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publ-15 State of the Art Reformer Tubes

Paper no. 2B
State-of-the-art of materials and inspection strategies
reformer tubes and outlet components
by :
R. Gommans
Dr. D. Jakobi
Dr. J.L. Jiménez
Gommans Metallurgical Services
Stevensweert, NL
Schmidt + Clemens
Kaiserau, D
S-C Spain S.A.
Murieta, SP
prepared for presentation at
the 46th Annual Safety in Ammonia Plants and Related Facilities Symposium
in Montreal, Quebec, Canada on January 14-17, 2002
copyright  R.J. Gommans, GMS and D.Jakobi, S+C Group
AIChE shall not be responsible for statements or opinions contained in papers or printed in it’s publications
State-of-the-art of materials and inspection strategies
reformer tubes and outlet components
Catalyst tubes in steam reformers are made of high-alloy spun cast materials.
Alloys of the present and past are presented with their specific advantages and
disadvantages. The development of spun cast alloys for outlet parts (e.g. manifolds)
are presented in comparison to wrought alloys. Also, new alloys for reformer tubes
and outlet parts are presented.
The main damage mechanisms for catalyst tubes and outlet parts will be
discussed. For catalyst tubes these are relaxation of thermal stresses by start/stopcycles and creep by steady-state operation. For outlet parts the main damage
mechanisms are creep by steady-state operation and creep-fatigue interaction
resulting from hindered thermal expansion during start/stop-cycles. Also, failure
cases will be presented to demonstrate the failure mechanisms mentioned above.
The influence of materials choice on tube wall thickness, tube weight, catalyst
volume and tube life will be discussed. Because of the damage mechanisms
involved tube life does not always profit from a thicker tube wall!
Also, the life assessment method and inspection strategy has to take these
damage mechanisms into account, thus providing a solid basis for a Risk Based
Inspection (RBI-) strategy. The preferred life assessment method and an overview
of the pro’s and con’s of the various inspection techniques are presented for
catalyst tubes. In many cases it has been observed that inspection of outlet parts
is overdone – a guideline is given to cut inspection costs.
1. Introduction
Steam reformer units are critical to many processes in refining and chemical plant. They are
used in the production of ammonia, but also for the production of hydrogen for oil refining,
and direct iron reduction, hydrogen and carbon monoxide for nickel reduction and purification, and syngas as a basis for producing methanol, acetic acid and various other chemicals.
The costs for the reformer furnace are a substantial part of the investment of the complete
plant. For a typical 1500 metric tons ammonia plant this is about 20%. Also, the materials
and assembly costs for the reformer tubes and outlet components make up a substantial part
of the reformer furnace (for the above mentioned plant size this is about 10 Million USD).
For economic reasons, but mainly because of maintaining high levels of safety, reliability,
and structural integrity, end-users want to use state-of-the-art materials and inspection
strategies for their reformer tubes and outlet components. This paper describes these aspects.
2. State-of-the-art and new materials
2.1. Reformer tube assemblies
Because of the severity of the operating conditions, reformer tube assemblies are fabricated
from centrifugally cast, thick section materials. For a high creep strength these alloys
contain around 0.4% carbon. A precondition for manufacturing wrought ttubes is the
malleability of the material, which ends at a carbon content of ~0.15%. At higher carbon
contents, the tubes must be produced by centrifugal casting. Compared to wrought tubes,
centrifugal cast tubes show subsequent advantages :
- high purity level because of the centrifugal casting process ;
- all diameters and wall thickness combinations can be made economically (also in
smaller quantities) ;
- concentricity is guaranteed by the centrifugal casting and the pull-boring process; and
- larger grain size for better creep strength .
After centrifugal casting tubes are internally machined by pull-boring to remove casting
porosity and impurities that have been driven to the ID by the centrifugal casting process.
The internal boring also prevents carburisation of the tube material by decreasing the active
surface for carburisation. Internal boring of the tubes also saves tube weight and decreases
fuel costs (less thermal resistance).
Because of the necessary oxidation resistance at temperatures up to 1000°C the chromium
content is about 25%. This chromium content is the standard for all modern centrifugal cat
alloys used as radiant tubes.
