Assuming this is a steam condensate system, is the system pressurized or open to atmosphere. Sometimes on systems open to atmosphere or operating at low pressures, the condensate temperature can be so high that the condensate is continuously flashing and condensing in the piping causing the water hammering. This can be dramatically increased at pressure reducing areas (control valves, throttling valves, etc.).
If this is the case, you need to find a way to either cool the condensate or increase the system operating pressure slightly, if possible.
For pressurized systems, sometimes the problem is leaking steam traps allowing steam into the condensate system. Or it may be a poorly operating heat exchanger (fouled or excessive flows) which is not condensing all the steam and maintaining a liquid level. Again this would let hot steam into the condensate system causing hammering.