Sistema fotovoltaico con baterías conectado a red

Possible

Total Loss Long-Term Effect Long-Term National Effect

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Severity of consequences is rated from Zero—No injuries, no damage, no environmental or reputation effect—to Five— Multiple fatalities, massive damage to the facility, and a huge long-term impact on the environment and on the company’s reputation.

A “Serious” effect, Three on the consequences scale, would be an event that produces many days of absence from work for affected employees, or that results in long-term disabilities; a release of large amounts of crude oil or of any reportable quantity of a hazardous chemical; an event that triggers an environmental fine; an event that incurs very high repair and mitigation costs; or an event that causes partial shutdown of a facility and generates extensive regional media coverage.

The likelihood of such an event occurring is also rated in five steps, from “Possible but unlikely,” as the lowest rating to “Occurs Frequently” for the most likely to occur. A simple chart of severity versus likelihood of an event produces the risk rating for that particular event. The higher the likelihood of an event is and the more serious its consequences are, the more closely and more frequently the equipment involved must be monitored to keep the potential event from happening.

The purpose of the RBI program is to reduce all such risks to a minimum “ALAP” (“As Low As Practical”), that is, to a level at which the cost and effort of further risk reduction are unaffordable or disproportionate to the risk reduction achieved. Once the Risk Assessment for all potential events has been completed, the actual evaluation of operating equipment begins in order to determine the required Risk Based Inspection process for assuring that operation of the equipment will not produce negative events beyond the “ALAP” level.

The second half of an initial RBI assessment involves personnel actually operating and maintaining the equipment being rated.

These are the people who actually live with the equipment day-in and day-out; they are most qualified to identify portions of the unit or piece of equipment most likely to fail, and whose failure is most likely to cause damage. They also are most likely to know what coincidental or collateral damage one failure might cause to other parts of the plant. This process allows a whole series of possible

“events” to be evaluated from each potential failure.

Plant maintenance records and equipment design blueprints are analyzed to determine the portions or pieces of equipment most likely to corrode and cause an “event.” Then, potential events are rated for their effect on plant operation and production, and the same potential events are rated against the company’s Risk Assessment charts.

This initial survey can be done by outside consultants, but, ultimately, it is the plant operating personnel who are familiar enough with plant components to know which are the most likely to fail, and local plant management who are best able to determine what and how severe damage such a failure will cause.

Commitment to an RBI Program

The engineer or manager chosen to design and implement a CUI-RBI program faces a daunting task. First, he or she must be assured of buy-in from upper management and from the field people who will be doing the site evaluation. After everyone

understands and agrees that an RBI program is a multi-year, continuing effort, not a one-time inspection, there comes the question of return on investment (ROI). On the one hand, the initial survey and risk assessment are expensive and time-consuming. On the other hand, preventing one “Moderate” event from the Risk Evaluation chart can mean a savings of $1,000,000; preventing a

“Major” event can save ten times as much. In comparison, the cost of the initial plant RBI survey may seem reasonable.

For a refinery or oil production facilities, and for many petrochemical plants, the in-plant risks—such as a vapor cloud explosion, petroleum jet fire, petroleum pool fire, or major toxics release—can all do grievous harm to the plant, to the surrounding environment, and to the company’s bottom line as well as to its reputation. The in-plant survey needs to identify specific high-risk areas or pieces of equipment whose failure might raise the severity of consequences on the “Equipment” column of the risk chart. Of course, such equipment should already be closely monitored as part of the plant maintenance program, but identifying (or re-identifying) key high-risk items helps the RBI initial survey become a defined risk-mitigation process.

Existing plant data on performance of unit vessels, piping, operating equipment, controls, and even electrical and electronic sub-systems can be used to develop an RBI continuing inspection schedule and calculate its expected cost in terms of dollars per square foot or dollars per linear foot of pipe per year of the RBI program. Remaining service life of an older unit, expected upgrades or replacement, and the part one unit plays in the overall operation of the plant all need to be evaluated against the risk evaluation for that particular unit.

