Hi,
Many things can happen, if there are metal particles in HP fuel pump...
And there are so many reasons, how those pieces can get in...
1. Please note, that most mono-block pumps need complete overhaul / adjustment at appx. 15000 - 16000 rhrs.
Good ones can make up to 30000 rhrs.
2. Probably engine was stopped for a long period before started ?
3. Just in case, check pump's inlet/outlet lines and filter cartridges, day tank.
4. Sometimes pump is combined with governor, problem can be there.
5. Make a good fuel analysis.
Could be great, if You post more information / pictures / description of Your system.
Engine's RPM ?
Fuel cartridge cleaning / replacement interval ?
How high is a day-tank, compare to HP pump ?
How old is Your MAN ?
What is pump's maker ? BO....? KI....? L'o.....?
rgds,
Andrei