Acoustic Emission On-line Monitoring of the Ammonia Plant NII Secondary Reformer Exit (Gas Channel) Jan Tonheim and Reidar Tveit Hydro Agri Porsgrunn, Norway

In 1987 and 1989 the Ammonia Plant Secondary Re- former Exit line experienced severe cracking which de- veloped in the fusion line between the CrMo low alloy and the high alloy CrNi (Incoloy 800m materials. Tbe major failure mechanism was identflied as “hydrogen disbonding, ”due to combinations of high material tem- peratures, high weld stresses and too high cooling rates during plant shutdown. An undetected development of defects could not be tolerated, and after a review of a p plicable monitoring techniques, it was decided to install a four-channel PC based AE monitoring system. Tbe o b jective was on-line detection of developing defects in the weld area. According to in-house evaluation criteria the acoustic activity was verified by on-line (high tempera- ture) ultrasonic examination.

m e safety philosophy behind the project, the AE moni- toring system and the analysis programs, and the sum- marized experience from nearly two years of operation is presented here.

INTRODUCTION

Norsk Hydro is operating a 1200 T/D Humphreys & Glasgow Ammonia Plant at Porsgrunn, Norway. The plant was commissioned in 1968. Basic feedstock is methane and ethane.

In 1987 and 1989 the Ammonia Plant Secondary Re- former Exit line experienced severe cracking which devel- oped in a weld (fusion line) between the low alloy (2,5CrlMo)-and the Incoloy 800H materials.

The major failure mechanism was identified as “hydro- gen disbonding,” due to high weld material temperatures combined with too high cooling rates during plant shut- down.

TECHNICAL DESCRIPTION

The gas exists the secondary reformer at approx. 600°C. A water quench system reduces the temperature to approx. 530°C through the gas channel and downstream to the HP Steam Superheater. Operating pressure is 30 Bar gauge, hydrogen partial pressure is 9.0 Bar gauge (Figure 1).

The gas channel materials are Incoloy 800H and low al- loy carbon steel of type 10CrMo9 10 (2,25Crl,OMo). The

weld material is Inconel 112. The weld OD is 1390 mm, with average thickness of 30 mm. The gas channel is re- fractory lined, in order to reduce the weld material temper- ature to maximum 2500C (Figure 2).

THEORY-FAILURE MECHANISM

The above mentioned findings indicated that the crack had opened up during the cooling of the weld metal. It was concluded that one of the major failure mechanisms was

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I h

PRIM.REFORMER SEC.REFORMER HP STEAM SUPERHEATER

550 dsgr. C

FIGURE 1 Ammonia plant NII secondary reformer.

FIGURE 2 Gas channel details.

Process Safety Progress (~01.16, No.2) Summer, 1997 101

Threshold level

A - Peak Amplitude N - Number of counts per Event D -Signal duration (Event) R - Rise time

Parameters for characterization of an

FIGURE 4 Typical AE stressware. FIGURE 3 Cross section of cracked area.

associated with the hydrogen. The phenomena is called “hydrogen disbonding,” and takes place during cooling of the materials. Generally it might be explained by the differ- ence in hydrogen diffusivity between ferittic (1OCrMo910) and high alloy austenittic (Incoloy 800H) materials. At a given temperature, the austenittic material has higher hy- drogen diffusivity than the ferittic material. During cooling, the imbalance of hydrogen diffusion will result in an in- crease of hydrogen pressure on the interface between the two materials. Combined with very high stresses in the weld area, the result might be the cracking which was observed (Figure 3).

CONDITIONS FOR CONTINUED OPERATION

Following the 1989 findings, the Plant Management de- cided to repair the weld without major changes in the gas channel design or weld procedures, and then in due course plan for a permanent technical solution. 7he decision was however based on the condition that the weld could be in- spected/monitored for developing defects during operation of the plant, and that this monitoring could alert the oper- ator about an impending failure.

It was decided to use Acoustic Emission as monitoring technique, combined with Ultrasonic Examination of the weld at fixed intervals.

