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Mitigate refinery influent water supply contamination

11.01.2011  |  Huchler, L. A.,  MarTech Systems, Inc., Lawrenceville, New JerseyGarrison, W.,  Valero Energy Corp., San Antonio, Texas

Resolving critical water supply and quality issues by managing risk and implementing successful solutions

Keywords: [water management] [wastewater] [refining] [environment]

Freshwater supply is critical to refinery operations, although the centralized design makes it a potential single-point source of failure. Contamination of the freshwater supply to a refinery is a significant event because it creates the risks of derating or complete shutdown of the refinery, as well as damaged equipment.

Managing these risks requires pre-planning to anticipate potential hazards, as well as intensive management of the pretreatment water system during these events to reduce the negative impact on units that use purified water, such as evaporative cooling water circuits and steam generators. This article discusses two events that compromised the influent water supply to a major Gulf Coast refinery and the refiner’s response to these incidents.

BACKGROUND

This Gulf Coast refinery receives freshwater from two rivers via a canal system managed by the regional water authority. The refiner stores over 460 million gallons (MMgal)—20 days’ supply—in a holding pond that serves the refinery influent water treatment facility located approximately two miles away.

The first event, a structural failure of a transfer pipe in February 2001, released measureable concentrations of refinery wastewater into the influent water supply, compromising the entire pretreatment system. This event sent off-spec water to the cooling circuits. Despite the malfunctioning demineralizers, refinery personnel delivered on-spec, demineralized water to the steam generators (50–850 psig) and power turbines without interruption, and there were no permanent consequences to the pretreatment system’s clarifiers, filters or demineralizers. The cooling water circuit experienced tube failures in two newly installed heat exchangers approximately one year following this event. The root cause of the failures was microbiologically induced corrosion due to poor bacterial control.

In the second incident, which occurred in September 2008, the Category 3 Hurricane Ike caused a Category 5-equivalent storm surge that forced seawater into the refinery’s holding ponds and into the freshwater supply canal for 18 miles. The seawater surge deprived the refinery of freshwater and prevented a timely restart of operations. The refinery, protected by a levee system, was not damaged.

As shown in Fig. 1, the holding pond is an unlined basin with earthen banks that supports water hyacinths and other naturally-occurring vegetation. The influent clarifier (Fig. 2) removes suspended solids from the river water, typically processing approximately 15,000 gallons per minute (gpm). The majority of the clarified water flows to the cooling towers, while a smaller portion flows through gravity filters followed by pressure filters to remove suspended solids (Fig. 3). Demineralizers then remove dissolved contaminants from the water. The demineralized water flows to fired and process heat boilers that generate steam to heat the process and drive turbines for power generation and other equipment drivers, such as pumps and fans.

 

  Fig. 1. Holding ponds.



 

  Fig. 2. Influent clarifier.



 

  Fig. 3. Process flow schematic.



THE FIRST INCIDENT

In early February 2001, a wastewater transfer line suspended over the earthen holding pond developed numerous leaks, releasing partially treated wastewater into the refinery’s freshwater supply. Refinery personnel compared the conductivity of the contaminated pondwater with typical results to assess the severity of the contamination, and then used the ratio to predict the increase in the ion loading on the demineralizers.

Other impacts included poor performance of the influent clarifier, severe fouling of the gravity and pressure filters, contamination of the cation and fluidized anion units, and dramatic increases in the fouling and bacterial activity in the cooling water circuits. Refinery personnel aggressively managed the operation of pretreatment system assets to prevent off-spec makeup water from entering the steam generators. However, controlling the risk of bacterial fouling in the cooling water circuit was not as successful.

Contamination of the freshwater holding pond continued for over a week while maintenance personnel repaired the numerous leaks. After the event, plant personnel replaced the entire wastewater transfer line.

Immediate response and corrective actions.

