DC/AC Solenoid

1) AC coils experience volt-ampere "inrush", the high momentary surge occurring at coil energization). The spec for volt-ampere "holding", the continuous draw following inrush, is also typically provided. In contrast, DC coils have no inrush current. That has to be taken into account at the 'PLC' end.

2) With regards to hazardous areas,

a) Service/duty
ASCO brand solenoid valves which are rated for I/S (intrinsically safe) operation are intended specifically for instrument air service duty only. They are not general purpose I/S solenoid valves.

Explosion proof (EXP) rated coils are available for many other valves whose service is limited by the application of the valve, classing them as general purpose solenoid valves.

b) I am only familiar with I/S solenoid valves that are available with 24Vdc coils. It is unlikely that an AC coil could get I/S agency approval.

The size of the valve and torque, (if by torque you are referring to the amount of force used to open and close the valve) are designed into the valve by the manufacturer. You need only ensure that you provide the voltage and current requirements on the name plate of the valve.

As far as area classification is concerned most Intrinsically safe or explosion proof valves I have come across are Pneumatically controlled.

The PLC output modules can control either voltage. It depends on your installed hardware.

Here are a couple of points you may want to consider.

(1) I prefer 24vdc controls because this voltage is safer and allows you to troubleshoot it without arc flash safety gear.

(2) On solenoids that are used frequently I place an interface relay between the PLC output and valve, and use surge suppressors or fly-back diodes to help contact life. That way if the contact fails, you need only swap out the interface relay and not the PLC output card. Interface relays also allow you to control different voltages with one output card.

(3) If you use 24vdc fuse your outputs and inputs. Too many machines are being produced that aren't fused sufficiently. When a device short circuits it pulls down the DC. power supply and the faulty component can be difficult to find.