BP has recently embarked on an upgrade to the oil monitoring systems installed at its lube oil storage facility in Ellesmere Port, Cheshire. For environmental, safety and operational reasons, the company sought higher accuracy and greater reliability of oil level measurement in large storage vessels at the site, and its aim was to achieve this economically, without having to resort to a bespoke solution that would likely introduce maintenance issues and compromise future upgrades - particularly those necessitated by future changes to site safety requirements.The Warrington based systems integrator, ATG was asked to draw up a plan for the new 'High Level Alarm System', including all new hardware and software, plus instrumentation, field wiring, controllers and a suitable fieldbus network serving both the North and South tank farms at the Ellesmere Port site. In particular, BP was keen to maintain an 'open' system (ie - more than one supplier's equipment on a common communications network), allowing it complete control over the equipment supply chain and avoiding an undesirable 'lock-in' with a single vendor.ATG's first task was to undertake an initial benchmarking trial to verify the performance of BP's chosen level measurement technology - radar - against periodic manual level gauging. Indeed, this project provided the first opportunity for BP to test the effectiveness of radar instrumentation for variable level measurement duties on an application of this kind. Radar devices were subsequently purchased from Siemens - the Sitrans LR400 - and from Endress+Hauser - the MicroPilot M Series, and these were fitted to two separate tanks using existing pipe-work and flanges, and wired to remote panels via a Profibus PA network. Process visualisation is provided by Siemens' WinCC SCADA connected via a Profibus DP link to the master PLC, a Siemens S7 400. Siemens high level probes are also incorporated into the scheme for alarm purposes, the frequency-shift type products being suitably housed for an Exe rated environment. Temperature probes complete the inventory of instrumentation.ATG proposed a system architecture that saw the S7 400 PLC as the main gateway for all I/O and fieldbus connections. This master controller is housed within a standard Rittal panel located in the Pump Motor Control Centre substation, and communicates with all 'outstation' nodes via Profibus DP. The outstation Profibus nodes are located centrally with respect to each tank farm and are housed within stainless steel enclosures of a 'standard' design that BP will be able to roll out to other areas of the site as future upgrades dictate. These panels are designed, built and tested by ATG using off-the-shelf components from Siemens, ABB or any LV supplier who is able to meet the specification. BP has been issued with the design electronically and could, if wished, get the panels built elsewhere, thus giving the company even more control of the supply chain.The temperature probes and level sensing instrumentation serving each tank farm are connected to their respective outstations via Profibus PA networks.Although the initial WinCC SCADA installation is only tasked with providing system information and alarm data for the central control operation, the future plan is to extend this system via Ethernet so that administrative site staff will have 'read only' access to the process data in an Excel format.Non-contact level measurement using microwave radar has proven itself in thousands of applications as a safe method for liquids in extreme process conditions. Among the advantages of level measurement by radar are: microwaves do not require a carrier medium and are largely independent of process pressure and temperature; application conditions can range from vacuum to 100bar and temperatures up to 400°C - ideal for aggressive media applications where gases or vapours are present.The Sitrans LR400 is a four-wire, 24GHz frequency modulated continuous wave (FMCW) radar level transmitter for monitoring liquid and slurry levels in vessels of up to 50m height. It is available in both standard and explosion proof formats. The device can be locally programmed using an intrinsically safe, infra red handheld programming tool, and communication options include HART and Profibus PA.Endress+Hauser's Micropilot M series of radar level measurement instruments has models to suit a diverse range of applications including a choice of frequency (6 or 26GHz), process connection and materials. Micropilot M utilises cost-effective two-wire technology with outputs that include 4-20mA, HART, Profibus PA and Foundation Fieldbus, and it is suitable for hazardous area applications.ATG has now completed the first phase upgrade work at Ellesmere Port, and the radar system has met all of BP's criteria. BP engineering supervisor, Ian Turner is delighted with the outcome. "We are passionate about maintaining high safety standards and we are always looking for opportunities to improve our systems further," he says. "ATG provided a cost effective solution using off-the-shelf components, which will in turn give BP greater flexibility in future upgrades of the system."
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