PURPOSE: To prevent expansion of concentrated ammonium nitrate in a centrifugal pump by arranging a thermal operation cutting means for opening an electric motor control circuit when a temperature attains a predetermined limit temperature, and an attaching means for attaching the thermal operation cutting means to a casing of the centrifugal pump.

CONSTITUTION: A one-shot thermal fuse 2 is arranged on a casing 22 of a centrifugal pump 20, and is inserted in a pipe coupling 14 on the casing 22, and attached by means of attaching means 6, 8. An electrical contact in a pump control circuit is arranged on the thermal fuse 2, and is held in a closing condition by a spring held in a compressed condition by a pellet made of a fusible material which is fused at a temperature lower than an expansion temperature of ammonium nitrate. When the pellet is fused with heat generated by the centrifugal pump 20, the electric contact is opened by releasing the spring, and the centrifugal pump 20 is stopped. Accordingly overheating and concentration of ammonium nitrate are prevented.