Got a bit of a problem, I am commissioning a pump circ Ammonia system running at -40c, I am struggling to get any sensible pump pressure. We are using Witt pumps, if I dead end the system I get pressure but as soon as I open it up again everyting goes imediately to suction pressure, I have tried increasing surge drum pressure and level neither of which seem to have had any effect, there is also no oil in the pumps (checked that first!) anyone got any ideas????