nipping of reformer tubes. - kandla · 2018. 8. 22. · pigtails to respectively the in- and outlet...

Paper 2c

Nipping of Reformer Tubes. Precaution and Mitigation of Incidents.

Yara runs a 1450MTPD ammonia plant in Porsgrunn, Norway as feedstock for further downstream processing into nitric acid and NPK (Yara’s nitro phosphate process). Due to unforeseen circumstances the plant experienced in 2008 a frequent leakage of the reformer tubes into the furnace. In order to avoid a full stoppage and because we had the availability of external access, the practice of nipping of the pigtails of the reformer was applied. The nipping of the pigtails is exercised and performed under slightly reduced plant load. Certain routines were established to do the nipping exercise in the safest way, having no leakages to the surroundings, nor endangering the people involved. However during one nipping exercise an outlet pigtail ruptured and a fire occurred, fortunately without injuries. A team was established to investigate the safety routines as well as the pigtail material and nipping tools.

Rob Stevens

Yara Norge AS, Porsgrunn

Introduction

ara, the world’s largest fertiliser company owns production facilities world wide. Its production organization

is organised within the Upstream division, whereas the Downstream and Industrial divisions take care of the sales and logistics of the finished fertiliser and industrial products. Within the Upstream segment, Yara Porsgrunn produces NPK-grade fertilizer. Raw materials, like phosphate rock and potassium salt are imported, whereas the nitric acid (feedstock to Yara’s fully owned nitro phosphate process) is being produced from ammonia produced on site, with the balance being ammonia import. The ammonia plant dates from 1968 and is a Humphrey’s & Glasgow design, revamped in 1997 by Kellogg. The synthesis and CO2

removal sections are own Yara design. The feed is based upon LPG (ethane\propane\butane) mixtures, whereas the fuel gas can include carbon monoxide rich gas, from a neighbouring steel additive plant.

Photo 1: Picture of the NII ammonia plant at Yara Porsgrunn, Norway.

Y

71 AMMONIA TECHNICAL MANUAL2009

As a result from overheating the reformer during the start-up (excess feed-gas), after a turnaround, the plant experienced in 2008 a frequent leakage of the reformer tubes into the furnace. In order to avoid a full stoppage, as well as the availability of external access, nipping of the pigtails at the inlet and outlet of the reformer is applied at slightly reduced plant load. The on-line nipping of the pigtails is a common practice, also as a result of the reformer-tube exchange philosophy followed by Yara Porsgrunn. All routines are established to do the nipping exercise in a safe way, having no leakages to the surroundings, nor endangering the people involved. However, as a result of the high nipping frequency in 2008, the chances of a less perfect nipping apparently increased. During one nipping exercise the outlet pigtail ruptured and a fire occurred, fortunately without injuries. History reformer and tube exchange philosophy Yara Porsgrunn’ s ammonia plant dates back from 1968, where it produced its first ammonia. Now, over 40 years later, ammonia is still being produced. Although the raw materials became much lighter, a mixture of ethane and LPG (propane\butane), the reformer is still the same. The only things that changed are the reformer tubes and catalysts. The 372 tubes however, are not being changed following fixed turnaround schedules. Tubes are changed, depending on the NDT inspection results, reformer performance history as well as those, which have failed already. The eldest reformer tubes, which are still present in the reformer, date back from 1978. The newest tubes are those, which arrived in summer 2008. This variance in age, as can be seen in Figure 1, clearly describes the philosophy on how the reformer, or more specifically, its tubes are run: until end-of lifetime. Of course the highest number of tubes, which are exchanged, are

related to scheduled turnarounds, following the inspection and estimation on future performance.

number of tubes placed in year

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Figure 1: Number of reformer tubes plotted against the year of operation. The philosophy of taking reformer tubes out of operation during an on-line process is shown in Figure 2. The figure also indicates the number of blinded tubes (in red), which have been physically separated off-line from the inlet and outlet header during a reformer stop, following the same NDT inspection methods.

