Turbidimeter system controls ash waste

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Model AP-VIE Indicator/Controller for Decanting Use

A McNab turbidity monitoring system was used to assure compliance with state pollution-control requirements for both the removal of flyash from stack gasses and the removal of boiler bottom ash


A municipal utility in Michigan took, among others, two specific steps to assure compliance with state pollution-control requirements: one step dealt with the removal of flyash from the stack gasses; the other with the removal of boiler bottom ash.  Each of these actions were facilitated with the use of a turbidity monitoring system.

To limit the escape of coal flyash from the stack gasses into the atmosphere, electrostatic precipitators were installed to remove the flyash depositing the ash in hoppers beneath the precipitator. The ash is then removed from the hoppers via a vacuum created with water eductors.

While most of the flyash is captured by mechanical filters, some escapes filtration and ends up in the eductor water. When the solids content of the eductor water reaches unacceptable levels the effluent is routed to a collecting sump.

Removal of boiler bottom ash is accomplished using high-pressure water jets, with the resultant slurry conveyed to inverted-cone holding bins.  The water is then decanted to the collecting sump from the bins.

From the collecting sump, the effluent is forwarded to the plant's waste-water treatment facility, where it is processed and discharged into the neighboring river.

To prevent ash from leaving the 375-MW power station's treatment facility and entering the river, the effluent had to be monitored for solids content. Plant designers determined continuous monitoring was preferable to periodic sampling.

Monitoring was accomplished by the installation of a Model AP-VIE continuous-reading turbidimeter, available from McNab, Incorporated.  Consisting of flanged flow cells installed directly within the process lines and monitor/control cabinets, the flow cells contain a light source and, mounted directly opposite, a photo-electric scanner. Signals from the scanner are transmitted via wire to each detector unit's monitor/control cabinet.

Cabinet electronics are calibrated to the signals generated by the scanner when clear water passes between it and the light source. The resultant signal is sent to a gauge on the cabinet front, which displays the results in parts-per-million (ppm) or other turbidity measuring units. Signals can also be sent to set-point relays which can be used to activate valves or sound alarms.

paper_A92-35b_cell_test_module.jpg (24164 bytes)In the case of this utility, when turbidity levels reach unacceptable levels an alarm is sounded to alert the operator that a solids content problem is developing. Such problems develop when the pH level of the water has gone out of balance, when the feed rate of treatment chemicals has been altered or when the sludge level in the settling units has risen too high and requires reduction.

Whatever the cause of the excess solid content, the continuous monitoring of the effluent allows the operators to shut-off or re-circulate the water under treatment until the cause has been determined and a solution enabled.

Optical Reference Filter

Many McNab optical units are available with an optional optical reference filter to verify span calibration.  This filter inserts directly into the light source filter slot.

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