Application of ELL ultrasonic external level switch in the SIS system of the spherical tank area

Thread Content

Abstract: Based on the physical properties of liquid hydrocarbons, as well as the working principle and output characteristics of ELL ultrasonic external level switches, the features of these switches in practical use are analyzed. For storage tanks of liquefied hydrocarbons that are hazardous chemicals, the control parameters of the SIS system for such tanks are determined through analysis based on the SIL level and the specific operating conditions at the site. A safety instrumented system that includes ELL ultrasonic external level switches was designed, and a two-out-of-three interlock control scheme was adopted. The ELL ultrasonic external level switch meets the required reliability standards during operation, is simple and easy to maintain, thereby ensuring safe production in liquefied hydrocarbon storage areas. Keywords ELL ultrasonic external level switch, liquefied hydrocarbons, remote transmission of switch failures, safety instrumented systems. Liquefied hydrocarbon substances are classified as fire-hazardous media of Category A and A1, posing a significant risk of fire and explosion. In accordance with the requirements of the \"Technical Guidelines for Light Oil Storage Tanks of China National Petroleum Corporation\", storage tanks shall be equipped with monitoring devices for liquid level, temperature, as well as alarm systems for high high liquid level, high liquid level, low liquid level, and low low liquid level. Article 13 of the Interim Provisions on the Supervision and Management of Major Hazard Sources of Hazardous Chemicals (**Order No. 40 of the State Administration of Work Safety**) stipulates that primary or secondary major hazard sources involving toxic gases, liquefied gases, or highly toxic liquids must be equipped with an independent Safety Instrumented System (SIS). For this project to address the safety hazards in the liquefied hydrocarbon tank area at Sinopec Tianjin Company’s refining department, a new safety instrument system has been installed. Each storage tank is equipped with an ultrasonic external level switch; together with the existing fork-type level switches and radar level gauges, these devices are used to implement a three-out-of-two interlock system for activating emergency shut-off valves. To ensure the normal operational requirements of the company’s various production units, taking into account the requirements of the tank areas and their integration within the SIS system, the newly added level switches must meet the following requirements: a. Installation in explosion-proof tank areas without the need for welding ； b. Install with the container filled, without opening it ； C. No need for tank cleaning; saves time and cost, easy to install, explosion-proof instruments for Zone 1 ； d. The instrument features intelligent self-check and remote fault transmission functions ； e. The measurements should be accurate and stable. The level switches that are widely used in industrial settings include float valve level switches, cable float level switches, capacitive level switches, and ELL ultrasonic external level switches. A comparative analysis was conducted on the principles and characteristics of various types of switches, and based on the conditions at the project site, it was decided to use the ELL ultrasonic external level switch. 1 Working Principle and Applications of the ELL Ultrasonic External Level Switch 1.1 Working Principle The probe of the ELL ultrasonic external level switch is capable of generating frequency-modulated ultrasonic pulses. These pulses travel a certain distance within the tank (0.8–1.5 m), are refracted at the tank walls, and simultaneously propagate through the liquid or gas inside the container. Due to the different transmission rates of ultrasonic waves in liquids and gases, which means different energy losses as the waves propagate, the remaining energy at the point under measurement differs when there is liquid present versus when there is no liquid. The ELL ultrasonic level switch detects whether liquid is present in the tank at the measurement point by receiving and analyzing this remaining energy; it then outputs a digital signal through its own relay, which is sent to the SIS system to enable interlock control of the liquid level. The propagation and refraction of ultrasonic waves within the tank wall are not affected by factors such as bubbles in the medium or pressure. The ELL ultrasonic external level switch features patented frequency conversion technology, which ensures that the switch is not affected by temperature changes, snowy or rainy weather, or medium accumulation on its surface. The switch features an intelligent self-check function; in the event of a fault, a fault signal can be sent to the control room, where the type of fault can be determined by checking the fault code displayed on the switch, allowing for appropriate maintenance to be carried out. 1.2 Installation Instructions: The installation of the ELL ultrasonic external level switch at the upper part of the spherical tank is shown in Figure 1. The container wall is made of Q345R, and the probe is fixed to the outside of the container wall via magnetism. The installation location selected must meet the following conditions: a. The two level measurement probes for one measurement point (the transmitting probe and the receiving probe) need to be installed on the same horizontal plane ； b. The distance between the transmitting probe and the receiving probe is 0.8~1.5 m ； Figure 1: Installation schematic. A 2\" (50mm) carbon steel vertical pipe, 1.0–1.5 m in height, is fixed near the on-site spherical tank using expansion bolts. The transmitter is fixed to the vertical pipe via a bend plate bracket using U-clips. The pipeline connection is shown in Figure 2. Figure 2 Pipeline Connection 2 Storage Tank Area SIS System. The SIS system provides alarms and interlocks for the factory’s production control system, and has gradually become an important component of factory automation control. The SIS system in the tank area provides audible and visual alarms as well as automatic interlock control for parameters such as the pressure, temperature, level, and flow rate of the storage tanks. After identifying and calculating the media within the tank area, it was determined that the liquefied hydrocarbon storage tanks constitute a Class II major hazard source. At the same time, a Hazard and Operability (HAZOP) analysis as required by the relevant departments was conducted, determining that the safety risk level for the liquefied hydrocarbon storage tanks is SIL2. To further enhance the intrinsic safety of the tank area, ensure safe production, and prevent accidents, an SIS system is installed. In the event of a safety incident involving the storage tanks, this system can act instantly and accurately to implement interlock controls for emergency shutdown, thereby stopping the production process safely or automatically bringing it into a predetermined safe state. 2.1 Main parameters for control In the control of storage tanks, the liquid level of the material inside the tank is one of the main control parameters. The liquid level in liquefied hydrocarbon storage tanks changes continuously due to material inflow and outflow, which may pose the risk of the liquefied hydrocarbon being drained completely or leaking from the tank. Therefore, the key control parameter for the SIS system is the tank’s liquid level. The sensing elements and measurement points of the instruments are installed separately; three types of level sensors—external ultrasonic level switches, fork-type switches, and radar level gauges—are used to create a two-out-of-three interlock system. The signals generated by these sensors are sent to the SIS system for comparison, and if both sensors reach their set values, the system sends a signal to trigger emergency interlock actions. 2.2 Design of the SIS system The SIS system consists of field instruments and actuators, and is designed to be fail-safe ； It can communicate with the DCS, but the DCS system is relatively independent from the SIS system. In the SIS system, the use of contactless level alarm devices such as ELL ultrasonic external level switches reduces the need for openings in storage tanks and for hot work, thereby lowering the probability of leaks and fires/explosions involving hazardous materials in sites with high risks. The instrument features intelligent self-checking and fault output alerts, which reduces the need for maintenance and repairs. It offers high stability and reliable operation, as well as improved safety performance. 3 Conclusion Since the design, construction, and commissioning of this project, the SIS system in the liquefied hydrocarbon storage tank area of Sinopec Tianjin Branch’s refining department has operated stably and reliably. Daily inspections and maintenance are straightforward. Moreover, the company has adopted various safety and reliability measures based on its own operational conditions, thereby ensuring the proper functioning of the system and guaranteeing the safe production of liquefied hydrocarbons, as well as environmental safety, along with long-term, stable, and continuous safe operation of the production facilities