Figure 2: Nipped and blinded tubes since 1999. Figure 2 clearly indicates that on a yearly basis an average amount of ten tubes is taken out of the process, using the on-line method of nipping. On-line pigtail nipping is performed without adjusting plant load. Neither feed- nor fuel-gas is adjusted, as the amount of gas leaking out of a reformer tube (in operation) or

Nipped tubes as function of year

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1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

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72AMMONIA TECHNICAL MANUAL 2009

pigtail (during an incident) is determined by the feed gas pressure and size of the hole. The absolute gas flow in the tube or pigtail does not influence this. The philosophy, that the tubes can be taken out of operation during full production, can be followed as the reformer tubes in- and outlet are sufficiently accessible from the outside of the reformer box. The accessibility of the tubes is related to the fact that they are fixed with in- and outlet pigtails to respectively the in- and outlet header. Figure 3 shows a schematic drawing of a leaking reformer tube, its pigtails and the nipping process. The pigtails do have a much smaller diameter than the reformer tubes, making them suitable to nipping. ‘Nipping’ is the squeezing together of a tube, until it is completely flat and therefore restricts the gas flow to zero. However, this nipping is not a straightforward operation, and some ammonia plants have abandoned the ‘live’ method and prefer to shutdown the reformer to isolate a tube by blinding, completely removing or replacing it immediately. Successful ‘live’ isolation, ‘nipping’, of reformer tubes can only be done if it is technically feasible.

Figure 3: Schematic view of a reformer tube, pigtails and location in the primary reformer. In addition to the accessibility, the pigtails need to be of a quality that facilitates external applied deformation without creating a risk of additional

leakages. The pigtails’ specifications, like size, diameter and thickness should be uniform and consistent. Carburisation should be limited. The quality of the pigtails determines the risk of the nipping procedure. The safety around the activity needs therefore to be ensured through procedures, training, protection, confidence or experience of those who do the job as well as the nipping tool available.

Nipping, nipping tool and clamps

What is nipping? As indicated earlier, nipping is nothing more than pressing\squeezing a tube together until it is completely flat, resulting in an entire stop of the flow. As we deal with reformed gas, high temperatures and related quality of pigtail material, the nipping involves material knowledge, suitable tools and proper safety measures. To avoid a complete or partial shutdown, the NII plant in Porsgrunn was designed and built with externally accessible pigtails. The plant was delivered with a nipping tool (Figure 4) as well as a detailed instruction on how to perform a pigtail ‘nipping’.

Figure 4: Nipping tool (20ton). The nipping tool, a spare parts list as well as the nipping instruction were delivered by ICI, Billingham, during the commissioning of the plant in 1968. The original instruction describes in detail how to assemble the nipping tool, as well as how to secure the connection between the 20-ton nipping tool, its connecting hoses and


the hydraulic hand pump. Safety precautions are only mentioned briefly.

Figure 5: Nipping tool with clamps (4) and (5) and hydraulic connection (14). The nipping is to be executed in two phases. Each phase uses a different clamp (see Figure 5, items 4 and 5). The first phase is bending the pigtail tube wall sufficiently by a large-radius clamp (Figure 6). This is to enable the second and last phase of closing, mounting the final, short radius clamp (Figures 7 and 8), which closes the pigtail completely. The instruction already refers to the possibility of a one-phase nipping, using the large radius clamp only. This is a possible method to be used with better quality materials like ‘carbon or CrMo steel’. When applying the final clamp, the clamps on both sides of the flattened pigtail should be bolted together.

Figure 6: Long-radius clamp plate. The procedure also states that the nipping of a reformer tube should be executed first at the inlet pigtail to reduce the amount of process gas to the outlet pigtail, before nipping the latter. The details of the nipping clamps also were provided, including detailed drawings of the two clamps in use, the long and short-radius clamp. Figures 6, 7 and 8 do show parts of Yara modified drawings (5F.74475, 5F.74476 and 5F.77098). The long-radius clamp plate (Figure 6) is used for the first round of squeezing together the pigtail tube. For both the inlet and outlet pigtails the clamp plate radius is identical. The only difference between the in- and outlet clamp plate is the distance (x) between the two clamp parts, which on the other side is adjusted by different length spool pieces (not shown).


Figure 7: Short-radius clamp plate inlet pigtail.

Figure 8: Short-radius clamp plate outlet pigtail. The second-round clamps differ significantly from the first-round clamps and are so-called

short-radius clamps. As can be seen in the figures 7 and 8 the short radius is determined by the width, which is only 4 mm, in contrast with the long-radius clamp, which uses the full 25 mm width. The difference between the inlet (2nd. round) clamp (Figure 7) and the outlet clamp (Figure 8) is of course determined by the size of the pigtail tube. In addition does the outlet short-radius clamp have a smoothened curve instead of a rectangular corner.

