Metering skids are turnkey, fully integrated metering systems for highly precise volume measurement of crude oil, natural gas and refined products. Typical applications include loading and unloading terminals for oil tankers, and measurements at the inlet and the outlet of transport pipelines.
Actuators and valves – in addition to flow meters and pressure and temperature sensors – are key components of such skids. For the most part, double-block-and-bleed valves are used since they are 100 % leakage-free and so prevent measurement errors due to leaking valves. A skid normally has several metering runs arranged in parallel, plus a master run for calibration. Actuators are needed for the flow control valves that direct the oil or gas flow to the individual metering runs and to the master run.
Krohne Oil & Gas B.V. in the Netherlands is one of the world’s leading manufacturers of such metering skids. The company, in cooperation with Auma, has implemented an automated metering solution for a major oil and gas group in the Middle East. Auma’s high-performance Sima² master station plays a key role in meeting the challenging requirements set by the customer.
For this project, Krohne designed and manufactured three metering skids incorporating 36 Auma actuators and one redundant Sima² master station for high-accuracy measurement of crude oil. For the same oil and gas group, Auma is supplying 17 Sima² master stations and several hundred actuators for other projects. Each of the skids is equipped with three Altosonic 5 ultrasonic flowmeters made by Krohne. Within each skid, 12 Auma actuators are used to control the flow rates and to switch between the different metering runs. They are connected to the Sima² master station in three separate loop topologies.
Sima² provides central control and monitoring of all 36 actuators across the three skids. To ensure that the valves can be operated reliably even in the event of a fire, Auma’s proven SAEx actuators and ACExC 01.2 actuator controls are equipped with fireproof shields.
Flow measurements from the metering skids are the basis for calculating the exact volume of oil or gas supplied and billed to the end customer. Any failure of the measurement system very quickly results in high economic losses for the operator. High availability was thus the customer’s prime requirement for the metering skids, and this extended to control and communication as well as the
reliability of the actuators themselves.
The Sima² master station is particularly beneficial here, thanks to its multiple redundancy options and its robust and proven communication via standardised Modbus protocols. For this project, Auma supplied a Sima² with hot stand-by system redundancy. Two Sima² subsystems – cost-efficiently located within a single housing – ensure that operation continues without interruption even if one subsystem fails.
Communication to the DCS is also redundant, via Modbus TCP/IP. The actuators are connected to the Sima² master station via Modbus RTU in a loop topology. Redundancy is also included at this level: if communication fails at a specific position within the loop, the Sima² considers both the resulting segments as individual lines, and all actuators remain accessible. Communication via Modbus is extremely fast and efficient, resulting in very short cycle and reaction times.
Plug and play
The metering skids, including the valves and actuators, were fully assembled and tested at the Krohne plant in Dordrecht in the Netherlands. For transport to the end user, only the external connections had to be separated. The objective was to minimise the steps needed to install and commission the pre-assembled modules at the end customer’s site. The metering skids only need to be connected to the product pipework, power supply and distributed control system (DCS), and they are ready to operate.
Using a master station as a central control hub for all the actuators considerably reduces the external connections needed at actuator level. Only two fieldbus cables are needed per skid to connect the actuators’ Modbus loop to the Sima². At the customer’s site, only the Sima² then needs to be connected to the DCS.
Benefits during commissioning
Commissioning of the actuators took place in the Krohne factory simply using the Sima², without needing connection to a DCS. The large integral multitouch screen allowed all the actuators to be conveniently and intuitively controlled and tested, and communication parameters set. The actuator networks for each of the three metering skids could also easily be pre-configured using Sima².
Commissioning was very fast, thanks to the Sima²’s automatic loop configuration feature, which facilitates actuator address assignment within the network. In this project, setting the parameters at the actuators and adding the devices to the Modbus loop was so easy and intuitive that the entire configuration for all 36 actuators was completed in less than 30 minutes.
Central diagnostic hub
During normal plant operation, using a Sima² master station reduces host communication to a minimum, thus reducing the workload on the DCS. Sima² distributes the individual operation commands to each actuator, receives status updates from all the actuators at cyclic intervals, and transmits only the concentrated data required for regular plant operation to the DCS. In addition, the master station offers a multitude of diagnostic functions. Status and availability are clearly visible at a glance at all levels: from the overall system down to each individual actuator. This speeds up fault localisation and repair.
Auma reacted quickly to additional customer requirements that evolved during the course of the project. For example, a new software feature to enable additional configurable feedback signals was implemented. Krohne uses this feature to monitor the external 24 V power supply to the actuators. Sima² also incorporates integral Ethernet interfaces, opening up new possibilities for enhanced data exchange, including IIoT applications.
Sima² master station has provided real added value for metering skids – in particular thanks to quick and easy commissioning, centralised control of all actuators, and a multitude of diagnostic functions. Efficient, high-speed Modbus communication and integral redundancy at all levels ensure reliable plant operation and optimised processes.
Aauma Riester GmbH & Co.KG, Müllheim, Germany