Centrifugal cast tubes are available to the market since the early 1950’s and have found
increasing use at the expense of wrought tubes since then. Since the mid-1960’s centrifugal
cast tubes were used for nearly 100% of all new reformers. Alloys of the past are :
- HK40 (used since the early 1960’s),
- IN519 (used since early 1970’s) ; and
- HP-Nb (used since the mid 1970’s).
Their composition is given in table 1. HK40 uses –apart from chromium– no carbide
forming elements. HK40 develops primary M7C3-type carbides (during casting, which are
transformed into M23C6 upon ageing) and secondary carbides of the M23C6- and M6C-type
(during ageing). The newer alloys IN519 and HP-Nb also precipitate Nb carbides (NbC,
both primary and secondary carbides). Because of the fine precipitation of secondary Nb
carbides, dislocation movement is effectively hindered, which increases creep strength.
These NbC-forming alloys have been used with much success.
Since the investigations by Zaghloul [ref.1] and the early work of some tube manufacturers
so-called micro-alloys have been developed since the early 1980’s. When strong carbide
forming elements are added, creep strength increases about 15-20% at temperatures of
interest for reformer tubes. This is because MC-type carbides are formed that are more
resistant to ageing (less Ostwald ripening) than NbC carbides. Such strong carbide forming
elements are Nb, Ti, Zr and others. The effect of this mix of elements is higher than their
individual addition, therefore, these alloys are called synergistic hardened alloys. Because
only small amounts of carbide-forming elements are used, the alloys are called micro-alloys.
The most successful alloy is the micro-alloy based on the HP-composition: HP-micro alloy
or HP-MA. Although the micro-alloy based on HK40 has been developed some time ago,
unfortunately this alloy has not gained much appreciation. This was due to the success of
HP-MA, which is obviously the stronger alloy, and the formation of σ-phase below 900°C.
On the other hand HK-MA is nearly as strong as HP-Nb and it can be a low-cost alternative
for existing reformers that need re-tubing. This needs to be considered case by case.
Table 1
Nominal chemical composition of centrifugal cast materials for reformers
S+C Märker  G /
Centralloy CA
high-carbon alloys ** for radiant tubes
4848 micro ****
4852 micro
HK 40
IN 519
Nb, Ti, Zr
Nb, W, Ti, Zr
low-carbon alloys *** for outlet components
H 101
H 101 micro ****
Nb, Ti, Zr
CFE = carbide forming elements
high carbon alloys contain typically 0.40-0.45 % carbon
low-carbon alloys contain typically 0.10-0.15 % carbon
new S+C alloy
Schmidt + Clemens
2.2. Outlet component assemblies
Outlet components such as manifolds, T-pieces, and cones can be made by centrifugal
casting. These components need to cope with expansion stresses, therefore, ductility is of
primary importance. Creep strength is of secondary importance. Because of these two
reasons the carbon content is generally limited to about 0.15%. The cast variety of Alloy
800H, thus of the 32Ni/20Cr type, has become very widely used in Europe and Asia since
the early 1970’s and later also in the America’s. The cast alloy with 32Ni/20Cr and Nb
has a creep strength which is about 50% higher at the temperatures of interest, while
ductility after ageing is on nearly the same (high) level as the wrought variety.
The chromium content of 20% limits the oxidation resistance and, therefore, the maximum
operation temperature is 950-1000°C (which is also dependent on the lifetime). For some
applications more oxidation resistance is required. For instance this is the case for thinsection components after long operating times, or for modern designs that have high outlet
temperatures. For these cases the chromium and nickel contents have been increased up to
the HP-composition (35Ni/25Cr). However, the ductility after ageing of HP-LC is less than
32/20+Nb. By increasing the chrome content from 20% to 25%, the carbide amounts are
increased which cause a decreased ductility after ageing [ref.2].
Small diameter pigtail tubing is usually made of wrought alloys such as Alloy 800HT.
However, dependent on pigtail dimensions, such small diameter tubing can be manufactured
by centrifugal casting in cast 32/20+Nb and HP-LC materials as well.
Table 2
Minimum 10 hours creep rupture strength (in MPa) for low-carbon alloys
Alloy 800HT
S+C name
(wrought) **
G 4859 *
H 101 *
H 101 micro ***
source : S+C.
source : ECCC [ref.3].
source : S+C. Preliminary values.
