Hot Tapping in Oil Refineries: Corrosion and Material Concerns

1 Introduction

Hot tapping aims to add or make new connections in an operational pipeline or equipment without disrupting the usual process operation. Therefore, in hot tapping, process equipment is hot tapped as the operation continues. Usually, hot tapping is not desirable as it makes the welding process complicated, but certain situations arise where the process cannot be stopped to perform the welding process. In these cases, hot tapping becomes inevitable. Hot tapping is regulated by certain safety protocols such as API RP 2201 Safe Hot Tapping Practices and ASME PPC-2 protocol. Despite this, certain concerns are associated with the hot tapping procedure including corrosion and material concerns. Hot tapping is a safe procedure if carried out according to the safety protocol. But in some cases, hot tapping is carried out in critical processes such as in refineries where a reactor or a pipeline working on flammable material is hot tapped. In such cases, the chances of corrosion or cracking of the oil/gas are present. Other such cases where corrosion and material concerns are present in hot tapping include taps of containers containing sour water and low alloy steels.

2 The need for hot tapping in refineries:

According to OSHA standards, hot tapping in oil refineries should be performed only when the following conditions are met:

1. The refining process cannot be stopped.
2. The shutdown is impractical.
3. The process is documented and all safety protocols are met.

In case any of the following standards are not met, hot tapping in refineries cannot be performed and alternate welding methods should be carried out.

3 Concerns about hot tapping in oil refineries

Due to difficult shutdown, hot tapping is a common process in oil refineries these days and it is performed routinely. However, some concerns are still relevant and need to be addressed. To avoid these concerns, the following three areas should be carefully addressed:

1. Development of a safe design.
2. Avoiding any problem during hot tapping and any after effect of it.
3. Make sure that the hot tapping process has acceptable corrosion and material resistance.

Figure 1. Hot Tapping is being performed

4 Hot tapping in oil refineries – corrosion and material concerns

Oil refinery industries have specific and unique considerations for hot tapping procedures. In oil refineries, when a hot tapping procedure is performed certain limitations and special requirements are added to the process which is not present in the other hot tapping procedures. These special requirements in the oil refineries are based on the following conditions:

4.1 Process Conditions

1. Flammable, exothermic, decomposing, and explosive material in the pipeline or equipment.
2. A process that is expected to cause environmental cracking includes stress corrosion cracking, stress due to hot tapping, or high hardness in hot tapping.
3. Very low or very high temperatures.
4. A system in a vacuum.

4.2 Material conditions

1. Materials that require post-weld heat treatment (PWHT).
2. Stainless steel cladding, weld overlay, lined refractory.
3. Special welding materials such as duplex SS.

5 Detail of the material and corrosion concerns

Following are the different concerns related to corrosion and materials involved in the hot tapping process in the oil refineries.

5.1 Flammable services

Flammable services have a major concern for hot tapping procedures. These include the involvement of oxygen, air, and gas mixtures, air services in the presence of lubricants or other oil mixtures, and the involvement of materials such as ammonia, acids, chlorine, and sulfur in which heat involved in welding can cause decomposition of the stock gases/oils. Moreover, as oil refineries carry exothermic hydrocarbons such as propylene and xylene, they can decompose due to the high temperature of the hot tapping process. To reduce the present risk of air ingress, the flair lines must be reviewed to rule out the presence of any trapped air before the hot tapping procedure is carried out. In case air is present, nitrogen could be purged in the system to remove the resident oxygen.

5.2 Environmental cracking during hot taps

Hot taps should be avoided where materials such as amine, carbonates, hydrofluoric acids, and other acids are used. In these cases, PWHT is usually required. Cracking is caused due to high temperature and internal stress on the surface. This cracking could result immediately after the welding process. When hot tapping is performed without PWHT, cracking chances are increased. The material conditions when cracking can occur after hot tapping is given in the material safety standards such as API RP 571 and NACE SP0403. In addition to these, other materials may also be prone to hazards during hot taps. These are but are not limited to nickel-copper alloys, austenitic SS, and chlorides. In case these materials are present, hot taps should be avoided.

