Liquid-phase pre-cooling of liquid oxygen pump

Operating and Maintenance Procedures for Cryogenic Liquid Pumps 1. Scope These procedures apply to single-cylinder double-suction reciprocating pumps used for pumping liquid oxygen, liquid nitrogen, and liquid argon. ⒉ Preparations before enabling version 2.1 2.1.1 Disconnect the pump from the drive gearbox. 2.1.2 Remove the cold-end flange, filter, cylinder block, and piston components; clean them thoroughly with carbon tetrachloride, dry them, and then reinstall them in place. 2.1.3 Open the transmission box cover to check for any foreign objects inside, and verify that the rod end shaft retaining ring is in good condition (as well as whether the tension of the gears in the gear-driven pump is appropriate). 2.1.4 Fill the transmission case with N68 mechanical oil (anti-freeze oil in winter), and the oil level should be at the center of the rod end shaft. 2.1.5 Inspect the belt drive section, adjust the tightness of the belt to the desired level, and turn the shaft to check for smooth operation (for gear-driven pumps, turn the coupling to verify smooth operation). 2.1.6 Check and adjust the controller according to the instruction manual for the electromagnetic variable-speed motor controller (this step is not required for gear-driven pumps with a fixed speed). 2.1.7 Connect all components between the gearbox and the pump, carefully adjust the connection bolts so that the piston’s travel limits meet the required specifications, and manually rotate the shaft to check for any signs of sticking, until the movement is smooth. 2.1.8 Check whether the connections at each pipeline joint are secure and whether the airtightness is good. 2.1.9 Pre-cooling of the pump body: The cryogenic liquid pump must be pre-cooled before it is started; it is not allowed to operate while still hot. Keep the pressure in the storage tank between 0.3 and 0.5 Mpa. Open the inlet valve on the storage tank to allow liquid to enter the cylinder, and open the pre-cooling valve to discharge the vaporized liquid. The temperature of the pump body gradually drops to the operating temperature (at this point, the pre-cooling valve continues to discharge liquid), after which rotate the pump shaft to check for any signs of jamming. 2.2 Startup and Operation 2.2.1 Open the drain valve on the full-open drain line, start the motor, open the pump’s vent valve, adjust the pump’s speed to 150 rpm (with the motor rotating at 400 rpm), and check whether the pump is operating properly. 2.2.2 Once everything is normal, gradually increase the speed to 600 rpm (motor). 2.2.3 Check whether there is any leakage in the piston rod seal ring and whether ice has formed on the rod; if leakage occurs, the compression nut can be tightened slightly to increase the preload. 2.2.4 Check whether there are any leaks in the gaskets of each joint. 2.2.5 Once everything is normal, gradually increase the pump speed until the desired flow rate is achieved. It is advisable to keep it at no more than 950 rpm (for the motor). 2.2.6 When stopping the machine, first shut down the motor; only then can the inlet, outlet, and return valves of the pipeline be closed. Open the pre-cooling valve to drain the liquid from the pump, and then close that valve as well. For liquid pumps with a fixed speed, steps 2.2.1, 2.2.2, and 2.2.5 are not applicable; instead, the discharge valve should be opened, the motor started, the return air valve opened, and it should be checked whether the pump is rotating properly. When filling with an argon-carbon mixture, during pre-cooling, when shutting down, and when the pressure in the tank is high, the diaphragm pump should be started simultaneously for filling; refer to the operating and maintenance procedures for the diaphragm pump. ⒊ Maintenance 3.1 Before each start-up, check the piston rod coupling nuts and turn the pump manually to verify that there is no sticking and that the cylinder is not being pressed. 3.2 Pay constant attention to the sound the pump makes during operation; if anything abnormal is detected, stop the pump immediately for inspection in order to resolve the issue. 3.3 Maintain the oil level in the transmission box; stop the machine if the oil temperature becomes too high. Check the oil quality and cleanliness. 3.4 Operators should constantly monitor the seals (including oil seals) for any signs of leakage; this is especially important when the working medium is liquid oxygen, as otherwise it will affect the safe operation of the pump. 3.5 When the vacuum level of the pump decreases, re-evacuation should be considered. 3.6 A new pump gasket should be installed immediately after the joint is removed to ensure reliable sealing. 3.7 The pump should be disassembled after it has reached room temperature; otherwise, the components may be damaged. 3.8 When the pump will not be used for an extended period, it should be disassembled and cleaned before being stored; it should be kept under nitrogen atmosphere, and the transmission parts should be lubricated for protection.