Once data is collected, the proposed RBI program needs to be prioritized, based on highest possible event consequences, age and replacement cost of equipment, turnaround schedules, and the ability to incorporate the RBI program into existing inspection procedures (if any exist). Because there is not enough budget for 100% frequent inspection of all insulated areas, a priority ranking program is set up, with the “riskiest” vessels, piping, and equipment receiving the most frequent and most thorough spot inspections, and lower-risk equipment being inspected less often, or with less of the insulation and cladding actually removed as part of the scheduled inspection. Lowest-risk or no-risk equipment may receive only the minimum required electronic wall thickness tests annually. Some critical refinery areas may require 100% removal of cladding and insulation and 100% visual inspection.

A key factor in the frequency of visual inspections is the equipment owner’s confidence in the CUI coating systems used on equipment included in the CUI RBI program. Where quality surface preparation, a suitable proven coating system, good application, and thorough inspection have been done on equipment under insulation, the number of inspection spots may be reduced to areas of known breakdown, and the inspection intervals may be extended. Table 2 shows a major global petrochemical company’s

“confidence level” for length of service life of coatings under insulation, where operating temperatures never exceed the maximum service temperature of the applied coating system.⁵

TABLE 2

Expected Service Life Performance of Typical CUI Systems

System Service Life Required Repair

Clad, insulated, uncoated bare steel

(Lose entire wall thickness corrosion allowance) 6 years; 100% re-do Clad, insulated, organic coating without abrasive blast

(Lose entire coating system, portion of wall thickness) 12 years; 100% re-do Clad, insulated, organic coating with SSPC-SP 10 abrasive blast

(Lose portion of coating system, portion of wall thickness) 16 years; 100% re-do Clad, insulated, thermal spray aluminum with SSPC-SP 5 abrasive blast

(Lose small portion of TSA, small portion of wall thickness) 40 years; 25% re-do Continuing the RBI Program

After the base plant (or unit) RBI survey has been done, and the risks and hazards have been agreed upon, quantified, and ranked by plant personnel, then the actual annual (or otherwise recurrent) field surveys can be done by an outside survey firm that has experienced, qualified inspectors, and follows the base survey. Many existing RBI programs actually combine electronic non-destructive testing (NDT) with insulation and cladding removal and visual inspection of selected small areas. Both parts of the survey may be done by the same firm, or NDT can be done by a specialist, and the results can be verified by a paint inspection company.

The findings of these recurrent surveys are summarized in electronic format, incorporating electronic testing results, digital photographs, and the field contract inspector’s “eyeball on the steel” evaluations. The plant’s corrosion engineer or maintenance manager now can examine the corrosion state of his facility on a computer monitor in his or her office, at his or her convenience.

Management personnel can review the survey results, match them against expected results based on the initial RBI survey, and decide on an appropriate course of action.

In simplified form, the recurrent RBI survey can have four possible results for a particular unit or piece of equipment.

• Less corrosion is found than was expected. This result is noted in the survey. If the result is found to repeat in the next scheduled survey of this unit, the unit or piece of equipment may be re-evaluated for lower risk or less frequent inspection.

Some owners also use such a finding to re-evaluate related equipment, working on the sound theory that if one unit or piece of equipment is rusting less than expected, something else related to the equipment may be acting as an anode and rusting more than expected.

• Corrosion is as expected. The survey is submitted and repeated as scheduled.

• A small increase in corrosion is noted over expectation. Additional portions of the unit are inspected at the same time to confirm the increase in corrosion. For CUI work, inspecting additional portions means removing additional small areas of cladding and insulation. The unit or area is marked, and the next scheduled re-inspection will determine whether unscheduled corrosion-preventive maintenance may be necessary.

• A large or unexpected increase in corrosion is noted. Additional portions of the unit are inspected at the same time to confirm the increase in corrosion, and plant personnel are brought in to try to determine a cause. Budget and scheduling are rearranged to give priority to corrosion-preventive maintenance on this unit or piece of equipment. The recurrent survey schedule is rearranged to closely monitor this problem until corrosion-preventive maintenance is done, and then afterward to determine whether the maintenance resolved the problem.

RBI programs for plants with large amounts of insulated piping and equipment require additional input during the initial set-up of the program to assure that the spots selected for recurrent survey are actually representative of the “worst case” areas of each unit or piece of equipment. The first few recurrent surveys done by a contract inspection or survey firm may actually include additional, redundant spot inspection points, which can be phased out later if survey results are as expected. Where electronic testing or thermal imaging produces reliable results and matches destructive spot testing over several recurrent survey cycles, the destructive testing spots may be reduced, thereby reducing the overall survey costs without affecting reliability.

