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Technical Discussions
Deepu Varghese
Other, Plant Inspector
SABIC, Saudi Arabia, Joined Jul 2011, 13

Deepu Varghese

Other, Plant Inspector
SABIC,
Saudi Arabia,
Joined Jul 2011
13
12:51 Aug-04-2011
Accuracy of ultrasonic thickness gauging while doing hydrostatic pressure testing

Will I get an accurate reading using ultrasonic thickness gauging while doing hydrostatic pressure testing (or a pipe which flows at full capacity during operation).

I know because of safety reasons it’s not practical to measure during hydro testing. But I have been carrying this doubt for a long time.

Your valuable comment will be appreciated.

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

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
16:34 Aug-04-2011
Re: Accuracy of ultrasonic thickness gauging while doing hydrostatic pressure testing
In Reply to Deepu Varghese at 12:51 Aug-04-2011 (Opening).

Sound velocity in metals does change with tension or compression. For example, the slowing of sound velocity under tension is a significant factor in applications involving bolt elongation measurement. However I have not heard of this as a practical problem when measuring wall thickness of typical pressurized tanks or pipes, since the effect is fairly small under most conditions. Whether the velocity changes during a hydrostatic test would be of functional significance in your case would depend on the pressure being applied, the elastic constants of the metal in question, and especially the measurement accuracy required.

 
 Reply 
 
Wieslaw Bicz
Engineering,
PBP Optel sp. z o.o., Poland, Joined Feb 2009, 276

Wieslaw Bicz

Engineering,
PBP Optel sp. z o.o.,
Poland,
Joined Feb 2009
276
16:34 Aug-04-2011
Re: Accuracy of ultrasonic thickness gauging while doing hydrostatic pressure testing
In Reply to Deepu Varghese at 12:51 Aug-04-2011 (Opening).

You can always attach the probe to the pipe and be in the safe distace. The accuracy of the reading can be limited, if the sound speed in the pipe will change. This can be measured and the result can be adjusted, but it requires a more complicated approach than a simple thickness gauge (testing must be made on multiple pathes). Using this type of approach almost any realistic accuracy can be achieved.

 
 Reply 
 
Gerhard Splitt
R & D,
Private, Germany, Joined Nov 1998, 38

Gerhard Splitt

R & D,
Private,
Germany,
Joined Nov 1998
38
19:31 Aug-04-2011
Re: Accuracy of ultrasonic thickness gauging while doing hydrostatic pressure testing
In Reply to Deepu Varghese at 12:51 Aug-04-2011 (Opening).

As Tom mentioned, the acoustoelastic effect, which describes the change in time of flight by stress, is very small. When the steel is strained to the elastic limit - which hopefully never will occur in a pipeline - the change in TOF is only 0.3% approx.. This means that at 100 mm wall thickness the error will be 0.3 mm at maximum. By the way, a temperature change of 30 degree C will have the same effect.

But:

Wall thickness measurements are normally performed with longitudinal waves. The TOF of this wave type is only changed by stress in the direction parallel to the sound direction, meaning that a circumferentiell stress in a pipeline will not change the TOF of the longitudinal wave. Therefore, there is no influence of the pressure on the result.

 
 Reply 
 
Lalit Mohan kothari
Consultant, -
On ..IOCL and BARC(Bhabha Atomic Research Centre).etc, India, Joined Jan 2003, 128

Lalit Mohan kothari

Consultant, -
On ..IOCL and BARC(Bhabha Atomic Research Centre).etc,
India,
Joined Jan 2003
128
08:20 Aug-05-2011
Re: Accuracy of ultrasonic thickness gauging while doing hydrostatic pressure testing
In Reply to Deepu Varghese at 12:51 Aug-04-2011 (Opening).

Deepu,
As most of the D-meters equipment reading is not fixed and due to some pressures some extra steers is there and this much impact is beyond of these common D-meter .
If you are measuring some flexible metal then this impact can be measure.
In general its depends on applied pressures and temp/thickness etc
As attenuation changed so impact is their .

Thanks & Regard

 
 Reply 
 
Deepu Varghese
Other, Plant Inspector
SABIC, Saudi Arabia, Joined Jul 2011, 13

Deepu Varghese

Other, Plant Inspector
SABIC,
Saudi Arabia,
Joined Jul 2011
13
18:57 Aug-09-2011
Re: Accuracy of ultrasonic thickness gauging while doing hydrostatic pressure testing
In Reply to Lalit Mohan kothari at 08:20 Aug-05-2011 .

Thank you all for your valuable comments

 
 Reply 
 
massimo carminati
Consultant, AUT specialist
IMG Ultrasuoni Srl, Italy, Joined Apr 2007, 691

massimo carminati

Consultant, AUT specialist
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
691
20:38 Aug-27-2011
Re: Accuracy of ultrasonic thickness gauging while doing hydrostatic pressure testing
In Reply to Deepu Varghese at 18:57 Aug-09-2011 .

To be most accurate you should:

1)fix probe to surface with adequate couplant which will remain stable for hydrotest duration
2)establish a measurement criteria which can be adequatelly satisfied in case of signal amplitude change (e.g. I would not use flank with fixed level)
3)Acquire data with high rate ( 1 per 5 seconds or so).

In this way you should be able to obtain a TOF curve which than should be evaluated, because the transit time is what we measure; if speed of sound change is expected, than it should be taken into account.

 
 Reply 
 

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