Tubular screen filters have a wide range of applications
throughout the process industries and are growing rapidly in general industry uses. Unlike
depth media liquid filtration systems that require replacement of the filtering media,
tubular screen surface filters offer economies of operation because the screen can be used
repeatedly.
Tubular screen filters are suited to pressurized liquid systems where trace amounts of
suspended solids are to be removed. The maximum solids removal is contingent on other
factors for each particular application, but 200PPM by weight is a maximum recommended
practical operating level for most applications. It is important here to differentiate
between total suspended instream solids and the amount of solids that the filter must
remove. The 200PPM guideline applies only to the concentration of solids to be removed,
not the total suspended solids. Therefore, a liquid stream with 400PPM total suspended
solids, for example, could be a suitable application for tubular screen filtration if only
half of the solids, or 200PPM, had to be removed.
For all practical purposes, application of tubular screen filters are not limited by the
nature of the liquid stream. Viscosities, temperatures, physical and chemical properties
of the liquid are seldom problems that cannot be solved through proper selection of the
filter housings, elements and screens. Proper screen selection for a particular
application involves the desired retention level or size of screen opening and the type of
screen or element best suited for the application. The retention level is determined by
careful analysis of the objectives of the filter application. The type of screen and
element to be used and their material of construction is selected on the basis of an
analysis of all the characteristics of the liquid system.
Selecting Screen Retention Level
Initially, it may appear that this should be a simple, straightforward decision
based upon the size of the solids in the liquid stream. In very few cases, the process of
selecting the size of the screen openings may be that easy. Rarely will all the suspended
solids be uniform in size. Given a varying particle size distribution, most processors
will find it neither economical nor necessary to remove all suspended solids. Therefore,
it is important to determine the level at which larger particles are removed and smaller
particles which are harmless can pass the screen.
Work with objectives...
What is the filter protecting? That is the first question that is to be asked in
defining the filter's job. A filter is not purchased just to remove solids from a liquid.
It is purchased to eliminate any problem those solids may cause. Some examples here will
be helpful:
A filter may be considered for the removal of dirt from cooling tower water. The objective is to eliminate and prevent plugging of heat exchanger tubes or to prevent settling and build-up in the tower basin. It is necessary then to remove only those particles that would collect in the heat exchange tubes or that would collect in the tower basin.
A filter may be considered to remove particulate from vegetable oil. The objective is to assure the visual clarity of the oil. Since the particulate is usually not harmful in any way, it is necessary only to remove those that can be visually detected.
On spray nozzle applications, it is usually only necessary to remove solids that would plug the nozzle orifice. Other objectives my be to protect pump seals from galling, or to protect meters and instrumentation from damage or malfunction from fouling.
These are all examples of applications where 100% removal of all
detectable solids is not required to accomplish the application objectives.
Use application precedents...
Sometimes a good starting point for the determination of the retention level needed
to satisfy the objectives is a similar successful application. Although the user must be
responsible for the ultimate specification of the screen retention, the filter
manufacturer usually has access to documented application parameters of some current
operating installations. This experience can be helpful as a guide in determining the most
effective and efficient level of solids removal for the job at hand.
Screen Types:
Woven Wire Woven Synthetic Perforated Slotted Wedge Wire
Frequently, there is a tendency to specify finer screens than necessary "just to be on the safe side". Though there may be some assurance in the added protection, the resulting added cost can be substantial. Not only is the initial cost of equipment apt to be greater because of the need for additional screen area to handle the flow at a low pressure drop, but the cost of screen maintenance and cleaning is apt to be greater as a result of removing more suspended solids.
RPA Process Technologies
9151 Shaver Road
Portage, MI 49024
Toll Free (US only):
1-800-656-3344
Voice: +1 269 323-1313
Fax: +1 269 323-0065
www.RPAprocess.com
info@RPAprocess.com
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