For a boiler MFT, what is the best practice of implementing the system?
1) just through the DCS where the critical field device i.e drum level connects to DI card and to the fuel relay or
2) a redundant line/cable going direct to the fuel relay.
There are regulatory requirements (legal) that require the BMS to be separate from the DCS.
For a simple system you could use SIL rated safety relays and signal monitors. But for more complex systems with many permissives, trips and multiple inputs, 2oo2 or 2oo3, you need a proper SIL rated PLC or SIS. You need to read and understand the NFPA code for boilers, ISA or CSA gas code and your local legal boiler code laws.
Many suppliers have a wide range of safety PLC and SIS systems based on size, number of inputs, SIL rating and level of redundancy. Rockwell has safety PLC versions and 2 2oo3 TMR systems from ACS Triplex. Triconex has 3 systems that are all 2oo3 TMR but vary in SIL rating, expansion size and 24 VDC I/O only or higher field voltages.