I am investigating alternatives, one is to use DC solenoid valves - but the need for a large DC power supply may not be cost effective, also robustness is an issue.
We have a hydraulic power pack in the system and thought of using low pressure hydraulic fluid to control the pilot valves. 
How low is your hydraulic valve system pressure? If its below 7 barg or you can reduce it down, why not use the same actuators. We've used standard pneumatic actuators with LP oil as the actuating medium in the past successfully. Although we did change the pneumatic directional solenoid pilot valves to true hydraulic ones. It helps if you current actuators are spring return too. You will find operation a lot slower. The output from the powerpack will also need to be taken into consideration as well, espacially if all valves are operating simultaneously.

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