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Access to the Basket
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A second consideration in strainer selection is ease of basket
removal. Since a basket strainer is used where cleaning may occur
often, it stands to reason that the basket should be able to be
removed and replaced as simply as possible. A yoke type cover will
accomplish this. The strainer cover is held down by a clamping yoke
which is sturdy enough to hold full line pressure and yet, which can
be quickly loosened or tightened by hand. Bolted covers are also
furnished with basket strainers and they cost less than yoke covers.
However, every time they are handled their nuts must be taken off and
replaced the same as with a flange. This is time consuming and, in the
long run, expensive.
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Materials of Construction
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A basket strainer should be available in several materials of
construction, the same as valves. A good manufacturer should offer
basket strainers made of iron, bronze, steel, and stainless steel.
Iron, of course is the least expensive. Bronze and steel cost about
twice as much as iron and stainless steel costs about four times as
much.
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Iron
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Since it costs, the least, iron should be the first choice of a basket
strainer user, and for this reason it is by far the most popular
material. Cast iron has good corrosion resistance in water service. It
is also used with many non-aqueous materials such as paint, fuel oils,
and plastics. Iron strainers are generally supplied with brass or
stainless steel baskets. The disadvantages of iron are its inability
to stand thermal or mechanical shock plus it's susceptibility to
corrosion in many applications.
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Why Open Area Ratio is Important
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The open area ratio in basket strainers refers to the amount of free
open of the strainer and basket. It is the ratio of the open area
through the strainer basket to the cross sectional area of the
pipeline. A well designed basket strainer should have an open ratio of
at least 4 to 1. Anything less may cause excessive pressure drop. The
ratio is calculated with a clean basket and as the basket begins to
clog the ratio will drip. Unless there is a wide safety margin the
area through the basket may quickly become smaller than the pipe area.
This will reduce flow through the strainer and necessitate very
frequent cleaning. A small open area ratio also means the holding
capacity of the basket is small - an important consideration if the
amount of solid matter to be removed is large.
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Other Things to Look For
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Another item to look at in picking a strainer is compactness of
design. Is the strainer unnecessarily bulky or tall? In many
industrial areas space is at a premium and the less room a basket
strainer takes the better. This is especially true on board ships
where every square foot counts.
A wide variety of basket perforations and mesh sizes should be
available. This is necessary to cope with the great range of particle
sizes which the sterner may be called upon to remove.
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Bronze
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Bronze is the preferred material for marine service. Bronze strainers
are widely used for handling sea water and also brackish or saline
ground waters. The best basket material in these areas is Monel.
Stainless steel will not resist sea water as well as Monel. Another
use for bronze strainers is where the product can be contaminated by
iron, but the cost of stainless steel is prohibitive. A good example
is handling of liquid sugar. The material is sensitive to iron pickup.
Stainless steel is expensive. Bronze is a good compromise and is
therefore used. Bronze can also be used with mildly corrosive
materials where iron is unsatisfactory and stainless might be too
costly.
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Stainless Steel
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Stainless steel strainers, of course, are used where high corrosion
resistance or where freedom from contamination are required. They are
popular in the chemical, food, and pharmaceutical industries.
Naturally, stainless steel strainers should have stainless steel
baskets.
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Carbon Steel
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Carbon steel strainers are used mainly in the oil and petrochemical
industry. They have excellent resistance to mechanical or thermal
shock and these are important considerations in the event of a fire.
Most oil refineries will not permit iron piping components for this
reason. Steel strainers are also used for higher pressure applications
because of their great strength. Basket material is generally
stainless steel.
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