Figure 9: Picture of the Yara Porsgrunn nipping tool, with a long-radius clamp mounted.

Figure 10: Picture of the Yara Porsgrunn nipping tool, with the short-radius clamp in place. The welding-wire is used as handgrip to position the spool piece. The pictures in Figures 9 and 10 show the head of the nipping tool with the clamps in it.


Procedures Since this first issue of the nipping procedure (according to ICI\Johnson Matthey, the procedure dates back to 1964), it has been revised many times. Historical documents, dated until 2000 indicate that, based on experience, the procedure has been modified, mainly focusing on the material choice of the pigtails. With respect to personal safety the original procedure ‘only’ refers to the use of gloves, safety goggles and working comfort (easy access platform and air fan). Later Yara revisions, however, are mainly related to the safety around the nipping as such, summarizing the original procedure, mechanical by origin, to a few bullet points. These mechanical bullet points or tasks are related to the nipping of the outlet pigtail, which is considered to be the most critical activity to take a reformer tube out-of-operation. This immediately indicates the main practical difference between the original procedure and the one in use now (8th revision since the beginning of this millennium). It is considered that by nipping the outlet pigtail first, the potential of a leakage at the outlet location reduces, which is a main difference from the original procedure. The reason behind this decision is that the temperature of the tube (and therefore its ductility) remains more or less constant during the whole activity. This ensures a better predictability of the execution of the nipping and therefore a better consistency in the preparations. Some Yara (related) ammonia plants do have less extensive and updated procedures, probably because of lesser experience in nipping. The latter might be related to a different philosophy in reformer tube experience or more homogeneous performance of the reformer due to the choice of feed and fuel gas. It is mentioned that a simultaneous nipping of the inlet and outlet pigtail is the safest. Others do have worked on further development as well.

Improvement of the clamps and methodology to avoid leakage during nipping is the main focus for further improvement. History in nipping failures in NII As shown earlier in Figure 2 the history is described from 1999 onwards. In the complete history of the plant, only four pigtail leaks have occurred during a nipping activity (including the one in 2008, which is described in the next paragraphs). Future expectations on the nipping frequency might be extrapolated from figure 1, which describes the age of the reformer tubes, taking into account reformer-tube lifetime data (based on reformer operations as well as inspections). Based on this information it is likely that the original average of below 10 nipped tubes per year might be achievable. After the July 2008 reformer tubes replacement 5 tubes have been nipped, matching these historic data. These are included in the 2008 data in figure 2. From the period before 2000 we do have three documented pigtail ruptures (of which two during nipping), which happened in 1986, 1996 and 2000. The main conclusions, also based on an examination of other nipped pigtails, are all related to the pigtail material or nipping clamps: 1986

- A to narrow gap of the nipping clamp, which resulted in stresses on the pigtail, leading to a fracture of the pigtail.

- A deviation on wall thickness of the pigtail from the purchase specifications.

1996 - Carburisation of the pigtail material.

2000 - New Incoloy 803 pigtail material, which

failed after nipping (to brittle). - 26 of these (carburisation resistant)

pigtails were exchanged in the next turnaround.

These findings have resulted in a more stringent material specification (Table 1) as well as an improved specification of the described nipping


clamps (see Figures 6 to 8). The verification of the latter has been included in the checklist for the maintenance operator as a part of the nipping procedure. Situation spring 2008 After the annual turnaround late 2007, the plant was started in December 2007. Due to an operational mistake during start-up the reformer tubes got coked, overheated and damaged. The plant was stopped early 2008 after a steam leak, during the reformer steaming operation. An evaluation and blinding of the worst tubes became possible and the plant was restarted. The production load was adjusted to keep the reformer at the lowest temperatures possible, waiting for the delivery of new reformer tubes. As more tubes reached the end-of-lifetime due to this overheating, the frequency of nipping increased drastically to a weekly average of 2 to 3, as can be seen in Figure 11.

