Inspection program evaluates HF-alkylation carbon steel piping

Leomar Peñuela, José Chirinos
Pdvsa Manufactura y Mercadeo
Judibana, Venezuela

In 1995, Petroleos de Venezuela SA (Pdvsa) implemented an inspection program in the hydrofluoric acid (HF) alkylation unit of its Amuay, Judibana Falcon, Venezuela, refinery. The program's purpose was to detect potential corrosion on carbon steel flanges and welds on critical lines.

That year, two failures in the 20,000 b/d HF alkylation unit at the Amuay refinery caused a shutdown of the unit. The failures occurred in one flange and one weld in the depropanizer-charge carbon steel pipe, which contained propane, isobutane, and anhydrous hydrofluoric acid (HF).

Examination of the flange revealed severe uniform corrosion at its inside surface while the adjacent elbow showed minimal corrosion loss. In the weld failure, preferential attack occurred in the weld without corrosion loss in the piping components.

A complete evaluation of the alkylation plant identified other lines in similar conditions that could cause future emergency shutdowns of the unit. The flange and weld corrosion problems in carbon steel piping components have occurred at relatively localized sections.

The principal factor that accelerated the carbon steel weld and flange-corrosion problems in HF service was the temperature. Of the 13 locations found with corrosion problems, 9 of them are in HF high temperature service, which is 150-200° F. or greater.

No clear correlation between copper, nickel, and chromium content and the corrosion mechanism for carbon steel welds and piping components was found.

More investigation is required to clarify the corrosion mechanism for flange and weld-metal loss in carbon steel components in HF service.

Unit background

The 20,000 b/d HF alkylation unit at the Amuay refinery started up in 1982. Fig. 1 [72,433 bytes] shows the process flow diagram. The unit is fed with olefin and isobutane to produce high-octane alkylate.

The process streams contain HF, organic compounds, and water (present as an impurity). Corrosion problems arise when the water content is above 1% and the anhydrous HF is dilute.

Carbon steel is the primary material of construction of alkylation units; Monel is used in the more corrosive areas in the unit. The specifications used for carbon steel are API 5LB, ASTM A-105, and ASTM A234 WPB for piping and fittings.

The two 1995 failures in the depropanizer-charge carbon steel pipe contained propane, butane, isobutane, and traces of HF operating at 190° F. and at 321 psig (22.5 kg/sq cm).

Evaluation of one failure revealed severe uniform metal loss at the inside surface of a flange while the adjacent elbow showed minimal metal loss. The other failure showed preferential attack in a weld without metal loss in the piping components.

A previous study¹ established that the corrosion susceptibility of carbon steel components in HF service is high when the sum of the residual elements, copper, nickel, and chromium is higher than 0.20 wt %.

A special on stream inspection program of this unit was performed to identify other piping with similar damage that could cause future emergency shutdowns.

Inspection program

The unit was divided into three sections for the inspection program:

1. Reaction section
2. Isostripper section
3. Depropanizer section.

All circuits in HF service and in hydrocarbons with trace HF (HC/HF) service in which water is present and that are subject to varying temperatures were included in the program.

The on stream evaluation consisted of the following components:

• Flange thickness measurements. The flanges were measured by ultrasonic tests (UTs) to detect general metal loss in the internal surface.
• Flange chemical analysis. The refinery attempted to quantify the amount of copper, nickel, and chromium in the flanges. This analysis required high precision to identify the corrosion susceptibility. The work was performed using a portable optical emission analyzer.
• Second flange thickness measurement. These measurements were done by UT to flanges which had a total copper, nickel, and chromium content higher than 0.40 wt %.
• Radiographic examination. This examination was conducted on welds to detect preferential metal loss on the internal surface of welds.

The results of the analyses are shown in Table 1 [193,086 bytes]. A total of 265 flanges were evaluated with UTs and three of those had significant metal loss. Of the 109 weld seams that were analyzed with radiographic examination, 10 seams with significant metal loss were identified.

The chemical analysis turned up less than 0.20 wt % total copper, nickel, and chromium content in 32 flanges, 0.21-0.40 wt % in 22 flanges, and 0.41-1.0 wt % in 14 flanges.

All the flanges evaluated by chemical analysis fit the ASTM specifications that established maximums of 0.40 wt % for copper, 0.40 wt % for nickel, and 0.30 wt % for chromium. The maximum sum content of these alloys is 1.0% (plus molybdenum).

