Pipe Current Measurement

The resistance per unit length can be calculated from the resistance formula of the linear conductor if the outer diameter d, the wall thickness s, and wall cross-section S = Tis (t/ - 5) are known  

A further measurement of current can be obtained from the pipe mass per meter, given in the standards, m = mil = S 

The specific electrical resistances usually depend on the material and the temperature [31]. For the most important pipe materials these are (in 10 Q cm)  

The values calculated using Eqs. (3-36) and (3-37) are only true for welded pipelines. Extension joints, fittings, and screwed or caulked joints can raise the longitudinal resistance of a pipeline considerably and therefore must be bridged over for cathodic protection. 

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The variation in the on and off potentials or the potential difference along the pipeline will usually indicate faults that prevent the attainment of complete cathodic protection. The protection current requirement of the pipeline may be estimated from experience if the age and type of pipeline is known (see Fig. 5-3). Figure 3-20 shows the variation in the on and off potentials of a 9-km pipeline section DN 800 with 10-mm wall thickness. At the end of the pipeline, at 31.84 km, an insulating unit is built in. The cathodic protection station is situated at 22.99 km. Between this and the end of the pipeline there are four pipe current measuring points. The applied protection current densities and coating resistances of individual pipeline sections are calculated from Eqs. (3-40) and (3-41). In the upper diagram the values of... 

Contacts with other pipelines or grounds can be localized to within a few hundred meters by pipe current measurements. Contacts with foreign pipelines or cables can also be found by measuring potential at the fittings of the other line while the protection current of the cathodically protected pipeline is switched on and off. While the potential of unconnected pipelines will assume more positive values when the protection current is switched on, the cathodic current may also enter any line in contact with the cathodically protected pipeline and thus shift its potential to more negative values. Should the contacting line not be located by this method, fault location can be attempted with direct or alternating current. 

Fig, 10-7 Aboveground test points, potential measuring points (2) and (3), pipe current measuring point (1/2). 

This simplification is only permissible with very low-resistance contact and if no other current flows in the pipeline. Currents flowing in the pipeline outside the measured span must be measured separately and taken account of in the calculation. This follows also indirectly when locating a contact with an unknown line. Figure 3-21 shows the necessary pipe current measurements on either side of the supposed contact point which, with U = t/C = IR, gives the following calculation of the fault distance [28] ... 

By substituting the well-known Blasius relation for the friction factor, Eq. (45) in Table VII results. Van Shaw et al. (V2) tested this relation by limiting-current measurements on short pipe sections, and found that the Re and (L/d) dependences were in accord with theory. The mass-transfer rates obtained averaged 7% lower than predicted, but in a later publication this was traced to incorrect flow rate calibration. Iribame et al. (110) showed that the Leveque relation is also valid for turbulent mass transfer in falling films, as long as the developing mass-transfer condition is fulfilled (generally expressed as L+ < 103) while Re > 103. The fundamental importance of the Leveque equation for the interpretation of microelectrode measurements is discussed at an earlier point. 

Keywords cathodic protection, correlation model, cylinder pipes, protection current, measurements, desert conditions. 

In it are detailed the waste management activities and toxic releases from facilities with more than ten employees that manufacture, process or use more than certain base quantities of any of over 640 currently listed chemicals and chemical categories. Following the implementation of the Pollution Prevention Act 1990 (PPA), TRI reporting requirements changed in 1991 in an attempt to encourage recycling and shift the emphasis towards pollution prevention rather than end-of-pipe treatment measures. 

Most electrification measurements use pipe flow like that shown in Fig. la where the current entering the pipe plus the current generated within the pipe are measured by electrometer and equals the current measured at the outlet by electrometer E2. For insulating tubes, the radial and axial electric field components are measured by non-contacting field probes in the wall of a grounded surrounding metallic enclosure as... 

In the discussion of cathodic protection monitoring, two important distinct areas can be identified. The first domain lies in monitoring the condition and performance of the CP system hardware. Monitoring of rectifier output, pipe-to-soil potential and current measurements at buried sacrificial anodes, inspection of bonds, fuses, insulators, test posts, and permanent reference electrodes are relevant to this area. The second domain concerns the condition of the pipeline (or buried structure) itself and largely deals with surveys along the length of the pipeline to assess its condition and to identify high corrosion-risk areas. 

One of the techniques capable of being used "on stream" is the novel INCOTEST technique, see figure 2, which measures wall thickness of piping and vessels through an insulation layer with a maximum thickness of 10 centimetres. This system is based on pulsed eddy currents, was first developed in the United States, and is now being commercialised and extensively validated for a number of applications. 

MFL is currently being used for the inspection of hundreds of kilometres of piping in the Oman desert, see figure 5., whereby an inspection rate of one kilometre per day is easily achieved. Try to realise what effort would have been needed if this would have been done with conventional ultrasonic wall thickness measurements. 

Gup and Vane Anemometers. A number of flow meter designs use a rotating element kept in motion by the kinetic energy of the flowing stream such that the speed is a measure of fluid velocity. In general, these meters, if used to measure wind velocity, are called anemometers if used for open-channel Hquids, current meters and if used for closed pipes, turbine flow meters. 

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