The filter cartridge and the fluid must be compatible. Variables affecting compatibility include service temperature, duration of exposure and concentration of any component capable of causing filter degradation. No table of compatibilities can be complete enough to cover all fluids or conditions of exposure. Experience in identical or similar systems can be highly valuable. Without such information, some sort of testing is required. "Bucket testing" -- in which a cartridge is put into a pail of the process fluid for a relevant amount of time -- can be useful. It suffers from difficulties in running the test with hazardous fluids, at elevated temperature, from change in fluid composition with time and from being a static rather than a flow-through test. If the filter cartridge seems somewhat affected but not destroyed, further testing is essential to see if it can provide adequate service life or if a more chemically resistant filter is necessary.

An identical cartridge should be tested to measure the extent of the attack. However, an appropriate compatibility test, which might involve a filter's compressive or tensile strength, may not be easy to develop. Despite the potential problems in devising it, such a test is worth pursuing, especially in systems for which there is little prior experience. Some filter housings have enough room internally to permit placing an extra filter cartridge somewhere in the housing, enabling exposure to actual process conditions. It is also possible to install a dummy test filter in one position within a housing. In such cases, the position should be blanked off, with no flow through the filter, to prevent downstream contamination should the test cartridge fail.

 

 

Filter housings.

These contain the cartridges and come in a variety of sizes and shapes. The number of "equivalent length" filter cartridges needed to accomplish a given task depends upon the required degree of filtration, contaminant characteristics and the viscosity, temperature and flow rate of the fluid. For a given micron rating, three 10-in. cartridges have the equivalent filtration capacity of a single 30-in. cartridge. Once the number of equivalent length filters has been determined, the housing can be specified. For a given number of equivalent lengths, it generally is more economical to choose a small-diameter tall filter housing rather than a large-diameter short housing. Another choice is whether to orient the housing vertically or horizontally. Operators changing cartridges generally find that horizontal housings facilitate cartridge removal and installation, especially if there are many cartridges and they are 40 in. or more in length. In all cases, available space plays a key role in determining housing dimensions and orientation.

Filter location.

Proper placement can assist filters in doing an effective job. Contaminants should be removed as close to the beginning of the process as possible. For instance, in a polymerization or a multi-ingredient paint-making process, it is far easier to remove dirt from solvents by treating the solvent alone than it would be to remove the same dirt after all components, including high viscosity resins, are present. The added benefits of filtering as close to the source as possible can include being able to treat smaller volume streams and the prospect that filtration may be easier in terms of temperature and absence of chemically aggressive materials.

Filters should be located within the process at the lowest temperature possible because this usually offers benefits in solids solubility and minimizes compatibility issues between the filter structure and the process fluid. The location should also provide adequate workspace needed for changing filter elements and should enable easy sampling of the fluid upstream and downstream of the filter to monitor its performance.

Full versus partial filtration.

Recirculating process streams can be filtered partially, usually via 10-20% side streams, or fully. Filtering only a side stream reduces the initial capital cost of the system. Unwanted solids in the recirculating fluid, however, are not effectively removed without full-flow filtration. In many systems, particularly refineries processing very sour crude oil and gas plants handling very sour gas, serious problems, including heat exchanger fouling, foaming, corrosion and fouling of the trays in the contactors and strippers, have arisen because contaminants from the circulated fluid became deposited in critical places. Upsets in the system can result in frequent shutdowns and loss of revenue. Stable operation is promoted by removing suspended solids as quickly as possible -- which calls for filtration of the full process stream.

Capital versus operating costs.

The installed cost of the filtration system must be balanced against operating cost. Budget restraints sometimes lead to installation of under-designed systems that bring unusually high operating costs. Filters in these systems will have short life because of high fluxes through them. They also may not effectively capture certain contaminants such as gels because of the excessive fluxes.

Filter versus filtration cost

The expense of a filter is only part of the overall operating cost for filtration [5]. Table 2 lists typical expenses, including the cost of the filter, plus a number of other important items.

Items 1 through 8 in the table cover a variety of filtration costs that are incurred short term, around the time the filters are placed in service. These are relatively easy to identify among a plant's operating expenses. Items 9 and 10 are costs that only become evident after a longer term, often many months later. They can markedly exceed the short-term costs but are not as easily identified among total plant expenses. The important point is that direct filter cost is only a part of total filtration cost.

Consider the case summarized in Table 3, which compares low-cost filter A to a more expensive but better-performing filter B. The table shows typical parameters describing the total filtration cost for each case. Filter B clearly offers significant cost savings even without taking Items 6-10 from Table 2 into account. These items are too case-specific to include in this example; their effect, however, would be to increase the savings realized by using the "more expensive" filter.

Summary

The key to effective and economic filtration is properly matching the filter to process conditions. Important considerations are the nature of the contaminant, choice of filter media type, compatibility between filter and fluid, flow rate through the filter, location of the filter within the process and partial versus full flow filtration. The simple cost of a replacement filter represents only part of filtration expenses. Controlling the total cost of filtration can lead to significant savings in overall process costs.

Ronald V. Repetti is a senior applications specialist and Alan R. Ponchick is a senior sales specialist for CUNO Inc., Meriden, Conn.