Chemicals are introduced into the boiler:

The introduction of chemicals into the boiler system is primarily done via the internal feedwater pipe. This method is crucial for ensuring that the chemicals are evenly distributed within the boiler water before it is heated. The feedwater pipe typically allows for a controlled and measured amount of chemicals to mix with the water, which helps in maintaining water quality, preventing scale formation, and controlling corrosion.

Using the internal feedwater pipe provides a direct path for treatment chemicals to enter the system, making it an effective and efficient way to ensure that the entire volume of boiler water receives the necessary conditioning.

In contrast, other options do not serve as practical paths for chemical introduction. The dry pipes are generally part of the steam delivery system and are not designed for introducing substances into the water. The injection tank could be associated with pre-treating water but is not part of the boiler's feedwater delivery system. The mud drum is designed to collect sediment and impurities but does not function as a means to introduce chemicals effectively into the boiler water.

Thus, the correct answer accurately reflects the standard practice for chemical treatment in boiler operations.