An actuator applies force not pressure. Pistons and diaphragms can both supply the same force but a diaphragm actuator usually uses lower pressure air on a larger area diaphragm than a piston. With spring return, it will be the spring that is the limiting factor and you cannot fit as large a spring on a small piston as you can with a larger diaphragm. I started my career in a chemical plant built in the 40's that was all pneumatically controlled using 3-15 psi control signals that directly operated diaphragm type control valves. The controllers were basic pneumatic computers. It was an amazingly robust and elegant system.

Hi Compositepro,
Thanks for your reply, but can we achieve class V shutoff at that high pressure with S&D actuator.
Some colleagues say the pounds per linear square inch for this case will be very high and S&D actuator may not achieve the seat tightness.

Thanks
Jack

Only the valve manufacturer can reliably answer your question. But there is no reason that piston or diaphragm is relevant. Also, most large, high-pressure valves should be pressure balanced so that system pressure does not affect the force required to operate the valve.