Assessing HRSG dependability
by admin | November 17, 2011 1:13 pm
The last thing a plant manager wants is for his generating facility to be forced out of service. This is especially true if the plant is a large combined cycle operating in a competitive market or a gas-turbine (GT)- based cogeneration unit serving an industrial process. The financial penalties for an unexpected outage when under contract to deliver electrical and/or thermal energy can be onerous. Worst case—personnel safety aside— is that one such incident can turn a profitable year into a loss.
Sometimes, despite your relentless diligence and best efforts, a forced outage can’t be prevented. But others can; in particular, those caused by water-chemistry upsets and tube failures in heat-recovery steam generators (HRSGs). One way to assess your HRSG’s dependability is to answer the questions below, add up your score, and see what your rating is.
This self-help evaluation was developed by Technical Executive Barry Dooley (bdooley@epri. com) and his colleagues at the Electric Power Research Institute, headquartered in Palo Alto, Calif.
Dooley is recognized globally for his leadership in guiding research to solve practical boiler problems related to water treatment (“FAC: Attention to detail, key to prevention,” incorporated in the report on the HRSG User’s Group annual meeting that appeared in the 2Q/2005 issue of the COMBINED CYCLE Journal; access at www.psimedia.info/ ccjarchives.htm).
Supplementary information for Factor 3
According to EPRI’s research, the minimum level (so-called “core level”) of cycle chemistry monitoring required at all plants/units is as outlined below. Note that instrumentation at each of the measurement locations identified should be of the continuous online type.
Cation conductivity: Condensate pump discharge, condensate polisher effluent (if installed), feedwater (or economizer inlet), drum blowdown (each drum), main or reheat steam.
Specific conductivity: Makeup, drum blowdown (each drum). pH: Drum blowdown (each drum).
Dissolved oxygen: Condensate pump discharge, feedwater (or economizer inlet), drum downcomer (drum unit on oxygenated treatment).
Sodium: Main or reheat steam.
Phosphate: Drum blowdown (each drum using a phosphate treatment).
Silica: Makeup.
The knowledge base that validates the accuracy of EPRI’s scoring methodology comes from the research organization’s 20 years of work thus far to reduce availability loss and costs associated with boiler tube failures in conventional utility boilers. A companion effort for HRSGs has been in effect for more than 10 years.
EPRI’s Boiler Tube Failure Reduction Program/Cycle Chemistry Improvement Program (known by the unwieldy acronym BTFRP/CCIP) has been conducted for more than 60 power producers worldwide. Over 100 organizations have been benchmarked and had their results tracked. Benefits of the program have been significant.
EPRI’s HTFRP/CCI program for HRSGs, already conducted for 25 organizations, considers the shorter operating history of these units compared to conventional boilers and the smaller staffs characteristic of the GTbased powerplants. Its aim is to identify those processes (chemical and thermal transients, and cycle chemistry optimization) known to influence mechanisms that cause unavailability. The leading failure mechanisms, in order, are thermal fatigue (including creep fatigue), flow-accelerated corrosion (FAC), corrosion fatigue, under-deposit corrosion in high-pressure evaporators, and pitting.
By way of example, Dooley points to the chemistry side, where it is most important for plants to eliminate those features that have been “built in” to cause failure from the specification and design phases. One of these is the practice at some facilities to mistakenly add a reducing agent to feedwater in systems with low air in-leakage. This virtually guarantees single-phase FAC in the low-pressure economizer and evaporator circuits.
Learn how to reduce tube failures
EPRI’s HRSG Tube Failure Reduction Program/Cycle Chemistry Improvement Program is conducted at individual combined-cycle plants and at locations central to several facilities.
It is designed to help an organization prevent tube failures and to identify the precursors to damage. Program
outline is as follows:
- Identifies the mechanisms and root causes of tube failures.
- Explains how cycle chemistry impacts tube failure rate.
- Explains how thermal transients (cycling) influence tube failure rate.
- Shows how to optimize cycle chemistry in the feedwater and evaporator circuits to prevent tube failures.
- Identifies locations where thermally driven tube failures can occur.
- Shows how to monitor thermal transients.
Dooley says he has clear evidence that HRSG tube failures essentially can be eliminated if a coordinated management-supported prevention program is adopted. He explains that the EPRI program moves an organization to the proactive or predictive mode conducive to success. Dooley fairly observes that the cultural change necessary is challenging even for organizations committed to the program. Without top-management commitment to prevent repeat failures, however, he adds, organizations are just waiting for failures to occur. CCJ OH
Source URL: http://www.ccj-online.com/3q-2005/assessing-hrsg-dependability/