Precautions for reversing a melt pump

When the molten urea pump is in operation, due to dead zones in the insulation, the discharge valves at the inlet and outlet or the bases of the flushing water valves are usually blocked by crystals. If the water is not removed during pump reversal and the pump body as well as the inlet and outlet pipes are not cleaned promptly after the reversal, it is easy to cause crystalline blockages. With a well-designed process, the pump’s inlet and outlet are equipped with flushing water; water is added immediately after the pump is turned off, which helps prevent pipe blockages.

Indeed, the speed at which crystallization occurs is very fast in certain areas. Generally, when using a pump for this purpose, the inlet and outlet valves are closed first, followed by flushing with water. If you fail to close the pump’s outlet valve, the urine exiting will contain an excessive amount of water, resulting in dilute urine. If you are using granulation and circulation, you can allow water to flow first before closing the outlet valve; however, with this type of pump, it’s necessary to dilute the urine before crystallization takes place.

Reply to 1# Hai Fei: When reversing the direction of operation of a melt pump, it is necessary to preheat the pipelines and the pump body with flushing water. The specific steps for reversing the pump’s operation are the same as those for a single-stage centrifugal pump. There are just a few points to note: after stopping the pump, do not flush both the inlet and outlet simultaneously, as this can easily clog the inlet flushing pipeline. When reversing the pump, it is best to keep the granulation in a circulating state to prevent the nozzle from becoming too thin. If it cannot be confirmed that the flushing pipeline is unobstructed, do not discharge water rashly, as this may cause blockages in the discharge pipeline. The pump should be stopped only after the outlet valve is closed

6.3.2 P-108A/B Pump Inversion Method (Taking A inverting to B as an example) (1) Preheat Pump B using the inlet LS and outlet CY of the pump; the preheating standard is that no condensate should be present in the steam discharged from outlet CY. (2) While preheating the B pump, conduct a verification check on it, including: ① The oil level in the bearing box should be at 1/3–1/2, and the oil level in the oil cup should be at 50%–60%. ② Rotate the crankshaft 3-5 times; it moves smoothly without any sticking. ③ The cooling water inlet and outlet valves for the bearing box are both open, and the return water gauge shows normal levels. ④ Turn on the seal purge steam; a small amount of steam flowing out is sufficient. ⑤ The outlet pressure gauge is in good condition. (3) After checking pump B and confirming that it is in good condition, as well as ensuring that the pump body has been properly preheated, close the inlet purge LS and the outlet CY. Contact the main control system to initiate the evaporation circulation process, in preparation for switching pumps. (4) Slightly open the outlet valve of pump B to allow fluid to flow in, and slightly open the inlet CY to expel air from the pump. Once the urine has been discharged through CY, close it and rinse it thoroughly. (5) Close the outlet shut-off valve of pump B, start pump B, quickly open the inlet valve, slowly open the outlet valve, and close the outlet valve of pump A. (6) Once Pump B is operating normally, stop Pump A and flush it according to the pump shutdown procedure.

In fact, in our factory, the melt return pump is usually installed during the evaporation and recirculation process, so it’s less likely to encounter problems. If an emergency pump reversal is indeed necessary, we should first switch to circulation mode and then reverse the pump.

Actually, reversing the pump isn’t that troublesome. 1. Check whether the purge steam pipes of both pumps are unobstructed, as well as the oil level of the pump that is to be shut down and whether it can be rotated. 2. Preheat the pump that is to be shut down using steam until its inlet and outlet become warm. 3. Turn off the steam, slightly open the outlet valve, quickly open the inlet valve, and start the pump. 4. Gradually fully open the outlet valve, while closing the outlet valve of the other pump. 5. Allow some steam to flow into the pump that is to be stopped; turn off the steam as soon as abnormal noises are heard from the other pump, and quickly close the inlet valve. 6. Let some steam enter to prevent crystallization inside the pump

Reply to 7# huangniansun: What process is used in your urea production facility? Won’t connecting a liquid lead from the outlet of the standby pump cause the pump to reverse? It seems that the reverse pump isn’t in circulation; will this have an impact on urea granulation?

Reply to 8# htcfkq: Aqueous solutions have no effect on particles