A misconception held throughout the industrial manufacturer sector is that "concrete is forever," therefore making it maintenance-free. For all of its seeming permanence, concrete comes under attack from both natural and man-made forces almost from the time it is first placed and finished. Often sulfur recovery unit (SRU) operations are most affected, because they represent the most challenging environment for a refinery's concrete infrastructure.

The implications are significant because concrete structures play an integral role in the infrastructure found supporting refinery processing units. From foundations, support beams and columns to containment and storage facilities, concrete is the construction material of choice. Every day, refinery plant managers deal with concurrent issues that affect the way they do business; higher yields, environmental issues, maintenance, and capital issues all affect many aspects of their operations, including civil infrastructure.

In a refinery setting, the response to the progressive levels of degradation, framed mostly within the context of economic repair issues, poses a set of challenges that are vastly different than those encountered in new construction.

FIGURE 1: Sulfur pits like this one face shortened life expectancy because of continual exposure to aggressive deteriorating conditions.

Sulfur Storage: Pits

In today's refinery operations, one of the most aggressive exposure environments for concrete are found in SRUs. SRU operations have structures known as sulfur pits that function as storage vessels for molten sulfur. Basically, two types of storage vessels are used. These vessels can be either above-grade tanks, or a sulfur pit, which are best described as large concrete boxes, built below grade. For this discussion, the focus is on subsurface concrete structures.

While in operation, sulfur pits are continuously exposed to elements that are detrimental to the integrity of the concrete, which leads to shortened life expectancy. Numerous types of aggressive deterioration mechanisms that can be found in the sulfur pits, and these need to be identified accurately to be neutralized effectively.

Typically, concrete in sulfur pits is exposed to operating temperatures of molten sulfur at or above 300° F. Long-term exposure of Portland cement-based concrete at these high temperatures can lead to sulfur impregnation of the concrete, which chemically alters the paste fraction of the concrete matrix. At these temperatures the concrete mass can desiccate, which causes cracking that can subsequently lead to acid attack. Generation of sulfuric acid - a chemical extremely detrimental to concrete - can be caused by water leakage into the pit from cracks and failed slab or wall penetrations. Also, exposed reinforcing steel bars above molten sulfur levels may corrode because of the electrochemical process of corrosion.

Another Culprit: Construction Defects

As with all concrete structures, defects in original construction can also jeopardize sulfur pit operations. The following defects can contribute to the reduced life expectancy of sulfur pits:

• Poor consolidation of concrete members resulting in cold joints, voids, and honeycomb;
  
  
• Improper placement or absence of reinforcing steel;
  
  
• Inadequate concrete coverage overtop of embedded reinforcing steel;
  
  
• Poor design detailing;
  
  
• Use of poor quality construction materials.

Repair Strategies to the Rescue

Obviously, strategies must be implemented carefully to prevent unnecessary loss in production, as well as for the proper identification of failures and determination of courses of action required to provide a sound concrete structure that is ready to be placed back into service. Complicating matters further, experience has shown that there are other forces at work that can also wreak havoc in SRUs. These forces involve repair failures, where the wrong material was chosen for repair (e.g., cementitious repair materials with high shrinkage characteristics were used, the substrate was not prepared properly, or there has been an ingress of groundwater, which eventually leads to sulfuric acid production). A faulty repair only leads to higher costs and the unwanted prospect of "repairing the repair."

History and experience have shown that each sulfur pit can pose unique challenges to a repair contractor. Regardless of whether it is a full-depth repair, a partial liner, or stopping water ingress, it is imperative to utilize an engineered solution. A proper repair strategy should consist of the following elements:

• Identifying and determining the root cause of the failed concrete;
  
  
• Employing proper materials in construction and repair techniques; and
  
  
• Using a qualified, experienced contractor who can provide a solution as well as a well-planned quality assurance and control program, for the repair.

FIGURE 2: This flow chart shows the decision-making steps and actions required to effect good concrete repair.

These three steps will assure the SRU manager that the repair-failure-repair cycle can be eliminated, and a sound structure put back into operation.

An unplanned shutdown of sulfur pit operations is a cost no facility can tolerate. That fact alone should dictate the manner in which structural concrete repairs are planned and implemented. The best repair strategies will make sense from both a technological and economic standpoint. They should be carried out by a team of technicians highly experienced in the repair of these structures, so that the project can be completed quickly and efficiently either during a planned shutdown period, or under emergency conditions.