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Proper
Basket Selection and Care
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| 1 |
What
Size Mesh Lining to Use |
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| 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. |
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| 2 |
Watch
Out for Pressure Differential |
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| 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. |
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| 3 |
When
to Clean Baskets |
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| 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. |
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to Hayward Back to PEP
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Copyright
HAYWARD Industrial Products, Inc. 1999
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Proper
Basket Selection and Care