PRINCIPLE OF ACOUSTIC EMISSION

According to the ASTM Spec. E610-77, Acoustic Emis- sion is defined as the class of phenomena where-by tran- sient elastic waves (stress) are generated by the rapid re- lease of energy from a localized source or sources within a material. This could for instance be initiation and growth of cracks caused by fatigue, stress corrosion, hydrogen em- brittlement, etc.

Figure 4 shows the parameters which are utilized to characterize an acoustic stresswave. The threshold level is preset to filter out unwanted signals (normally background noise). “D” is called an Event and is a measure of the sig- nal duration above the threshold level. Totalling the num- ber of events during a period of time is a measure of the acoustic activity (or how a crack is progressing). Each Event has several measurable parameters. The duration might be used to separate real events from false (electromagnetic in-

terference) which have very short durations. The number of counts (“N”) for each event is equal to the number of times the signal rises above the threshold value. The total counts during a test period is a measure of the amount of emissions. “A’ is the signal strength (peak amplitude) and the rise time (“N”) is the time from the first threshold cross- ing to the peak amplitude. All the abovementioned param- eters give a quantitative measure of the amount of energy which is released, and is used to characterize a source. The signals can be detected by sensitive transducers, placed on the surface of the object. The signals are preamplified and then recorded by sensitive transient recorders and com- puter programs. The use of several transducers makes it possible to localize the signal source by comparing the sig- nal arrival times to the different transducers. The most uti- lized source location technique is “triangulation,” in which the transducers are placed in triangles. In order to locate a source, the signal must hit three transducers.

ACOUSTIC EMISSION ON-LINE MONITORING PHILOSOPHY

The most probable defect mechanism was hydrogen dis- bonding during cooling. One had to assume that this would generate very strong AE-signals. However, at this stage (1989), one could not rule out additional defect mecha- nisms during normal operation. Such mechanisms might also produce detectable AE-signals. From a safety point of view it was therefore strongly recommended to pwform continuous monitoring for possible identification of any development of defects. As possible defect signals were un- known, it was also decided to use Ultrasonic Examination to verify the results of the AE monitoring. This examination had to be carried out on-line, at surface temperatures up to 200°C.

DESCRIPTION OF THE AE-EQUIPMENT

The selected AE-equipment configuration was based on four transducers, two on each side of the weld, mounted to the gas channel via welded 240 mm stainless steel wave guides, which reduces the transducer temperature from 200 to 40°C. The transducers were connected to explosion proof preamplifiers (via standard 50 ohm coax-cables) close to the gas channel. The signals were transmitted about 150 me- ters to a Acoustic Monitoring System (located in the Plant

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Control Center), featuring an analog trigger unit, a four channel digitizing transient recorder, and an IBM PC/AT compatible computer.

The trigger unit, which also supplied power to the preamplifiers, filtered on frequency, signal level threshold and signal sequence, before the signal was sent unmodi- fied to the transient recorder.

MONITORING CRITERIA

To meet the monitoring requirements it was necessary to ensure maximum sensitivity of the AE-system since the de- tailed mechanisms were unknown. This meant that the threshold for detection had to allow acceptance of a rea- sonable number of noise signals. These noise signals had to be rejected during subsequent analysis. Criteria for fur- ther ultrasonic examination on basis of the AE-monitoring were:

Location within 250 mm from the weld, location concentration exceeding the average by a

short term location concentrations, and high amplitude signals.

factor of 5,

MONITORING EXPERIENCE

Prior to the 1989 plant startup the repaired weld was ex- amined/mapped by P-Scan Ultrasonic Examination, both to ensure the structural integrity of the welds and to serve as reference for the forthcoming periodic Ultrasonic Examina- tion.

The AE-system was commissioned and put into service during the pressure test of the gas channel. There was no particular correlation between AE-activity and pressure. The observed activity rate corresponded well with that observed at lower pressures, and could be classified as non-active. An active or critical active source would show linear or exponential development in the activity vs. pres- sure diagram.

