Precise Level Measurement and Simple Engineering in Extensive Tank Farms

Measuring the liquid in the large tank farms used in the oil and gas industry is essential for accurately determining the volume of the product in the large storage tanks. The tank measuring systems must ensure that the fill level, temperature, pressure, and flow can be precisely measured as part of inventory control. To obtain accurate results, more variables are required for measuring the mass flow than just the PV value of the throughflow.

The legally regulated measurements for accounting purposes are often controlled by special calibrated PLC systems for the tank fill level. In the application example described above, a specially developed “CUBE” PLC from Agidens was previously used. This was based on mass flow measurements with MODBUS communication. However, this measuring system required an additional gateway from MODBUS to PROFIBUS DP— previously a critical component in the control loop.

The MODBUS communication used by the previous system needed be replaced with PROFIBUS DP communication via a new CUBE PLC. The field devices were connected digitally to ensure drift-free and highly precise data transfer. All products had to be approved for use in hazardous areas. Furthermore, the legal requirements regarding system accuracy and certification had to be strictly adhered to. For example, it was crucial that the measurements were accurate to ensure precise accounting and correct invoicing in the future.

This required an efficient solution with simple engineering, meaning it could be easily used for other projects. In addition, product selection would need to be tailored to the scope of the application to avoid oversizing and ensure a high level of cost efficiency.

A DP/PA basic Segment Coupler from FieldConnex® was selected to ensure that the solution was precise and efficient. The combination of a gateway and fieldbus power supply enables a transparent connection to be formed between PROFIBUS PA measuring instruments and PROFIBUS DP. The FieldBarrier® from Pepperl+Fuchs was also used for taking intrinsically safe mass flow measurements on PROFIBUS PA networks. FieldConnex® Segment Protectors were selected for taking Ex d mass flow measurements on PROFIBUS PA networks.

The new solution allows data to be transmitted digitally, considerably improving the level of accuracy compared to the previous connection via 4 – 20 mA. The increased measurement accuracy also ensures greater certainty and clarity when invoicing business parties.

The chosen solution with a DP/PA basic Segment Coupler removes unnecessary interfaces and translates the protocol from PROFIBUS DP to PROFIBUS PA. The number of field devices can be reduced since a single instrument is now able to transmit multiple values. It is also possible to collect additional data and significantly improve the measurement accuracy.

The use of the Segment Coupler for a PA segment means the solution is ideally tailored to small networks. The simple engineering is a key feature of the solution: The DP/PA coupler is fully transparent to the system, so each device can be configured directly as a PROFIBUS DP node—there is no need to configure the settings of the Segment Coupler itself. The distinguishing features of the selected products include simple integration and minimal engineering effort.

The solution is ideally suited for ATEX applications with Ex i or Ex d degree of protection. Since the technology, installation, and operation of FieldConnex® solutions are similar for all hazardous areas, users will find the them familiar and easy to handle.