I don't know whether this is a simple Q or not.
Why use ball valves on Gas pipelines instead of gate & or globe valves, am I missing something?

I am new to Gas pipelines having mostly worked in petrochem & powerstations where gate & globe valves are the standard for isolation because they are cheap & reliable.

I have used small 150# ball valves in Pharmaceutical plants but these are 30" 2500# and must cost way more than gate valves.

Ball Valves versus Gates and Globes?
1) Rotating in the packing instead of reciprocating the process from within or the environment from the outside through the packing gland along with the stem.
2) Elimination of the bonnet gasket and potential leak path.
3) Sealing member rotates in it's own volume instead of having to displace a volume.
4) A full ball is a balanced trim. A half ball, gate or globe valve plug would be unbalanced. This would be important with actuators if you lost your gas or air supply.
5) No collection zone in the bottom of the valve. There has to be room for the gate to move to and may have difficulty displacing what ever has collected there.
6) Smaller overall envelop dimension.
7) Ease of automation - ninty degrees to go full open to full closed. Not multiples of turns.
8) Ball valves for control must be sized by Cv not line size. They can have the same capacity as a globe that is twice their size. In other words, they can be half line size. Ball valves have equal percentage inherent characteristics. So when applied to systems where 30% or less of the system drop is applied to the valve, will provide a "linear" installed characteristic.

It appears the general trend of this thread  is going in the right direction, but I will add my 2-cent's worth (after adjusting for inflation)

Gate valves were invented many years ago. They could be manufactured with the machinery and materials available in the early industrial revolution.They had a head-start on the market.  Ball valves were invented in 1949. This was made possible  by the availability of some of the polymers created in the 20th century.  Although PTFE was invented in 1928, its development and availability as an industrial material enabled the development of the ball valve.  
  
Ball-Valves shut off tight.  "Bubble-tight" means nothing, but API6D does.  

Ball valves are more compact.  Generally manufactured to the same lay-length as gate valves (ANSI B16.10), but look at the dimansions of the assembly: Gate valve has to have a "chest" or bonnet, equal to the valve diameter for the gate to occupy when the valve is open.  Then there's a packing box, and a yoke that has to provide for full diameter travel of the actuator stem.  Then there's the actuator which has to be beefy enough to provide the "Hammer blow" feature necessary to seat and unseat the gate. Then above the actuator it is necessary to provide room for one diameter of stem to stick out when the valve is open (frequently inside a tube to protect the grease on the threads).So a motorized gate valve is typically around 5 diameters tall, and almost always has to be installed with the stem vertical to reduce bending loads on the "tower". Compared to a ball valve which has a short packing box, a short bracket, and an actuator.

Environmental concerns.  Cycling a gate valve drags a diameter's worth of valve stem through the packing box, and drags a diameter's worth of environmental contamination ofn the valve stem back into the packing box. All the towering mechanism above the packing box puts a hopefully small but finite side load on the packing due to imbalanced mass. line vibration, etc.   Packing in a gate valve therefore wears very quickly.  Compare to a ball valve where the microscopic roughness on the valve stem moves packing particles through a rotational path that remains inside the stuffing box. Net migration of mass from/through the packing assembly in a ball valve is orders of magnitude lower than the gate valve.  With a less expensive, simpler packing design a ball valve can have zero leakage to the environment for much longer than a gate valve.  

Speed: A large gate valve requires minutes to close with a big electric actuator grinding away. Ball valves need speed controls on pneumatic actuators to slow them down to prevent water hammer.  Which would you want to isolate your piping in the event of a fire?

Throttling:
Gate valves are guided very loosely except when the gate is wedged into the seat.  If the operator tries to throttle with a gate valve all these loosely-affiliated pieces will jingle together due to flow eddies. Control will never be precise because the relationship of the trim pieces is not precise.  Wear will be rapid, and catastrophic failure is likely. Ball valve: Guided positively.  Small ball valves have the ball secured in an interference fit between two seats.  "Floating" balls don't float very much, and careful fitting of the stem tang into the ball slot ensures accurate positioning of the ball.  Larger trunnion valves are guided positively by top and bottom bearings (trunnions) so the ball is not deflected bt process effects, and the ball-to-stem joint is usually a positive connection.  


Conclusion: if you want a cheap valve that is seldom if ever cycled and can live with a small trickle when it is closed or if you have large maintenance apes with long cheater bars to try to get tight shutoff, then gate valves are OK.  If you cycle the valve with any regularity, or ever throttle with it, and want a high probablility of zero leakage either from upstream to downstream or from inside to outside, than a ball valve is a better choice.

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