Figures 1-3 that accompany this article show an in-house RBI program in action at a petrochemical plant in South Louisiana. The facility is an older plant, but equipment is meticulously maintained, and a very thorough RBI program is in place. Sections of insulated piping, vessels, and equipment are inspected annually on a rotating basis, with a typical section being re-inspected every three years on average. The plant uses a combination of organic coatings and thermal spray aluminum for CUI work; annual survey results tend to confirm the plant’s RBI base surveys and the service life expectations for the systems used. Confidence is high that the CUI-RBI program is working as it should.

A Houston-area industrial gas facility, which produces various gases by cryogenically refrigerating air and then separating its components, has an entirely different approach to RBI for the company’s piping for transfer, storage, and loading. The facility doesn’t do any RBI. Analysis of maintenance and operating records on these low-temperature piping systems in the plant has shown that failures are always due to cracking of piping in cyclic service from cryogenic to ambient temperatures. A failed pipe is quickly discovered through unexpected pressure loss; the insulation and cladding over the pipe act as an effective containment over the ruptured pipe; and the only loss is of the product in the pipe, which, as a gas component of air, is inherently non-polluting.

The plant has been designed to allow effective isolation of failed pipe run sections, so when such a failure occurs, the affected pipe run is shut in, insulation and cladding are removed, and the failed pipe section is replaced. Loss of product and loss of productivity are minimal. The plant runs several parallel air separation trains, so the downtime required to replace a fractured length of pipe in the transfer, storage, and loading piping produces only a small reduction in plant output and does not require other shutdowns.

Corporate management has determined that for these portions of the plant, this policy of neglect presents low enough risk and is more cost effective than an intense RBI program.

Conclusion

Unfortunately, a great deal of corrosion-mitigation plant maintenance, both for CUI and for atmospheric corrosion damage, is done reactively, rather than proactively. There is an “Oh Sh**” moment that comes in almost every unscheduled CUI inspection. That’s when the plant corrosion engineer or maintenance manager looks at the large area of newly exposed corroding steel where insulation and cladding were removed after serious corrosion was seen in a smaller exposed area, and the engineer says “Oh Sh**.

Fixing this is going to take my entire maintenance budget for the year.”

For these plants, CUI repair work is scheduled and done only after a serious problem is unexpectedly found. This work often involves unscheduled shutdowns; loss of production; manufacturing bottlenecks or backlogs; and, occasionally, even fires, explosions, or toxic product releases. This maintenance process is unnecessarily costly and can be easily improved. Improvement requires only a small increase in budgets and no long-term increase in plant personnel, using RBI with an initial survey by plant personnel and recurrent inspections by outside contract inspectors or surveyors.

Although commitment to a CUI-RBI program requires a substantial initial investment of time and effort, and a multi-year continuing commitment, the relative security, peace of mind, and confidence in the plant corrosion state offer a positive return on investment even before factoring in the cost savings of not having an unexpected event that might shut down the plant, pollute the

neighborhood, and irreparably injure the company’s reputation.

RE F E R E N C E S

1. George F, Hays P. E . “Now is the Time,” White Paper, World Corrosion Organization, Houston, TX,corrosion.org, 2007.

2. “Control of Corrosion Under Thermal Insulation and Fireproofing Materials, a Systems Approach,” NACE SP 0198-2010, NACE International, Houston, TX, nace.org, 2010.

3. “Corrosion Under insulation and Fireproofing,” Currently circulating draft of API RP583, First Edition, First Ballot, American Petroleum Institute, Washington, DC, api.org, 2012.

4. Keith E. McKinney, Fred J. M. Busch, Andre Blaauw, Andrea M. Etheridge . “Development of Risk Assessment and Inspection Strategies For External Corrosion Management,” Paper No. 05557, NACE Corrosion 2005, NACE

International, Houston, TX, nace.org, 2005.

5. William C. McRae, Nalton Thompson . “CUI Project Development,” Bring on the Heat 2013, NACE International, Houston, TX, nace.org, 2013.

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