Points to note in the operation of centrifugal liquid oxygen pumps: Centrifugal liquid oxygen pumps are primarily used to increase the pressure of liquid oxygen in order to enable its transportation. It is commonly used as the liquid oxygen circulation pump in the main condensation evaporator, the process liquid oxygen pump arranged in parallel between the upper and lower towers, the product liquid oxygen pump in internal compression processes, and the liquid oxygen pump for storage tanks. Its working principle is similar to that of a regular pump, utilizing centrifugal force to increase the pressure of liquid oxygen. However, it operates at low temperatures, and liquid oxygen is both the product and a powerful oxidizer; therefore, there are special requirements for operating liquid oxygen pumps. First of all, it is necessary to ensure that the pump reaches its rated flow rate, head, speed, power, and efficiency. Before starting, the liquid oxygen pump must be thoroughly cooled with liquid oxygen. Open the exhaust valve at the top of the pump and the discharge valve at the bottom of the pump; once frost forms on the discharge pipe and liquid appears, close both valves ； Make sure to check whether the pump impeller is stuck. Open the outlet bypass valve of the pump, and intermittently start the liquid oxygen pump ； After it is officially started, pay attention to whether the outlet pressure of the pump is stable. If the pressure fluctuates or does not rise (i.e., cavitation occurs), it is necessary to open the exhaust valve at the top of the pump again, continue cooling the pump, and then start it up. “The occurrence of the \"cavitation\" phenomenon not only subjects the impeller material to severe impacts, resulting in erosion spots on its surface, but it also disrupts the stable flow of the liquid inside the impeller as well as the normal distribution of velocity and pressure. This leads to a sudden drop in the pump’s head, a sharp decrease in efficiency, and intense vibrations in the pump body that can cause it to stop operating. This is a fault that must be avoided at all costs during operation. Additionally, during the operation of the liquid oxygen pump, attention should be paid to the pressure changes inside the liquid oxygen tank. Since some liquid or gas returns to the liquid oxygen tank during the startup process or while the pump is in operation, the flow rate will inevitably affect the internal pressure of the tank; therefore, a dedicated person must monitor this pressure gauge during operation. For liquid oxygen pumps used in storage tanks, when it is necessary to evaporate liquid oxygen to supply it to oxygen users, after pre-cooling and starting the pump, and once the outlet pressure reaches the desired level, it is necessary to check whether there are any leaks in the pump body and pipelines, whether the discharge valve is closed, and to confirm that vapor has been introduced into the evaporator. Only after the manual valve at the oxygen outlet is opened can the liquid oxygen be sent out, with the flow rate of oxygen being adjusted gradually. Check the oxygen outlet pipeline for frost; if the temperature at the oxygen outlet is found to be too low, the outlet valve should be closed immediately to stop the evaporation of liquid oxygen. The evaporator oxygen outlet temperature controller is usually set at around -15°C; below this temperature, the oxygen supply valve should automatically shut off to prevent the carbon steel pipes from freezing and cracking.

Cooling process: • When cooling of the pump starts, the DCS displays “Pre-cooling not completed”, and the timer shows 0； •In the DCS, press the start button (“HS6754”) to initiate the pre-cooling procedure ； •The inlet valve and bypass valve open automatically, and the timer starts counting time ； •When the exhaust temperature is below –170°C and the timing reaches the set value (150 MIN). Then it can be considered that the cooling is complete ； •Once cooling is complete, the DCS displays “Pre-cooling finished”; the pump is ready to be started ； •It was confirmed on site that the pump was not running ； Note: The cooling time and temperature are determined by the commissioning engineer based on the actual conditions on site; these values are provided here for reference only ； There is no need to open the blow-off valves before and after the pump; only the exhaust valve at the pump outlet and the drain valve at the bottom need to be opened, and that’s it once liquid starts to flow out.

The general pre-cooling method is as follows: Open the inlet valve and the reflux valve, slightly open the pump’s inlet and outlet valves as well as the blow-off valve on the pump body, so that the pump can be pre-cooled. Pre-cooling usually takes about 1 hour; after that, liquid is drained, and if liquid continues to flow out, it is considered that the pre-cooling process is complete.

During pump pre-cooling, first open the pump inlet valve and the pump body purging valve to carry out pre-cooling. Pre-cooling is completed when continuous liquid discharge occurs.

The blow-down drain can be turned on to speed up the pre-cooling process and to remove air as well. Regarding the drain valve at the back of the pump you mentioned, pre-cooling occurs as soon as liquid is detected by the pump; I don’t think that’s necessarily the case. We close the drain valve only after liquid is detected, and it takes around more than 2 hours for the pump to be fully pre-cooled.