Monitoring probe installation

An electrical resistance methods which directly measures loss of metal from a probe installed in the corrosive system under study is described in Section 19.3. It is reported that corrosion equivalent to a thickness loss of as little as 2-5 X 10 cm can be detected . This technique is most useful as a means of monitoring steps taken to reduce corrosion, e.g. by inhibitors, or to detect changes in the corrosivity of process streams. Electrical methods of determining corrosion rates are considered subsequently. 

Polarisation resistance This technique, sometimes referred to as linear polarisation resistance (LPR), has been applied widely in industrial monitoring because of its ability to react instantaneously to a corrosion situation or change in corrosion rate " " . The limitation of the technique arises from the necessity to have a defined electrolyte as the corrosive (the author has seen an LPR probe installed in a dry gas-line in an oil refinery). 

Waterside problems that lead to decreases in efficiency and material deterioration can be caused by a variety of mechanisms, such as electrochemical corrosion and deposition of foulants. These problems can be exacerbated by low flow, poor operational practice, process contamination, or specific stresses. It is also important to try to determine cause and effect relationships in order to provide a logical and practical water treatment solution. Such a solution will usually involve some form of cleaning, plus a combined engineering and chemical action plan. Inspection may be made easier by the use of a Boroscope or similar optical/video recording device. The color, texture, and quantity of all deposits should be noted, measurements of pits taken, and microbiological contaminants analyzed. It may be useful to conduct biocide efficiency tests on bacterial slimes. The period when a heat exchanger is open for inspection may be an opportune time for the permanent installation of ports for corrosion-monitoring probes. 

Execution. In the execution phase, the CP system is applied to the structure following the design. During the work, the complete concrete surface is checked for cracking, delaminations, cover depth, steel continuity and the presence of metal objects that might cause short circuits. If necessary, continuity is provided and metal objects are connected to the reinforcement. Subsequently the cracked and spalled areas are removed and repaired. The reference electrodes and other monitoring probes are embedded. Then the anode is applied, with overlay or top coat as designed. All electrical coimections are made and the power source is installed. 

In existing structures corrosion monitoring may be by regular inspection using the techniques described in Chapter 4, or by probes installed in the concrete or on its surface. These will have wired or wireless connections to monitoring equipment. 

Figure 5.1 A permanent LPR monitoring probe being installed in a precast deck unit of the Dartford Tunnel (Broomfield et ol, 2003) photograph. Courtesy BGB Projects Ltd.

Figure 5.2 A corrosion monitoring probe consisting of a reference eiectrode and an auxiliary electrode potted up in mortar in the process of installation into a continuously reinforced concrete pavement where corrosion monitoring is required due to the high level of chloride found in the mix water after laying several kilometres of concrete (Broomfield et a/., 2003) photograph. Courtesy BGB Projects Ltd.

Downhole specimens are rarely used for monitoring because of the costs and complications involved. Indirect monitoring of downhole corrosion conditions has been done with specimens or electrical resistance probes installed at the wellhead. These have been used for monitoring inhibition treatments, always with the reservation that such exposure does not accurately simulate downhole conditions. 

It is possible to test ventilation systems by installing monitoring probes for post-installation validation If a 1 200 g DPM material is to function as a gas barrier It should be installed according to BRE 212/BRE414, being taped and sealed to all penetrations. 

An updated controlled temperature irradiator ( 5l) incorporated an asbestos housingj multiple cartridge heaters installed within an aluminum sample oven individual ports for a maximum of 5I samples Internally mounted thermistor sensor and thermocouple monitor probes an adjustable resistance bridge Trlac controller ( ) sensitive to 0.005°K and prior calibration ( 0.05°k) as well as continuous monitoring ( 0.15°k)... 