The most immediate concern was stopping the contamination by repairing the ruptured transfer line and maintaining a high-quality demineralized water supply for the steam generators. Refinery personnel decided not to flush the contaminated water out of the holding pond because the flushing procedure would have dramatically increased the concentration of suspended solids from turbulence created by high flowrates in an unlined earthen pond. This action could have compromised the influent clarifier operation.

A preliminary assessment of the plant indicated that the demineralizer run lengths would be one-sixth of the normal run length, or four hours. This run length was slightly longer than the normal regeneration procedure, indicating that operators would have to continuously regenerate a unit to meet the current cold-weather demand for demineralized water and steam generation.

Influent clarifier. The influent clarifier was performing poorly, with high effluent turbidity causing high pressure differentials in the downstream filters. The wastewater contaminants were interfering with the clarification process.

The most logical corrective action, feeding additional coagulant, was difficult due to a dysfunctional automated feed system that made precise feedrate adjustments difficult. Operations personnel depended on the automated system and had lost the skill and habit of conducting jar tests, the most common diagnostic test for manual feed control. Another complication was the lack of reliability of the online turbidity analyzer, which forced operators to frequently measure turbidity. Consequently, operators had difficulty predicting the correct feedrate, and constantly overfed and underfed chemical. The clarifier efficiency was compromised, and the downstream filter and demineralizer efficiency were negatively impacted.

Several other limitations on the clarifier operation occurred. During normal operation, this refinery did not feed an oxidizing biocide (chlorine donor chemical) upstream of the clarifier to kill bacteria and counteract the dispersant characteristics of organic contaminants. This refinery also did not have a chemical feed system for a flocculant that would have improved the clarifier operation. The cold temperatures further compromised the efficiency of the coagulation and settling reactions in the clarifier, and the plant had no capability to inject steam or divert condensate to the clarifier inlet stream to improve the efficiency of the clarification reaction.

Additional investigation revealed two other causes of poor clarifier performance: a recent pond dredging procedure and a change to a freshwater source that had higher concentrations of naturally occurring organic contaminants. Immediately prior to the wastewater transfer line leak, plant personnel were conducting an annual dredging procedure to remove vegetation from the holding pond. The dredging process increases the concentration of suspended solids in the water. Since the vegetation normally acts as a “sink” for hydrocarbon contaminants, disturbing and removing the vegetation can release additional hydrocarbon contaminants into the freshwater.

The regional water authority draws water from a primary source, with occasional additions from a lower-quality, secondary source. During this event, the freshwater supply had a larger-than-normal proportion of water from the secondary source, which increased the concentration of naturally occurring organic contaminants that reduce the efficiency of the clarification reaction.

Filters. Pressure differentials across the gravity and pressure filters increased dramatically during this event, indicating the accumulation of suspended solids from the poorly functioning clarifiers. The spent filter backwash water had a strong septic odor and brown foam, indicating severe biological contamination.

Operations personnel understood the risks of fouled filters passing suspended solids downstream to the demineralizers. In response, they created and immediately implemented a filter cleaning protocol.

Demineralizers. Despite the intensive remediation efforts on the upstream filters, some suspended solids and bacteria reached the cation units. Normal backwashing during each regeneration cycle was sufficient to remove the suspended solids from the fluidized cation units. The packed-bed units required a non-routine, external resin cleaning procedure. The bacterial contamination was severe; in one packed-bed cation, the bacteria created a large, black mass on the surface of the upper resin bed that operators physically removed during resin cleaning.

Visual inspection of the anion resin and evaluation of the effluent quality indicated that the “old” or conventional fluidized anion units—which have only strong base anion (SBA) resin—had a high rate of organic fouling from the wastewater contaminants. The packed-bed SBA resin had low rates of organic fouling because these units have a weak base anion in a separate compartment that acts as an “organic trap.” Refinery personnel designed an offline cleaning protocol to remove the organic contaminants from the SBAs. The cleaning procedure was conducted on one train per day during the event and as needed after the wastewater transfer line was repaired.

