New Liquid Fuel check valves were changed in combustion inspection, but when starting the unit with distillate fail to ignite several time. LF pump pressure is 8.7 bar at 440 rpm when the firing, small amount of smoke coming from stack and no liquid fuel coming from false start drain. We checked all things causing fail to ignite, but no fire. We test the vck opening pressure, it open at 7.8 bar.
After that we fired the unit with natural gas and did transfer from gas to distillate at full speed no load. the transfer succeeded. the LF pump pressure was 15 bar during transfer so the check valves opened. Then we stopped the unit and restarted with liquid fuel.The unit fired normally.
Why the check valves not opened From the beginning? what if we haven't natural gas?
Pretty well I reckon your machine needed purged somewhere. I reckon if you had tried a few more liquid starts, you would have got it away.
I agree with glenmorangie; air had gotten into your liquid fuel system at some point(s). If you changed fuel filters but didn't purge the air out of the filter vessels, that could be one area. You said you changed the liquid fuel check valves; certainly some air would have made it's way into the liquid fuel lines downstream of the liquid fuel flow divider.
If you don't run the unit very often on liquid fuel, the liquid fuel check valves have been known to lead in the reverse direction and that causes combustion gases to get into the liquid fuel piping downstream of the liquid fuel flow divider, and upstream of the liquid fuel flow divider, as well, all the way to the liquid fuel stop valve. If the check valves were leaking in the reverse direction badly enough, the liquid fuel stop valve has even been know to be opened by combustion gas pressure (which is essentially CPD pressure) and air can get all the way back to the liquid fuel storage tank.
After a maintenance outage you can start the liquid fuel forwarding system and then force open the liquid fuel stop valve (with the unit at rest or on cooldown) and use liquid fuel forwarding pump pressure/flow to purge air out all the way to the high pressure liquid fuel filter. I have even had the liquid fuel check valves removed, and installed temporary hoses to barrels/drums in their place and used liquid fuel forwarding pump pressure/flow through a forced-open liquid fuel stop valve to purge all air out of the liquid fuel lines downstream of the liquid fuel liquid fuel flow divider prior to a start on liquid fuel. This doesn't remove all the air (because when the hoses are removed some air will inevitably make its way back into the lines downstream of the liquid fuel flow divider as the liquid fuel check valves are being re-installed), but it's very effective.
The other procedure I use is exactly the one you followed--start the unit on gas fuel, and while at FSNL (Full Speed-No Load) transfer to liquid fuel. This will purge all the air out of the liquid fuel system--but, trips have occurred because of air in the line causing problems with liquid fuel flow divider feedback. One can use the MIX fuel option to stop the transfer at, say, approximately 30% liquid fuel (the MIX is always expresses in terms of the amount of liquid fuel--NOT gas fuel!) and let the air get purged more slowly. On occasion, I have forced the liquid fuel flow divider trip on these transfers because I know there will be air and problems, and don't want to trip the unit. BUT AS SOON AS THE FUEL TRANSFER IS COMPLETE AND AT 100% LIQUID FUEL AND SPEED IS STABLE, THE TRIP IS UNFORCED!
Then the unit can be shut down, on liquid fuel, and re-started on liquid fuel with NO problems. Just as you experienced.
But, remember--there is ALWAYS going to be air entering the liquid fuel supply system of dual fuel units that operate primarily on liquid fuel. ALWAYS! It can't be avoided. And, exercising the liquid fuel system regularly (the OEM recommends at least once per week!) is always best for keeping air in the system to a minimum and for keeping all of the liquid fuel components in working order.
Hope this helps!