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Figure 11: Nipping frequency on a specific date (blue) since turnaround 2007 to stop in July 2008. The monthly subtotal is indicated in red. The operational and maintenance staff managed to keep the plant running until one week before the scheduled ‘reformer tube replacement’-turnaround. A total of 42 tubes were taken out of operation in half a year by nipping (in

addition 8 tubes were blinded during un-scheduled stops during this half-year period). Nipping incident June 2008 On the 23rd of June it was attempted to take a leaking reformer tube out of operation. During the nipping of the outlet pigtail, it started leaking around the nipping clamp. In addition one of the bolts was broken. Under Nitrogen atmosphere it was tried to replace the bolt. However, the size of the leakage was easily noticed and it was tried to isolate the inlet pigtail first. In parallel gas-explosivity (EX) tests were executed at the outlet pigtail. After the isolation of the inlet pigtail all safety measures were checked once again (radio contact with the control room and the fire-fighters standby). When switching off the nitrogen to enable an accurate EX measurement, the gas ignited and the plant was manually tripped immediately. Nobody was injured and all personal protection functioned well (no signs of damage to the nipping dress, gloves or helmets). The plant equipment was undamaged. Root cause analysis An investigation team was established to investigate both the safety measures around the nipping routine as well as the pigtail itself and its leakage. Safety With respect to the safety, the nipping procedure (AMM-MEK-12) was modified again:

- 4 people instead of 3: in addition to the 2 persons executing the nipping it is now ensured that the person who operates the nitrogen is not involved with the nipping tool (pump).

- EX measurement will be done, at a distance from the pigtail, by using a lengthened pipe to take the gas sample


- During outlet pigtail nipping a water hose will be available (for personnel protection) in addition to the already present nitrogen and powder in use as fire extinguisher.

It has to be mentioned that the fire department with one fire-truck always is on standby, fully prepared, during a nipping operation. Communication lines between those who execute the nipping, and those who are standby as well as the shift leader are ensured through tested radio contact. Pigtail investigation The investigation compared the leaking pigtail (tube 142) with the neighbouring nipped pigtail (tube 143), which was nipped during the same period. The temperature at which the pigtails were nipped was around 720 °C, but can vary between 600 °C and 800 °C. Both the leaking and the reference pigtail are shown in the figures 12 and 13.

Figure 12: Nipped pigtail nr 142 and the leaking pigtail nr. 143. A cross-sectional cut from both pigtails was taken. As can be seen in the Figure 14 and 15, the leaking pigtail (142) has been sheared off.

Figure 13: Sheared pigtail (nr 142) to the right and reference pigtail (nr. 143) to the left.

Figure 14: Sheared pigtail (nr 142).

Figure 15: Reference pigtail (nr. 143).


The metallographic examination of both pigtails did not show any differences or abnormalities. A normal microstructure for Incoloy 800, with respect to exposure at elevated temperatures has been concluded. Incoloy 800 is supposed to be the right pigtail material at the operational temperatures. Normal (annealed) Incoloy 800 (Figure 16) exhibits higher tensile properties compared to the heat-treated variants (Figure 17) and should be used where creep is of less importance.

Figure 16: Mechanical data for Incoloy 800.

Figure 17: Mechanical data for Incoloy 800H/HT.

A higher creep resistance is achieved by a coarser grain structure, as a result from heat treatment. However, pigtail material should have a good ductility in the temperature range where the nipping takes place (700 °C). The elongation (Fig. 16 and 17) is a measure for the material’s ability to absorb the deformation during the nipping process. The lowest, worst value lies in this temperature range. Non heat-treated Incoloy is therefore recommended. In Table 1 (appendix) the material specifications of various Incoloy grades of different suppliers can be found. A review of the specifications shows that the content of aluminium and titanium may have a significant influence on the ductility in the temperature range where the nipping takes place. Two of the suppliers recommend, restricting the sum of aluminium and titanium to a maximum of 0.7%, when operating in the temperature range of 500 to 700 °C (932 °F – 1292 °F). Nipping tool optimization The investigation also focussed on the nipping tool itself. A computer model (by SWECO) calculated the stresses in the pigtail. Three different calculations were executed for nipping clamps as depicted in Figure 22.

Figure 18: Model of the stresses in the pigtail using the 2nd. -phase small-radius clamp.


Figure 19: New nipping clamp at start of nipping of a pigtail

Figure 20: First deformation of the pigtail

Figure 21: Final deformation and stresses in the pigtail.

