kPa or 2.72 atm). That is because the minimum line pressure in the U.S. is usually about 40 psi, which means that when the ∆P is also 40 psi, there may be very little flow of water. All or nearly all of the pressure provided is consumed by the system, with little or nothing left over to provide “push” to make the water flow. Thus, a 6P of 40 psi is usually considered the hydraulic end of life. It should be easy to see that in any system designed to transport water, a filter in the line will probably be the major contributor to pressure drop. And it is self-evident that:
a) finer-grained media provide better mechanical filtration than coarse media, and
b) finer-grained media also produce greater pressure drop than coarse media.
Thus, it should be easy to see that there is a trend from coarse filters with low pressure drop to fine-filters with higher pressure drop, and that there is a practical limit to the thickness. or depth of filter media because of pressure drop. For granular media, the practical limit is about 30 inches (76 cm) of bed depth: beds of granular carbon, ion exchange resins or filter sand deeper than that can be expected to have excessive initial 6P at the usual flows and pressures. It should also be easy to see that, as the filter media particle size becomes increasingly smaller, the pressure drop for a given media depth increases. Or to put it the other way, when the media particle size is decreased, the media depth must decrease if the same initial 6P is to be maintained. This trend is taken to the logical limit in precoat filters, which are Everpure’s specialty.