What is the minimum size connection for a sight glass drain?

The minimum size connection for a sight glass drain is correctly identified as 1/4". In piping systems, sight glasses are used to visually monitor liquid levels, and their drain connections must facilitate the proper discharge of the fluid within the system.

A 1/4" connection is sufficient for a sight glass drain because it allows for adequate flow to prevent backpressure and ensure that the liquid can drain effectively without causing obstruction or delays. This size is commonly standardized in the industry for sight glass applications, enabling easy maintenance and operation.

In contrast, larger sizes such as 3/4", 1", or 1 1/2" are typically unnecessary for a sight glass drain and could lead to the potential for excess backflow or create unnecessary complexity in the system without providing significant benefits to functionality. Thus, the choice of a 1/4" connection aligns with best practices and standards in pipefitting.