Figure 18 shows the stresses in the pigtail, using the 2nd-phase small-radius clamp. The 1st-phase model is not shown. The figures 19 to 21 show how the new clamp deforms the pigtail. The calculated stresses in the pigtail during the final deformation are calculated to be less than 80% of the stresses when using the large and small radius clamps during the two phases of nipping.

Figure 22: Schematic drawing of the three different nipping clamps. Recommendations and Conclusions

- The failed pigtail’s (Special Metals) Aluminium and Titanium content is analysed to be 1.03%. The spare pigtails on stock (Sandvik) are specified to be 1%. It is recommended to specify the composition to a maximum of 0.7%.

- The nipping clamps design can be improved for one-phase nipping

- The nipping procedure and its safety aspects should continuously be updated, although the shift supervisor remains the key-player in the decision-making.


- In case of a burning leak, it might be the best to keep it burn, until the plant has been shutdown to avoid an ignition at a later stage.

- On-line temperature and\or external IR temperature measurement of the pigtails provides valuable information to evaluate a safe nipping.

References - Examination of pigtails after a failure during pinching, Norsk Hydro F-senter, 86D.B09, 1986 - Undersøkelse av sprukket pigtail fra ammoniakkfabrikk N-II, Norsk Hydro F-Senter, 96P.CW7 - Cracking of pigtail during nipping, Norsk Hydro F-Senter, 00C.CI9 - Nipping av utløps- og innløpspigtail i reformer, Yara, AMM-MEK-12, rev.3, Brynjar Svensen, 2008. - Yara Porsgrunn, Ammonia plant N-2, Pigtail Leaking During Nipping, 16.01.2009, Bernt Swensen, internal report - Yara Porsgrunn, Ammoniakkfabrikk N-2, Nipping av Pigtails - En Oppsummering, 13.02.2009, Bernt Swensen, internal report - Kurs i nipping av inlet og outlet pigtail, F0201, 11.03.2009, Brynjar Svensen, Yara intern notat - Nipping av reformerrør, SWECO, Olav Aakre, 6 mars 2009 - Johnson Matthey nipping training presentation. Acknowledgement Our sincere thanks go to the staff of the NII- ammonia plant at Yara Porsgrunn for safely handling the described incident, and to the maintenance team of BIS for the safe execution of all the past and future nipping. Special thanks go to: Mr. Bernt Swensen, materials specialist; Mr. Morten Kamperhaug and Mr. Kjetil Bakli, material inspectors; Mr. Bryjnar Svensen, maintenance planner and guard of the NII historic documents.


Table 1: Incolloy Specifications

UNS designation

N08800 N08810 N008811 N08880 N08800 N08810 N008811

Special Metals (INCO-Wiggins) Sandvik ThyssenKrupp VDM

Grade 800 800H 800HT Sanicro 31HT

3220LC 3320 3320H 3320HP

Nickel (%) 30.0-35.0 30.0-35.00

30.0-35.0 30.5 32.0-34.0

30.0-32.0

30.0-32.0

30.0-32.0

Chromium (%)

19.0-23.0 19.0-23.0

19.0-23.0 20.5 20.0-22.0

19.0-21.5

19.0-21.0

19.0-22.0

Iron (%) 39.5 min 39.5 min

39.5 min balance balance balance balance balance

Carbon (%) 0.10 max 0.05-0.10

0.06-0.10 0.07 <0.025 0.04-0.08

0.06-0.08

0.06-0.10

Aluminium (%)

0.15-0.60 0.15-0.60

0.25-0.60 0.5 0.15-0.40

0.20-0.40

0.20-0.40

0.30-0.60

Titanium (%) 0.15-0.60 0.15-0.60

0.25-0.60 0.5 0.35-0.60

0.20-0.50

0.20-0.50

0.30-0.60

Σ Al+ Ti (%) 0.30-1.20 0.30-1.20

0.85-1.20 1 <1.0 <1.0 <0.7 <0.85

ASTM grain size

Not specified

5 or coarser

5 or coarse

>5 >5 4-2 4-2

Heat treatment

(°C)

920-980 920-980

1150 1150-1200

Application temp. (°C)

<500 <600 600-950

700-1000

Carbon* (%) 0.08 max

Σ Al+ Ti* (%)

0.4-0.7


nipping of reformer tubes. - kandla · 2018. 8. 22. · pigtails to respectively the in- and outlet...

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