Overpressures must have a place to go, so it's great that every process unit is fitted with relief valves, typically connected to common piping terminating in a flare stack or other safe discharge point. A typical refinery will have hundreds of relief valves, and Singapore Refining Company's 290,000 bbl/day facility on Jurong Island has more than a thousand. The joint venture between Chevron and Singapore Petroleum Company includes three crude distillation units and catalytic reformer, visbreaker, hydrocracker and residue catalytic cracker (RCC) complexes.
Relief valves present significant challenges for loss prevention, maintenance and emissions management. "Hundreds of valves are not monitored," said Jian Ting Kwan, process engineer, energy and loss control, Singapore Refining Company, to attendees of his session, "Refinery Cuts Losses with Continuous Relief Valve Monitoring" at the Emerson Global Users Exchange 2015, October 13 in Denver. "You can't tell which ones are passing. It's hard to correlate releases with process and equipment events."
Even when they're closed, relief valves often leak. "Small leaks by many valves over long periods of time add up to significant losses," Kwan said. His plant measures and calculates total flare amounts, and they're highly visible. "Our management tracks total losses closely because it wants to minimize costs."
Leak checking inadequate
The facility performs frequent leak testing with a portable sonic leak detector and using infrared imaging, "but checking hundreds of valves is very time-consuming and costly, and it must be done frequently to be effective," Kwan said.
By automatically monitoring valves, it should be possible to quantify and reduce flaring and vent losses, effectively inspect valves and schedule overhauls, and correlate passing to process conditions to improve operations.
"We want to know when a valve lifts, and which one is lifted," Kwan said. "When a valve seats back, it often doesn't seat back completely, and we want to know that, too."
Mission: Smart relief valves
The plant defined a mission to turn ordinary relief valves into smart relief valves by installing leak detection instruments. "A wireless acoustic transmitter detects ultrasonic emissions due to passage of liquid or gas, with a secondary indication by temperature," Kwan said. "There's no need to upgrade the DCS or historian, and no special software required."
The acoustic instruments are easy to install, and transmit data to the wireless receiver, which routes it to the DCS, where operators can see the overpressure release as an indication of process problems. It also feeds the historian, where Energy and Loss Control can use it to monitor losses, Maintenance can use it to plan and prioritize repairs, and EH&S can use it for reporting.
Phase 1 demonstrated value
An Emerson acoustic leak detector trial was carried out from January 2013 to May 2014. The trial involved real-time monitoring of both steam traps and the flare system. The installation includes a gateway, management software and 50 wireless transmitters spread across the site: three on relief valves, two on control valves, 29 on stream traps and 16 as repeaters for the WirelessHART mesh network.
The results on steam traps were very encouraging. Verified against a portable tester, 97% gave the same result. Seven of the 29 team traps were found faulty instantly on installation, with six leaking and one trapping condensate.
Phase 1 showed that the transmitters could detect passing and leaks, but high background noise affected the accuracy of results, especially on relief valves.
Phase 2 promises payback
The refinery is now planning to install 224 additional transmitters, including 184 on relief valves. This phase also includes three flowmeters to pinpoint areas of high flare flow. Kwan calculates 31 kg/hr potential flare reduction, or 30% of the facility's historical releases. He estimates the resulting savings at US$114,700 per year, plus increased maintenance efficiencies.
Using the transmitters' temperature indication will help overcome noise problems, and the latest version of the 708 has the ability to suppress background noise as well.
"This application uses Modbus [to connect the wireless gateways with the DCS], but we can use HART-IP or other network protocols," said Jonas Berge, director, applied technology, Emerson Process Management. The same wireless system is being used for many other things in the plant, such as monitoring vibration, corrosion, inventory and materials movements.
Wireless networks and acoustic transmitters have broad applications in many plants for steam trap and other equipment monitoring, as well as for relief and pressure control valves, Berge said. "All plants that use relief valves have these challenges—to cut flaring and prevent losses by pinpointing which valves are lifting or leaking."