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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. "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."