What is the maximum limit for dissolved oxygen in the de-aerated feedwater used in hot de-aerated backfill lay-up?

The correct answer of 15 PPB (Parts per billion) for the maximum limit of dissolved oxygen in de-aerated feedwater used in hot de-aerated backfill lay-up is essential for maintaining the integrity and efficiency of the systems involved. Keeping dissolved oxygen at this level is crucial to prevent corrosion in boiler systems and other components where water is utilized at high temperatures. High levels of oxygen can lead to oxidation reactions that produce rust and other corrosive byproducts, which can significantly affect the performance and longevity of the equipment.

In this context, 15 PPB represents a threshold that balances operational safety with the technical limits of water chemistry in such systems. Levels above this can initiate detrimental effects on metal surfaces, potentially leading to system failures, increased maintenance costs, and unplanned downtime.

Other answer choices suggest maximum limits that are either too low or too high, potentially not aligning with industry standards and best practices for water treatment in marine engineering. The selected maximum of 15 PPB reflects an understanding of the limitations and requirements of managing dissolved gases in critical operational environments.