EVALUATION OF HTHA EFFECT ON CARBON
STEEL & CARBON 0.5M0 EXCHANGER USING
VARIOUS NDE TECHNIQUES FOR REFORMER
WASTE HEAT BOILER GAS EXCHANGER IN
HYDROGEN UNIT
By
Musaed Mohammed Al-Aradah
Mr. Mefleh Al- Otaibi
Mr. Mohmoud E. Moh’d
INSPECTION & CORROSION DIVISION
MINA ABDULLA REFINERY
KUWAIT NATIONAL PETROLEUM COMPANY
STATE OF KUWAIT
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Inspection Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Inspection Narrative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Detection of HTHA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Inspection Plan & Use of Advanced NDT . . . . . . . . . . . . . . . . . . . . . . . . . 6
Schematic Process Flow Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Schematic Diagram of E-18-205 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
LPT Photographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
The Advanced Ultrasonic Backscatter Technique . . . . . . . . . . . . . . . . . . . 14
Traditional Inspection Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
High Temperature Inspection Approach and Method Validation . . . . . . . 14
Schematic diagram of Inspection Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Data Acquired during TOFD Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Radiography Reports during Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . 28
Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
INTRODUCTION
HTHA has been observed in petroleum refining and
petrochemical equipment used in the environment with high
hydrogen pressures at elevated temperatures. This attack is
caused by the ingress of hydrogen into steel. Hydrogen atoms
react with dissolved carbons or carbides in steel and form
methane gas in accordance with the following chemical
reaction.
4H + C → CH4 or 4H + MC → CH4 + 3M (M: metals)
This gaseous methane forms voids mainly on grain boundaries
or inclusions, and the increase of gaseous methane pressure
in the voids may result in the formation of micro fissure,
blistering or cracking. The attacked material significantly
deteriorates its mechanical properties in tensile strength and
ductility, and finally causes catastrophic failures.
Depending on the combination and number of the above
variables, the hydrogen damage may be classified as shown
below:
•
•
•
•
Hydrogen Embrittlement
Hydride Embrittlement
Solid Solution Hardening
Producing Internal Defects (Cracks)
ABSTRACT:
Existing C-0.5Mo steel in hydrogen service is still our concern
in industries. High Temperature Hydrogen Attack (HTHA) has
been one of the major problems in petroleum and
petrochemical industry because of its effect. Since the original
Nelson Curves was suggested in 1949 to define the operating
limits for steels used in hydrogen service to avoid HTHA, a
number of research and investigation activities on HTHA have
been carried out around the world.
In USA, API summarized these data as Publication 941 –
“Steels for Hydrogen Service at Elevated Temperatures and
Pressures in Petroleum Refineries and Petrochemical Plants”
in 1970 and, since then, it has been widely used for material
selection in hydrogen service, operation and maintenance in
petroleum and petrochemical plants.
After completing the NDT activities the inspection results was
revealed that some difference between the original NDT result
report and onsite report
Inspection Outline
During hydrogen unit shut down E‐18‐205 (Waste Heat Boiler
Exchanger) was opened for Maintenance & Inspection.
This heat exchanger operates at high temperature in hydrogen service.
The 10” Ø bypass flue gas pipe made of C‐0.5Mo in this reformer waste
heat exchanger in hydrogen unit was found leaking while hydrotest of
the equipment during plant shutdown (Jan 2009). Subsequently, various
NDE techniques were used to determine the location and extent of the
crack on the base metal of the center pipe.
The dish end of the inlet chamber (Hot End) of the waste heat gas
exchanger is made of C‐0.5Mo material having refractory lining on the
internal surface. Due to high temperature operation in the presence of
hydrogen service, this material is subjected to HTHA (High Temperature
Hydrogen Attack). Hence to investigate that no more damage has been
done to the equipment & to conform the equipment is fit for the service
it is meant to be, the mechanical integrity of the equipment was
established by using various NDE techniques like TOFD, Phased Array,
In‐Situ Metallography and Hardness etc.
Key Words : NDE – Non Destructive Examination, HTHA – High
Temperature Hydrogen Attack, TOFD – Time of Flight Diffraction.
Inspection Narrative
E-18-205 (Waste Heat Boiler Gas Exchanger)
While carrying out shell side Hydrotest, heavy leak was
observed from the wide open crack occurred on base metal of
10” Ø by pass flue gas pipe at location near to the its west side
end between 4 and 5 O’ Clock position. Subsequent, visual
inspection and Liquid Penetrant Examination showed a 3”
long wide open crack 2” away from the west end of pipe and
also multiple cracks originating from the west end of pipe
which were extending inside the pipe over area of 2 sq in.
All the circumferential and longitudinal welds of exchanger
shell were subjected to TOFD (Time of Flight Diffraction)
additional advance ultrasonic technique is applied . During
TOFD inspection Phased Array Technique was applied , some
linear indications were reported in BFW shell circumferential
weld joint CS-3 having a approximate length of 1 meter.
Subsequent to Hydrotest of the shell side, this section of the
above mentioned circumferential weld was again subjected to
TOFD NDE in order to reassess its condition with respect to
any further deterioration due to high stress encountered
during Hydrotest. However, no significant deterioration to its
original condition was reported.
