Hi again Pengze,
To properly clean the exhaust gas economiser takes many hours of water washing, I remember there was a dicussion about this earlier, in this Forum. The problem being that you can not discharge the dirty water from washing the economiser overboard when in Port and you can't waterwash the the Economiser when you are at Sea because the engine is running. If you let the water go into the Bilges you will have to pump it overboard via the OWS and the carbon/soot from the economiser will totally destroy the filters in the OWS.
The best system would be to have a tank holding acouple of tons of water and pump that through the Economiser letting it drain back to the tank and keep circulating for 24 hours or longer, pressure is not important but you must have a good flow rate to ensure that the water washes all the tubes thoroughly, I think Jimmy pointed out that if the soot was only partially washed off there was a very high risk of a Carbon fire starting in the tube nest.
Once you are well out to Sea you could pump your circulating tank overboard because it does not contain oil, only Carbon and it has never been in the Machinery Space Bilges. BUT NOT in USA waters because it would cause a discolouration of the surface which is illegal there.
Hope all this helps.
It is always better to ask a stupid question than to do a stupid thing.