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Key to the Future: Part 2

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World Fertilizer,


Wally Schaefer, Jorvic Vital and Melissa Callejo, Nalco Water, USA, explain the importance of heat exchanger monitoring data in enabling the ammonia plants of the future.

Case study 2

OMNI data was used to make a business decision regarding whether to open a critical heat exchanger during a planned maintenance outage or earlier. Historical U-Value trends indicated the heat exchanger was maintaining its operation without any scaling or biofouling issues. However, the cooling water flowrate showed a constant, slow decline. Based on these metrics, and after confirming that the cooling water network operation did not change, it was highly likely that the water inlet tube sheet was being plugged by debris and restricting the flow of water.

Having the ability to translate these variations to critical information, such as velocity of the water on the tubes, led to the conclusion to open and clean this exchanger during the turnaround and not three months earlier where the market conditions were more favourable for the customer. The plugged tubes were confirmed during turnaround.

The plant observed that the decision for cleaning a heat exchanger is based on business drivers. Having the data from OMNI is key for making decisions about scheduling a cleaning procedure.

Case study 3

A key component of the OMNI program is a test heat exchanger that replicates cooling water performance at the skin temperature and velocity of the most critical exchangers in the plant. The simulator in this particular case study with CF Industries’ Courtright Nitrogen Complex used KnewValue’s Sentinel, which provided several methods of monitoring and assessing fouling, scale and corrosion. The clear polyacrylic shell allows for visual observation of the annular exchangers at any time. The annular exchanger tubes can also be pulled for laboratory analysis. Pre-weighing of the heat transfer tube allows for determination of scale accumulation, as well as actual metal loss due to corrosion at the end of the exposure period. Metallographic and deposit analysis further defines the type of mineral scale, as well as whether corrosion is velocity-induced or a result of under-deposit pitting.

In this case, a rapid build-up of scale and corrosion was observed on the test exchanger tubes during a 46-day exposure period. Standard corrosion coupon results on the cooling water supply and exchanger outlet were 1.32 mpy and 1.9 mpy, respectively. The corrosion rates for the test exchanger tubes were 1.93 mpy for Exchanger 1 and 3.88 mpy for Exchanger 2. The data points were not unusual since corrosion rates are more aggressive under heat exchanger conditions. Corrosion rates typically double with every 18°F increase in metal temperature. However, further examination of the exchanger tubes revealed localised corrosion with pit depths greater than 20 mils. Photomicroscope and deposit analysis showed a build-up of mineral scale, primarily composed of iron phosphate, which resulted in under deposit corrosion.

Based on this new understanding of the nature of the scale and performance under actual heat flux conditions, the cooling water treatment program was adjusted, which included an overlay of pyrophosphate to sequester iron and minimise deposition. After these program changes, only slight corrosion was observed, and pit depths were 0.1 mpy, a 99% improvement and a quantum change in asset protection. Ensuring the designed asset life of a tube bundle can save an average of US$50 000 – US$75 000 per year for each exchanger (based on bundle replacement cost of US$450 000 and 20-year lifecycle).

“The test heat exchanger technology [Sentinel] is a paradigm shift – a new way of thinking about how we monitor corrosion in a cooling system,” explained Brian Bloxam, Ammonia Plant Superintendent, CF Industries – Courtright Nitrogen Complex. “We continue to evaluate the simulator’s results as they correlate data about our critical asset performance to help drive action to maintain our plant’s uptime and reliability.”

Conclusion

In conclusion, enabling the ammonia plant of the future requires a comprehensive solution that includes the ability to quickly correlate KPIs on the water chemistry, operation and mechanical factor to accelerate decisions to deliver step-change on reliability and profitability. Gathering information that impacts critical heat exchanger assets will continue to support the nitrogen processing industry to scale up production for years to come.

Read the article online at: https://www.worldfertilizer.com/special-reports/17102018/key-to-the-future-part-2/

 

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