DETECTION OF HTHA
Material degradation caused by high temperature hydrogen damage
occurs in three distinct stages. During the first stage hydrogen reacts
with carbides located in the material leading to decarburization and the
formation of methane bubbles located at the grain boundaries.
With time the methane bubbles will lead to micro-cracks, stage two,
which, affect the mechanical properties of the material, these microcracks can propagate, stage 3 and may lead to failure.
Detection of HTHA is reliably performed non-destructively using
advanced back scatter ultrasonic techniques early during the stage 2
degradation.
The most cost-effective and often specified approach involves the
advanced backscatter ultrasonic technique which includes the following
and is appropriate for detection and assessment:
Backscattering
Velocity Ratio
Ultrasonic Spectrum Analysis
The advanced ultrasonic back scatter technique is based on the detection
and subsequent analysis of the backscattered ultrasonic signal. Clearly
as the size of the micro-cracks increase the amount of energy reflected
increases and the amplitude of the reflected signal increases
proportionally. As the micro-cracks develop deeper into the material the
depth of penetration can be measured and monitored. Interpretation
must however rely on the pattern rather than the absolute
backscattering amplitude in order to differentiate between HTHA and
inclusions and impurities
The primary detection technique is the pattern based back scattering
technique and involves the use of a normal UT flaw detector with a 0
degree, 1/2”, 5Mhz broadband, longitudinal wave transducer. Suspect
areas are scanned and the ultrasonic signal shape is observed for
evidence of “scatter” from the grains.
The purpose of the inspection technique is to reliably detect the presence
of micro-cracking as well as accurately and effectively measure and
report the depth of penetration such that this information can be used to
determine fitness for service and remaining life.
The exact technique deployed as the primary detection technique is
dependent on whether the parent material or welds are to be inspected.
None the less the primary detection tool is the advanced backscatter
ultrasonic technique supported by the velocity ratio and Fast Fourier
Transformation techniques both of which are used to support the
backscattering technique.
Inspection Planning and the use of Advanced NDT
Before putting the equipment back into service it was necessary to make
sure that no more damage has been done to the equipment & to
confirming the equipment is fit for the service it is meant to be, the
mechanical integrity of the equipment was established by using various
NDE techniques like TOFD, Phased Array, In-Situ Metallography and
Hardness etc.
An initial assessment of the vessel was carried out to identify susceptible
regions of equipment for inspection. It was decided to inspect a band
50mm wide on either side of all longitudinal, circumferential or nozzle to
shell welds, as well 6 locations with area of 1m x 1m subjected for
inspection by back scatter technique. Further to the above all
longitudinal and circumferential welds would also be inspected using the
Time of Flight Diffraction ultrasonic technique & Phased Array
Technique.
P-18-2305-CH1-6'’
FROM SULFINOL UNIT
P-18-2308-CH1-6'’
FED GAS TO R-18-201
V-18-206
STEAM DRUM
H-18-201
REFORMER
BY
VENDOR
GAS INLET FROM
REFORMERTHROUGH
TRANSITION CONE
(VENDOR SUPPLY)
P-18-2311-CHC1-8'’
FOR INSPECTION
14"
CONTROL
CONNECTIONS
R-18-201
P-18-2309-CHC1-8'’
STEAM & WATER
TO STEAM DRUM
V-18-206
8"
R-18-202B
R-18-202A
E-18-205
24"
MW
10"
10"
16"
TUBE BYEPASS
REFORMER
WASTE HEAT
EXCHANGER
24"
MW
EXCHANGER TUBES
HYDROGENERATOR
4"
2"
REFORMER EFFLUENT TO R-18-203
P-18-2312-CHC1-20"
14"
BOILER FEED WATER
FROM STEAM DRUM V-18-206
2"
ABSORBER
FlowSULFUR
Diagram
for
14"
2"
2"
SM-18-2303-C1-1½”
1½”
INTERMITTENT BLOWDOWN TO V-18-208
BB-18-2304-S1-3"
V-18-208
INTERMITTENT
BLOW
DOWN
DRUM
Schematic Process Flow Diagram
Schematic Diagram for Waste Heat Boiler (E-18-205)
Crack indications observed During Liquid Penetrant Test
ًُE-18-205
Cracks observed on Tube sheet to internal sleeve pipe weld of a Waste Heat
Exchanger of Hydrogen Reformer Unit
E-18-205
Cracks observed on Tube sheet to internal sleeve pipe weld of a Waste Heat
Exchanger of Hydrogen Reformer Unit
`
E-18-205
Cracks observed on Tube sheet to sleeve weld fusion area & face edges of
the sleeve as well as internal surface of the sleeve
E-18-205
Cracks observed on Tube sheet to sleeve weld fusion area & face edges of
the sleeve as well as internal surface of the sleeve
E-18-205
Cracks observed on Tube sheet to sleeve weld fusion area
E-18-205
Cracks observed on Tube sheet to sleeve weld fusion area
E-18-205
Cracks observed on face edges of the sleeve
E-18-205
Cracks observed on internal surface of the sleeve
Multiple transverse cracks observed on the sleeve
THE ADVANCED ULTRASONIC BACK SCATTER TECHNIQUE
The advanced ultrasonic back scatter technique is based on the detection and subsequent analysis of the
backscattered ultrasonic signal. Clearly as the size of the micro-cracks increase the amount of energy
reflected increases and the amplitude of the reflected signal increases proportionally. As the microcracks develop deeper into the material the depth of penetration can be measured and monitored.