S+C has developed a new alloy with high oxidation resistance and high creep strength.
This HP low carbon version contains 25%Cr for high oxidation resistance, while a modified
composition and micro-alloy additions improve both high creep strength (see Table 2) and
ductility after ageing. The ductility after ageing of the HP-LC micro-alloy is much better
than that of the standard HP-LC. This alloy is specially recommended for use at higher
operating temperatures (900-1000°C) in order to profit from the increased oxidation
resistance. Furthermore, the new H101-micro can be used at lower temperatures as well.
3. Damage mechanisms
3.1. Reformer tubes
The main damage mechanism for reformer tubes is the combination of thermal stresses
across the tube wall and internal pressure stresses. This combination causes that creep
damage typically develops at the inner diameter or just below the ID surface. Also, the creep
damage occurs over the complete circumference (or at least a large part of the circumference) and over a longer (axial) part of the tube. The damage process results in diameter
increase and creep damage (cavitation) at the inner diameter. Final rupture occurs in a
longitudinal direction.
Another main damage mechanism can be overheating by catalyst degeneration or by
operating upsets. Typically, catalyst degeneration results in creep damage over a small part
of the circumference and over a short (axial) part of the tube. This means bulging and the
final rupture occurs also in axial direction.
Besides these two main damage mechanisms, other damage mechanisms can be important as
well. These are, for example, metal dusting (in cold areas with T<800°C), galvanic
corrosion between cast austenitic material and forged ferritic flanges, formation of brittle
phases at the fusion line between austenitic and ferritic material. Sometimes bending can be
a problem because of a failure in the tube hanging system. The last mentioned damage
mechanisms result in failure in circumferential direction.
3.2. Outlet component assemblies
The damage mechanisms of outlet components (manifolds, T-pieces, cones) are generally
much simpler than that of reformer tubes, because the outlet components are not subject to
firing conditions. Thermal gradients across the tube wall are not significant and do not cause
thermal stresses.
The main damage mechanism for outlet component is hindered thermal expansion. The
outlet system cannot expand (or shrink) freely and causes Low-Cycle Fatigue (LCF)
problems during start-up and shut-down. Very often, there is an interaction with creep,
because of the long hold-times involved. The damage starts at the outer diameter and
concentrates near the welds. The final rupture occurs in circumferential direction.
Another damage mechanisms is creep under internal pressure resulting in diameter increase
and creep damage (cavitation) at the outer diameter. Final rupture occurs in longitudinal
Sometimes, Ni-base welding consumables cause problems because of the difference in
thermal expansion coefficient. The Ni-base alloy has an expansion coefficient of 2/3
of that of the base metal. This may cause a problem by hindered thermal expansion.
4. Influence of materials choice on reformer tube assemblies
By increasing the materials creep strength the tube wall thickness of the reformer tube can
be decreased. This saves money by :
- decreased thermal gradient over the tube wall resulting in :
- increased tube life
- decreased fuel costs
- increased resistance to thermal shock
- extra catalyst volume (if the OD is kept the same) resulting in :
- increased output and/or increased efficiency
Sometimes the end-user decides the leave the dimensions unchanged. The stronger alloy
will permit the new tubes to operate at higher pressures and/or temperatures than before.
The minimum sound wall thickness (MSW) of the reformer tubes can be calculated by
simple equations, such as the ones given in API RP-530. This takes account of the internal
pressure stresses, but not for the thermal stresses caused by start/stop-cycles. The equations
given in API RP-530 are used here as a guideline. Further optimisation of the MSWcalculation can be obtained by taking the thermal cycling stresses into account (see
paragraph 5.1). This subject is out of the scope of this paper.
The MSW is given by the equation :
Here is :
P · OD
-------------- + CA
( 2 · Sa ) + P
design pressure [MPa]
outer diameter [mm]
allowable stress [MPa]
corrosion allowance [mm]
Some operations require a corrosion allowance (CA), but generally this is not applied.
The allowable stress (Sa) is defined in API RP-530 as minimum 100,000 hours creep
rupture stress. The minimum is 80% of the mean stress or the lower limit of the 95%
confidence interval (if available).