Cooling water systems. The cooling towers used contaminated clarified water as makeup. The hydrocarbon contaminants acted as nutrients for bacteria, increasing the risk of fouling and under-deposit corrosion in the heat exchangers. The appropriate response, feeding additional oxidizing biocide, proved to be difficult because, as in all refineries, the responsibility for water treatment for cooling towers is decentralized. Each operator at each unit had a different understanding of the requirements for monitoring and controlling the oxidizing biocide feedrates, increasing the risk of bacterial fouling and microbiologically induced corrosion.

Recovery.

After seven days, maintenance personnel repaired all leaks on the wastewater transfer line and stopped the contamination of the holding pond. The clarifier operation rapidly returned to normal. The chemical supplier conducted feasibility tests for alternate chemical treatment protocols for the influent clarifier to optimize the effluent quality and to reduce the risk of compromised effluent quality during future upsets in raw water quality. The refinery trained operators to conduct jar tests to improve chemical feed control for seasonally changing water quality and during system upsets. Operations personnel developed a preventive maintenance procedure for the online turbidity instrumentation and installed new chemical feed systems.

After the termination of the leak, the filters continued to operate with high foaming and poor-quality effluent. Operators conducted an offline sterilization procedure using an oxidizing biocide. Following the completion of the final cleaning and disinfection procedures, the filter operation returned to normal.

Plant personnel requested an analysis of samples of resin from every demineralizer vessel during the event. Unfortunately, the results were not available quickly enough to aid corrective actions. The resin analyses results indicated that most of the resin was at or near the end of its useful life and would require replacement to maximize effluent quality, minimize water and chemical usage, and maximize system reliability. Following the event, operations personnel created a proactive resin management plan. Post-cleaning, the demineralizers operated normally and produced water according to specifications.

THE SECOND INCIDENT

On Sept. 13, 2008, Hurricane Ike made landfall near Galveston, Texas, causing a Category 5-equivalent storm surge that flooded the coastal areas of East Texas. In anticipation of the storm, the refinery terminated operations and evacuated the facility. A levee system protected the refinery from inundation by seawater, but it could not protect the canal supplying freshwater, the freshwater storage pond and the adjacent wastewater storage pond outside the levee boundaries. Like all refineries using freshwater, the pretreatment plant could not purify seawater or freshwater contaminated with measureable quantities of seawater.

Immediate response and corrective actions.

The refinery’s first priority was to restore the freshwater supply to the influent water treatment plant to initiate steam generation. Refinery personnel investigated several parallel options: restoring freshwater in the canal, draining the storage pond, desalinating the contaminated water in the pond and/or canal, and creating a connection to one of the canal pump stations using temporary piping to bypass the pond. They decided on the most direct method: bypassing the large holding pond.

Canal flush. While the canal authority pumped freshwater into the short section of canal outside the levee system, the refinery supplied diesel pumps to pump contaminated water out of the canal. The initial assessment indicated that the canal flush procedure would require several days.

Freshwater storage pond. The refinery also began draining this storage pond by breaking the containment wall near the second pump station and using temporary diesel pumps to drain the contaminated water. The limited availability of diesel pumps and the size of the transfer pipe meant that the draining and refilling process would require at least a month before returning the freshwater storage pond to service. Consequently, refinery personnel investigated alternatives to bypass this holding pond or desalinate the water in the pond.

Desalination. Refinery personnel contacted mobile water treatment vendors to evaluate desalination technologies to expedite the restoration of all or part of the freshwater requirements. However, the capacity of the available equipment was smaller than the minimum required capacity of several thousand gpm. The refinery began to investigate bypass piping options.

Bypass piping. Plant personnel decided that the installation of piping to bypass the contaminated holding pond was the fastest solution to the freshwater problem. Irrigation pipe, although not a perfect solution due to the relatively small diameters and thin walls, was readily obtainable and would provide a portion of the water required to operate the refinery until larger, stronger pipe became available.

