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Water is filtered primarily to remove particulate impurities. If the water is pretreated with flocculants, filtration also removes floc particles (used to suspend solids in the clarification process). There are many techniques used for filtration. Filtering is conducted with several types of media, which are the materials used to trap particles as water is forced through the filter.
Filters use either gravity or pressure to force the raw water through the filtration media.
In gravity systems, such as a sand filter, water is fed into a basin or tank from troughs. The water passes over the troughs and into the filter media. Water flow into the tank is regulated based on level. Ultrasonic sensors and hydrostatic pressure sensors are the two most popular ways to control level for this type of filter. As flow from the tank increases, based on consumer demand, the level sensor provides a continuous (4-20 mA) signal to the outlet valve. The signal either can be direct to a modulating motor, or indirect, converted from current to pneumatic pressure (I/P transducer), then to an air actuated valve.
Pressure based systems utilize a tank or "pressure vessel". Water is fed in at the top of the vessel under substantial pressure. Water is then forced through the filter media to an outlet valve. Pressure is monitored at the inlet and outlet valves. Pressure regulation determines filter flow rate. Sensor options include blind pressure transmitters, digital or analog pressure gauges, or differential pressure (DP) transmitters with retransmission of pressure via
4-20 mA. |
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| Amplified Output Pressure Transmitters |
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Available ranges from vacuum
up to 500 psi and 0 up to
10,000 psi |
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Standard outputs: 4-20 mA,
0-5 VDC, 0-10 VDC, 1-5 VDC,
and 1-6 VDC |
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High overpressure capability |
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Single piece stainless steel process fitting and sensor |
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Zero and span adjustments |
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1/4-18 NPTM fitting standard |
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True 0-5 or 0-10 VDC, can transistion from vacuum to positive pressure |
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±0.25% accuracy of full scale
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