In order that the data acquisition system can obtain information about the spatial location and orientation of the probe, a four-channel incremental encoder interface board is installed. Three channels are used to define position in three-dimensional space, while the fourth monitors the skew of the probe (skew is defined as rotation about an axis normal to the probe face). Although six measurements are required to completely define the location and orientation, it is assumed that the probe remains in contact with the inspection surface. 

Leakage current monitor - this can be connected perma-nently for continuous reading or periodic monitoring. The normal practice is to measure only periodically for a short period to take average measurements on an hourly, daily, monthly or yearly basis. When not connected permanently, the instrument can also be used as a portable kit to monitor the condition of other arresters installed in the vicinity. Field probe - to compensate the third harmonic of the system voltage to make the IZnO free from the third harmonic of the system voltage. This method of /r measurement therefore provides more accurate and closer monitoring of the arrester. 

Axial proximity probes are another means of monitoring rotor position and the integrity of the thrust bearing. A typical installation is shown in Figure 21-11. In this case two positions are being monitored one at the thrust runner, and one at the end of the shaft near the centerline. This method detects thrust-collar runout and also rotor movement. In most cases this ideal positioning of the probes is not possible. Many times the probes are indexed to the rotor or other convenient locations and thus do not truly show the movement of the rotor with respect to the thrust bearing. 

By using the positional information available from a proximity probe, another valuable parameter can be measured. The axial position or axial shift detection can be added to the monitored information. Probes are generally installed at the shaft end in pairs to provide redundancy. Fhe probes, preferably, should be sensing the axial position from two differ ent surfaces. A typical arrangement can be seen in Figure 8-37... 

Install and monitor corrosion and hydrogen probes in key areas,... 

A factor which previously limited installation of automatic corrosion monitoring systems was the cost of cabling between sensors and control room instrumentation-this was particularly relevant to the electrical resistance (ER) systems. Developments to overcome this have included transmitter units at the probe location providing the standard 4-20 mA output (allowing use of standard cable) for onward transmission to data systems or the use of radio linkage which has been successfully used for other process-plant instrumentation. 

Performance data Two moisture monitoring systems were installed, one at Disposal Area A and one at Disposal Area AB plus in May and November 1999, respectively. Each monitoring system has two stacks of time domain reflectometry probes that measure soil moisture at 24-in. intervals to a maximum depth of 78 in., and a station for collecting weather data. Based on nearly 3 years of data, there is generally <5% change in the relative volumetric... 

In order to quantify the physical environment of a bioreactor, fluorescence assays can be applied for on-line monitoring of the mixing time behavior of all types of bioreactors. In this case the fluorosensor probe can be installed in the bioreactor at different locations of interest. Afterwards, selected fluorophores can be injected in order to study the overall mixing time. These fluorophores must fit to the excitation and emission behavior of the probe and should be selected in regard to the pH-dependency of the bioprocess, and when used during cell cultivation experiments they should not interfere with the cells. Scheper and Schiigerl reported on the use of different coumarins for mixing time experiments under bioprocess relevant conditions [49]. 

Flow assemblies are recommended for housing a conductivity cell, pH/ORP probes, or other monitoring devices. However, they should be designed and installed for easy physical access and cell/electrode cleaning. Flow assemblies should also incorporate a sample port. 

ERM methods give a direct measure of metal penetration but can be of low accuracy, especially in slow-moving cooling systems, thus requiring long periods of measurement. As a result, for most systems the probes are permanently installed and the multichannel monitor transmits data directly to a control room computer, although hand-held equipment has become available in recent years. 

The renewable energy processes will be monitored and controlled by small, fast, and economical detectors, and fiber optics will play a major role in their designs. The operation and applications of fiber-optic probes have already been discussed in connection with Figure 3.2, so only a brief summary is provided here. Fiber-optic probes can be installed in situ, whereas their readout instruments can be several hundred meters from the probe. The probe can be located in toxic, corrosive, radioactive, explosive, high- or low-temperature/ pressure, and noisy environments. Because the measurement signal is optical, the cables are immune to microwave or electromagnetic interference. 

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