Request interlock settings for liquid oxygen pump redundancy

This post was last edited by wcq1010 on 2015-10-21 at 19:58. It is still difficult to perform an emergency switch to a liquid pump without disruption in emergency situations. Using the operation signal to stop as a switching signal is certainly not reliable; if the pump pressure is low, cavitation occurs. This is a situation where the liquid pump must be switched over urgently. As signals for switching, pump inlet and outlet differential pressure information and pump outlet pressure are generally used. Differential pressure is generally the more common method. But automatic switching can still achieve a complete switch. All require manual intervention and adjustment. As far as I know, there are hardly any that automatically switch. All require manual adjustment and intervention.

Is the interlock value appropriate? Also, check the cold standby status of the backup pump to see if it meets the startup requirements~~

Can this issue be considered from the following aspects: 1. Failure of the oxygen pump to operate (such as power outages or equipment problems), which would cause the pressure in the pump to drop suddenly; in such cases, the backup pump must start immediately to ensure that the pressure in the oxygen supply network does not drop significantly, thereby avoiding disruptions to the customers’ use of oxygen. In general, when such a situation occurs, the spherical tank should be able to provide a temporary solution, which is just enough time for the backup pump to start up and reach normal operating conditions. Or the carburetor is controlled based on low pressure, starting automatically when pressure is low and stopping automatically when pressure is high. 2. The oxygen pump experienced unstable pressure due to cavitation, but the pump did not stop operating. In this situation, if separate pressure interlocks are used for the backup pumps, could it happen that both pumps operate at the same time? In this case, it is possible for the two pumps to affect each other, resulting in more unstable pressure. Additionally, oxygen venting at the back when the pressure is high can cause further interference, making it even harder to control the operating conditions. In such cases, theoretically the pump should be equipped with a discharge valve designed to prevent cavitation; to avoid pump cavitation, the oxygen pump needs to be discharged regularly. This should eliminate the cavitation problem. If it really cannot be eliminated, should another condition be added, such as current? When cavitation occurs, the pump’s current should decrease; therefore, if this level remains low for a certain period of time, could the pump be shut down automatically? In that case, the backup pump would be switched in, just as in the first scenario.

Personal opinion: Theoretically, it is possible to achieve seamless switching between two oxygen pumps, but in practice this is quite difficult to accomplish. For example, once the backup pump starts, the return valve adjusts itself based on pressure; similarly, the vent valve also adjusts automatically according to pressure or flow rate. With both valves operating automatically, the pressure and flow of oxygen tend to become unstable. Generally, the interlock system for backup oxygen pumps involves the main pump triggering various interlocks such as low pressure at the outlet of the oxygen pump (or a low pressure difference between the inlet and outlet), low pressure of the seal gas used by the oxygen pump, high or low current levels in the oxygen pump, high or low temperatures of the pump’s bearings, low temperature due to seal gas leakage, and the disappearance of the signal indicating that the main pump is not operating. These conditions cause the backup pump to start automatically and then operate using the same parameters as the main pump. In some cases, frequency converters and return valves are used for automatic adjustment; in others, adjustments are done manually. Still in other situations, one pump operates manually while the other does so automatically. Once the backup pump is functioning properly, the main pump should be stopped manually. If the main pump continues to operate, the simultaneous operation of both pumps will make it impossible to adjust the operating conditions. I feel that operating in this way might be more stable: when the main pump triggers the above-mentioned interlock to start the backup pump (copying the parameters of the main pump), the frequency converter and the return valve remain in manual mode, while the oxygen vent valve adjusts automatically. If necessary, the operator can switch to manual mode for adjustments, and the return valve of the backup pump can also be manually fine-tuned promptly to ensure proper operation. Once the above interlock is triggered, the main pump can automatically reduce the load to its minimum level after a 15–20 second delay (this prevents difficulties in adjusting the operating conditions when both pumps are running simultaneously, and it also reduces the workload on the operators, allowing them to focus on adjusting the operating conditions). If the main pump fails and is unable to maintain the minimum load level, it can be stopped manually.

Thank you for the responses from the experts. In my opinion, the interlock condition based on the operation signal has significant limitations; it only triggers in the event of a power failure. Most accidents are actually caused by cavitation in the oxygen pump. I believe that using the pump outlet pressure and current signals as the interlock conditions would be more appropriate, as this would also eliminate the risk of both pumps operating simultaneously due to faults in the outlet pressure transmitter.

Hello, thank you for your reply. I’m still not quite clear about the interlock conditions for the liquid oxygen pump, such as shaft temperature and seal gas pressure. As for the inlet and outlet pressure difference, we have a pressure gauge on the inlet side; this can be modified to meet the requirements.

The interlock values are appropriate, and the standby pump operates in idle mode; mainly, the operating signal conditions are not met, so the mutual backup condition cannot be achieved.

Use inlet and outlet pressure difference interlock! When the outlet pressure of the operating pump is low and the interlock due to a low pressure difference between the inlet and outlet is triggered, the manual controller of the standby pump’s frequency converter switches from manual mode to automatic mode. Starting from the original idle speed, the controller increases the valve opening by 5 percentage points per second until it reaches 100%. At this point, the operating pump has already stopped operating due to the low pressure difference, and the return valve is fully open. The operator must manually reduce the opening of the standby pump’s return valve and adjust the valve setting via the controller in order to ensure that the outlet pressure meets the process requirements. The fundamental reason for triggering the interlock that causes the standby pump to increase its load is still the shutdown of the operating pump! However, in addition to various equipment-related reasons, the reason for the pump stopping operation also includes the pressure difference between the pump’s inlet and outlet!

Then create another interlock: when the pressure difference between the pump’s inlet and outlet is low, the pump should shut down via this interlock! Add a transmitter to the field gauge and connect it to the main controller, implementing a pressure two-out-of-two interlock; this will prevent erroneous operations caused by a failure of a single transmitter! It’s simple to avoid dual-pump operation – just have the pump with lower pressure shut down automatically as a result of this.