What device prevents raw steam from entering and damaging pressure gauges and other components?

The pigtail is a device specifically designed to prevent raw steam from directly entering pressure gauges and other components in a steam system. It is typically a curved or coiled piece of piping that serves two main purposes. First, it acts as a cooling device, allowing steam to condense into water before reaching the gauge. This condensation helps protect sensitive instruments from high temperatures that could cause damage or inaccurate readings. Second, the pigtail traps a small amount of condensate, creating a barrier that prevents steam from flooding the gauge, which would otherwise lead to malfunction or failure.

In contrast, the other choices serve different functions in steam systems. Sight glasses are used to monitor the level of liquids within a vessel but are not designed to protect gauges from steam. Pressure valves are crucial for regulating pressure but do not address the issue of steam damaging gauge components. An expansion tank is utilized to accommodate fluctuations in pressure and volume within a steam system, primarily dealing with condensation and thermal expansion rather than protecting gauges from raw steam. Thus, the pigtail stands out as the most suitable device for the specific purpose of safeguarding instruments from hot steam.