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LyondellBasell eliminates SIS trips with differential pressure flow solution

Jim Montague

It takes a lot of petrochemical production capacity to make the building blocks for the world's clothing, packaging, construction materials, auto parts, furniture and bedding. And at 3,900 acres and about 2,000 employees, LyondellBasell's Equistar Chemicals complex in Channelview, Texas, is one of the biggest on the U.S. Gulf Coast.

The complex has two world-scale olefins units that manufacture ethylene, propylene, butadiene and benzene, and in 2015, two new cracking furnaces were added that were three times the size of the older units. Within each furnace, gas to be cracked passes through eight tubes instrumented with eight pressure transmitters per tube—four for the feed and four for dilution steam. Each tube also has two orifice plates, one each to measure the hydrocarbon feed rate and the other to measure the dilution steam flow rate. Each of the orifice plates has four differential pressure (D/P) transmitters installed across it, including one for the basic process control system (BPCS) and three for the safety instrumented system (SIS).

"Each of the eight tubes runs hydrocarbons through it for cracking into other products, but failure to control the flow could cause a fire and burn the furnace down," said Wes Nance, senior I&E and IEA reliability engineer, LyondellBasell, who presented "Run and don't trip: LyondellBasell improves SIS on cracking furnace feed and dilution steam flows" at the 2019 Emerson Global Users Exchange in Nashville, Tennessee.

Overcoming some flaws

To operate the furnaces safely and reliably, LyondellBasell had to compensate for some design and engineering flaws, Nance said. "The EPC firm we bought the license from built the furnaces to their specifications, not ours, so important details concerning the process were either not known or not considered," he explained. "These included excessive impulse line lengths greater than 50 feet in distance and more than 30 feet in elevation, as well as poor choice for scaling upper range value (URV) that was originally set at 25 inches of H2O.”

"This poor design and engineering resulted in massive problems with furnace reliability. We had multiple furnace trips due to inaccurate measurements and excessive variability from both feed and dilution steam meters. Startups required calling out a technician to blow down transmitters and readjust the zero. And we had recurring Priority 1 work orders to correct problems, which interrupted maintenance schedules and was detrimental to productivity."

Nance added that early corrective actions included changing the initial orifice place to create larger differential pressure, re-ranging transmitters to 0-100 inches of H2O. "These early measures reduced frequency of failures and deviations, but not to an acceptable level," said Nance. "We were concerned that these errors could threaten the reliability of our whole furnace expansion project.

Nance reported that he and his colleagues also found several other root causes of poor furnace performance.

For example, in the dilution steam meters, taps were fouling due to sludge in the dilution steam, and blow downs of the meters would blow particulates into their diaphragms causing premature failure. The orifice plates also had inadequate diameters of upstream straight pipe run, and the long impulse lines allowed large variations in the static head.

"The 15 diameters we had weren't enough," said Nance. "We also had two elbows in the line, and needed closer to 45-50 diameters.” Also, condensate was collecting unevenly in the impulse lines because they were too long.

“In all, less sweat, less stress and more money." LyondellBasell’s Wes Nance discussed the successful installation of a Rosemount D/P flow measurement solution that has eliminated cracking furnace trips due to feed or dilution steam flow-metering errors.

A better alternative

Nance added his team considered several flowmeter alternatives to address its furnace performance issues:

  • Strap-on ultrasonic flowmeters would increase costs, but weren't widely used or proven in this application;
  • Coriolis flowmeters were known to be quite accurate, but this most expensive option would require piping modifications and incur excessive pressure drop;
  • Vortex flowmeters also would require piping modifications, but importantly only were available in dual-transmitter configurations insufficient to meet necessary safety integrity level (SIL) requirements;
  • Meanwhile, revamping the original D/P approach promised to be the least expensive option as well as simplest to maintain, even though it wasn’t the most accurate.

In the end, they chose to stick with a revamped D/P solution. "D/P was accurate enough for our application, even though it wasn't as accurate as the other options,” Nance said. “Plus, they were the easiest to maintain and least costly, and we could maintain three out of four (3oo4) voting. Our other design goals were to eliminate the fouling due to the sludge in the dilution steam; eliminate the head errors from the impulse lines; improve ease of maintenance; and increase our flow measurement accuracy and reliability."

Because the straight-run piping associated with the original orifice plates was too short, Nance and his team selected Emerson's Rosemount D/P solution with a 1595 four-bore conditioning orifice place (COP) that combines a flow conditioner with an appropriate orifice plate. This approach works to create a uniform flow profile in a relatively short pipe run, improving flow measurement accuracy.

"Four holes give the best possible flow profile in 2-in. to 24-in. or larger lines," said John Scott, executive account manager, Emerson. "Rosemount 1595 conditioning orifice plate only requires two diameters upstream and downstream of any flow disturbance, while up to 50 diameters are required for conventional orifice plates."

The Rosemount D/P solution adopted by LyondellBasell also used a remote-seal assembly with 1199FFW flush-flanged seals, and Duplex 2507 SST diaphragms that are durable in slurry applications where 316L steel can be damaged. It also included 2-meter (6.6-ft) capillary lengths, and Tri-Therm 300 fill fluid that can handle -40 °F to 572 °F temperature extremes, and met specs without heat tracing for Houston’s relatively mild ambient temperatures.

A plan comes together

"Using remote seals on the transmitters eliminated damage from the sludge, while the 2-meter capillaries got the transmitters far enough away from the heat, but still kept them close to the process,” Nance said.

Meanwhile, the feed transmitters used a design that inverted the typical pipe mount. "We also learned it was best to mount the tap for a transmitter at an angle that positioned it between the holes in the orifice plate. If we hadn't done this, it would have made a big difference in our readings and we wouldn't have known why. Partnering with Emerson really helped prevent this issue."

Thanks to its improved flow designs, conditioned flows, and other measurement improvements, Nance reported the dilution steam meters variability was reduced from about 22.5% to about 3.75%, while the feed meters variability improved from 23.8% to about 1.25%.

"Since installation of our Rosemount D/P solution in 2018, we've had no ethylene furnace trips due to feed or dilution steam metering errors," added Nance. "We've also had significant reductions in I&E maintenance to check and repair meters, and increased furnace availability and profitability. In all, less sweat, less stress and more money."