hen purchasing a pressurized irrigation system, such as a sprinkler or drip/micro system, one of the primary decisions to make is which filter to use. Aside from the obvious cost factor, the type of filter you choose will primarily depend on the emission device and the water source/quality of your system.
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Choosing the appropriate filtration system, or improving your existing filter station to ensure it is working effectively, is important for keeping your irrigation system clean and ensuring the distribution uniformity remains high. The following points outline recommendations to consider when selecting and/or maintaining a filtration system for surface or groundwater use.
Always purchase more filtration capacity than you think is needed for your quality of water.
For example, when buying sand media tanks, a common guideline is that one 48-inch diameter tank is capable of filtering 250 gpm. This rule assumes that the flow rate through the tank is 20 gpm/square foot, which is suitable for average water quality. If your water is dirty, however, you should only use a target flow rate of 15 gpm/square foot. This would mean that each tank should only filter a flow rate of about 185 gpm. Really dirty water would even be lower. Screen filters should also be derated with dirty water.
Inspect your filters regularly.
The sand in a media tank should be checked monthly to ensure an adequate amount is present. It should be replaced annually because it will become smooth over time, reducing its filtration potential. Tubular screen filters, vacuum filters and disc filters should also be regularly inspected to ensure that there is no buildup or crusting taking place, especially if the filter does not frequently have water running through it.
Verify that you have the proper backflush flow rate.
This is very important for sand media tanks because a flow rate that is too high will pull the sand from the tank, and a flow rate that is too low will not remove the debris. One method to verify the proper flow rate is to install a flow meter on the backflush line (see fig. 1). Other filter types require a certain pressure to achieve proper backflush flow rates, so check with the manufacturer.
Don’t forget about the backflush flow rate.
Oftentimes, a designer may size a filtration system for the system flow rate but forget about the backflush flow. In these cases, the irrigation system typically sees a reduced flow rate because the higher flow rates cause a larger pressure drop during backflush. This is another reason to have more filtration capacity.
For example, a four-tank sand media system would be capable of about 1,000 gpm at 20 gpm/square foot. However, when a tank goes into backflush, it is no longer supplying water to the field, and it requires about 190-200 gpm to properly backflush. This means that the three remaining tanks that are still in operation must supply 1,000 gpm to the field plus about 200 gpm for the tank that is backflushing. In this situation the three tanks would be at about 32 gpm/ square foot. For this reason, you should never have less than a three-tank system. In addition, an extra tank in this scenario would reduce the demand on the filtration system during backflush.
Figure 1. Magnetic flow meter on the backflush line to set the proper
flow rate during backflush Photo credit: Cal Poly ITRC
12 Irrigation TODAY | January 2019
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