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Technologies and Hardware of Ultrasonic Testing of welded Joints of Steel and Polyethylene PipelinesAuthor -Dr. G. Giller
Co authors -Dr. L. Mogilner, Dr. V. Khomenko (JSC "POLYTEST", JSC VNIIST)
|Fig 1a :|
At the same time there is an effective method of ultrasonic testing of welded joints of such pipelines circumscribed by us, in . It use so-called "CHORD" scheme of inspection of the welded joints with separate functions of transmitting and receiving of ultrasonic waves. Main parameters of transmission and receiving of ultrasound (the angles of input - receiving, angle between transducer and receiver etc.) are selecting in a such manner that the main energy of an emitted beam is concentrated in a working cross section of welded joint (fig. 1b). Due to this the detection of various defects in any zone of the joint within the limits of a thickness of connected pipes is ensured.
|Fig 1b :|
In limits of reinforcing bead of a weld (for joints executed by fuse welding ) or near to a zone of fusion(for joints, executed by butt welding) the ultrasonic waves distribute parallel to surfaces of pipes, and the plane of a falling (reflection) on defect is perpendicular to a reflecting surface of planar defects. The projections of beams, reaching a defect (and reflected from it), on an axial plane of the welded joint lie on a chord of a cross-section of a pipe - it is the reason for the name of sounding.
The practical realisation of "chord" scheme of sounding is hampered by a number of effects connected to excitation of high-power surface waves along an inspected surface.
Thus, getting rid of parasites of reverberation character and signals reflected from irregularities of joints (reinforcing bead or weld burr), "chord" scheme, generally speaking, can generate acoustic parasites of other nature. It essentially limited application of the considered scheme of examination to pipes by a diameter up to 100 mm.
At the beginning of 90 -s the JSC VNIIST and JSC "Polytest" made it possible to solve the problems. The new design of "chord" type double probe with an elastic protector and technology of their manufacturing have been offered. This engineering solution has allowed to use "chord" scheme for inspection of welded joints of steel pipes of various classes (including - austenitic) with diameter from 10- up to 530- mm and thickness of a wall from 2 up to 10- mm. The batch production of " chord" type double probe with an elastic protector was run in.
The given approach has allowed to solve and problem of inspection of welded joints of polyethylene pipelines, first of all - gas pipelines with diameter from 63 up to 315 mm with a thickness of a wall from 6 up to 25 mm.
The acoustic characteristics of joints of polyethylene pipes have been not studied before. Therefore we recognise it expedient to present in tab. 1 and in a fig. 2 figures, of acoustic characteristics of polyethylene used for manufacturing of such pipes which where measured by us.
|The mark of polyethylene||velocity of longitudinal waves C, 103 m\s||density. p 10 kg\m3||Acoustic impedance, z 106 Pa s/m||Factor of attenuation (f=2,5 Mhz) db/mm|
From the tab. 1 it is visible that the factor of attenuation of ultrasonic waves in the given material even for longitudinal waves considerably exceeds figures, typical for steel. The use of transverse waves for inspection is practically impossible. Besides that it is visible, that the difference in acoustic characteristic of polyethylene of different marks can be up to 10 %. It means, that for different marks of polyethylene the parameters of inspection shell be changed, what results in necessity to manufacture specialised probes for inspection of pipelines from polyethylene of various marks.
The researches have also shown, that the inspection of joints of polyethylene pipes is expedient on frequencies which are not exceeding 2,5 Mhz (fig. 2)
The accumulated experience of use of "chord" type probes has shown, that it is possible to ensure the ratio of a signal / noise not less than +20 dB for steel pipes and +12 dB for polyethylene pipes. Term "Signal" here is understood amplitude of a signal reflected from the control reflector of a calibrated size in standard, and under a term "noise" - amplitude of noise of any nature in a zone of location of this signal or near to this zone. As the control reflector for work with "chord" type probes usually used an orifice with flat bottom, whose axes is parallel to a wall of the pipe.
The normative diameters of a control reflectors (orifices with flat bottom), acting now in some industry, are listed in table 2:
|Diameter of a pipe, mm||thickness of a of a pipes wall thickness mm.||material of a pipe||diameter of an orifice with flat bottom, mm||the normative document|
The modern industry use some thousands of "chord" type double probes with elastic protector. With their help the effective control of butt joints of pipelines on thermal and atomic power stations, gas and oil pipelines, at the oil refining enterprises is carried out.
In a fig. 3 the double angle probes with an elastic protector designed for inspection of welded joints of polyethylene pipes of various diameters are shown.
While studying different technological solutions for inspection of butt joints of pipes by "chord" type probes the following details where taken into account:
Analyses of this and other reasons makes it possible to propose the following philosophy of US inspection:
These solutions have passed more than 10- years long approbation in various industries for inspection of welded joints of steel pipelines.
Experience of their application for inspection of butt welded joints of polyethylene pipelines obtained in 1999 and 2000 , has shown, that this technological solutions with some clarifications are applicable and in this case.
It is important to mark, that the inspection of butt joints of pipelines by using " chord" type double probes with elastic protector can be realised with any domestic or imported ultrasonic flaw detector of general purpose. However above mentioned technological principles can be easily realised with the help of specialise portable ultrasonic flaw detectors, developed by us.
The inspection of butt joints of pipelines by using of above mentioned ultrasonic probes can be realised with serial ultrasonic flaw detectors as well as with original portable ultrasonic flaw detectors of UD series, specially designed by joint-stock companies VNIIST and made by JSC Polytest
These devises have no the customary display with pulse indication of A-scan type. Instead of display sound and light indicators are used . Practically, when carrying out the US inspection, such instruments, in particular UD-09, are used for reaped inspection of welded joints of steel and polyethylene pipelines.
Rather interesting prolongation of works connecting to creation of portable flaw detectors is development of a new type of device, - flaw detectors - registrars. Their construction, for the first time in practical works makes it possible to provide the following:
In1999 the new instrument which is called UD-21R was tested in laboratory and in field conditions. The special testing was carried out by GOSSTANDARD RF. UD-21R has been approved for quality control of welded joints of polyethylene pipelines.
Beginning of 2000 JSC "Polytest" has began batch production of flaw detectors
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