Monitoring technique

As discussed in Chapter 4, chemicals can be a nuisance or pose health risks if they become airborne as a result of inadequate process control, operation and maintenance malpractice, inadequate maintenance, incomplete understanding of the process etc. Hazards may arise if the oxygen concentration in the air fluctuates beyond its normal level of 21% by volume, and fire/explosion dangers may arise from the presence of flammable gases, vapours or dusts in the atmosphere. Thus air quality tests may be required for a variety of purposes such as  

Environmental analysis entails sampling ambient air (Table 9.1) by means of a pump with subsequent analysis of the collected sample. Pumps vary from large, stationary high-volume versions to pocket-size devices used in personal dosimetry. Samples may be collected in airtight bags, or absorbed in liquids or on solid adsorbents for subsequent analysis. 

A common approach for personal dosimetry is collection of pollutant on, e.g., silica gel, organic resins or activated charcoal in small tubes worn on the operator s lapel (Table 9.2). Silica gel is useful for polar chemicals charcoal finds wide use for non-polar substances. The pollutant is then solvent-extracted or thermally desorbed for subsequent analysis by, e.g., chromatography. 

Advantages and disadvantages of thermal desorption are listed in Table 9.3. (Charcoal badges are also available which require no pump these are termed passive samplers .) Instruments are also available to give direct readout of atmospheric levels of pollutant. 

The range of analytical techniques is extensive and usually requires the help of a hygienist. The following illustrates a selection of common techniques. 

Oxides of sulphur and other sulphur compounds Oxides of nitrogen and other nitrogen compounds Oxides of carbon 

In addition to pollution episodes, risks may arise due to atmospherie oxygen eoneentrations fluetuating beyond its normal level of 21% posing health (page 72) or fire hazards. Eire and explosion dangers may also arise from the presenee of flammable gases, vapours, or dusts in die atmosphere (Chapter 6). 

as illustrated by Table 17.13 monitoring emissions of hazardous ehemieals into die environment may be required for a variety of reasons sueh as  

Assessing fire or explosion risks from atmospheres eontaining flammable gas, vapour or dust. Determining oxygen eontent of the working atmosphere. 

Determining sourees of leaks of toxie, flammable, or nuisanee pollutants. 

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None of the foregoing methods will tell the frequency or duration of exposure of any receptor to irritant or odorous gases when each such exposure may exceed the irritation or odor response threshold for only minutes or seconds. The only way that such an exposure can be measured instrumentally is by an essentially continuous monitoring instrument, the record from which will yield not only this kind of information but also all the information required to assess hourly, daUy, monthly, and annual phenomena. Continuous monitoring techniques may be used at a particular location or involve remote sensing techniques. 

The principal requirement of a sampling system is to obtain a sample that is representative of the atmosphere at a particular place and time and that can be evaluated as a mass or volume concentration. Remote monitoring techniques are discussed in Chapter 15. The sampling system should not alter the chemical or physical characteristics of the sample in an undesirable manner. The major components of most sampling systems are an inlet manifold, an air mover, a collection medium, and a flow measurement device. 

Disposal Discharge of wastes to air, water or land in properly controlled, or safe, ways such that compliance is achieved. Secure land disposal may involve volume reduction, encapsulation, leachate containment and monitoring techniques a c j 5 j ... 

Schafer, K. (ed.) (1999) Spectroscopic Atmospheric Environmental Monitoring Techniques, SPIE Society of Photo-optical Instrumentation Engineers. 

Emission measurements are required for many purposes. They can be used as the basis for emission and air quality studies, as well as for process control and specific technologies to reduce emissions. The reliability of the measured values is constantly improving with developments in monitoring techniques. 

This section discusses normal failure modes, monitoring techniques that can prevent premature failures, and the measurement points required for monitoring common machine-train components. Understanding the specific location and orientation of each measurement point is critical to diagnosing incipient problems. 

The failure of plant by corrosion can be gradual or catastrophic. Gradual failure has few implications for safety providing it is monitored. Direct corrosion-monitoring techniques are described in Section 53.8. Indirectly, the correct interpretation of records relating to metal contamination of products or the loss of efficiency of heat exchangers, etc. can provide useful information. 

A number of corrosion-monitoring techniques based on electrochemical principles are available. These give an indication of the instantaneous corrosion rate, which is of use when changing process conditions create a variety of corrosion effects at different times in a plant. Some... 

This technique is based upon the detection of corrosion products, in the form of dissolved metal ions, in the process stream. A thin layer of radioactive material is created on the process side of an item of plant. As corrosion occurs, radioactive isotopes of the elements in the construction material of the plant pass into the process stream and are detected. The rate of metal loss is quantified and local rates of corrosion are inferred. This monitoring technique is not yet in widespread use but it has been proven in several industries. 

Hydrogen probes are mainly used in refineries to detect the onset of conditions when HjS cracking of carbon-steel equipment could become a real risk. As a qualitative monitoring technique, it has a long and proven service of worth. 

User experience of monitoring techniques in oil and gas production has been reviewed and indicates success and failure for the same methods by different operators. A survey of current monitoring practice in UK offshore fields has been published and other experience related to oil/gas production has be reported . A draft document has been prepared by CEA Task Group E2-5 providing guidelines for monitoring sea-water injection systems. 

Asher, J., Conlon, T. W., Tofield, B. C. and Wilkins, N. J. M., Thin Layer Activation-A New Plant Corrosion Monitoring Technique , Proc. Conf. On-Line Surveillance and Monitoring , entitled On-Line Monitoring of Continuous Plants, Ellis Horwood, London (1983)... 

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