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Technical Discussions
Dr. Shoef
NDT Inspector
Gabi Shoef Ltd, Israel, Joined Dec 2005, 19

Dr. Shoef

NDT Inspector
Gabi Shoef Ltd,
Israel,
Joined Dec 2005
19
00:19 Mar-01-2006
Too low radiographic density

a radiography lab has to meet EN 1435 requirements ( 2.0 minimum densuty) and was requested to make evaluation of welds of densities lower than that ( down to 1.2) . How low would you go if at all. Do you know what is the impact of this request on the detection? The concern is cracks in the root
The pipes are high pressure gas piping - 70 bar


    
 
 Reply 
 
Frank
Frank
01:32 Mar-02-2006
Re: Too low radiographic density
----------- Start Original Message -----------
: a radiography lab has to meet EN 1435 requirements ( 2.0 minimum densuty) and was requested to make evaluation of welds of densities lower than that ( down to 1.2) . How low would you go if at all. Do you know what is the impact of this request on the detection? The concern is cracks in the root
: The pipes are high pressure gas piping - 70 bar
------------ End Original Message ------------

Too low of densities could cause indications to not be visible. Especially as low as 1.2. The density should be at least over 1.8. 1.2 is too low.




    
 
 Reply 
 
Scott
Scott
09:39 Mar-02-2006
Re: Too low radiographic density
Some codes that are used for structural will allow density dowsn as low as 1.5. ASME specifies 1.8 min for x-ray and 2.0 for gamma. I like radiographs darker, around 2.8-3.0 personally.


----------- Start Original Message -----------
: : a radiography lab has to meet EN 1435 requirements ( 2.0 minimum densuty) and was requested to make evaluation of welds of densities lower than that ( down to 1.2) . How low would you go if at all. Do you know what is the impact of this request on the detection? The concern is cracks in the root
: : The pipes are high pressure gas piping - 70 bar
: Too low of densities could cause indications to not be visible. Especially as low as 1.2. The density should be at least over 1.8. 1.2 is too low.
------------ End Original Message ------------




    
 
 Reply 
 
David
David
01:04 Mar-03-2006
Re: Too low radiographic density
Who requires evaluation at a lower density and why? If the lab goes outside the code and there is a failure I hope they have good insurance.
Any "special" requests like this must be fully documented with liability clearly defined.
Personally I would not evaluate critical welds such as these at a low density, especially for root cracking.
Are you talking of the actual root area being of low density due to the cap and root extra thickness? Density should be within code in the area of interest. With densities that low the contrast must be poor leading to less chance of seeing a crack.

----------- Start Original Message -----------
: Some codes that are used for structural will allow density dowsn as low as 1.5. ASME specifies 1.8 min for x-ray and 2.0 for gamma. I like radiographs darker, around 2.8-3.0 personally.
:
: : : a radiography lab has to meet EN 1435 requirements ( 2.0 minimum densuty) and was requested to make evaluation of welds of densities lower than that ( down to 1.2). How low would you go if at all. Do you know what is the impact of this request on the detection? The concern is cracks in the root
: : : The pipes are high pressure gas piping - 70 bar
: : Too low of densities could cause indications to not be visible. Especially as low as 1.2. The density should be at least over 1.8. 1.2 is too low.
------------ End Original Message ------------




    
 
 Reply 
 
Uli Mletzko
R & D, Retired
Germany, Joined Nov 1998, 89

Uli Mletzko

R & D, Retired
Germany,
Joined Nov 1998
89
06:16 Mar-03-2006
Re: Too low radiographic density
----------- Start Original Message -----------
: a radiography lab has to meet EN 1435 requirements ( 2.0 minimum densuty) and was requested to make evaluation of welds of densities lower than that ( down to 1.2) . How low would you go if at all. Do you know what is the impact of this request on the detection? The concern is cracks in the root
: The pipes are high pressure gas piping - 70 bar
------------ End Original Message ------------

Due to the maximum density (silver content) of films for technical radiography (Dmax about 8 or 10) the inflection point of the density curve will be very high (about 4 or 5 or 6 or more). Therefore you will have a higher contrast (density difference) between a given Material thickness and a given flaw size, if you choose high density and a viewer with bright illumination, instead to use a low density and a viewer with low illumination intensity.
This is in contrary to medical applications, were you have films with a Dmax of about 3.
Therefore in technical radiography according to European standards we are demanding a minimum density (above fog!!) of D = 2 for high quality films. In addition, to avoid blinding during evaluation, the maximum density difference due to different material thickness in an evaluation area should be not more than 1.5 (of course this is not valid for a flaw itself). So, you could try to have a density of 3.5 for the base material and a density of 2.0 for the root (plus superimposition of ungrinded crown!). This contrast requirement sometimes could be fulfilled by increasing the energy and/or by pre-filtering of the weak energy parts of the spectrum, but sometimes could be not fulfilled resp. be not allowed.

A minimum density (above fog) below 2 (down to 1.5) could be allowded only, if it is clear, that you have not to make high quality radiography (i.e. low safety requests, no fine cracks, only large volumetric flaws, use of gamma sources, use of high speed coarse grain films etc.). But this has to be expressed clearly in the contract, the procedure and in the evaluation report.

Regards
Uli Mletzko
NDT Group, Materials Testing Institute (MPA),
University of Stuttgart, Germany


    
 
 Reply 
 
John O'Brien
Consultant, -
Chevron ETC , USA, Joined Jan 2000, 278

John O'Brien

Consultant, -
Chevron ETC ,
USA,
Joined Jan 2000
278
07:12 Mar-03-2006
Re: Too low radiographic density
There are a number of issues here. The density is an issue of can you see the required flaw size at these low densities. Also what is the weld prep and welding process. Weld prep angle will have a a contributing feature to the location of critical flaws. The welding process may be sensitive to particular flaw types and their detectability with RT. The codes specify minimum density for a reason i.e that in most real world circumstances is the detection threshold. If you want to deviate you need to prove detectability at these density levels and then raise a deviation from code requirements.




    
 
 Reply 
 
Frank
Frank
05:29 Mar-04-2006
Re: Too low radiographic density
----------- Start Original Message -----------
: There are a number of issues here. The density is an issue of can you see the required flaw size at these low densities. Also what is the weld prep and welding process. Weld prep angle will have a a contributing feature to the location of critical flaws. The welding process may be sensitive to particular flaw types and their detectability with RT. The codes specify minimum density for a reason i.e that in most real world circumstances is the detection threshold. If you want to deviate you need to prove detectability at these density levels and then raise a deviation from code requirements.
------------ End Original Message ------------

Well said John. You hit the nail on the head.


    
 
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