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| How Strainers Can Improve a Process | |
The problem of unwanted material in pipelines is a never ending one. Whether the flowing material I seawater, oil, paint or a variety of food or chemical products, there is often something present that can cause trouble. Dirt, foreign matter, or even clumps of the product itself can clog or damage pumps, spray nozzles, condensers, and similar equipment. Sometime a finished product has to be rejected because of the presence of undesirable solid matter. Basket strainers remove unwanted particles from pipeline flow. Furthermore they are relatively inexpensive compared to the equipment they protect or compared to the down time, inferior products, or loss of production if they were not doing their job in the piping system. | |
| What is a Basket Strainer? | |
An official definition adopted by the Fluid Controls Institute is : “A closed vessel with cleanable screen element designed to remove and retain foreign particles down to 0.001 inch diameter from various flowing fluids.” Note the term “foreign particles”. Strainers do not necessarily remove only dirt. They take out material which is not wanted in the fluid and this can sometime be a valuable product which may be saved. | |
| The Difference Between Strainers and Filters | |
What is the difference between a strainer and a filter? Actually there isn’t any since a strainer is, in reality, a coarse filter. The question is then one of semantics. Generally it is assumed that if the particle to be removed is not visible to the naked eye, the unit is filtering, and if the particle is visible, the unit is straining. The average human eye can detect a specific particle between 50 and 70 micros. Most people cannot see anything smaller than 325 mesh, or 44 microns. Since 200 mesh is equivalent to 74 microns, a general rule would be that if the screening device is coarser than 200 mesh, it is a strainer and if it is finer than 200 mesh it is a filter. One of the best uses for a strainer is in conjunction with a filter. By installing a strainer directly ahead of a filter, the large heavy pieces which would quickly clog the filter are removed. The filter is then free to do its major job of fine particle removal and does not have to be cleaned so often. | |
| Why Open Area Ratio is Important | |
| 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 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. | |
| How to Choose the Right Strainer Basket | |
| The basket is the heart of a strainer since this is where the unwanted material is trapped. Strainer baskets are made of perforated sheet metal and a wide range of opening sizes is available. The size of the basket perforation should be slightly smaller than the minimum particle size to be removed. Using a smaller perforation opening than necessary only means the basket will fill and clog more quickly and have to be cleaned more often. Well made strainer baskets should be brazed or welded. The use of solder to hold the basket together cuts the cost, but it is not recommended. Soldered baskets are inherently weak and can break easily. | |
| Fine Straining Applications | |
| If finer straining is needed, then the gasket is fitted with a wire mesh liner inside the perforated sheet. Here the mesh lining does the actual straining, while the outer metal sheet acts only as a mechanical support. For this reason mesh lined baskets are generally supplied with an outer perforation having about 50% open area. This gives the best combination of maximum flow rate with the least loss of strength. Perforated materials with more than 50% open area are too weak to provide adequate mechanical support for the mesh lining. It is important that the wire mesh be integrally and tightly fastened at the tip and bottom of the basket, other wise material can bypass the mesh lining by getting behind or under it. For this reason baskets with removable mesh lining should be avoided. A mesh lined basket should be a one piece welded unit. | |
| 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. | |
| Basket Cleaning | |
| Baskets should be handled carefully. They should be cleaned by using a brush or by soaking them in a solvent or cleaning solution. They must not be rapped on table or struck to loosen something in them. This will cause them to dent out of shape and eventually their welds will break. It is a good idea to clean baskets promptly after removing them from a strainer. It prevents the product from drying and hardening and thus becoming harder to clean. Most important, a spare basket should be on hand for every strainer. Baskets have a habit of breaking at the most inconvenient times and the strainer cannot function unless there is a replacement. | |
| Installation Considerations | |
For protection of pipeline equipment under relatively clean service conditions where there is little debris loading for both vapor (natural gas, air, steam etc.) and liquid service, a y-strainer is generally the strainer of choice. In liquid service applications however, where the debris loading is higher and collection of solids is required, basket strainers should be considered. This is because basket strainers, both simplex and duplex, have more gross strainer basket area than a y-strainer has gross screen area, and the ratio of free area to pipe area for basket strainers is greater than for a similar size y-strainer.
Y-strainers can be installed in either a horizontal or vertical pipeline, whereas basket type strainer are designed for installation in horizontal lines. The flow must always be into the inlet – into the screen or basket (s) – for satisfactory operation. If y-strainers are installed in a vertical line, the flow must be from top to bottom (downward). The blow down (drain) may be angled 45 degrees maximum.
The following items should all be considered in selecting the proper strainer of an application:
Cast iron and carbon steel construction basket strainers are generally acceptable for fresh/city water, and lube oil services. Bronze and stainless steel are generally acceptable for brackish/salt water use. Carbon steel is generally acceptable for straining fuel oils. Stainless steel is generally acceptable for most chemical applications.
Y-strainers constructed of cast iron, bronze, carbon and stainless steel are generally acceptable for steam, air, natural gas or liquids.
Don’t overstrain, the opening in the basket or screen should never be smaller than one-half the size of the largest particle whose presence in the system is considered harmful. | |
| Selection and Sizing of Basket Strainers | |
| When installing basket strainers, it is sometimes a good idea to use two of them in tandem. This is especially true when there is a great deal of material to be taken out of the line and the particles vary in size. If a single strainer with a fine mesh lining were used alone, it would fill quickly and necessitate very frequent cleaning. By using two strainers in series, the first with larger opening to trap the large particles and the second with a fine mesh lining, the load is spread out and time between cleanings is also spread out. | |
| A Good “Rule of Thumb” | |
| Selecting the proper size basket strainer for a particular application is most important. Most reliable manufacturers have pressure drop information for all sizes. A general rule of thumb is that pressure drop through the strainer should not be greater than 2 psi. If it is, the strainer selected is too small. Factors such as viscosity, specific gravity, and mesh lining size all influence pressure drop through the unit and they must be taken into account. | |
| Don’t Be Misled By Pipeline Size | |
| Thus, the strainer size is not automatically the same as the pipe size. It may be larger. In the case of highly viscous liquids, for instance, where pressure drop through a mesh lined basket can reduce flow considerably it is sometimes necessary to use a strainer several sizes larger than the pipeline in order to insure adequate flow. However, this is often desirable because, aside from the slightly larger initial investment, there are no subsequent added costs. Basket strainers last almost indefinitely and there is almost nothing to wear out. If there is an unusually large amount of material to be taken out of the line, then a larger strainer is almost certainly called for. Too small a unit will fill too quickly for adequate straining and will require frequent basket changing. This can be expensive in the long run. If a basket has to be changed more than once an hour, the strainer is probably too small. | |
| Application Considerations | |
| The nature of the product also has an influence on the strainer size. Soft, gummy materials like tars and resins are more difficult to remove and clog baskets more quickly than hard crystalline materials. In these cases, a larger strainer size is also recommended. The size of a basket strainer, therefore, is a function of the job to be done. It should not automatically be the same size as the pipeline. There are no industry wide standards on face to face dimensions for flanged basket strainers, such as there are for gate valves. For this reason, the strainer of one manufacturer may not fit in a space designed for a different brand of strainer. If there is doubt on this point, it is best to check the face to face dimensions of the particular unit which is going into the line. | |
| 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 are 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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FACTORY DIRECT PIPELINE PRODUCTS, INC. Your Single Source for Industrial Filtration and Valve Products Copyright 2001-2008 |
phone: | 908.496.9020 x-302 | |
| fax: | 908.496.8080 | |||
| email: | fdpp@fdpp.com |
