Plant management at the paper products plant were always on their guys to save money. Therefore, when it came time to do the maintenance on their desiccant air dryer, the replacement of their dryer filters was delayed. This was not thought to be a problem because the differential gauge on the filter was showing in the green — in fact it showed no pressure drop at all. This continued year after year, the differential gauge never once registering in the red, where it might indicate it was time to change the filter.
A compressed air auditor placed data loggers across the dryer and was surprised to see significant pressure loss of 7 psi on the filter. The dryer differential gauge had become stuck and was not moving despite high pressure loss.
Maintenance pulled the filters and were surprised to see a pile of desiccant dust and pipe scale pour from each of the three filter elements. Replacement returned the filter differential to less than 1 psi.
It should be noted:
• Filter differential gauges are often inaccurate or stuck at zero,
• There should be a method of testing the gauge without taking down the filter, use this to test the unit,
• Red/green indicating filters should be avoided, purchase high quality instruments,
• The differential across the filter varies with the square of the flow, if the flow is never at the rating of the filter, the gauge will never read correctly, even if it needs changing,
• Consider changing filters on a time-based schedule, or an in-service-no-longer-than schedule.