Review of DCS data can pinpoint avoidable, damaging thermal transients in HRSGs

Heat-recovery steam generators (HRSGs) are sometimes ignored by personnel at plants powered by GE Frame 7 E-class gas turbines—primarily 7Bs, 7Es, and 7EAs. The 7EA Users Group, with which you are most familiar, focuses on “things engine,” leaving solutions for other major equipment—including HRSGs and steam turbines—up to others.

There are more boilers and steam turbines serving in plants with Frame 7 E-class GTs than you might imagine. At the 2017 7EA Users Group meeting, GE said the fleet totaled 1168 engines, with about two-thirds of those in simple-cycle service. About half of the remaining machines are in cogen systems. Another 140 units are integrated into combined cycles, the remainder mostly drive compressors in LNG plants. Given this information, the editors put the number of HRSGs supporting the 7B-EA fleet at about 325; two-thirds of those are in the US.

Perhaps the best open source of practical O&M problem-solving information on boilers is the HRSG Forum with Bob Anderson, which meets next at the Hilton Orlando, July 22-24, 2019. This meeting will bring you up to date on HRSG issues and solutions worldwide given the Forum’s affiliation with user groups in Australia/New Zealand, Canada, Europe, and Russia. Plus, Anderson, a respected and active industry consultant specializing in boiler work, brings first-hand insights to Forum discussions based on the scores of HRSG surveys he has conducted around the globe.

Anderson told the editors that HRSG owners and operators, in particular those affiliated with plants built prior to and during the gas-turbine “bubble” (2000-2004), often traced pressure-part failures—superheater/reheater tubes and steam-pipe girth welds, for example—to attemperator overspray. A common root cause: Controls configured as a simple feedback loop instead of the cascade control scheme typically required, and operator intervention by lowering the attemperator setpoint.

Anderson recalled that a few years ago, after many operators had improved their control logic, he noticed leaking spray water when an attemperator was supposed to be out of service and believed that was causing most of the trouble. Anderson found many attemperators with chronic and/or severe leaking problems used valve-protective logic that opened and closed the block valve each time after and before, respectively, the control valve opened and closed.

The intention of this “master control valve/martyr block valve” logic was to protect the seat in the expensive control valve from damage caused by opening and closing against the high differential pressure across the valve. In other words, let the block valve take the damage in an attempt to protect the control-valve seat.

Anderson has for years been telling anyone who would listen (including attendees at the 2018 HRSG Forum) that use of such logic in a unit where the attemperator comes into and out of service frequently ensures the block valve’s seat will be damaged quickly, allowing leak-by. Damage to the control valve’s seat then occurs because the block valve can no longer isolate it from high differential pressure. The large number of leaking attemperators Anderson has found doing the HRSG surveys mentioned earlier supports this hypothesis.

He suggests that owners use the following “master block valve/martyr control valve” logic to avoid wear and tear on valves serving in cycling units: Open the block valve once during unit startup when the gas-turbine exit gas temperature increases to about 950F and leave the valve open until EGT decreases to 950F during shutdown.

The improved logic assures the block-valve seat will remain sound, protecting superheater/reheater tubes, headers, and piping from damage far more costly to repair than control-valve seats. Anderson believes owners will not see significantly higher maintenance costs on their control valves because their tight shutoff is not critical provided the block-valve seats are protected.

At the recent HRSG Forum, Anderson urged users to periodically review DCS data to see when their valves are opening and closing and to see if there is any leakage.

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