Final answer:
The Double dry gas seal is generally the best option against toxic and flammable gases due to its dual-sealing arrangement and buffer gas system, which provides enhanced containment and safety measures.
Step-by-step explanation:
The effectiveness of a dry gas seal in handling toxic and flammable gases depends on the level of containment and redundancy required. Among the options provided, the Double dry gas seal generally offers the best performance against toxic and flammable gases. This type of seal is composed of two independent seals and often includes a buffer or barrier gas system between them. The buffer gas typically has a higher pressure than the process gas, preventing any leakage of the process gas to the atmosphere.
In comparison, a Tandem dry gas seal consists of two seals in series, with a lower-pressured buffer gas between them, which acts as a safety measure in case the primary seal fails. However, if the buffer gas fails, there might be some leakage. Lastly, a Single dry gas seal is the simplest and provides only one seal, which is not typically recommended for highly toxic or flammable applications due to the increased risk of leakage.