Using this equation for the alloys of interest, the MSW can be calculated for each alloy
by taking the OD constant. Thus the ID and the catalyst volume increase and the tube weight
decreases. As an example, two scenarios are presented with HK40 as a base case (100%).
Case A : Ammonia reformer : P = 4.0 MPa ; T = 925°C ; and OD = 114.3 mm (4")
Case B : Methanol reformer : P = 2.0 MPa ; T = 975°C ; and OD = 101.6 mm (3½")
The results are given in table 3 and figure 1. As can be observed in these tables and graphs,
considerable improvements can be achieved. For instance, by changing from HK40 to HP
micro-alloy the catalyst volume can increase by more than 50% ! Wall thickness and tube
weight also reduces significantly. For new reformer tubes the strongest alloy (HP-MA) is the
primary choice. For re-tubing of existing furnaces also HK-MA could be an attractive
Table 3
Influence of tube materials on MSW and other parameters
Case A : Ammonia reformer : P = 4.0 MPa ; T = 925°C ; and OD = 114.3 mm (4")
Case B : Methanol reformer : P = 2.0 MPa ; T = 975°C ; and OD = 101.6 mm (3½")
Sa *
tube weight
Sa *
tube weight
Influence of tube materials on MSW and other parameters for case A
MSW (mm)
increase (%)
decrease (%)
Sa (MPa)
Figure 1
source : S+C
5. Life assessment
Any life assessment procedure should take the relevant damage mechanisms into account.
5.1. Reformer tube assemblies
Tube life is primarily limited by creep, driven by a combination of internal pressure and
through-wall thermal stresses that are generated during start-up cycles and operating
transients. Creep life exhaustion is evidenced by progressive grain boundary cavitation
which, due to the significant influence of thermal stresses generated during operating
transients, initiates within the tube wall towards the bore.
Thus, the life assessment technique used, should take both loading mechanisms (steadystate creep and thermal cycling) into account. In the past some attempts were made to
incorporate the influence of start/stop-cycles [ref.4,5] , but they were not successful.
Later attempts were made by DNV [ref.6], MPT [ref.7], and ERA Technology [ref.8].
ERA Technology has developed a model and a software program called REFORM. The
model has many specific features to make the model work. These features are described here
shortly, but more extensive (and scientific) background can be found elsewhere [ref.9-11].
S+C and ERA have made a collaboration agreement to serve reformer tube users.
Materials creep behaviour The simplest established materials model, that simultaneously
predicts strain and damage with time, is a continuum damage mechanics model developed
by Kachanov and Rabotnov. It reflects the high stress start-up situation as well as the lower
stress steady state regime.
Creep life consumption A strain based life fraction rule is employed here, which is more
realistic than a time based approach. Given the evidence of samples taken from service, see
figure 2, which shows that multiple, parallel cracks are formed, all of similar length.
Therefore, a damage front propagation model is used rather than creep fracture mechanics.
Figure 2
Cross section showing
damage front propagation
Input for the model The REFORM model needs design and operating input data. The
operating data need to be available as a function of time. From experience it is known that
some parameters have a large influence on the calculated life. Specially tube skin
temperatures, and initial dimensions (diameter and wall thickness) have a large influence.
If operating data are available from digital control systems or advanced data loggers, this
greatly improves the speed at which the REFORM-analysis can be performed.
Probabilistic procedure In order to obtain a realistic life prediction for a particular unit,
a probabilistic treatment is employed. Statistical distributions of all input variables are
determined and sampled using a Monte-Carlo method. The results can be presented as CPcurves (Cumulative Probability curves) for time-to-crack initiation, time-to-failure, time-to
reach a certain strain level, and as time to reach a certain damage level.
Simulation The greatest advantage of the REFORM model is that it allows simulation of
changes in operation before the change is actually effected in the plant. This can save much
tube life and thus availability of the furnace.
An example of the results of an assessment by REFORM is presented by a Cumulative
Probability (CP-)curve for crack initiation and tube failure (see figure 3) [ref.12].
The operating hours at the moment of assessment was 215,000 hours. Each small line
represents one year of operation. It can be observed that crack initiation had already
occurred, but that tube failure is not to be expected within a short period of time :
- the first tube is expected to fail after another 50,000 hours (~6 years);
- 10% of the tubes are expected to fail after another 200,000 hours (~25 years); and
- 50% of the tubes are expected to fail after another 400,000 hours (~50 years).
