September 2008

Corrosion/Maintenance

Caustic injection relocation project

Corrosion and fouling in the second preheat exchangers were eliminated

 

Yanbú Refinery Department (YRD) operates a 235,000-bpd hydroskimming refinery. The crude oil passes through three preheat stages (series of heat exchangers) before entering the crude heaters. After the first preheat stage, water is added to the preheated crude oil prior to entering the desalters. A 3°Be caustic is injected upstream of the desalters to neutralize acids formed. Also, a 3°Be caustic is injected downstream of the desalters for corrosion control. It was observed since YRD startup in 1983 that the second preheat stage exchangers have a history of active corrosion and fouling problems. It was strongly recommended by the Consultant Services Department (CSD) to implement the best-practice idea, which is relocating the downstream caustic injection from downstream of the desalters to upstream of the crude heaters to eliminate the corrosion and fouling problem in the second preheat exchangers. On average, this project is saving $80,000/yr in maintenance cost. The cost avoidance of dumping diesel to fuel oil is around $1.5 million. Furthermore, this article will discuss the challenges faced after the relocating project and the lesson learned.

Process overview. The crude distillation process flow in plant V04 carries a crude charge through preheating, desalting, charge heating, crude column distillation and naphtha fractionation (Fig. 1). The crude unit consists of two identical preheat trains (A and B).

 Fig. 1   

Yanbu' Refinery CDU overview.

The crude charge passes through the first-stage preheat train, which is a series of four heat exchangers:

  • E-1 crude/circulating naphtha exchanger
  • E-2 crude/kerosene product exchanger
  • E-3 crude/LDO product exchanger
  • E-4 crude/no. 1 circulating LDO exchanger.

After that, the crude charge flows to the desalters at about 120–125°C, and is mixed with wash water before entering the desalters. From the desalters, the crude passes through the second-stage preheat train that heats the crude charge to flash temperature in a series of three heat exchangers:

  • E-5 desalted crude/no. 2 circulating LDO exchanger
  • E-6 desalted crude/HDO product exchanger
  • E-7 desalted crude/cold reduced crude exchanger.

Liquid from flash drum V2 is pumped by the flash drum bottoms pump P2 to the final preheat section:

  • E-8 desalted crude/circulating HDO exchanger
  • E-9 desalted crude/hot reduced crude exchanger.

The preheated crude oil finally enters crude charge heaters H-1A/B where it is heated to crude column flash zone requirements. Heater outlet temperature is controlled at 375°C.

Caustic injection at CDU. After the first preheat section, water is added to the preheated crude oil by using a preset mixing valve and is thoroughly mixed with the crude prior to entering first-stage desalter V6A. The oil leaves the desalter and is further mixed with fresh water prior to entering the second-stage desalter V6B (Fig. 2). The 3°Be caustic is injected upstream of the desalters to neutralize acids formed during the hydrolysis of salt. The caustic injection is controlled to maintain a pH of 6–7 of the brine water effluent. The 3°Be caustic is also injected downstream of the desalters for corrosion control, and to control the chloride content in the O/H of the column.

 Fig. 2   

Caustic injection locations.

Fouling and corrosion problems. Since Yanbu' Refinery (YR) startup in 1983, the middle preheat exchangers V04-E5, E6 and E7 had a history of active corrosion and fouling problems, caused by under-deposit, inorganic deposits and high-temperature sulfide corrosion that lead to pressure buildup and tube leaks.

The under-deposit corrosion products are activated, mainly by the elevated temperatures of the inlet shell sides, leading to under-deposit corrosion due to partial vaporization of the water and light ends in the crude. The high temperature of the crude passing through the second preheat train is another factor that accelerates fouling. With the sulfur content of AL crude at 1.5–2%, and with the high operating temperatures, the environment is set for high-temperature sulfur corrosion.

In terms of the inorganic deposit inside the tubes, it was noticed as per the R&D analysis on V04-E7, that around 88 wt% of the plugging material was iron sulfide corrosion product (pyrite and pyrrhotiteit), 9 wt% was water-soluble salt (halite-NaCl) and 3 wt% was calcium carbonate. These inorganic deposit materials are considered to be the major contributors for fouling inside the tube, which leads to pressure buildup.

The high temperature of the crude passing through the second preheat train is another factor that accelerates fouling. A two-stage desalter after the first preheat train was designed to minimize the salt content in the feed (outlet design for salt is less than 1 ptb). This target could not be achieved due to low desalter efficiency, caused by inadequate water wash rate and a low desalter residence time.

The water wash injection, as per industrial best practice, should be 3–5 volume% to ensure that the required salts are dissolved compared to the existing injection rate, which is around 2.9% of volume. Desalter residence time is also important to make sure that there is no water carry-over, but the calculated residence time of the operating desalter is 11.5 min. @ 235 mbd, which is under the design (15 min @170 mbd).

It was concluded that downstream caustic injection acts as a catalyst for fouling and under-deposit corrosion, raising the pressure drop and leakage in the middle preheat exchangers.

Inspection records for middle preheat exchangers. It was found that most of the tube leaks were in the top portion of the bundle, especially for V04-E5 and V04-E6 and this is due to the high heat flux at that location (Fig. 3). In terms of V04-E7, most of the problems associated with these exchangers are mainly uniform thinning, localized attack and erosion corrosion as per the CSD.

 Fig. 3   

Most of the leaks were in the top portion of the bundle.

