A vertical phosphoric acid feed pump failed within weeks of being put into service. Duty - pumping phosphoric acid from the digester to the filters, MOC - CD4MCu Rated pump speed - 1440 rpm Failure - near complete erosion-corrosion of the semi open impeller and cracks similar to IGC on the inner surface of the pump casing/discharge bend and delivery pipe. The inner surface of the discharge bend displayed a fish scale apperance. Note - 1) phosphoric acid (by the wet process) contains traces of chlorine and flourine (as free radical or in combined form / acid) and with around 40% gypsum crystals. 2) No cracks or other signs of damage were observed on the external surface of the pump casing / discharge bend or the discharge pipe.

why was the cracking observed only on the inner surface of the pump casing / discharge bend ? Is it possible that the chlorine and / or flourine present in the acid was released by the churning action of the pump leading to SCC only on the inner surface. The external surface of the casing / discharge bend and discharge pipe showed no signs of cracking thougn essentially immersed in the same fluid.