From these results it is clear that –with similar operating conditions in the future–
replacement of the reformer tubes are not to be expected for a long time. Furthermore,
with proper inspection techniques the reformer tubes can be replaced on schedule.
Figure 3
CP-curves for time to crack initiation and tube failure
Cumulative Probability
Initiation Time
95% Confidence Interval
Failure Time
95% Confidence Interval
Current Operational Hours
Subsequent Years Of Operation
Total Service Time (Hours)
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In some particular cases the combination of process conditions, tube dimensions and
material properties appear such that compressive effects in the outer part of the tube are
sufficient to dominate over tensile effects towards the tube bore. In such cases the
compressive stress field at the OD “wins” from the tensile stress field at the ID. This
causes that the OD decreases, which should not be seen as “negative creep” [ref.12].
With the above mentioned results of ERA's REFORM model recommendations can be
made about :
- advised future inspections (inspection moment, frequency and moment of inspection,
and inspection method) ;
- advised availability of spare parts
- availability of the catalyst tubes and the reformer furnace as a whole.
5.2. Outlet component assemblies
Remaining life calculations of outlet components can be done by an inverse design
procedure using actual material properties and actual service conditions (internal pressure,
metal temperatures). In most cases this works well; however, in some cases also axial
stresses have to be taken into account.
For outlet components such as manifolds a deterministic approach is suitable. For many
similar components, such as pigtails, a probabilistic approach can be used [ref.10].
6. Inspection stategies
Any inspection procedure should take the relevant damage mechanisms into account.
6.1. Reformer tube assemblies
The damage mechanisms indicate diameter changing and creep damage (starting at the inner
diameter). It is important that the inspection techniques used detect these diameter changes
and creep damage. Several inspection methods are commercially available for reformer
tubes assemblies. These include ultra-sonic, eddy-current and dimensional measurements
on a crawler unit traveling up and down the reformer tube.
The ultra-sonic measurements are performed from the outer tube diameter and are based
on sound attenuation. The ultra-sound travels through the most damaged area, which are
the burner sides. The signal is transmitted into the tube by a transmitter and received by
a receiver. Creep damage in the tube results in sound attenuation, that can be measured
quantitatively. The angles of the in-coming and out-going sound are critical to the success
of this method.
A TOFD-based ultra-sonic back-scattering technique has been developed as well, and it is
claimed that this TOFD-based technique suffers less from false-calls due to the influence of
a bad surface condition giving the impression of creep damage.
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Eddy-current measurements can be performed inside and outside. The method is based on
the principle that an electro-magnetic field is induced by the test coil. If, for instance creep
damage is present, the magnetic induction changes and generates a counter electro-magnetic
field, which is detected by the measurement coil.
EC is less sensitive to creep damage located towards the inner tube diameter, because the
primary magnetic field has a limited penetration depth. Also, EC-measurements suffer from
changes in the magnetic properties of the alloy, such as the oxide layer, the de-carburised
zone at the outer diameter, the presence of σ-phase, and carburisation.
The EC-technique can be used more reliably from the inside. The magnetic field is then
more close the most damaged area.
Dimensional measurements can be performed from inside and outside. As a general
guideline, HK40 is thought to be at the end of it’s service life when 1-3% diameter increase
has occurred; for HP-materials this is about 5-7%.
Automated measurements of the outside diameter along the tube length are offered
by various companies. Chiyoda argues that the outer diameter is not suitable to determine
diameter increase by creep [ref.13], because of oxidation of the flue gases. Consequently,
Chiyoda recommends that the inner diameter should be used to determine diameter increase.
Another explanation for decreasing outer diameters has been provided by ERA and DSM
[ref.12]. However, also here it was recommended that diameter increase could be monitored
easier using the inner diameter.
Diameter measurements from the inner diameter can be performed by using laser
profilometry. Apart from diameter measurements, defects such as pits, cracks and
manufacturing defects can be detected. Internal diameter measurements can also be
performed by capacitative displacement measurements.
