Issue regarding the need to interlock pump shutdown in case of low-low liquid level alarm in the raw material storage tank of the unit

It’s not beneficial for the storage tank; it’s beneficial for the pump. At low liquid levels, the pump cannot feed properly, it can’t build up pressure, and as a result, the material cannot be transported, leading to a disruption in supply. If the pump runs idle for a long time, it will get damaged.

“It is not mandatory; it can be decided according to the circumstances.

Whether to shut down the pump via interlock should be coordinated with the plant process. If the unit has measures to handle emergency pump shutdowns, interlocked pump shutdown in the tank farm is acceptable. Otherwise, it is best not to perform interlocked pump shutdown.

1. The material feeding system for the storage tank is equipped with a low liquid level alarm and a low-low liquid level interlock； 2. In actual production operations, the operating liquid level in storage tanks is always above the low-level alarm threshold; for conventional internal floating roof tanks this level is around 2 meters, while for dome-shaped tanks it is around 1 meter. This prevents the pumps from shutting down as a result of interlock mechanisms ； 3. The low liquid level interlock for storage tanks is not a mandatory requirement, whereas the high liquid level interlock for storage tanks is mandatory.

In fact, when the low liquid level interlock triggers the pump to start, it’s not just an alarm; it should be treated as an accident. Therefore, it was absolutely unacceptable to fail to take any action despite the low liquid level alarm having persisted for 15 minutes before it turned into a low-low liquid level interlock. The purpose of the interlocked pump shutdown is to prevent fires that could occur as a result of the pump operating in an empty state for an extended period. Even if the pumps shut down due to this interlock, the device’s buffer tanks can ensure a supply of fluid for 15 minutes, so that the device does not stop operating immediately. Therefore, when the liquid level in the device drops below a certain threshold, an alarm is triggered to warn of a lack of fluid supply. In my humble opinion, one should not give up on the idea of implementing interlocks just because the term “suitable” is mentioned. If an accident does occur, those who decided not to use interlocks will have to face the consequences. At the very least, those of us involved in storage and transportation should take measures to protect ourselves, informing the equipment about the risk of material shortages; this way, the equipment might be designed with better buffer tanks or alarm systems.

I think a low liquid level alarm is necessary, as it can remind operators to change the tank, although the tank should be changed even before the low liquid level alarm triggers. But it’s not necessary to shut down the pump automatically when the liquid level is low. Relatively speaking, the plant is after all more important than the tank area; a low liquid level can still allow the plant to receive supply, and if the operators take timely action, normal operation of the plant can still be ensured. But if the pumps stop due to a low-level interlock, the supply of raw materials to the device is immediately cut off, resulting in even greater hazards. So it is advisable to set it up rather than necessary to do so. For the purposes of this specification, I personally think this item is unnecessary.

Reply to the person above: Actually, everyone faces the problem you mentioned, and it’s quite difficult to solve it. We always consider the operators and the equipment, worrying about interruptions in the supply of material. In my opinion, since the regulations require it, we should follow those rules. At the same time, we need to inform the operators that if an interlock causes the pump to stop, it should be treated as an accident, so that they pay attention to this. This also serves as a warning to the operators, as situations where the pump stops almost never occur. But if, in the unlikely event that the pump runs dry and an accident occurs, the accident investigation will first focus on why interlocks to stop the pump were not installed in accordance with the regulations; at that point, there will be no discussion about whether it is \"appropriate\" or \"necessary\" to do so, and the management will definitely bear responsibility. “Whether it is \"appropriate\" or \"necessary\", I believe it is mainly to take into account the older tank areas that lack automatic valves; it’s not possible to apply a one-size-fits-all approach. If conditions permit the installation of interlocks, it is recommended to do so, even if they aren’t used at all times. It is possible to increase the time interval between low-low liquid levels and low liquid levels. What’s most concerning actually is the possibility of false readings from the instruments leading to interlocks; it is likely necessary to set up a system in which both sets of instruments must indicate the interlock level before the pump is stopped.

It’s not very suitable in some cases; a specific analysis is needed. Crude oil entering the plant usually first goes into the electrodesalination and dewatering tank. That tank operates at full capacity; even when the pump stops, there is still crude oil in the electrodesalination tank. If an interlock system is in place to stop the pump, a low liquid level alarm will be triggered, and the pump itself also has alarms for such conditions. The electrodesalinated water tank should likewise have liquid level indicators and alarms. If specific procedures and responsibilities are established, with dedicated personnel in charge, issues can be detected promptly; however, it should still be possible to detect them in a timely manner even without such measures. . . Perhaps it’s time to analyze the event tree to examine the potential accident consequences and probabilities in cases with interlocks and without them. There are interlocking scenarios: Interlock-1, liquid level alarm in the tank-2, pump with display or alarm-3, liquid level display and alarm in the electrodialysis tank, and even interlock. . . -As a consequence, the supply of raw materials is interrupted, and the plant stops operating. Plants are generally equipped with complete interlock shutdown systems; in the event of a material shortage, alarms and interlocks should be activated for conditions such as overheating and low liquid levels, and manual shutdown procedures are also available. The device features a high level of human control as well as automatic control. No interlock situation: No interlock – 1; Liquid level alarm in the tank – 2; Personnel conducting on-site inspections – 3; Float disk at the bottom; Pump running idle, pump damaged or leaking; Flammable gas alarm present on site; Possible transmission of pump pressure; On-site pressure gauge; Detected by personnel during inspections. . . ==Let’s analyze it further to see which one is better and has a lower cost. . .

There is no mandatory requirement in the standards; it can be designed according to actual circumstances. But for the purpose of protecting storage and transportation facilities, it is still designed that way. Material is fed into the device; generally, the device itself has a small buffer tank that can hold it for a while. The low-low interlock for storage tanks serves to protect the safe operation of the tanks as well as the pumps. If the level of automation is high, two sets of level gauges and fork-type level switches can be installed, employing a \"two-out-of-three\" system, which effectively prevents accidental shutdowns of the pump due to false signals from a single switch, thus avoiding interruptions in material supply. In control systems, spare storage tanks are installed to enable automatic switching of tanks when the liquid level is low, thereby preventing interlock-induced pump shutdowns due to delayed tank switching.

We encountered this issue when designing new tank areas before as well; mainly, we often have to empty the contents of the tanks, which is problematic: either we need to disable the low-liquid interlock in advance, or we have to leave some material inside the tanks.