Figure 2 Corrosion Challenge in Hot Taps

Environmental cracking could also happen due to a high-temperature hydron attack (HTHA) before performing a hot tapping procedure at the oil refineries. Therefore, before using the hot tapping process in such a situation, a complete review should be carried out to rule out the chances of HTHA. To perform a review, the existing pressure and temperature conditions could be matched with the safety guidelines mentioned for HTHA in API RP 941. However, in most cases, the chances of HTHA due to welding heat are negligible as it has a specific incubation time in which the favorable conditions for HTHA are developed.

The presence of hydrogen sulfide in the pipeline can also cause cracking and degradation if hot tapping is performed. Pre-existing H2S blisters could poop up when the tap is performed. To avoid these blisters, a prior inspection could be carried out. Sulfide stress cracking is also a similar issue that can arise during a hot tapping procedure. This type of cracking is more prone in pipelines and vessels. Several hot tapping modeling software is available that can help in the identification of this type of cracking before performing the actual operation. Hot tap modeling requires parameters such as thickness, flow rates, temperature, and heat capacity.

Another type of material concern is stress-oriented hydrogen-induced cracking (SOHIC). This type of cracking occurs in weld stress zones. To avoid SOHIC, after the welding has been carried out, a PWHT could be performed. This results in lower stress from welding and reduces the risk of cracking.

5.3 Low and high-temperature service

It is challenging or impossible to hot tap lines that operate at lower temperatures because, even with preheating, the minimum temperature for welding of 10°C (50°F) may not be able to be reached. There is a significant risk of hydrogen cracking while welding at lower metal temperatures. All hot tap welding should be done with low hydrogen electrodes, which help to lessen the chance of cracking at lower temperatures but do not completely prevent it. Depending on the stock temperature, heat capacity, flow rates, etc., and the number of heating coils utilized, it may be possible to achieve preheat to 10°C (50°F), or above the ambient dew point if it is lower than 10°C (50°F).

Before beginning to weld, the preheat should be measured. If the desired preheat cannot be accomplished, either more insulation and coils can be added while the temperature is still being monitored, or the hot tap attempt should be given up. Welding is once more impractical at high temperatures, and safety considerations exist for shielding the welder from hot surfaces. The hot tapping contractor will decide on a case-by-case basis what the high-temperature restriction is.

5.4 Vacuum Services

Under vacuum conditions, hot work on the piping or equipment may not be appropriate because the heat from welding could cause the pipe or vessel wall to buckle, collapse, or let oxygen (and possibly flame) into the vessel. Before performing a hot tap on vacuum services, API RP 2201 mandates that a particular engineering analysis be performed to assess these risks. Numerous operating businesses forbid welding on active or potentially vacuum-functioning pipes or equipment.

5.5 Post-Weld Heat Treatment to avoid material concerns

Hot tapping is still not advised for other hydrocarbon services without hydrogen because the welds won’t receive PWHT. Hot taps, though, have been used in some carefully planned situations in the past, but they were only meant to be “temporary.” One important aspect was that the high hardness weld was protected against hydrogen cracking and/or brittle fracture as long as the welds were maintained at the service operating temperature. PWHT should be carried out after the initial shutdown after the system has cooled (with the pressure being lowered throughout the cooling phase). Until the shutdown, the owner must make sure that the weld is kept above at least the alloy’s usual preheat temperatures. To guarantee the hot tap is treated at the following shutdown, special tagging and documentation should also be made (planned or unplanned)

5.6 Material concerns related to lined steel, weld overlays, organic coating, or refractory

One issue with hot tapping coated pipes or containers is that even if the new attachments are made of higher alloys, the bore of the cutout area, where the coupon or cookie was removed, would leave bare carbon steel exposed to the process. Concerns about corrosion, cracking, or exposure to temperatures over design can result from this. There are circumstances in which these worries can be resolved and/or temporarily accepted, but this should be done case-by-case with a comprehensive engineering examination. The corrosion rates can be estimated (with a safety margin) to establish a service duration restriction if the only corrosion rate issue is high corrosion rates on the exposed carbon steel. The hot tap needs to be marked as “temporary” and properly documented. The hot tapped item must be turned off and a proper repair must be completed within the allotted time frame.

