Trying to find a solenoid valve capable of sealing off liquid flow even if a large differential pressure exists between the outlet and the inlet or vice versa.  In other words, the valve should remain tightly closed in both of the following situations:
1) 100 psig on the inlet, 0 psig on the outlet
2) 0 psig on the inlet, 100 psig on the outlet
Most solenoid valves I'm aware of will open if outlet pressure greatly exceeds inlet pressure. 
Furthermore, I need a valve with a Cv of 1.5 or greater to keep my pressure drop in a reasonable range.  I have yet to find a valve that meets all these criteria (on, and material compatibility with DI water and glycerin).  I've used keywords like "bi-directional" but I'm not sure if that's what I want or not.
Use a small solenoid valve on low-pressure instrument air to pilot the actuator on the ball valve.
You might look at Maxon for solenoid valves with more horse-power.  Unless there is something special, I would go with the actuated ball valve.
The footprint and weight can vary greatly.  Search for a "Whitey" valve with a pneumatic actuator from the Swagelok site.  This should work for a Cv below 2.  If the valve becomes too large then you might look at an Apollo valve - perhaps with a small rack and pinion actuator.  Assure that the actuator is single acting (spring return) if there is a failure position.

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