The second UTs were performed for 25 flanges. No significant metal loss was found.

Metallurgical evaluation

Part of the circulating pump discharge pipe in the reaction section that showed metal loss in the weld seam was replaced.

One sample was taken for metallurgical examination. The sample consisted of a flange-reducer weld joint, showing significant metal loss in the weld internal surface (Fig. 2 [140,979 bytes]).

The objective of this analysis was to evaluate the preferential attack in the internal weld seams. Chemical analysis, metallographic examination, and hardness measurements were performed.

A ferrite and pearlite microstructure typical of carbon steel was observed in this sample. Grain sizes were ASTM No. 6 for the reducer and ASTM No. 5 for the flange.

The microstructure of the weld consisted of typically long dendrites and equiaxial grains. For the heat-affected zone (HAZ), the microstructure showed a normal widmanstatten structure. Fig. 2 illustrates the microstructures found in different sections.

The results for the microhardness of the section were also in accordance with the specifications for carbon steel components. On average the hardness was 138 HB for the reducer, 133 HB for the flange, 150 HB for the weld, and 148 HB for the HAZ. The maximum specification for hardness for carbon steel is 220 HB.

UT results

Flange UTs showed only three locations with significant metal loss: one in the isostripper-tower overhead pipe and two in the HF-stripper tower overhead pipe.

Both lines operated at 160° F., were in HC/HF service, and were 14 years old. The corrosion problem was localized only in circuits with high-temperature service. Temperatures above 150° F. are considered high for HF service with carbon steel.^1-3 According to a previous study,¹ there is a potential problem related with galvanic corrosion if the total contents of copper, nickel, and chromium in the carbon steel components is above 0.20 wt %.

Some HF alkylation process licensors suggested this number in their recent project piping specifications. Chemical analyses showed that 70-80% of the flanges evaluated were above 0.20 wt % (in copper, nickel, and chromium content), and all the material were in accordance with the ASTM A-105 specification.

The flanges with high contents of these residual elements did not show significant metal loss, however. Thus, there is no direct correlation between a high content of copper, nickel, and chromium (0.20 wt %) and carbon steel corrosion by galvanic effects in HF service.

Radiographic examination results

Weld-joint radiographic examination, showed 10 locations with significant localized metal loss: 4 from the discharge pipe of the acid circulating pump (100° F. and 14 years old) and 6 in the depropanizer-charge pipe.

One section from the circulating discharge pipe was selected for metallurgical evaluation. This pipe was the only one in low temperature service (less than 150° F.) showing corrosion problems.

The laboratory chemical analysis showed that the components (weld and metals base) were in accordance with ASTM A-105, ASTM A-234, and E-7018 specifications, and the copper, nickel, and chromium content was less than the established 0.20 wt %.

Also, the metallographic characterization and the hardness were consistent with the typical microstructure and values for carbon steel-piping components. Thus, again, there is no clear correlation between the chemistry, the microstructure, and the weld hardness, with respect to carbon steel weld corrosion.

The localized corrosion problem possibly could be associated with weld defects. That is, lack of penetration that promotes turbulence or weld contamination in the root pass. Additional investigation is required to clarify the corrosion mechanism for both flange and weld metal loss in carbon steel components in HF service.

Special consideration was taken with the depropanizer-charge pipe that failed. It was replaced in 1995 as a result of flange and weld-metal loss. In 1997, the same type of weld-metal loss was detected by the inspection program.

The principal factor that accelerated the corrosion in this pipe is the temperature (190° F.). According to a different paper and previous study, the corrosion problem for carbon steel in HF services arises when the temperature is above 150° F.

Acknowledgment

The authors wish to thank Petroleos de Venezuela, SA, for permission to publish this article.

References

1. Phillips, W.L, Hashim, H.H., and Valerioti, Bill, Effect of residual copper, nickel, and chromium on the corrosion resistance of carbon steel in hydrofluoric acid alkylation service. NACE Corrosion 93 Conference, Paper 623.
2. Dobis, J.D., Clarida, D.R., and Richert, J.P., A survey of plant practices and experience in HF Alkylation units. NACE Corrosion 94 Conference. Paper 511.
3. Forsen, O.B.W. and Tavi, M., Materials performance in HF-alkylation units. NACE Corrosion 95 Conference. Paper 342.

The Authors