Wall thickness

The wall thickness of the coil can be chosen by using any number of recognized codes and standards. In the United States, the most commonly recognized are American National Standard Institute (ANSI) B31.3 and B31.8, or American Petroleum Institute (API) Specification 12 K. Volume 1 has the tables for ANSI B31.3 and ANSI B31.8. Table. - -1 

It has become apparent that preparative columns will have significantly greater diameters than analytical columns in order to accommodate the greater sample loads. As a consequence, the wall thickness of the columns 

It should be noted that equation (13) is for a specific stainless steel and a particular type of tube. Before employing the equation to fabricate a column the original reference should be reviewed to ascertain the correct constants or function to employ for the type of steel and tube that is intended for use. 

One further property of the preparative column is often required and that is its unpacked weight (W(coi)). This is simply given by, 

The equation for the optimization of a preparative LC column can be summarized as follows. 

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For the repetitive inspections the required hydrotest can only be performed for a limited number of the small cylinders, and even then the drums have to be removed from the line and the cylinders will be supported in defined distances for the weight of the water and the pressurisation. For the new and long cylinders even this is impossible, because they loose due to the additional weight of the water and the over-pressurisation their roundness and balances. Therefore the law in the most countries within and outside of the EU accept as a replacement of the hydrotest an additional application of different NDT methods, which were often done by an ultrasonic measurement of the wall thickness of the cylindrical part and a MT of the flat covers. 

It was pointed out, that the periodical inspection of the steam drums has been become an absolute must especially under the circumstance, that the economical pressure results in smaller wall thickness, higher steam pressure and higher rotation speed. The conventional periodical inspection (hydrotest and visual inside inspection) is on one hand time consuming and therefore expensive and on the other hand the results of the hydrotest are doubtful and can result in a seriously damage of the roundness and balance of the steam drum. 

Cracks, Corrosion, Pitting, Wall thickness, IGSCC... 

Whereas in the scope of plastic deformations differences are observed Arc welding of pipes <6 32 mm, wall thickness 6,5 mm has caused own tensile stress of 260 MPa in the jont, relief at 720°C during 4 hours, has caused a lowering of stress to 60 MPa. 

Fig, 5 Course of stress in pipes with 0 32 mm and wall thickness 6,5 mm after welding (1) and after annealing (2). 

With this testing method an evaluation is possible within shortest time, i.e. directly after the heat impulse. The high temperature difference between a delamination and sound material is affected - among other parameters - by the thickness of the layer. Other parameters are size and stage of the delamination Generally, a high surface temperature refers to a small wall thickness and/or layer separation [4],... 

The large temperature difference of the remarkable borehole, opposite other boreholes and their environment is significant. This high temperature difference is a typical feature for a small wall thickness between borehole and blade surface. For technical reasons, precise eroding of the boreholes is difficult. Due to this, the remaining wallthickness between the boreholes and the blade surface has to be determined, in order to prevent an early failure, Siemens/Kwu developed a new method to determine the wallthickness with Impulse-Video-Thermography [5],... 

For wall thicknesses of typically 2-30 mm of steel Selenium has meanwhile become well accepted in international radiography with the clearly legible trend to be the preferential choice in the near future. 

The results undoubtedly show a significant improvement of eontrast of Selenium radiographs compared to Iridium in the eomplete range of wall thicknesses as can be foreseen by the difiference in radiation energies. 

Practical applications [2] of a GammaMat M model using the new Selenium crawler camera loaded with approx. 1 TBq (30Ci) on a pipeline of diameter 12 and wall thickness of 0.25 showed 6-7 m axial distance to the exposed source as limit of the radiation controlled area (40pSv/h) and 22m perpendicular to the pipeline. Other authors [3] have reported about a comparison for Ir-192 and Selenium source on a 4.5 diameter pipe and 0,125 steel thickness they have found for 0.7 Tbq (18Ci) Selenium a value of 1 Om behind the film (in the unshielded beam) comparing under same conditions to approx. 40m for Iridium. 

The layout of the system is given in figure 1. The system has two X-Ray sources to accommodate for the variation in wall thickness of the products to be inspected. The 160 kV tube is placed in top of the 320 kV tube (Figure 2). 

The sensitivity of the luminescence IP s in the systems employed here decreases with increasing x-ray energy more strongly than in the case of x-ray film. Therefore, this phenomenon must be compensated by using thicker lead front and back screens. The specific contrast c,p [1,3] is an appropriate parameter for a comparison between IP s and film, since it may be measured independently of the spatial resolution. Since the absorption coefficient p remains roughly constant for constant tube voltage and the same material, it suffices to measure and compare the scatter ratio k. Fig. 2 shows k as a function of the front and back screen thickness for the IP s for 400 keV and different wall thicknesses. The corresponding measured scatter ratios for x-ray films with 0,1 mm front and back screens of lead are likewise shown. The equivalent value for the front and back screen thicknesses is found from the intersection of the curves for the IP s and the film value. 

