Oct-2013

Reactor effluent air cooler safety through design

Quality-controlled replacement of carbon steel with Duplex 2205 for revamps can increase the service life and reliability of the REAC in the high-pressure loop

ERIC LIN and PETER RISSE
Chevron Lummus Global

Article Summary

In hydroprocessing units, the reactor effluent cooler (REAC) is one of the most vital pieces of equipment, and any hindrance to its smooth operation immediately impacts the whole high-pressure loop. Older REAC designs used carbon steel, but these required low concentrations of ammonium bisulphide and/or polysulphide sulphide injection together with frequent and thorough inspections. As feeds in most heavy oil hydroprocessing service have become more laden with sulphur and nitrogen, the concentrations of ammonium bisulphide with economic levels of water injection have risen to a point where carbon steel tubes have routinely been substituted by alloy tubing.

Duplex 2205 and Alloy 825 are used, with the former being very popular because it is relatively less expensive. Initially, there were several problems associated with Duplex 2205, which were a result of poor fabrication techniques. These included a rapid cooling rate associated with thick header boxes, which could result in high ferrite and thus poor corrosion resistance; welding of thick tubes to tube sheets with joint leaks; and lack of control of welding, resulting in high hardness and thus susceptibility to sulphide stress cracking. Many of these initial problems seem to have been overcome. However, problems persist around the REAC, primarily because of a lack of attention to detail and not adhering to licensor specifications. In this article, we will illustrate real REAC problems from recent projects and consider the remedies that were recommended

Background
In the hydroprocessing industry, the REAC is one of the most important components in the high-pressure recycle gas loop. It typically provides the final cooling solution before separating the vapour (recycle gas) from the oil effluent and the sour water. The outlet temperature directly impacts recycle gas molecular weight as larger hydrocarbon molecules drop out of the vapour phase. The same mechanism also affects the hydrogen partial pressure, which directly impacts reactor catalyst life. The importance of this piece of high-pressure equipment cannot be overstated. However, operating under high pressures and low temperatures can bring a host of issues into the equation, not the least of which includes ammonium bisulphide (NH4HS) and ammonium chloride (NH4Cl) precipitation (leading to pressure drop build-up), corrosion and/or erosion-corrosion. Some refiners have also experienced cracking in REAC welds, leading to fires, and loss of both time and money.

Tube metallurgy has a huge impact on a REAC’s life expectancy. Materials currently used in REAC systems include carbon steel, Type 400 series stainless steels, Type 300 series stainless steels, duplex stainless steel Alloys 3RE60 and 2205, Alloy 800, Alloy 825, Alloy 625 and Alloy C-276. Early refiners used carbon steel and this was found to be effective with NH4HS concentrations up to 3 wt%. As sour opportunity crudes became available, the resulting NH4HS crept up into the double digits. In cases where the resulting NH4HS was between 3-8 wt%, polysulphide injection was a very cost-effective solution for the continued use of carbon steel (especially for revamps). However, over time, the persistent plugging/operation problems, frequency of inspection and the foul smell of the polysulphide liquid greatly diminished its use. Increasingly sour feeds demanded more corrosion-resistant materials such as nickel-based Alloys 625, 800 and 825. Capable of handling up to 15 wt% NH4HS, Alloy 825’s corrosion resistance is normally matched by an equally hefty price tag. As the cost of materials continued to rise, a more economical alternative with comparable protection against corrosion was sought.

Duplex stainless steels are often successfully used in these systems because they offer advantages from both the ferritic and austenitic stainless steel families. They are often cost effective due to their higher strength and reduced alloy element content compared to other higher alloys (up to one-third the cost of Alloy 825). However, since these materials consist of dual-phase microstructure, heat-treating, fabrication and welding techniques need to be carefully reviewed and monitored to assure that the balanced microstructure is not compromised. In the past, Duplex 3RE60 was used, but it had inferior corrosion resistance and toughness at the welds (it is no longer available). The most commonly used grade today is Duplex 2205.

Early implementations of duplex REACs failed to show significant reliability improvements, and a few units failed by hydrogen embrittlement cracking or sulphide stress cracking (SSC). Advancements in steel manufacturing have minimised microstructural deterioration during fabrication. Weld procedures and practices have been developed to assure balanced ferrite and austenite content, thereby improving reliability. API TR 938C provides guidance on materials and fabrication practices to achieve good corrosion resistance in duplex stainless steels.

Case studies
Many of the reported incidents involving Duplex 2205 REAC failure have a common thread: pressures greater than 1000 psig and a NH4HS concentration of 6 wt% or greater. The following two recent case studies highlight these conditions.

A licensee in Asia (“Company A”) had a carbon steel REAC running for a number of years. An expansion of the unit required a REAC metallurgy upgrade to Duplex 2205. The design pressure of the REAC was ~2400 psig and the NH4HS concentration was expected to be 5 wt% during the modelling phase. A new Duplex 2205 REAC was installed and ran without issue for about two years, then fire erupted due to REAC failure, and nearby equipment and piping were damaged. Cracks were observed on the weld joints between the top plate and tube sheet, as well as the bottom plate and tube sheet of the floating header. Fin tubes were deformed and the walkway was nearly unusable. An investigation was commissioned to find the root cause of the REAC failure. The investigation discovered that during the REAC fabrication, Charpy impact testing was conducted at 0ºC instead of -40ºC (as specified by CLG). This oversight led to inadequate toughness and low ductility for the welds as well as less than favourable microstructural phase balance. Hardness values in the heat-affected zones (HAZ) and at the weld of failed specimens were higher than those recommended (in the range of 313-359 HV10 vs 310 HV10 maximum). As a result, CLG concluded that REAC failure in this case was due to sulphide stress cracking (SSC).

Another licensee based in the US (“Company B”) had a two-stage hydrocracker and hydrotreater ready for startup. The unit required a Duplex 2205 REAC. The design pressure of the REAC was ~2400 psig and the NH4HS concentration was expected to be 8 wt% based on modelling. During initial startup, a decision was made to skip the high-pressure tightness testing. A fire erupted from cracks formed in the REAC outlet temperature indicator thermowells in two separate trains. These cracks were found to be a result of improper manufacturing practices (of the failed duplex material) and would have been detected during a high-pressure tightness test. By not following standard operation practices, the unit startup was delayed.

REAC safety through design
Issues that plague the industry’s REACs can be mitigated or eliminated altogether by considering “Safety Through Design”. In preparing a basic engineering package for licensees, CLG highlights certain areas and design considerations that are considered strong recommendations. These considerations include:
• Using balanced, symmetrical piping
• Limiting tube velocities based on metallurgy, NH4HS concentration and H2S partial pressure
• Specifying fabrication and welding guidelines for Duplex 2205 above and beyond those specified in API TR 938C
• Using single-point water injection instead of multi-point water injection.