|NDT.net - October 2002, Vol. 7 No.10|
Field radiography based on portable X-ray units with all its important applications such as weld inspection of pipelines, bridges and vessels in all hot and cold areas of the world still is and will remain a very important inspection technique. This X-ray technology requires special components such as integrated tube heads and adapted tube inserts, all aiming at low weight and highest possible power. These goals normally are fulfilled by using an air- cooling mechanism. Typical X-ray tubes used for this application have a focal spot of 3.0 mm (according to EN12543). Up to now, all available commercial portable X-ray systems in the world were limited in power by 900 W or less on the 3.0 mm focal spot. The limitation to the power is given mainly by the cooling mechanism; with water cooling the power on the same tube insert can be brought up to higher values.
Based on new construction principles for the air-cooling mechanism and for the power electronics, YXLON International has developed a new type of portable unit called XPO which clearly breaks the previous power limit. XPO is able to provide a continuous power which exceeds 1000 W by far; in laboratory tests values of up to 1500 W have been achieved.
In this presentation, the new portable system XPO with focal spot 3.0 (EN12543) will be described; technical data given here are based on prototype models and should be considered as being preliminary.
YXLON International is well-known for its SMART product family of portable X-ray units which covers the whole X-ray tube voltage range from 160 kV to 300 kV; directional beam as well as panoramic beam systems are available. Crawler systems are an important part of the family.
The big advantage of portable X-ray systems is the low weight which is achieved by using an air-cooling mechanism and by integrating the gas-isolated HV generator into the tube head. Even at 300 kV, the tube head has a weight of less than 35 kg, thereby providing full radiation safety according to several international standards. Due to the design principles of the X-ray tube insert and of the cooling mechanism, the tube power for systems with 100% duty cycle up to now was restricted to 900 W with air cooling (for typical focal spots of nominal value 3.0 according to EN12543 standard). Higher power could only be achieved by using additional heavy-weight water cooling or larger focal spots.
Portable X-ray systems like SMART in the majority of cases are used for field radiography like e.g. inspection of pipelines, bridges and vessels, but also for inspection in power plants. In some cases they are used for stationary radioscopy applications such as casting inspection etc.
Standard water-cooled X-ray tubes
For a given design principle of the cooling mechanism, there is a linear relationship between the power of standard water-cooled X-ray tubes and the focal spot size. This can be seen in fig.1 where data are given for a series of X-ray tubes plus a linear fit line. In principle, the linear function in fig.1 can be extrapolated to Microfocus X-ray tubes on the low-power side as well as to oil-cooled bipolar tubes on the high-power end.
HP-type water-cooled X-ray tubes
Only recently, improvements have been achieved with the cooling system of water-cooled stationary X-ray tubes. With the so-called HP-technology an increase of approx. 50% in power can be achieved (see the black triangle in figure 1).
|Fig 1: X-ray tube power versus focal spot size.|
Standard air-cooled X-ray portables
For standard air-cooled tubes normally used in portable units like SMART the permitted permanent power is much less than for water-cooled stationary systems (see open circle in fig. 1). This is mainly due to the less efficient cooling mechanism of air-cooled portables. In some cases, the HV power supply has also been a limitation in the past.
XPO-type air-cooled X-ray portables
With the new XPO technology, YXLON clearly demonstrates that improvements in cooling technology are possible. With XPO the power of air-cooled tubes can be brought up far beyond 1000 W instead of the previous 900 W. With prototype models, 1500 W has been achieved so far (see full black circle in fig.1).
The new XPO portable stands in line with the previous SMART units. XPO consists of the following main parts: a lightweight tube head , a control unit and connection cables. XPO has been designed to meet the IP class IP65, and it will work at very rugged environmental conditions. The temperature range will be at least from –20°C to +40°C; at very high temperatures a sensor device will cause a protective temperature switch off. The low- temperature range can in principle be extended by adding a heating device.
XPO tube head
|Fig 2: XPO tubehead (prototype).|
XPO control unit and power electronics
The XPO control unit comprises a µ-processor based control system, a display unit and the DC-DC power electronics (see fig.3). The permitted line voltage range will be from 110 to 230 Vac with a permitted frequency range from 50 to 400 Hz. The weight is approximately 15 kg. There are connections for external safety devices such as warning lamps, safety circuits etc. to be fully compatible with stationary X-ray systems. The control unit is equipped with a tube head recognition for flexible operation.
|Fig 3: XPO control unit (prototype.|
XPO software functions
XPO has a menu-driven user-interface for selection of operating parameters, for displaying exposure diagrams etc. Software update can be performed via standard interfaces (CAN- bus and RS232). Except others, there are also history functions and radiation protection data available.
Exposure diagrams of XPO are very similar to those of stationary X-ray systems. In fig.4 preliminary exposure curves for the voltage range from 100 kV to 250 kV are presented.
|Fig 4: Exposure diagram for XPO-250 (preliminary).|
Precision and stability
The tube voltage is measured inside the tube head with a direct method. Based on the results with prototypes, it is expected that the stability of the tube voltage will be approx. 1% w.r.t. the rated value at constant temperature; within the specified temperature range the temperature drift of the voltage is expected to be of the same order.
As can be seen from fig.5, 1500 W portables of the XPO-type have two important advantages compared to previous 900 W types : a) the exposure time of the films can be reduced by 40 % at comparable tube voltage, and b) at constant voltage and exposure conditions the penetrated material thickness will increase by approx. 10%. The reduced exposure time enables a faster and cost-saving inspection in the field.
Besides field radiography, the higher power level makes the new portable systems also attractive for stationary applications such as casting or food inspection. Regular production of XPO is planned to begin in summer 2002, starting with a 250 kV model. For XPO, a broad range of accessories such as tube stands, laser beam finders, filters etc. will be provided.
|Fig 5: Comparison of exposure data for 1500 W XPO and previous 900 W models (preliminary).|
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