In summary, there are many inspection techniques available from various inspection
companies that can detect and quantify the effects of the damage mechanisms (diameter
increase and creep damage). These are available on crawler units that are able to travel up
and down the reformer tube. Some inspection techniques are available from the outside,
some from the inside. Each inspection technique has it's specific advantages and
disadvantages, also on removal of the catalyst. These advantages and disadvantages are
well known by S+C and GMS, therefore they can advise the end-user to choose the best
inspection technique(s) for his situation.
A complementary approach where inspection results can be implemented in a life
assessment model, can be of great benefit to the end-user. S+C and GMS can help
the end-user in such a complementary approach and in the decision making of the
relevant inspection strategy, including method, frequency and moment of inspection.
- 12 -
6.2. Outlet component assemblies
The relevant damage mechanisms for outlet assemblies indicate diameter changes (by creep)
and cracking by hindered thermal expansion. It is important that the inspection techniques
used detect these.
The diameter of the manifold can be measured by strapping or with a calliper rule along the
tube length. By doing these measurements at regular time intervals (e.g. every turnaround) a
trend can be established. As an end-of-life criterion a diameter increase of 5-10% can be
taken for low-carbon alloys such as wrought Alloy 800H and cast 32/20+Nb.
Thermal expansion damage occurs mainly at the welded joints. These welds should be
inspected by liquid penetrant testing (LPT); however, special care should be taken by proper
surface preparation. Since each reformer design has another outlet system design, no general
guidelines for LPT-inspection can be given. An example is given for a typical “hot”
manifold. The welded joints between manifold and T-piece and between T-piece and cone
are critical and should be inspected, see figure 4.
Figure 4
Outlet component assembly and advised inspection items
(example for a "hot" manifold)
diameter measurement
LPT (liquid penetrant testing) of all the manifold and Tee- butt welds
LPT (liquid penetrant testing) of 10% of the sockolet welds
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7. Conclusions
The state-of-the-art regarding materials for reformer tube assemblies and
outlet components has been presented. There is a trend towards the use
of micro-alloys for both reformer tube assemblies (HK-MA and HP-MA)
and outlet components (HP-LC-MA).
Compared to HK40 much smaller wall thicknesses can be obtained by using
HP micro-alloy. Therefore, HP-MA is the primary choice for new reformer units.
This enables higher catalyst volumes (up to 50% more) and lower tube weights
(up to 40% less). HK-MA can be low cost alternative to HP-Nb for re-tubing
of existing reformer furnaces.
The main damage mechanism for reformer tubes is the combination of thermal
stresses across the tube wall and internal pressure stresses. This combination
causes that creep damage typically develops at the inner diameter or just below
the ID surface. The damage occurs at the location with the highest thermal loading
and final rupture occurs in axially.
The main damage mechanism for outlet components is hindered thermal
expansion. The outlet system cannot expand (or shrink) freely and causes LowCycle Fatigue (LCF-) problems during start-up and shut-down. The damage
concentrates near the welds and final rupture occurs in circumferential direction.
Besides the above mentioned main damage mechanisms many other damage
mechanisms may limit the life of reformer tube assemblies and outlet components.
Any life assessment and inspection procedure should take the relevant damage
mechanisms into account.
For reformer tubes the REFORM life assessment model is capable of predicting
tube life. Inspection techniques are available on crawler units to detect diameter
increase and creep damage along the tube length. Such inspections can be
performed both from the outside and from the inside of a reformer tube.
For outlet components inverse design procedures can be used by using actual
material properties, actual service conditions (internal pressure, metal
temperatures) and eventually axial stresses. The diameter of outlet components
can be measured easily. Hindered thermal expansion damage occurs mainly
the welded joints. This can be inspected by liquid penetrant testing (LPT); special
care should be taken by proper surface preparation.
A complementary approach where inspection results can be implemented in a life
assessment model, can be of great benefit to the end-user. GMS and S+C can help
the end-user in such a complementary approach and in the decision making of the
relevant life assessment and inspection strategy (including method, frequency and
moment of inspection).
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The authors would like to thank the inspection companies and ERA Technology who
provided information about their respective inspection techniques and their life assessment
model. Both S+C authors would also like to thank their management for permission to
publish this paper.
Rob Gommans is working as an independent metallurgical consultant under the name of
Gommans Metallurgical Services (GMS) in Stevensweert, NL.
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