Proper Basket Selection and Care

What Size Mesh Lining to Use

The question of which mesh lining size to use comes up regularly. Here the basic rule is to use the coarsest mesh which will strain out the product to be removed. Using a finer mesh than needed will only result in premature clogging. When in doubt about which of two mesh sizes to use, it is best to choose the larger. Once the mesh fills with dirt it is sometimes impossible to completely clean it since material will sometime pack into the square corners of the wire weave. The result is that the cross sectional area of the mesh opening is lightly reduced. Thus, for instance, if uncertain whether to us a 40 or a 60 mesh lining, use the 40 mesh.  After several fillings and cleanings, if dirt packs into the mesh corners, the mesh will wind up being equivalent of a 60 size anyway.

Watch Out for Pressure Differential

While strainer housings are deigned to withstand pressures far in excess of their rated maximum operating pressures, this is not true of baskets. Very few strainer baskets will stand high pressure differentials through them.

In a strainer rated at 150 psi, for example, if used with a line pressure of 150 psi and if the basket is completely clogged, the pressure differential across it is equal to the full line pressure, in this case 150 psi. Under these conditions, the basket will probably break. Generally the amount of pressure baskets will stand when they are fully plugged is considerably less than the maximum strainer rated operating pressure. The larger the basket, the less differential pressure it will take. There are specially made, reinforced baskets available which will take higher differential pressure.

When to Clean Baskets

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 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 basket ate now working faster and faster to decrease the flow passage and to increase drag on the liquid flowing through the 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 basket are ready for cleaning or not.

Of course, the most obvious indication that a basket need changing is a drop in flow through the line.  In closed systems this I 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 basket needs cleaning.