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Blog post published May 10, 2016 Blog: Go with the flow

Reliability for burner shut-off and furnace decoking

This is Part 3 of our series of three posts on valve selection for ethylene cracking. Gas/liquid flows into the steam cracking furnace burners through a series of two ESD valves that have a vent (ESV) between them. The vent is used to prevent pressure buildup, and gas/liquid flows through the second isolation valve when the system is isolated.

The ESD valves automatically shut off the supply of fuel when de-energized by a combustion safety control, safety limit control or loss of actuating medium. This is followed by burner shut-off valves, some of which are shut during decoking operations. It is imperative that the ESD valves operate properly even after extended periods of non-operation. Type approvals are becoming a standard requirement by local authorities. Typical process conditions are similar to those for the control valve.

Metso’s Jamesbury soft-seated ball valves and ValvGuard VG9000 safety solenoid valves ensure operability in case of an upset. These shut-off valves provide excellent tightness during shut-off:

  • Field-proven seating capabilities in both continuous and on-off (switching) heating
  • Safe and reliable bubble-tight shut-off even after a million cycles and the self-relieving feature (cavity relief) provides safe operation after a long time of non-movement
  • Partial stroke testing capability with the VG9000 safety solenoid valve
  • Fire-safe design according to API 607 or ISO 10497
  • Low fugitive emission approvals by third-party authorities
  • Certified up to SIL 3 by third-party certifications
  • Gas burner valve type approvals according to EN161, EN264, ISO 23553-1, AGA, FM and CSA

Regular decoking is required to remove coke from the furnace coils, typically every 3-4 months per furnace, depending on the type of feed and the reaction severity. Decoking is typically done by steam or air. When the end-of-run conditions are reached, the hydrocarbon feed is discontinued, the furnace is isolated and the firing rate in the burners is adjusted. After this, the controlled amounts of dilution steam and decoking air are admitted. The decoking effluent is routed to decoking drums, where the coke fines are separated from the decoking effluent gases. Decoking valves have to withstand high piping forces due to large temperature variations. The downstream process must be protected from fire, high-temperature steam and coke to ensure plant operability and safety, making 100% tightness essential.

Metso’s unique ethylene cracker valve, the Mapabloc butterfly valve, has been successfully used in several steam cracking plants since the late ‘90s, ensuring reliable decoking operations. The valve provides:

  • Double block and bleed sealing system, giving the valve a safe, extra tight seal
  • Reduced CAPEX and OPEX costs for piping design, transportation and maintenance, when compared with a traditional gate valve, thanks to the compact and lightweight design
  • Minimal emission rates by utilizing rotary technology
  • Resistance to piping forces, eliminating the risk of the valve getting stuck
  • Temperature shock resistance, allowing the valve to be installed before the quench oil feed
  • Fast change from production to decoking, as the closing element is moving freely, ensuring quick and accurate reactions to system signals

 

Related posts: 

Part 1: Flow control solutions for ethylene cracker furnaces

Part 2: Control valves for feedstock, dilution steam and fuel to furnaces





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