Inspections of more than 500 units over the past 10 years reveals common challenges faced by mid-life HRSGs, particularly those used in combined cycle plants to offset renewable generation and other aggressive operating strategies.
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Peter Jackson, President of Tetra Engineering Group was invited to present Tetra's experience on Purge Credit implementations at the 501F User Conference this February (2019). Combined cycle power plants use fuels and other materials that can cause fires or explosions in the combustion turbine, ducting, or heat recovery steam generator. Purging that equipment with ambient air to displace residual combustible gases before starting is a normal safety practice. But when plants are cycled, the disadvantages of purging often outweigh the advantages.
Tetra Engineering provides a full-scope service for the modelling and operation analysis of all types of natural or forced circulation Power Boiler and HRSGs, including supercritical and once-through designs. This can be integrated into a full plant model to incorporate the steam turbine and steam cycle balance of plant.
The level of detail can be adapted to focus on local effects (e.g. row-to-row temperature differences) and can be steady-state or time-dependent, the latter to model transients such as startups. The state-of-the-art Power Plant Simulator and Designer™ (PPSD) software package developed by KED software is the primary tool for this work. Frequent applications at Tetra Engineering include:
- Calculating steam cycle efficiency (heat balance) according to ASME PTC or other codes
- Determining natural circulation rates in support of FAC risk assessment
- Generating metal temperature profiles for inputs to finite-element stress calculation
- Assessing the impact of design changes or modification to original plant operating regime
How low can you go? That’s the question owners of gas turbine combined cycle plants are asking these days as they are being called upon to operate those units for rapid response in markets where load following is becoming the norm. The resulting cyclic operation introduces challenges that can result in damage to steam cycle components if you aren’t careful.
Many combined cycle power plants have experienced thermal shock damage to bypass power piping, especially in the HP steam and Hot Reheat systems. Severe shocks occur when inadequate drainage exists and liquid water accumulates in bypass piping, for example during periods of operation when the bypass is closed or during shutdowns. Startups where the bypass is opened then allows the accumulated liquid to be picked up and carried at high velocity downstream where it impacts on the internal surface at a change in piping direction, imparting a significant load and associated stress. Low load operation is another means by which very high attemperator spray can fail to be absorbed by the steam and consequently come in contact (in liquid form) with the pipe wall.
During the startup of the triple-pressure HRSGs, plant staff noticed steam coming from the stack and a discrepancy between feedwater and steam flows. On shutting down the unit a T23 superheater tube was found with a complete circumferential failure. The failed tube was removed and the bore was plugged with some difficulty due to poor access and the uncommon material.
Very large differences in tube wall thinning rates by flow-accelerated corrosion (FAC), or in some cases fluid erosion, are observed in the low-pressure (LP) evaporator tubes of certain HRSG designs. The tubes located near the duct wall and occasionally near the gaps between module bundles have more rapid thinning. Tubes in a given row nominally should have very similar process conditions, both on the gas-side and on the waterside. Different wall thinning rates mean that process conditions differ across the tube row.
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- HRSG Condition Assessments Identify CAPEX, Maintenance Priorities
- Role of Boiling Mode and Rate in Formation of Waterside Deposits in Heat Recovery Steam Generator Evaporator Tubes
- ASME B31.1 Covered Piping Systems' Mandatory Requirements
- Impact of Startup Purge Credit on Combined Cycle Plant Operation