Center for Irrigation Technology
- Irrigation Notes -
Sand Problems Call for Irrigation Technology
by Edward Norum
CATI Publication #990801 © Copyright August 1999, all rights reserved
Sand is a contaminant in water supply systems. Even in minor concentrations in residential systems, it can plug shower heads and distort the pattern of faucet aerators. Single grains of sand can cause leakage of toilet flush valves and backflow prevention devices. Water is wasted directly by leakage and indirectly by degrading the effectiveness of spray patterns. Corrective action can range from the homeowner attempting to fix the problem to seeking the services of a plumber or other specialist. Sand also has a devastating effect on agricultural drip and sprinkler systems, causing plugging, abrasion and fouling.
The Center for Irrigation Technology (CIT) is located in Fresno, California. The city’s water is obtained from wells located in a grid system spaced at approximately one-mile intervals. A regular characteristic of the water supplied by this well grid system is sand contamination. Sand contamination occurs from at least two sources. First, it can result from damaged or improperly constructed wells. Properly designed wells require the use of wedge-wire screens and graded gravel packs. Screen opening size and gravel pack gradation is best specified by a hydrogeologist with an intimate knowledge of the water bearing strata. Even then, natural phenomenon such as earthquakes, droughts, floods, etc. can cause sand to enter the well water. Second, sand can be inadvertently introduced when pipeline repairs or modifications are made.
With more than 18 years of experience in testing irrigation equipment (sprinklers, drip emitters, etc.), the Center for Irrigation Technology (CIT) has become expert at evaluating and testing various screen, disc, sand media and sand separator products. This work has resulted in establishment of protocols for testing performance and operating characteristics of filtration systems. CIT researchers also have observed the similarities of sand problems in both potable water systems and irrigation systems. Such similarities suggest that the CIT irrigation-related findings could be considered viable data for solving sand problems in potable water systems.
Following the logic of the drip irrigation system experience, the first design parameter to be determined would be the largest sand particle size the system can tolerate. For example, a representative shower head baffel plate has 0.045 in. diameter holes. This is the equivalent of 16-mesh screen. Since sand particles are not round (see photo on Page 3), a finer mesh should probably be used to insure that "length wise" wedging does not occur. Other commonly used system components should be tested to determine their susceptibility to sand. This would lead to a system filtration specification that defines both maximum concentration and particle size that can be tolerated. Plugging tests on drip emitters show a range of tolerance for particle size from 80 to 200 mesh. Fine emitter inlet "screens" reduce the plugging susceptibility.
When testing ring and screen filters, CIT uses a "manufactured" contaminant sand sample consisting of equal amounts of particles screened to 80-100 mesh, 100-120 mesh, etc. The contaminant sample includes particles both larger and smaller than the manufacturer’s rating. Under stable hydraulic conditions, the sample is injected into the upstream pipeline. Material trapped by the filter and collected downstream of the filter are dried and rescreened. Representative results for a screen filter rated 150 mesh are shown on Figure 1.
The arbitrary nature of the screen filter rating can be noted. In this case, the screen passes about 30 percent of the sample larger than the 150 sieve mesh size rating. Since an absolute ability to remove 100 percent of the particles larger than the mesh size rating is probably unrealistic, some acceptable standard of say five percent may have to be set. For this screen filter, a rating of 100 to 120 mesh is justified based on these performance test results.
A further concern would be whether the manufacturer used wire cloth with dimensions corresponding to U.S. standard sieve series (ASTM E-11).
Disk filters have also been tested with representative results shown on Figure 2.
Given that both filters are rated by the manufacturers at equivalent to 150 mesh, the following observations can be made:
• The disk filter concept is far more effective at particle removal than the screen filter.
• The advertised ratings were evidently not made based on actual performance testing.
Our previously suggested standard of 95 percent removal is met by the disk filter at the advertised rating of 150 mesh. The comparison of removal efficiency at 230 mesh is also of interest. The disk filter removed 87.5 per-cent of the total sample while the screen filter removed only 40.5 percent. Apparently there is significant bridging of contaminants in the disk filter that restricts the movement of the finer particles.