Recovery.

Within 10 days of Hurricane Ike’s landfall, the refinery installed three 13,000-ft strands of 10-in.-diameter aluminum irrigation pipe between the canal pump station and the pump station that had transferred water from the holding pond to the refinery’s influent water treatment plant. The pipe installation made possible the supply of approximately 40% of the refinery’s boiler makeup water (Fig. 4).

 

  Fig. 4. Bypass pipe routing—irrigation pipe.


The water treatment plant began making demineralized boiler feedwater on Day 11, and the refinery introduced steam into the primary headers on Day 12. The refinery installed three parallel strands of irrigation pipe (Fig. 5) on Day 14 to provide freshwater for cooling applications, allowing the refinery to return several of its units to operation.

 

  Fig. 5. Irrigation pipe.


Concurrent with the installation of the irrigation piping, the refinery made plans to install a 36-in.-diameter, high-density polyethylene (HDPE) water pipe to temporarily meet the full demand for freshwater to the refinery. Refinery personnel floated the HDPE pipe across the wastewater pond adjacent to the freshwater pond and installed the pipe on the retaining wall that separates the two ponds (Figs. 6 and 7). 

 

  Fig. 6. Bypass pipe routing—HDPE pipe.


 

  Fig. 7. HDPE pipe.


Refinery personnel commissioned the HDPE pipe on Day 23 to provide over 15,000 gpm of freshwater and allow the remaining refinery units to return to operation. Refinery personnel continued to flush the freshwater holding pond to reduce salinity, and returned this pond to service on Day 73.

CONCLUSION

The awareness of risk usually begins during a crisis. The key to reducing risk is not experience, but rather proactive management of risk. Managing risk does not start with planning; it starts with envisioning scenarios that would lead to a crisis. Operations personnel sometimes regard scenario planning as pure speculation because it requires personnel, grounded in their rear-view mirror of experience, to consider the possibility of events that have never happened and have a low probability of happening. The process of scenario planning challenges the boundary between reality and imagination and requires a temporary suspension of disbelief.

In both cases of water supply contamination, refinery management had not developed plans for these types of failures. In the first incident, contamination of influent water supply by a leaking wastewater transfer pipe, refinery personnel had not imagined or conducted scenario planning for an event of this severity. In the second event, refinery personnel had evaluated the risk of inundation of the freshwater supply system with seawater in their emergency planning efforts and identified an option to install a pipe within the levee system from the refinery water treatment facility to the freshwater canal. The refiner did not install this bypass piping because personnel defined this risk as very low; there had been no known severe seawater inundation in the refinery’s 100-year history.

Following each of these events, plant personnel analyzed shortcomings in the refinery water supply systems and implemented changes to mitigate the risk of future events. In response to the first incident, refinery personnel installed more reliable instrumentation and feed systems, and conducted training in water chemistry, testing techniques and troubleshooting. After the second incident, plant personnel upgraded the 36-in. HDPE pipe to a permanent installation for use as a future contingency.

More importantly, management successfully fostered a change in culture. Refinery personnel now have a greater appreciation of the value of scenario planning and a better understanding of effective crisis management methods. HP

The authors 

 
  Walker Garrison is the technology advisor for utility infrastructure at Valero Energy. He directs new project development activities and stewards reliability and efficiency improvement initiatives that impact refinery steam, power and water systems. Dr. Garrison has a PhD in chemical engineering from Auburn University and is registered as a professional engineer in Texas. 

 
  Loraine A. Huchler is president of MarTech Systems, Inc., a consulting firm that provides technical advisory services to manage risk and optimize energy and water-related systems including steam, cooling and wastewater in refineries and petrochemical plants. She holds a BS degree in chemical engineering from the University of Rochester. She also has professional engineering licenses in New Jersey and Maryland and is a certified management consultant.  




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