The ethylene oxide and derivatives (EOD) unit at LyondellBasells Bayport complex is capable of producing multiple products by operating the fixed process equipment on a campaign basis. A campaign typically runs four to six weeks, and the differential pressure (DP) flowmeters are re-ranged between campaigns to accommodate different flow rates.
The unit includes a loop reactor that reacts material with ethylene oxide (EO) and maintains the reaction in a liquid phase. The reactor has an elaborate trip and alarm system that met process requirements for safety integrity level (SIL) 3 and two-out-of-three (2oo3) voting.
The existing arrangement used an orifice plate with four DP flow transmitters, three connected to the safety instrumented system (SIS) and one to the distributed control system (DCS). "The DP flow transmitter impulse lines would plug an average of six times per year," said Sergio Vincencio, senior instrumentation and electrical engineer, Lyondellbasell Industries Bayport Complex, Pasadena, Texas. "We had more than 350 deviation alarms per month."
"The flowmeters required monthly preventive maintenance (PM) and had to be re-ranged for campaign changes, potentially exposing personnel to EO. Ethylene oxide is really dangerous stuff," Vincencio said. "As a result, the plant was spending $85,000 per year in PM and troubleshooting activities."
In an effort to reduce that cost, engineers first reviewed the process hazard analysis (PHA) and SIL calculations. According to Vincencio, "The process people said, SIL 3! SIL 3!' but we did the PHA again and found we did not need SIL 3, that SIL 2 would be OK," with flow-ratio trips serving as an independent layer of protection.
They compared SIL level, proof test interval (PTI) and cost of multiple configurations using two or three Coriolis flowmeters, with or without pump trips or additional on/off valves, and determined that SIL 2 requirements could be met with two Coriolis meters, which would extend the proof-test interval (PTI) from one to 36 months at a cost of $110,000.
Why choose Coriolis?
The former flowmeter installation included 36 potential leak points. Further, impulse lines tended to plug or freeze, measurements were inconsistent, turndown was limited, and accuracy degraded due to plate wear. On the other hand, a best-practice Coriolis installation reduced the potential leak points to two, had no moving parts, required no pressure or temperature compensation, reduced maintenance and extended calibration intervals. The transmitter has a real-time clock with historian and audit logs for regulatory compliance, and in-situ smart meter verification can self-check meter integrity. A USB plug allows data transfer, an SD card backs up the configuration, and the meter is SIL-rated. Lyondellbasells flowmeter requirements were met by Emersons Micro Motion CMFS/5700 Coriolis flowmeters, including:
"We selected CMFS/5700 flowmeters with two outputs, one connected to the SIS, the other to the DCS," Vincencio said. "We have no plugging issues because we have no impulse lines.Our PTI is now 36 months instead of 30 days, and there is no need to re-range due to the Coriolis meters 25:1 turndown, which also increases safety by reducing potential exposure to EO. "We met our process requirements for SIL 2 with 2oo3 voting, and were saving $85,000 per year."