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Technologies and Hardware of Ultrasonic Testing of welded Joints of Steel and Polyethylene Pipelines Author -Dr. G. Giller Co authors -Dr. L. Mogilner, Dr. V. Khomenko (JSC "POLYTEST", JSC VNIIST) Contact

The ultrasonic inspection of butt joints of crucial steel and polyethylene pipelines of small diameters (up to 500 mm) with a small thickness of a wall traditionally has been considered as a complicated technical problem. Till now such joints in many spheres of industry are inspected only by radiographic methods, and butt- welded joints of polyethylene pipelines are not inspected at all by physical methods.
At the same time the pipelines of the indicated type are applied practically in all industries, including power (steel pipelines of various purpose), pipeline transport systems (steel and polyethylene gas and oil pipelines), chemical industry etc. Total amount of joints of pipes of small diameters with a small thickness of a wall is estimated in millions pieces. Therefore problem of quality control of such joints can be considered as an independent section of the theory and practice of ultrasonic NDT.
Let's consider features of this problem on an example of inspection of welded joints of steel pipelines. By using of traditional technologies of ultrasonic testing, reflection of ultrasonic waves from walls of a pipe and roughness of a welded seam usually create so high-power interference, that on their background it is possible to detect only defects of significant sizes, and, as a rule, of planar type. For example, by using of the single probe scheme ultrasonic inspection of welded joints of pipes with wall thickness from 4 up to 6 mm in boilers only poor fusion, located in the root of a welded seam is detected confidently. Even the open blowholes frequently are not detected.
The similar situation takes place for attempts to detect defects in joints of polyethylene pipelines. For them, and also for flash butt welding joints of steel pipes , is characteristic that all dangerous defects have a planar character and are located extremely perpendicularly to surfaces of connection. Such defects practically con not be detected with single probe scheme.
The typical screen picture of a vertically oriented planar defect in a welded seam detected with the single probe scheme is shown in a fig. 1a. In this case reflection of ultrasonic waves from defect or their diffraction on its edges create signals, which amplitude is comparable to parasites of reverberation character. At the same time it is much lower than amplitude of signals reflected from roughness of a welded seam. That is why the detection of both volumetric, and planar defects in considered welded joints is not always authentic.

Fig 1a :

At the same time there is an effective method of ultrasonic testing of welded joints of such pipelines circumscribed by us, in [1]. 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
P-63	2,41	0,95-0,96	2,29-2,31	0,46
P-80 (Yellow)	2,25	0,94	2,11-2,13	0,62
P-80 (Black)	2,32	0,94	2,18-2,19	0,55
P-100	2,39	0,96	2,29	0,44
Table 1:

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)

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
10-51	2,0-3,0	Steel	0,9	[2,3]
18-114	3,0-4,0	Steel	1,0	[2,3]
32-219	4,0-6,0	Steel	1,2	[2,3]
114-325	6,0-9,0	Steel	1,5	[2,3]
32-51	3,0-5,0	Steel	1,1	[4]
57-76	3,0-5,0	Steel	1,3	[4]
89-102	4,0-6,0	Steel	1,4	[4]
108-159	4,0-8,0	Steel	1,5	[4]
168-529	5,0-9,0	Steel	1,6	[4]
63(SDR11)	5,8	Polyethylene	1.0	[5]
110(SDR11)	10,0	Polyethylene	1,8	[5]
160(SDR11)	14,6	Polyethylene	2,6	[5]
225(SDR11)	20,5	Polyethylene	3,7	[5]
Table 2:

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.

Fig 3:

While studying different technological solutions for inspection of butt joints of pipes by "chord" type probes the following details where taken into account:

• The ratio signal / noise, at least, for steel pipes in a small level depends on depth of position of defect in the joint;
• for welded joints of considered standard sizes depth of position of the defect, as a rule, has no big significance.

Analyses of this and other reasons makes it possible to propose the following philosophy of US inspection:

• Detection of defects only in a working cross-section of a welded joint;
• Detection of defects in a welded joint without evaluation of the occurring depth
  (we shall remind, that the wall thickness is only few mm.);
• Carrying out the weld joints inspection using two levels of sensitivity, i.e., for defects search and for defect rejection;
• Use of an alternative principal of inspection when applying the rejection sensitivity. This means: "in this area there is no defect with the amplitude criterion above the inadmissible level" or "in this area the defect is identified for which the amplitude criterion is inadmissible in accidence with pre-set "rejection" level";
• Use of the next inspection principle in case of applying of the "search" sensitivity. This means: "the length(extension)of identified defect is not more than maximum admissible Value" or " the length(extension)of identified defect is more than maximum admissible value". At the same time amplitude criterion(excess of the reach level of sensitivity) provides a basis for measurement of the conventional length of defect..

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 [5], 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:

• To register all signals reflected from the defects of welded joint during its scanning under two sensitivity levels;
• To determine linear co-ordinates of defects (co-ordinates of beginning of each defect and its extension);
• To determine the rejective parameters of the welded joint as a whole. The evaluation is based on the combination of detected flaws and is in compliance with existing regulating documents;
• To enter parameters of the inspected pipeline and all necessary data into the device memory in prompt mode to make a final report. The report comprises "Conclusion based on inspection results";
• To store all accumulated information in the memory of device and print a report with final "conclusion" in standard form, when required, or to create a database for inspected objects
• The US flaw detectors of UD-R type are introduced for practical inspection of pipelines at the number of enterprises of fuel and energy industry and are manufacturing by JSC "Polytest"

The primal problems, which were put by us for designing of new ultrasonic flaw detectors, are based on practical experience of use of above mentioned instruments and on references experts using indicated devices.
First of all, it would be desirable to mark, that, despite of that prize, which we have received, excluded the traditional display -reduced size, weight, power consumption, its presence expands possibilities of the flaw detector. In particular, display with pulse indication of A-scan type allows to use the flaw detector not only for work with chord type probes, but, if necessary, and with the usual single probes. The information shown on display makes the tuning more easy, provides possibility to change characteristics of signals, to measure co-ordinates of defect in a cross section of weld, thickness of a wall etc.
Therefore designing the new ultrasonic flaw detector we took into account necessity to extend its possibilities by maximum automation of tuning procedure, measurements and calculations.

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

REFERENCES

1. Dr. G. Giller, Dr. L. Mogilner Ultrasonic Inspection of welded pipeline joints: new technologies and instruments
   Non-destructive testing, 1, 2000. RAS, Ekaterinburg.
2. 2. Normativ document RD 34.17.302-97. Steam and water Boilers Steam and hot water pipelines, vessels. Welded joints. Quality control. Ultrasonic inspection. M. "Norma". 1997.
3. Instruction on technology of ultrasonic inspection of butt welded joints of pipelines and pipes heat exchangers with double angle probes with elastic protector. JSC "Polytest". M. 1995.
4. Manual on technology of ultrasonic inspection of butt welded joints of pipelines with diameters 32 up to 529 mm. by flaw detectors UD I series.
   Joint-stock company "VNIIST",M. 1995.
5. Technique of ultrasonic inspection of butt welded joints of polyethylene gas pipelines
   JSC "VNIIST", M., 1999.

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