Interpretation must however rely on the pattern rather than the absolute backscattering amplitude in
order to differentiate between HTHA and inclusions and impurities
TRADITIONAL INSPECTION APPROACH
The inspection options and approaches available to assess C-0.5Mo equipment susceptible to HTHA
have been documented in many areas, including API 5816. API 581 outlines several levels of
inspection effectiveness options for base metal and weld inspection. Base metal is often inspected
using ultrasonic back scatter (with 0 Degree compression wave) or by utilizing a suite of methods
sometime referred to as AUBT (Advanced Ultrasonic Backscatter Techniques). These methods have
been extremely difficult to implement at high temperatures because they depend on the detection of
micro-fissures and high frequency attenuation. Both of these parameters can be difficult to assess
during high temperature UT.
Welds may be inspected for HTHA using shear wave, TOFD, or radiography. Shear wave inspection
may be done with conventional pulse echo methods, or the more specialized approach known as ABSA
(Angle Beam Spectrum Analysis). In either case, working at 750oF (400oC) is considered to be
impractical. Radiography can be performed at elevated temperatures; however significant precautions
must be taken (e.g., insulating the film and/or large stand-off) to avoid heat damage to the film. Since
radiography is not ideally suited for detecting the early stages of HTHA, it becomes even less attractive
with these precautions that tend to degrade performance. TOFD has been used for elevated temperature
crack inspection. However, neither the ability of TOFD to detect initial HTHA, nor its ability to detect
small defects associated with HTHA has been widely published.
HIGH TEMPERATURE INSPECTION APPROACH AND METHOD VALIDATION
Based on the anticipated challenges of performing an HTHA inspection at 700F (370 C), Backscatter
spectrum analysis was utilized for the base metal inspection and TOFD used for the weld and HAZ
(heat affected zone) inspection. The ambient temperature application of the backscatter spectrum
analysis method is based on the attenuation increase of a backwall (ID) reflection7. It has been shown
that attenuation increase with increasing frequency is a function of scattering associated with HTHA
damage (e.g., micro-voids, micro-fissuring, etc.).
Data acquired from TOFD Inspection
Indication Showing
in 54.8mm
Indication
showing from
44.8mm up to
the Back
Indication
showing from
44.8mm up to
the Back surface
Indication
showing from
44.8mm up to
the Back surface
(linear
THE FINDING
The Internal 10 Inch Diameter internal pipe found to having different cracks at different locations.
Weld
Ident.
Total
Length
Scanned
Defect
Idnt.
1
CS-03
l
153
d
Defect Size in mm
H
a/t
A/l
47.1
11.2
.088
.037
Criteria
.020
5600mm
2
542
47.9
18.1
.141
.017
.020
Type
Slag
Mixed
with
Porosit
y
Slag
Mixed
with
Porosit
y
Welder
No.
Assessment
To be rejected
according to
ASME section
8 Division 1
W
N/
A
To be rejected
according to
ASME section
8 Division 1
Scan with 52 Degree TOFD probes with 125mm Space mainly focus on the half T and below
and 45 degree Phased array 128 Element Probe. Both TOFD and PH Reports attached.
Inspectors Comments / Remarks /Attachments (If any)
Total 2 scan covered 100% of the weld Length.
‘L’- Length of the defect,’d’- Depth, ‘h’- Height of the defect, ‘a’= height of the defect for surface flaws and
‘2a’ height of the defect for subsurface flaws.
Channels and Transducers settings
Channel –01 -52° -5 MHz 10mm -48 dB-- TOFD
1st Defect starting from 3045mm to 3198mm
2nd defect starting from 3341mm to 3883mm
4. CONCLUSIONS
1. Hydrogen attack is caused by exposure of steel to a hydrogen environment. The severity of the damage
depends on the time of exposure, temperature, hydrogen partial pressure, stress level, steel composition and
structure.
2. Hydrogen undamaged and damaged samples of steel used in plant equipment should be available for the
hydrogen attack testing purposes.
3. Recommended methods for detection of hydrogen damage are AUBT - Advanced Ultrasonic Backscatter
Techniques, methods based on TOFD, thickness mapping, backscatter and velocity ratio and in-situ
metallography - replicas. Results of methods like AUBT – Phased Array can be used for estimation of life of
hydrogen attacked equipment.
4. Non-destructive methods based on ultrasonics are able to quantify the hydrogen attack and estimate
mechanical properties of hydrogen-damaged steels. The results of such tests can be used in life assessment
calculations.
5. It would be recommended to review all NDT reports before receiving any new equipment specially which
designed for hydrogen Environment.
6. Spot NDT activity 1%-10% internal testing is recommended
7. Most of the sleeve cracking because cement refractory was damaged