Particle removal efficiency, although important, is, however, only one factor involved in the filter purchasing decision. Further, there is no agreed upon performance testing protocol that manufacturers can use to accurately represent their products.
Disk and screen filters are generally used in drip irrigation systems as a final stage of filtration before water enters the distribution system. The primary filtration process used depends on the type of contaminant. If the contaminant is organic matter, the preferred technology is the use of high rate sand media tanks. Technological innovation for media tanks has centered around development of more effective under-drain systems. In particular, the under-drain must uniformly direct the backflow so as to completely purge the media of contaminants. Also, as with the screen and disk filters, there is no recognized testing protocol to characterize backflushing uniformity. The center assisted a graduate student at California State University, Fresno in the development and application of a proposed protocol. Although judged effective, the protocol is cumbersome to use and has not been accepted.
If the contaminant is relatively large amounts of sand, hydrocyclone separators are the preferred technology. CIT has pioneered the development of a testing protocol patterned after the one developed for disk and ring filters. The test facility is shown schematically in Figure 3.
In this case, the test contaminant is #120 Feldspar by P.W. Gillibrand of Simi Valley, California. The particle size distribution is characterized in Figure 4.
This sand size distribution covers the range of interest in emitter plugging studies. Any smaller particles are classified as silt or clay and will not plug emitters unless some form of particle aggregation occurs.
For silt and clay, there is no practical removal technology and these fine particles are flushed through the system.
Below at right is a photograph of #120 Feldspar, giving an impression of the size and shape of the individual particles.
Performance testing of sand separators is conducted in the following manner. First, a head loss test is run over the range of flow rates of interest to the manufacturer. The flow rates are typically set to give a range of headloss values from 3.0 to 12.0 psi. Each specific removable efficiency run is made by controlling the flow rate to a fixed value.
After establishing stable hydraulic conditions, a measured sample of #120 Feldspar is injected into the pipeline.
The portion removed by the sand separator is purged from the unit. The portion passing through the separator is caught by a downstream disk filter rated at 600 mesh. Both samples are dried and weighed. In this test, there is no underflow, and the contaminant makes a single pass through the sand separator. Solids are trapped in the separator’s lower chamber and purged periodically. This duplicates the usual mode of field operation. Typical results for a range of headloss values are shown in Figure 5. During the efficiency removal test, flow rate is monitored closely so as to maintain the integrity of separation process.
Figure 5 documents the trade-off between headloss and the removal efficiency characteristic of many sand separators. In this case, the removal efficiencies range from approximately 91 percent at nominal headloss values to 94.5 percent at high headloss values.
While the removal efficiencies can be high, the sand separator is not absolutely discriminating as to maximum particle size removal. Particles smaller than 75 microns have been found in separator samples.
If the design requires removal of all particles larger than a specific size, a disk filter is more likely meet this design objective. If, however, significant amounts of sand are present, the sand separator is preferred because of its ability to store trapped sand and purge it free of the system with a minimum of wasted water. Disk and screen filters require major backflows to insure complete purging. The best overall combination involves the use of a sand separator followed by a disk or screen filter.
Over the last several years, CIT has tested a number of sand separators. The range of performance results is shown in Table 1.
The performance documented in Table 1 undoubtedly reflects the range of technological innovation practiced by the manufacturers. It suggests that the current state of the art provides separation efficiencies approaching 97 to 98 percent at least for smaller units. Our experience with larger units is limited and the tests get more expensive.
Conclusion
If sand is a problem in your water supply system, it should be possible to specify equipment that provides the best possible commercial answer. This starts with a sand separator capable of removing at least 95 percent of total sand contaminants. The sand separator can then be followed in-line by a disk or screen filter capable of removing at least 95 percent of remaining sand contaminants retained on a 140 mesh sieve. This standard can be met by commercially available hardware. A performance standard on filters and sand separators could improve overall system operation.
{ CATI , CIT , CIT - Research Publications }
