CP X-ray sets are used for carrying out field radiography in petrochemical plants, the oil and gas sector, mining and automotive.
It has been, and will continue to be, an undeniable method of testing that the industry has tried to get rid of over the years. While there is a reasonable replacement, it will be there for the foreseeable future, and this technology will continue to evolve around industry needs.
Until relatively recently, almost all NDT x-ray generators applied alternating current to the tube when generating x-rays. The major components to produce an X-ray are: X-ray generator (the tube, the high voltage generator), the control console, and the effective cooling system. The basics of a Pulsed X – Ray: Alternating current x-ray generators produce x-rays only during onee half of each 1/60th second alternating current cycle ( the portion off the waveform produces “ burst” of radiation rather than a constant stream . Only a portion of these “burst “ are usable radiation .
The lower voltage portion” blue line “produces low energy, unusable radiation
A constant potential generator produces constant stream of radiation as well as greater % of higher energy, “useful” radiation
Based on the industry it is used in, key factors need to be considered:
General Application Thickness range to determine your KV and Ma
Frequency used to look at duty cycle
Location of application to evaluate the mobility of the unit • Safety aspects that include compliance Today, the NDT market has a full complement of different sizes and
Safety aspects that include compliance.
Today, the NDT market has a full complement of different sizes and shapes , bells and whistles, which suppliers have to offer.
Generally if you have the resources, you would rather consider looking at procuring a ceramic tube because that is what you were taught. But is ceramic better that glass for an insert? The key factors for reliability and long lifetime of any x-ray insert, whatever the insulating material used, is its design.
The key factors for reliability and long lifetime of any x-ray insert, whatever the insulating material used, is its design, its cooling, and its high voltage environment. As discussed above, an effective cooling system is a must. Therefore, a good glass tube design shall be, by far, superior to a bad metal-ceramic design. The opposite situation being of course also true; the quality of any x-ray insert is NOT simply dependent on the insulating material used… if this was the case, the medical field would not use 90% glass.
The quality of any x-ray insert is NOT simply dependent on the insulating material used… if this was the case, the medical field would not use 90% glass inserts . To put this in perspective, in general, the powerful NDT units have about 900w vs medical CT ( glass insert) at 115Kw!
Why are NDT users sometimes reluctant to use glass inserts?
It is based on 2 simple facts:
1. Over the past 20 years, ceramic and/or metal-ceramic inserts have been mainly used in CP systems with an efficient water-cooling system and were almost never used on sites – these are the best conditions to extend the lifetime of any insert.
2. During the same period, glass inserts have been exclusively used on portable generators, with poor air cooling, AC voltage supplied, within harsh on site environment and, although being BIPOLAR designed, they have been mostly used in grounded configuration, which is the worst thing you can do… Glass inserts have existed for 100 years and easily reach 3.000.000 Volts…while ceramic inserts are still practically limited to 225 – 270 kV… It is for instance; well known that many NDT customers have experienced an incredible reliability and lifetime of those glass inserts when they are used in as soft conditions as metal-ceramic are (CP, water cooling, indoor use, etc…)
Why is glass a superior material for making X-ray inserts?
1. It has a MUCH better capability to evacuate electrical charges (which are responsible for 85% of the problems met with any x-ray inserts.) This is the reason why ceramic inserts are no longer used and why metal-ceramic have been developed (the metal being supposed to be the only material within the insert collecting these electrical charges).
2. It is NOT a POROUS material as ceramic still remains (which leads to a slow decrease of the vacuum level and reduce the theoretical lifetime).
3. It is almost not affected by the presence of x-ray photons while ceramic rapidly loses its characteristics in presence of x-ray. It is however possible to design a good metal-ceramic insert, but that will ALWAYS BE MUCH MORE EXPENSIVE because
1. Special expensive screens must be integrated to protect the ceramic from charges.
2. Other special screens and external lead protection must also be installed to protect ceramic from direct or scattered radiations output.
3. More sophisticated pumping procedures are currently used to prevent a rapid destruction of the vacuum level due to the porosity of the ceramic and its “capacity” to slowly release gas.
Dropping X-ray units are not recommended and very much frowned upon, but does happen .
The likelihood that a ceramic insert would break when dropped from the same distance as a glass insert is very likely .
In a ceramic insert there filament that is much more likely to break and require full insert replacement Therefore: A well designed glass insert has a 3 to 5 times longer lifetime than a metal ceramic one and are about 60% cheaper to replace if necessary.
-References NDT Resource centre
Science of NDT Radiography
Teledyne ICM resources – Martin Raets and support team
GERT VENTER GTECH NDT PTY LTD