Questions about shutting down large units due to a disruption in circulating water?
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The ethylene plant was originally designed with an interlock that activated when the low pressure difference in the circulating water (less than 120 kPa between the inlet and return water) was reached; however, this interlock was later removed for various reasons. This results in the unit continuing to operate when the circulating water pressure difference is low or approaches 120 kPa. The remedial measure is to inject fire-fighting water into the unit’s lubricating oil cooler to cool the lubricating oil to its normal temperature, ensuring the unit remains operational for a certain period of time; once the water pressure is restored, production can be resumed promptly. Everyone analyze the advantages and disadvantages of this change. My questions are as follows: 1. When the circulating water is interrupted and the low pressure differential interlock is activated, it will inevitably cause the unit to shut down; at this time, the emergency oil pump or the high-level oil tank supplies the amount of lubricating oil required for the unit to operate in idle mode. In this way, the device remains in a safe state. If the interlock that shuts down the generator due to a disruption in the circulating water is disabled, other interlocks such as those related to bearing temperature or the vacuum level of the surface cooler may come into effect. The unit will continue to operate during this time, which leads to an increase in the temperature of the lubricating oil (due to the lack of circulating water), posing a risk to the unit. 2. If the circulating water pressure fluctuates for a short period of time, and in line with the objectives of the modification, fire water is promptly introduced into the oil cooler to control the temperature of the lubricating oil, the unit can continue to operate. Restoring the circulating water pressure in a timely manner allows production to be maintained, preventing a shutdown of the entire plant. Everyone analyzed the pros and cons and spoke freely.
As long as the remedial measures are implemented in a timely manner, I believe it is feasible from the perspective of maintaining production stability. For large-scale units, it is sufficient to ensure that they can be cooled down; there are no excessive requirements regarding cooling water. The circulation of water may be interrupted, but fire-fighting water belongs to the category of critical supplies and will not be interrupted, so this should work. The key is that the remedial measures must specify responsibilities and establish operation guidelines or key procedures to prevent mistakes or delays in action that could lead to accidents
This is only a temporary emergency measure; it cannot be treated as a systematic approach to safe operation. Interlock contacts ensure safe operation. So it was temporarily lifted, and it will be reinstated once conditions permit.
As for the shutdowns caused by fluctuations in pressure, you can avoid this by adjusting the parameters; for example, you can set a longer time interval of 10 seconds, 20 seconds, or one minute, or you can widen the range allowed for pressure differences. This requires your manufacturing process to meet the optimal requirements.
The number of interconnections should be as low as possible, but this is subject to the level of harm caused and its severity; in the case of circulating water in machinery systems, I think it’s sufficient to just issue an alarm!