Basket Strainer Selection Tips
How to Choose a Basket Strainer
NOTATION: This is general basket strainer information, and does not reflect Eaton specific info.
Why Open Area Ratio is Important
The open area ratio in basket strainers 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 drop. 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.
Access to the Basket
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.
Other Things to Look For
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 strainer may be called upon to remove.
Materials of Construction
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.
Iron
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 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.
Bronze
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.
Carbon Steel
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.
Stainless Steel
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.