Liquid level interlock for small containers
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In accordance with the requirements of documents issued by regulatory authorities such as the Safety Supervision Bureau, storage tanks that are classified as major hazard sources of level 1 or 2 shall be equipped with appropriate alarm and interlock protection measures for low-liquid-level and high-liquid-level conditions. In a chemical plant’s hazardous chemicals storage area, there is a wide variety of tanks, with each tank having a capacity of less than 100 m3. Based on calculations, these tanks are classified as secondary major hazard sources. Is it necessary to install a level interlock for all of these tanks? If set, there isn’t much medium between the high liquid level and the very high liquid level in these small tanks, making it difficult to meet the flow rate requirement of 10–15 minutes of fluid inflow.
A tank of 100m3 is referred to as these small tanks. . . . If it is mandated by safety regulations, then it must be installed. Is it in conflict with the flow rate requirement of infusing over 10~15 minutes? What do you mean by the 10–15 minute feeding requirement?
It mainly depends on whether there is enough time for manual intervention and adjustment after a high liquid level alarm or an extremely high liquid level alarm is triggered.
1. For storage tanks classified as secondary major hazard sources, if the medium is a hazardous chemical subject to strict supervision, a high and low liquid level interlock system must be installed, and it must be integrated into the SIS system. If it does not fall under the category of hazardous chemicals subject to strict regulation, it is not necessary to include it in the SIS system; however, it is still recommended to install a high liquid level interlock. 2. For the flow rate requirement of 10~15 minutes, the standard term is “preferably”. If the pump has a high flow rate and the tank is small, it can be reduced as appropriate; otherwise, the effective volume of the tank will become very small.
According to AQ 3036-2010 \"Code for the Installation of Safety Monitoring Equipment at Sites of Major Hazard Sources of Hazardous Chemicals\", section 6.3.7 stipulates that large flammable liquid storage tanks (with a capacity of over 5000 m3) and pressure storage tanks for hazardous chemicals (with a capacity of over 400 m3) shall be equipped with separate high-level liquid level monitoring, alarm, and interlock control systems. In fact, interlocking is only implemented for various types of tanks that meet certain size requirements. The standards set by departments such as CNPC are actually even higher; however, after a series of regulations were issued by the Work Safety Supervision Bureau, some of those thresholds were lowered. Later documents from the Safety Supervision Bureau state that the size of the tanks should be determined in accordance with relevant standards, while other documents do not mention this at all. Does this mean that all tanks are required to have high and low interlocks?
The regulation means that large storage tanks require high-high and low-low interlocks, while conventional storage tanks only need high and low interlocks.
The current standards are very strict; it is necessary to follow them during fire safety inspections, and anything required by the standards must be included
For small tanks, the values between low and very low liquid levels are also small; either the reaction time is insufficient, or the low liquid level threshold needs to be set higher, which results in frequent low liquid level alarms.
Volume alone cannot be considered: Clause 5.4 of SH/T3007-2014 requires that tanks with toxicity levels I and II be equipped with high-high interlocks, while raw material tanks already have low-low interlocks to shut down the pumps.