Caustic injection relocation project. It was strongly recommended by the CSD to implement the best-practice idea, which was relocating the downstream caustic injection from downstream of the desalters to 66 m upstream of the V04H1A/B heater, to enhance the overhead corrosion control on the chloride content, and to eliminate the corrosion and fouling problem in the second preheat exchangers (Fig. 4).

 Fig. 4   

Caustic injection relocation.

The design includes adding a crude slipstream to the caustic stream before injection to the 14-in. heater charge line to eliminate water vaporization and caustic deposition. In addition, an interlock system was provided to trip the caustic pumps when the crude slipstream flow is low. The premixed caustic injection point location is 66 ft upstream of the crude charge heater passes. This location is to allow the premixed caustic to mix well with the bulk crude. In addition, Monel 400 was used to construct the piping downstream of the crude slipstream/caustic mixing point including the quill and the static mixer. The caustic injection relocation was started in May 2006.

Post relocation project. It was observed that frequent fouling of the second preheat exchangers was eliminated. Testing and inspection of the desalted crude heat exchangers V04-E5 and E6 were completed in July 2007 (a total of eight exchangers). Initial visual inspection revealed all components were in good condition without any significant mechanical damage. A 100% magnetic particle inspection (MPI), conducted on the internal weld seams of the shells, channel box and covers, did not reveal any defects. All exchangers passed the initial and final hydrostatic tests.

YR has major savings after this project. Before, on average, 10 exchangers were fouled and leaked every year. Mechanical cleaning, hydrotesting and maintenance labor cost $8,000. On average, this project annually saves $80,000 in maintenance costs.

Furthermore, this project prevented the frequent isolation of V04-E6 A/B. Isolation of this exchanger resulted in dumping heavy diesel oil product to the fuel oil stream, reducing diesel production (Table 1).

On average, the V04-E6 isolation was experienced 10 times a year, with an average of three days of maintenance work. The cost of dumping heavy diesel oil to fuel oil is calculated to be $1.50 MM/yr (Table 2).

On average, this relocation project is annually saving $1.8 million.

Challenges. After relocating the caustic injection in May 2006, inspection data at the new caustic injection locations revealed unacceptable corrosion rates in the 14-in. crude lines upstream of the heaters for both trains A and B. Estimated localized corrosion rates were 0.6 mm per five months for train A (at the bottom) and 0.4 mm per five months for train B (at the top, Table 3).

A root-cause analysis was conducted by the CSD, P and the YR plant personnel to focus on potential causes and actions necessary to minimize corrosion. This included looking at the detailed inspection data (ultrasonic testing, radiographs, scans and infrared) as well as design, fabrication and operational parameters. It was concluded that the damage mechanisms for each train were different and the best scenario for the attack is:

• Metal losses in the 14-in. crude lines are attributed to general corrosion attack by caustic, which attacked the pipe wall prior to the caustic being diluted by the main crude. This would explain the highly localized corrosion seen immediately downstream of the injection point in the 6 o'clock position for train A and in the 12 o'clock position (downstream top-of-the-line weld-o-let) for train B.

• For train A, it was suspected that there are leaks of partially undiluted caustic on the 14-in. carbon-steel line from the roots of the quill end-plate fillet welds that are causing corrosion (Fig. 5). The infrared surveys showed localized areas of much lower temperatures.

 Fig. 5   

Train "A" injection quill assembly.

 

 Fig. 6   

Train "B" injection quill assembly.

• For train B, it was suspected that improper mixing was allowing some of the caustic to drift upward to the crude piping, leading to localized attack. The end-plate weld in this case appeared to be solid and intact. Also, we cannot be certain if there could be a small leak in the flange weld that could allow top-of-the-line corrosion (Fig. 6).  HP

BIBLIOGRAPHY

Yanbu' Refinery Instruction Manual.
YR Caustic Injection Relocation Project's Package (RFE-2004-291).
YR 14-in. Crude Line Investigation Report.

 

 

Mohammed Eid is a process engineer working in the Operation Engineering Unit in Yanbu Refinery. He has five years' experience in refining processes. Currently, Mr. Eid is working in the crude distillation unit in the refinery. He holds a BS degree in chemical engineering from Arizona State University and a master's degree in business administration from Regis University, Colorado.


Rabea Saggaf holds a BS degree in chemical engineering from King Abdulaziz University of Jeddah in the Kingdom of Saudi Arabia. He works with the Saudi Aramco Yanbu refinery department operation engineering unit. Mr. Saggaf has 10 years' experience in refining and was involved in the initial start up of the Platformer/CCR unit.


Sajeesh Padmanabhan holds a BS degree in chemical engineering from the University of Calicut in India and works with the Operations Engineering unit at Saudi Aramco's Yanbu refinery. He has 12 years' experience in refining and petrochemicals and was involved with the commissioning and startup of the CCR-Platformer at Yanbu Refinery.


Neelay Bhattacharya is a process engineer working with the Saudi Aramco Process and Control Systems Department. He has 17 years' experience in refineries and petrochemical plants. Mr. Bhattacharya holds a BE degree in petroleum and petrochemicals from Pune University, India, 1990. He joined Saudi Aramco in 2006.


Hamzah Z. Abuduraihem is a process engineer working with the Saudi Aramco Process and Control Systems Department. He joined Saudi Aramco in 1992 and has 15 years' experience in refinery operation and project management. Mr. Abuduraihem holds an MS degree in chemical engineering and petroleum refining from the Colorado School of Mines, US. He spent six months training with Chiyoda Corporation in Japan and a one-year internship with IFP/Axens in France.

 

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