Minimum exposure times must be observed in order to reach the requisite S/N ratio. As per EN 1435 and EN 584-1, for the different ranges of utilization (energy, wall thickness), definite film elasses are prescribed. They are characterized by the minimum gradient-to-noise ratios. Based on this, one can calculate the minimum values for the S/N ratio based on the IP systems. The exposure time and the device parameter sensitivity and dynamics (latitude) must be adjusted accordingly, with an availability of an at least 12 bit system for the digitalization. 

The IP s were used in combination with appropriate Pb-filters (see 121). For wall thickness inspection we used standard medical IP s (AGFA MDIO), which exhibited an inherent unsharpness of ca. 310 pm. This should be compared with the inherent unsharpness of the film for an Ir-192 exposure which amounts 230 pm 111. For corrosion assessment we used IP prototypes with an inherent unsharpness < 230 pm. 

The corrosion inspection and wall thickness measurement of pipes was performed in the classical way both on film and on the monitor using simple software measuring tools. Additionally algorithms were developed for an interactive, computer supported evaluation. 

The wall thickness estimation in tangential projection technique is based on the evaluation of profile plots along the pipe diameter as shown in fig. 1 (lowest row). 

Effect of Different Filters for Wall Thickness Estimation... 

Fig. 1 Simulated, profiles across a pipe in the tangential projection technique and the results of different filters for determination of the projected wall thickness...

Fig. 3 Measured CR wall thickness profile and the influence of noise to the filters...

The detection of the profile edges gives the projected wall thickness in pixels of the image data. The next step of data processing is the compensation of the magnification factor in the used tangential projection method. 

To consider the geometrical magnification of the pipe image on film or IP in the tangential exposure technique according to the set-up shown in fig. 4 a correction of the measured wall thickness (w ) must be performed. The true wall thickness (w) depends in a rather complicated way on the film-focus-distance (f), the radius of the pipe (r) and the radius of the insulation (R) as shown in equation 2 ... 

Fig. 5 systematic error of the simple formula (3) compared to the correct model according equation (2) depending on the ratio of film focus distance to pipe diameter. The wall thickness calculated according to (3) is smaller then (2) by the given error. 

User Interface for computer supported wall thickness estimation... 

Fig.6 User interface for automated wall thickness estimation in tangential projection technique...

The accuracy of the presented methods was verified using test pipes with various diameters and wall thickness of which the wall thickness was measured independently using a slide-rule. Both on film and on the monitor the wall thickness could be determined within an accuracy of ca. 0.2 - 0.3 mm. 

Figure I represents a two-dimensional damage distribution of an impact in a 0/90° CFRP laminate of 3 mm thickness. Unlike in ultrasonic testing, which is usually the standard method for this problem, there is no shadowing effect on the successive layers by delamination echos. With the method of X-ray refraction the exact concentration of debonded fibers can be calculated for each position averaged over the wall thickness. Additionally the refraction allows the selection of the fiber orientation. The presented X-ray refraction topograph detects selectively debonded fibers of the 90° direction.

Wall Thickness Estimation from Digitized Radiographs. 

Projection radiography is widely used for pipe inspection and corrosion monitoring. Film digitisation allows a direct access to the local density variations by computer software. Following to a calibration step an interactive estimation of local wall thickness change based on the obtained density variation is possible. The theoretical model is discussed, the limitations of the application range are shown and examples of the practical use are given. The accuracy of this method is compared to results from wall thickness measurements with ultrasonic devices. 

Projection radiography has long been used for pipe inspection and corrosion monitoring. In this traditional tangential wall thickness estimation the distance of border lines of the projected wall shadows of a pipe onto the film is a direct measure for the wall thickness. This method is not considered here, newer developments can be found in / /. 

Beer s absorption law (1) for the penetrating radiation is used to correlate the penetrated wall thickness (w) and the radiation dose I (lo - radiation dose at w=0, p - absoiption coefficient) ... 

Fig. 2 shows the response of a C2 film system on a step wedge (wall thickness range 2. .. 18 mm) exposed with a X-ray tube at 160 kV. For the exposure withy-rays (Irl92 or Co60) corresponding linear relationships are obtained. From this linear relationship it is followed, that the influence of the scattered radiation and the energy dependence of the absorption coefficient can be considered by an effective absorption coefficientPcff in equation (1). 

Basically it is only possible to determine a wall thickness change in penetration direction from a density variation of a radiographic film. No absolute wall thickness values can be obtained in this way contrary to the tangential method. 

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