Monitoring and Control in Boilers with All Solid State Fluorometers

Godfrey, Martin R. (ONDEO Nalco Company)

OnePetro 

ABSTRACT
Recent advances in the field of light emitting diodes have enabled the production of new all solid- state fluorometers. High intensity light emitting diodes are used to excite the fluorophores and photocells are used to detect the emission. These fluorometers are small, with low power consumption, but they are highly sensitive with exceptional stability and low noise. The instruments are used in conjunction with water treatment chemicals that contain an inert fluorescent tracer. Handheld, battery- powered units are used for grab sample analysis and provide greatly improved accuracy with less testing time for systems where chemical addition is controlled manually. On-line fluorometers can provide continuous signals for a variety of automated control schemes. Important system diagnostics such as cycles of concentration and polymer recovery measurements are easily performed. The low cost and versatility of these units brings tracer technology to a broad range of facilities where it was previously not viable.

INTRODUCTION
Manpower is a constantly dwindling resource in today's boiler house. Boiler operators are continually called upon to spend more time with other maintenance tasks. As the job market becomes tighter, personnel with less experience and training occupy positions as boiler operators. In response to this pressure, boiler treatment programs must become more forgiving and require less operator time for testing and adjustment.

The use of fluorescent tracers for system diagnostics and dosage control of treatment chemicals is a proven method for reducing operator work load and improving system performance. 1' 2 However, application of tracer technology was to some degree limited by the type of equipment that was necessary for accurate and reliable measurement of fluorescence in the field. Previous generations of fluorometers used gas discharge lamps as excitation light sources and photomultiplier tubes to detect the light emitted from the fluorophore. These devices have the advantages of long life and very high sensitivity. They are also quite versatile and can be adapted to fluorophores utilizing wavelengths throughout most of the ultraviolet and the entire visible spectrum. Careful design and painstaking construction techniques made these instruments extremely rugged and reliable in virtually any industrial environment.

Although the previous generation of fluorometers was highly valuable there is always room for improvement. Both the gas discharge lamp and the photomultiplier tube are high voltage devices requiring a large and heavy transformer. They also consume too much power to be operated with a battery of convenient size. In addition, these devices require several minutes of warm-up time to reach their ultimate level of stability. While photomultiplier tubes are extremely sensitive, they have a limited dynamic range of response. This means that the detection circuit must be balanced either optically or electronically to be sensitive at the fluorophore concentration range of interest. For boiler applications, the practical implication of this limitation in the detector was that the fluorometers sometimes needed to be recalibrated when switching from feedwater to blowdown samples. All these factors conspired to limit the convenience of fluorometers designed for grab sample analysis since they would be fairly large and heavy devices that required line power and a warm-up period.

All the components of the previous generation of fluorometers represent well-known and readily available technology, but the ones mentioned in the preceding paragraph are not used in consumer electronics applications. Specialty devices generally carry a higher price tag than th