Since the start of the Plant in June 1989, the AE-system was in continuous monitoring mode until the plant was stopped for a major shutdown in April 1991.

The AE activity was checked on a daily basis using the analysis program “Online source location” (Figure 5). Here the position of each event is marked. The amplitude range of each event is displayed by color codes. During weekly

analysis the location plots show a repeatable pattern, which was identified as “process noise.”

If there were significant changes in this pattern with re- spect to location, amplitude and event rate, then further data analysis were initiated. If the activity exceeded the ac- ceptance criteria, ultrasonic examination was initiated.

All transient data was stored, so in case of an actual de- velopment of defects, it was possible to reprocess the data with special filters, and hence more accurately indicate the source of emission. Data analysis was carried out on a weekly basis by one of the technical staff engineers (a non-expert), and took about 10 minutes each time. During the analysis it was soon established that the general noise emissions were caused by interaction between the gas channel shell and a suspension ring (Figure 6), as well as general emissions from the inside refractory. At three dif- ferent occasions both location and transient analysis indi- cated “cracklike” activity from the weld, but this was not found to be related to cracks/disbonding which was de- tectable by ultrasonic examination.

In case of a significant temperature drop in the reformer section (for instance as a result of a plant trip), the AE-ac- tivity pattern was checked out as soon as possible after the incident.

In April 1991 the plant was stopped for a major shut- down. During the cooldown of the plant the AE activity was closely monitored, due to uncertainty about thermal stresses, which eventually might cause active cracking. Ex- cept from “more than normal” randomly distributed “noise,” which was identified as activity from the inner lin- ing and the refractory, there was no special activity from the weld area. As previously indicated, high weld tempera- tures was one of the prime reasons for the cracking. Dur- ing the failure analysis in 1989 it was discovered that the gas channel was not supposed to be insulated. This was erroneously carried out during the 1987 major shutdown, without anybody noticing. The weld temperatures might have been as high as 500°C during normal operation. The actual stress in the weld exceeds the allowable stress for the weld material at 270°C and the yield stress at 450°C. Combined with the cooling during the plant stop, these conditions most certainly caused the weld to crack.

During the 1989 shutdown the gas channel external in- sulation was brought back to its original design, and the internal refractory design was improved (Figure 7).

FIGURE 5 Online source location.

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FIGURE 6 Source location, AE activity in weld area. High concentration.

Summer, 1997 103

FIGURE 7 Principle of gas channel internal refractory

After the 1991 shutdown, the AE project was reviewed. As the defect mechanisms had been identified, and reme- dial actions carried out, it was decided to conclude the AE monitoring.

CONCLUSION

The article describes a way to apply AE-technology to carry out on-line monitoring of a potential hazardous plant equipment. Even if the end result was “no cracks,” we be- lieve our monitoring concept was well worth the effort, based upon the following considerations:

The uncertainties about the failure mechanism and the integrity of the gas channel made it necessary to

come up with a monitoring program. The AE sys- tem was an important part of this program. The scope of the program was to monitor the installa- tion while further investigation was carried out. The program was accepted by the authorities (Directo- rate for Fire and Explosion Prevention). It was demonstrated how a simple four-channel AE-setup could be installed with the objective to observe a possible hazardous development in a closely defined plant equipment. By subsequent analysis the AE activity in the weld area was continuously monitored. A development of cracks would most probably have been detected, and then verified by Ultrasonic Inspection. After initiation by experts, the plant technical staff kept the AE-system running and performed the fairly simple daily and weekly analysis. A modem con- nection was established to the expert company in order to allow for remote advanced analysis and troubleshooting. This ensured a “low cost” profile not previously associated with AE.

LITERATURE CITED

1. Norsk Hydro A.S., Internal report. 2. Ringstad, O., and A. S. Robit, RAM System Opera-

3. Eriksen, M., and A. S. Robit, Akustisk Emisjons- tion Manual.

overvdking av gasskanal.

%his paper (26 was presented at the 41st Ammonia Safety S y m p sium held in Boston, Massucbusetts, September 9, through 12, 19%.

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