I wondering if someone knew what type of maintenance could be performed in maxon electromechanical valves (double block and bleed) put in boilers. Also how reliable is the maxon compared to jamesbury ball valve for safety critical systems.

Maxon's web site is
http://www.maxoncorp.com/

The site offers installation guide downloads for all its shut-off valves.  
Each guide includes a procedure for a periodic test.
One such test (of two in the guide), for the 4700 valve is below.   
You should find the test for your specific valve.

Suggested leak test procedure for
double-blocking shut-off valves
(without vent line)
(a) Shut down combustion system per manufacturer