Hi rsv,
From your question "Why use ball valves on Gas pipelines instead of gate & or globe valves, am I missing something?", I would like to ask you,
"What do you want this valve to do?"
Depending on what you need the valve to do, you need to select the valve appropriately. The valve is selected "fit for purpose".
Having said that, there are usually multiple options for valves for each application. There are also many factors in valve selection (valve sizing, cost, etc. are each a component to be considered in determining the selected valve).
As with anything, go to the expert/professional for advise. If one needs information on pumps, consult a mechanical rotating equipment resource. If one needs information on valves, I strongly suggest consulting your I&C resource (that's what they do) and ask them for further info and advice.
All of the above posting provide valuable information, insight, descriptions and value. However, they are not sufficient to select a valve as is, without further data and information.
One other note, the term "Bubble Tight" is sort of misleading.
ANSI B16.104-1976/FCI 70-2-1976, defines 6 seat leakage classs (note that they actually say "seat leakage" classes). All classes have an allowable "leakage rate".
Class VI is intended for resilient-seating valves (or soft seat), and provides that the leakage limits depends on valve size and ranges from 0.15 to 11.5 ml per minute for valves sizes 1" to 12", and 1 to 81 bubbles per minute.
The difference between the class include for different valve type, the allowed leakage, and testing conditions.
If you do not have an I&C resource available to you, this forum may be of help to you. I have seen other postings where people have provided sufficient information that someone was able to provide a solution.
Cheers!