Authors: Robert Connor Rojina and Richard Juskowiak, Process Filtration division of Donaldson
Timely replacement of process filters is an important condition for ensuring the safety of food and beverages, as well as for making a profit. Replacing elements too late creates a risk of contamination, from mold to bacterial buildup. Replacing elements too early leads to unnecessary downtime and expense. The optimal filter replacement schedule is created for each specific production.
A basic maintenance schedule can be created by setting calendar dates for filter replacement. These dates are usually chosen arbitrarily. To optimize the maintenance schedule, it is necessary to follow the manufacturer's recommendations and carefully assess the condition of the filters using differential-pressure monitoring and/or filter-element integrity checks. This approach requires more time but makes it possible to improve the safety and cost-efficiency of the system.
Below are recommendations for assessing filter condition and planning replacements.
Follow the filter manufacturer's recommendations.
Hot-steam sterilization places a load on the element and causes wear of the filter material after a certain number of cycles. The recommended filter replacement interval is usually related to the average number of sterilization cycles the filter can withstand. Most of the elements available on the market withstand from 80 to 90 hot-steam sterilization cycles. New technologies used in LifeTec™ filters from Donaldson Co. make it possible to increase this figure to 100 cycles or more.
The recommended replacement interval can be taken as a basis and adjusted depending on the intensity of filter use. The nature of filter use may also change depending on the season or the materials being filtered. For example, under conditions of increased ambient humidity, more intensive use of a cyclone separator and coalescing filters may be required. Water from underground sources places a greater load on pre-filters than tap water.
Monitor the differential pressure.
The first way to improve the accuracy of the filter replacement schedule is monitoring differential pressure. Differential pressure, measured in pounds per square inch (psi), is the difference in pressure at the filter inlet and outlet. Such a pressure change indicates the degree to which the filter restricts flow. When the differential pressure reaches a set value, the filter will need to be replaced. A significant pressure drop after the filter indicates the need for its regeneration or replacement in order to prevent costly downtime. However, at plants with complex filtration systems, using this method can be more difficult.
The filter manufacturer may specify a recommended maximum pressure drop, but the user can make the final decision on filter replacement based on economic considerations. If at the start of operation the differential pressure across the filter is 1 or 2 psi, its replacement can be postponed until it reaches a level of 10 psi. An increase in differential pressure leads to an increased load on the pumps, which have to compensate for the pressure drop across the filters and maintain a constant process output. It is therefore necessary to determine the optimal balance between the increase in energy costs and the cost of replacing filters. If the filters are not replaced, they will eventually become completely clogged and lead to costly production downtime.
Carry out integrity tests.
The second way to determine the intensity of filter use is checking their wear. If the air, water or steam is more contaminated than usual, filter replacement based on inspection results may, for example, be required every four months instead of the recommended six.
Standard integrity tests of a filter element are carried out using equipment that measures differential pressure and/or efficiency (the percentage of contaminant filtration). The methods for testing air, steam and liquid filters may differ. A common way to test membrane filters for liquids is to apply compressed air to a wetted membrane and check for the formation of bubbles on its opposite side (the «bubble point»). If several filter elements performing the same function are used in a process, a selective check of two or three such filter elements is permitted.
The individual maintenance schedule must include the frequency of hot-steam sterilization. For example, if membrane elements are used at the final stage of the water-filling process, the integrity of the filters can be checked at the end of a chosen shift and then the data extrapolated over several weeks or months to determine the proper replacement interval. This approach gives you a clear advantage: you have several months to plan the replacement and purchase spare filters, so that by the time the filters need replacing you have all the necessary components on hand and can prevent downtime.
Conclusion
When creating a maintenance schedule, also take into account the losses from downtime during filter replacement. Donaldson offers help with such an assessment and with defining an individual preventive-maintenance strategy that will minimize downtime by improving filter performance. Our staff can also carry out several initial filter inspections for new customers to help them determine an appropriate filter replacement schedule.
See the full text of the article in the October issue of the magazine Plant Engineering for 2017.