The heat from the hot tap welding will likely cause the lining of organic linings (such as integrally bonded epoxies) and other lining materials to deteriorate on the internal surface across from the hot tap welding (including across from the fillet welds at the borders of the hot tap reinforcing). The base metal will likely no longer be shielded from use and be vulnerable to moderate to high rates of corrosion. To establish a service duration restriction as previously discussed, the corrosion rates should be predicted (with a safety margin). Only non-hazardous utilities, like water, normally permit hot tapping of components with organic linings.

If the organic liner is “loose”—that is, not completely attached to the base metal—hot taps shouldn’t be used. The risk of the loose liners—organic or metallic—distorting during cutting and maybe getting caught in the hot tap cutting machine is very significant. Additionally, if an inner liner perforation is established, the stock could go well beneath the liner with a significant danger of increasing corrosion or other degrading processes.

5.7 Material with special welding requirements

Hot tapping should typically not be permitted and should only be done after a thorough technical review on an individual basis for materials that have unique welding needs that would be challenging to regulate with hot tap welding. One instance is duplex stainless steel, which has specifications for minimum and maximum heat input as well as interpass temperature limits. These specifications are easy to meet with conventional welding, but hot taps may make them impractical or impossible to meet, depending on the stock temperature, flow rates, heat capacity, etc.

Figure 3. Material and corrosion concerns in hot tapping

6 Final Inspection of Hot Tapping

The hot tap welds should be inspected in accordance with the relevant codes and standards, the Owner’s specifications, and the hydro testing that is normally necessary (provided that the safety precautions indicated in API RP 2201 are followed). Visual inspection, magnetic particle testing (for magnetic materials), and dye penetrant inspection ought to be the absolute minimum. This inspection must be performed on both the root and cap sides of branch connection welds.

The required inspection must frequently be performed at least 24 hours after the welding is finished if there are concerns that the steel may be charged with hydrogen and may be susceptible to delayed hydrogen cracking (during this time, no cutting or actions that restrict access to the weld shall be taken). Depending on their sizes and configuration, some branch connection welds can also be tested using ultrasound. Although it is usually impractical to test the weld heat-affected zones on production welds, hardness testing can be done on the weld deposit.

7 Summary: Hot Tapping in Oil Refineries, Corrosion and Material Concerns

When hot tap welding, the safety concerns should be carefully considered both during the welding process and during any following servicing. Burn-through and hydrogen cracking are the main welding safety issues for all services. Various industry standards, including API RP 2201 and ASME PCC-2, are covered as well as ways to evaluate and reduce these hazards. Whenever these dangers cannot be avoided (e.g. if the pipe is too thin), hot tapping should not be done.

There are many particular services and/or materials used in refineries that shouldn’t be hot-tapped, while others should only be done after performing a thorough engineering review and maybe being deemed “temporary.” There is a table of these unique products and services along with the arguments against them. Hot taps in stress corrosion cracking services or on Cr-Mo materials are a couple of the situations in which they may present concerns during subsequent service. To make sure that temporary hot taps are correctly examined, repaired, post-weld heat-treated, or replaced as necessary, at the following turnaround, they need to be labeled and included in worklists for the next turnaround.

8 References

Shargay, C., Daru, K., & Desai, J. J. (2017, July). Hot Tapping in Oil Refineries: Corrosion and Material Concerns. In Pressure Vessels and Piping Conference (Vol. 57991, p. V06AT06A078). American Society of Mechanical Engineers.

Shargay, C., Daru, K., & Desai, J. J. (2017, July). Hot Tapping in Oil Refineries: Corrosion and Material Concerns. In Pressure Vessels and Piping Conference (Vol. 57991, p. V06AT06A078). American Society of Mechanical Engineers.

Buller, A. T. (2009). Oil and gas technologies. In Technology Guide (pp. 318-323). Springer, Berlin, Heidelberg.

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Akselsen, O. M., Fostervoll, H., & Ahlen, C. H. (2009). Hyperbaric GMA welding of duplex stainless steel at 12 and 35 bar. Welding Journal, 88(2), 21-28.