What are the advantages of the T-outlet design? With two 90-degree elbows, does it significantly increase back pressure during venting?

The main advantage of a “tee” style design is that the thrust loads at the vent exits are balanced. This means that an unequal force that might push the vent stack over is not present. Generally, the tee is also of the same size as the main vent line, thereby doubling the vent area for less pressure drop. The main disadvantage of a tee stack is that they generally vent with a horizontal discharge which means a hydrogen cloud closer to the ground and higher radiation exposure at grade. The tee is also often equipped with a downward facing miter cut to reduce the probability of rain or snow from entering the stack. However, a downward miter cut can direct the exit flow downwards, normal to the miter, if the vent is at high velocity. This can propel the hydrogen even closer to the ground than anticipated. If miters are used, the vent pipe should be oriented slightly downwards, but with the miter cut facing upward. This will help prevent moisture from entering the stack and if supersonic flows occur, the vent flow will be directed further upwards. CGA G-5.5 Hydrogen Vent Systems shows recommended orientations of vent stack outlets.