Strainer baskets should be cleaned on a regular basis, not when they become clogged, to insure that they are not damaged by too high a differential pressure. Another reason for frequent cleaning is a phenomenon known as “runaway buildup”. As dirt in a strainer basket accumulates and as the mesh or perforations plug up, pressure drop increases. The curve of this pressure difference is not a straight line. It starts out as a low slope, but as the strainer basket clogs more and more it turns upward faster and faster. As the dirt builds up the free open area in the basket gets smaller and smaller.
All conditions in the strainer basket are now working faster and faster to decrease the flow passage and to increase drag on the liquid flowing through the strainer basket. Dirt is being brought faster to the passages that are open and they are being shut faster. Liquid velocity and pressure inside the basket build up faster and the whole thing keeps accelerating. It is similar to a fire in that it progresses with increasing speed.
It means that a good flow of liquid can slow to a trickle or stop quickly. It also means that full line pressure is now brought to bear across the basket, and as pointed out before, if this is high, breakage can occur. Good maintenance procedure dictates that basket be cleaned or changed before they can become clogged. Most strainer users do this at standard intervals, whether the strainer basket is ready for cleaning or not.
Of course, the most obvious indication that a strainer basket need changing is a drop in flow through the line. In closed systems this is not always so easy to notice. In these cases installation of a pressure gauge on the downstream side of the strainer will point out drop in flow or pressure. Better yet, two gauges, one on either side of the strainer, will indicate pressure drop through the unit and show whether the strainer basket needs cleaning.