What type of fittings should be avoided in fuel oil piping?

In fuel oil piping systems, using cast-iron fittings should be avoided primarily due to their susceptibility to corrosion when in contact with fuel oil. Cast iron can deteriorate over time, especially when exposed to the chemicals and impurities commonly found in fuel oils. This deterioration can lead to leaks and compromised system integrity, posing safety and environmental hazards.

While other materials like brass and copper are sometimes used in specific applications, they also may have their limitations based on the type of fluid, system pressure, and potential for galvanic corrosion when dissimilar metals are in contact. Plastic fittings, though they are sometimes suitable for certain applications, may not be appropriate for carrying fuel oils due to their potential weakness under high temperatures or pressures. Therefore, among the given choices, cast-iron should be actively avoided for fuel oil piping applications to ensure safety and reliability.