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Technical Discussions
JONI H T
JONI H T
07:56 Apr-29-2015
HOT & COLD STEEL THICKNESS INSPECTION

Dear All,

I just realized that I measured the same pipe with different temperature. In ambient temperature this pipe has 25 degree of Celsius while 185 degree Celsius in normal operation. Generally the thickness of hot pipe was bigger than cold pipe.

Any body can explain this phenomenon? My opinion is, it can be occurred due to thermal expansion where the thickness of pipe will increase due to heating up of process fluid. Perhaps some of us have any likewise experiences.

 
 Reply 
 
Steven Doc
Other, Quality Manager
Siemens Energy, Egypt, Joined Feb 2011, 187

Steven Doc

Other, Quality Manager
Siemens Energy,
Egypt,
Joined Feb 2011
187
11:11 Apr-29-2015
Re: HOT & COLD STEEL THICKNESS INSPECTION
In Reply to JONI H T at 07:56 Apr-29-2015 (Opening).

Joni, if you have an item of a known thickness and it expands with heat, the thickness will reduce.

What kind of difference in thickness are we talking about? Could it be simply that it is very unlikely that the thickness is uniform and you didn't measure both readings from exactly the same spot?

1
 
 Reply 
 
Bill
USA, Joined Jan 2010, 52

Bill

USA,
Joined Jan 2010
52
20:16 Apr-29-2015
Re: HOT & COLD STEEL THICKNESS INSPECTION
In Reply to JONI H T at 07:56 Apr-29-2015 (Opening).

JONI H T,

Yes there is some expansion but at these tempertures the real issue is velocity.

The main difference that you noticed is the change in velocity due to the rise in temperature. The speed of sound is determined by the density and modulus elasticity of a material. As you heat a material it becomes more elastic. This increases attenuation but also lowers the speed of sound in that material. Hence your thicker numbers at the higher temperature. At that temperature range the difference in steel is likely to be about +3% or +4% of the ambient numbers.

1
 
 Reply 
 
Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
20:43 Apr-29-2015
Re: HOT & COLD STEEL THICKNESS INSPECTION
In Reply to JONI H T at 07:56 Apr-29-2015 (Opening).

As Mr. Bill notes above, the velocity in steel and all other metals decreases with increasing temperature, and if you don't compensate for that effect you'll get an inaccurately high thickness reading. That's because your room temperature calibration velocity is higher than the actual velocity in the hot metal. In carbon steel, the velocity drop is about -1% per +55 degrees Celsius.

We have an application note on high temperature ultrasonic testing that discusses this and other considerations in more detail. See section 3 at this link:
http://www.olympus-ims.com/en/applications/high-temperature-ultrasonic-testing/

1
 
 Reply 
 
JONI H T
NDT Inspector, Condition Monitoring Analyst
Star Energy Wayang Windu LTD, Indonesia, Joined Apr 2015, 3

JONI H T

NDT Inspector, Condition Monitoring Analyst
Star Energy Wayang Windu LTD,
Indonesia,
Joined Apr 2015
3
02:54 Apr-30-2015
Re: HOT & COLD STEEL THICKNESS INSPECTION
In Reply to Steven Doc at 11:11 Apr-29-2015 .

@ Steven, I took samples for about 50 points which 50 % of them still have previous measurement points marking and all of my data indicated the increasing of pipe thickness.

I have what Bill and Tom explained in mind. Since the thickness measurement using ultrasound, the change material density directly be influenced by atomic expansion of material ( in term of macro called density). Finally it will affect the travel velocity of ultrasound converted to thickness.

However , it is interesting as Steven explained. Physically, perhaps, the real thickness of the material is getting thicker. If it is so, it will the same with the strain effect isn't it?

 
 Reply 
 
Bill
USA, Joined Jan 2010, 52

Bill

USA,
Joined Jan 2010
52
16:53 Apr-30-2015
Re: HOT & COLD STEEL THICKNESS INSPECTION
In Reply to JONI H T at 02:54 Apr-30-2015 .

Joni H T,

It is getting thicker. However the coefficient of expansion for steel is approx. 0.00000645 in./in./deg F so at the temperatures you discussed, in the thru wall direction, it’s not a significant contributor to your change in readings.

The main factor is the modulus elasticity changing as previously discussed. If you do not compensate for the change in velocity as the temperature changes you will get incorrect readings (time x speed=distance).

 
 Reply 
 
joel nyaga
joel nyaga
06:46 May-03-2015
Re: HOT & COLD STEEL THICKNESS INSPECTION
In Reply to Bill at 16:53 Apr-30-2015 .

I totally agree with my colleagues, the material thickness in this case increases in UT measurement technique owing to the contribution made by changes in density of the material. Basically the heat introduction into the material has great effects to material velocities in metallic structures as an increase in temp tends to slow down the sound propagation through materials, coarse grained structure have lower material velocity than fine grained structures thereby more time for sound to travel thro' the material and hence greater thickness measurements when material temperature in increased..

 
 Reply 
 
collin maloney
NDT Inspector, - Plant Inspector
Applus RTD, Australia, Joined Nov 2000, 147

collin maloney

NDT Inspector, - Plant Inspector
Applus RTD,
Australia,
Joined Nov 2000
147
09:31 May-03-2015
Re: HOT & COLD STEEL THICKNESS INSPECTION
In Reply to JONI H T at 02:54 Apr-30-2015 .

A general rule for hot testing is to leave your calibration block on the test piece to equalise in temperature then do your calibration. Bear in mind that with all the discussion re change in velocity, the diference is rarely outside the quoted order of accuracy for UT thickness!

 
 Reply 
 
marco lancellotti
NDT Inspector,
IST NDT , Italy, Joined Mar 2011, 56

marco lancellotti

NDT Inspector,
IST NDT ,
Italy,
Joined Mar 2011
56
14:23 May-22-2015
Re: HOT & COLD STEEL THICKNESS INSPECTION
In Reply to JONI H T at 07:56 Apr-29-2015 (Opening).

After ten years of inspections of piping hot, we could estimate that the anomalies are due in part to the physical changes is the piece in question, but also to the physical changes of the probe!
In particular, for thicknesses up to 10 mm, the variation of the delay of the probe, due to the thermal expansion and the change in the speed of the protective wedge generates an error that is much greater than that due to the variation of the ultrasonic properties of the test material (typically carbon steel).
It remedied these issues, using a tool that allows you to perform a reset of the probe and where we can introduce a suitable correction factor of temperature (ASME V art. 23 SE 797 par. 8.5
greetings

 
 Reply 
 

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