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SIS System Level Instrument in Tank Area
For a long time, tank farms in petrochemical projects have been designed and implemented mainly according to the relevant design specifications of petrochemical industry. There is no compulsory requirement for the installation of safety instrumentation system in tank farms. At all stages of life cycle, such as design, installation, operation and maintenance management, there are still insufficient risk and risk analysis, improper design selection, unreasonable redundant fault-tolerant structure and lack of clear inspection and measurement. The problems such as trial period and lack of pertinence of preventive maintenance strategy are particularly prominent.
In 2014, the State Administration of Safety Supervision promulgated document 116, which pointed out that there were still insufficient risk and risk analysis, improper design selection, unreasonable redundant fault-tolerant structure, lack of clear inspection and testing cycle, and inadequate pertinence of preventive maintenance strategies in the life cycle of design, installation, operation and maintenance management of China's safety instrumentation system and related safety protection measures. At the same time, the document clearly pointed out that from January 1, 2018, all newly built chemical plants and hazardous chemicals storage facilities involving "two key points and one major" should be designed to meet the requirements of safety instrumentation system. From January 1, 2020, safety instrumentation systems for other newly built chemical installations and hazardous chemicals storage facilities shall implement relevant functional safety standards and design safety instrumentation systems that meet the requirements.
"Two priorities and one major" means the supervision of hazardous chemical processes, hazardous chemicals and major hazard sources under the key supervision of government safety supervision departments, referred to as "two priorities and one major hazard source".
The catalogue of the first batch of key supervised hazardous chemical processes can be consulted.
Secondly, the list of the first batch of hazardous chemicals under key supervision can be consulted.
A major hazard source: a unit that produces, processes, uses or stores hazardous chemicals over a long period of time or temporarily, and the quantity of hazardous chemicals equals or exceeds the critical quantity.
Safety Instrument System
Safety instrumented system (SIS) is mainly the alarm and interlocking part of the plant control system. It is an important part of the automatic control of the factory and enterprise to implement alarm action or adjustment or shutdown control for the detected results in the control system.
Dangerous chemicals tank farm basically constitutes a major hazard source
Tank area is an important part of production equipment in petrochemical enterprises. It is mainly used for storage and transfer of chemical raw materials, intermediate products and finished products. The complexity of chemical production and the relatively dense variety of dangerous chemicals in storage area of chemical plants make it easy for dangerous chemicals to exceed the critical value listed in "Identification of Major Hazardous Sources of Hazardous Chemicals" in storage area and become major hazard sources. It needs to focus on risk analysis and safety monitoring.
Current Situation of Level Instruments in Tank Area
For a long time, tank farms in petrochemical projects have been designed and implemented mainly according to the relevant design specifications of petrochemical industry. There is no compulsory requirement for the installation of safety instrumentation system in tank farms. At all stages of life cycle, such as design, installation, operation and maintenance management, there are still insufficient risk and risk analysis, improper design selection, unreasonable redundant fault-tolerant structure and lack of clear inspection and measurement. The problems such as trial period and lack of pertinence of preventive maintenance strategy are particularly prominent. In order to meet the basic technological requirements in the selection of liquid level gauge for dangerous chemicals, two sets of liquid level gauges are usually installed on a single tank, one for remote transmission and one for on-site display. There is no independent alarm interlocking device for high, high, low and low level liquid level.
Common remote level gauges: radar level gauge, magnetostrictive level gauge; common in-situ level gauge: magnetic tilting plate level gauge
Level Instrument Setting of SIS System in Tank Area
Usually, the installation of instrument in tank farm should consider such aspects as reasonable economy, mature technology, wide selection range, convenient maintenance and calibration, maintenance-free or excellent after-sales service, etc., combined with the investment situation of medium characteristic project and the automation level of the whole project. In order to set up SIS system instruments in tank farm, it is necessary to determine the safety risk level of each tank group, i.e. SIL level, through the analysis of hazard and operability (HAZOP), and determine the setting of tank instruments according to SIL level.
In the detection and control of tank area of major hazard sources, the temperature of tank, pressure and level of tank are the main control parameters. The liquid level of tank may be dangerous of material leaking and evacuation due to the constant changes of production materials and feeds. Therefore, the main control parameters of SIS system in tank area are liquid level. The instrument detection elements and the source point should be set separately, so liquid level should be adopted separately. Two different measuring methods of level gauge and level switch constitute redundancy. The level gauge and level switch alarm high level respectively, and the signal is transmitted to SIS system for comparison. As long as one of them reaches the set value, the system sends out the signal for emergency interlocking.
At present, SIS systems are mostly built in tank farms. There are usually several problems in increasing level gauges or level switches.
1. No installation location: Storage tank does not reserve enough installation interface;
2. Avoid affecting the normal production of tank farm as far as possible, unclear tank, no replacement and less modification;
3. Low investment: direct investment or auxiliary construction cost should be controllable under the premise of meeting the requirements;
4. High reliability requirement: The reliability and stability requirement of the level gauge or switch involved in interlocking is very high.