Question 31 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is B **Why option B ("Double mechanical seal") is correct:** A double mechanical seal is specifically designed for applications where zero or near-zero leakage into the atmosphere is critical, such as when handling toxic, carcinogenic, or highly flammable liquids. This configuration consists of two primary sealing faces arranged back-to-back or tandem, separated by a barrier fluid (seal pot) circulated under pressure. This pressurized barrier fluid serves two essential functions: 1. **Lubrication and Cooling:** It keeps the seal faces lubricated and cool. 2. **Containment:** The pressure of the barrier fluid is maintained higher than the pressure of the pumped product. If the primary (inboard) seal fails, the barrier fluid leaks into the product, preventing the hazardous pumped fluid from escaping to the atmosphere. If the secondary (outboard) seal fails, only the non-hazardous barrier fluid leaks out. This redundant system provides the highest level of safety and containment required for dangerous media. **Why the other options are incorrect:** * **A) Conventional stuffing box:** This uses braided packing (soft packing) and relies on controlled leakage of the pumped fluid to lubricate and cool the packing. This inherent, constant leakage makes it unsuitable and unsafe for toxic or highly flammable materials that must not escape into the atmosphere. * **C) External mechanical seal:** This is a single seal arrangement where the seal components are mounted outside the stuffing box. While easier to maintain, it is still a single point of failure and relies on the pumped fluid for cooling/lubrication, meaning leakage of the hazardous product is the primary failure mode. It lacks the critical redundancy needed for highly hazardous service. * **D) Rubber bellows mechanical seal:** This refers to the secondary drive mechanism (using rubber bellows instead of springs or metal bellows) for a single mechanical seal. Like other single seals, it offers no redundancy or pressurized barrier fluid system to guarantee containment in case of seal failure, making it inadequate for severe toxic/flammable service.