supply of PLC modules

Hi guys,
In a project I have several analog and digital beckhoff I/Os. The problem is after a while my analog output module (KL4408)stop working. All the channel maintained at a constant voltage. After I changed the module with a new one again after 2 days it happened again. The module wired to some actuators with 24 Vac. I doubt on the ac and dc ground which they are separated.Should I connect the grounds to solve the issue??
Thanks

the power 0 needs to be connected to the gnd, 0 of the valves(siemens)
so the zero is connected to the gnd of the DC supply, and to one side of the transformer for the AC supply.
the transformer of the DC supply, should not be to gnd.
in critical situations make a connection in the valve.

Define "stops working". Have you measured the output directly at the KL4408 using a multimeter? If you do have a ground reference issue, you would expect to measure a "pegged" signal on the receiving end (10 VDC or +/-2047 A/D cnts (12-bit). Without a ground current return path, slight current flow into op amps (pA) slowly drifts the signal to the rails.

using analog output 10V to activate 24VAC valve?

yes hoeywell and danfoss have valves you can use simple 24 Vac and the setting value is 0-10 volt DC.
you can also use 24 volt DC as power.