Boiler Troubleshooting and Forensic Services
Our capabilities and services include boiler design, troubleshooting and failure analysis with respect to the following boiler areas:
Boiler performance optimisation;
Boiler circulation and Downcomer stability analysis;
Boiler vibration analysis;
Superheater and convection bank tube failure analysis;
Failure analysis of backend fires and boiler explosions;
Oil to gas boiler conversions;
Marine and offshore LNG steam raising plant.
Pulverised coal firing systems;
Biomass and woodwaste combustion optimisation and analysis;
Overall boiler performance assessment by computer modelling and simulation.
CASE STUDY 1: HRSG FINNED TUBE BURN OUT
Heat Recovery Steam Generators [HRSGs] are typically installed behind Gas Turbines to recover low grade heat from the turbine exhaust stream, converting it to steam for use in power generation or industrial processing. A common misconception is that the relatively low operating gas temperatures protect the boiler from operating under conditions that exceed the metallurgical limits of boiler tube and tube fin materials.
In fact the finned tubing used in HRSG’s can generate extremely high heat fluxes which can quickly burn out finned tubes, leading to poor heat recovery and overall tube failure. This is more likely when upstream duct firing systems are installed to boost the steam output of the boiler. Finned Tube burn out is often a consequence of poor boiler circulation design and inappropriate duct burner configuration.
Description of problem. In this case study an HRSG that had recently been installed in a paper recycling plant experienced premature finned tube burnout shortly after the boiler was commissioned. We were engaged to investigate this problem and determine the cause of the finned tubes burning out.
We travelled to site to identify and collate relevant technical information and performance data. Operating personnel and engineers were also interviewed.
On Site HRSG Testing:
Temporary instrumentation was installed on selected tubes and performance tests conducted. The tube wall DT was measured under specified controlled operating conditions and recorded for later analysis.
Analysis of Results:
A computer model of the HRSG was setup. This analysis suggested that poor boiler circulation was probably occurring and that finned tube burnout may have indeed been possible.
The tube temperature measurements revealed a very distinct ‘telltale’ temperature difference across the tube wall indicating that the boiler was experiencing flow stratification issues, consistent with results from the computer model. Both the computer model and temperature measurements pointed to the problem being caused by excessive finning on the primary stages of the HRSG.
The boiler circulation system was reconfigured to compensate for the excessive tube finning and the flow stratification problem was effectively eliminated. The boiler was returned to service without any further finned tube burn out problems.
CASE STUDY 2: OIL TO GAS CONVERSION ON AN FPSO
Floating Production and Storage Offloading (FPSO) facilities consist of retired oil tankers located near oil rigs to provide pre-processing and oil storage facilities prior to transportation to onshore refining plant. Part of this processing is separation of gas from oil which is discarded to waste. It is therefore cost effective to convert the boilers on these FPSO tankers so they can fire on waste gas instead of oil.
However when an oil fired boiler is converted to gas firing, furnace heat absorption can reduce significantly, limiting the output of the boiler by thermally overloading the superheater. For this reason computer modeling is required to evaluate and address this issue, which may require the design of the furnace or the superheater to be modified. A typical conversion process involves the following activities.
Collect operating data, technical information and specifications.
Boiler Performance Modelling:
A computer model of the boiler is set up using in house boiler performance software. The computer model is calibrated on oil against the boiler’s original design performance and available test data over a range of outputs.
Analysis of Results:
Relevant issues of potential concern which could limit boilers output on gas are identified.
Assess possible solutions to problems identified in the computer modelling exercise.
After other conversion tasks have been implemented, carry out commissioning of the converted boiler and provide test documentation.
CASE STUDY: DOWNCOMER INSTABILITY ANALYSIS
Downcomer instability is one of the most common and least recognized problems experienced in natural circulating boilers. It occurs due to the incorrect positioning of baffling within the steam drum and is a fundamental boiler circulation defect. Symptoms include excessive wet steam carryover, steam turbine blade fouling and pitting, drum level control instability, superheater fouling, boiler bank and superheater tube failures, and boiler priming.
The problem can be fixed by re-configuring steam flow baffling inside a steam drum in order to improve the distribution of steam and water flow to downcomer pipes. To implement this fix, it is necessary to study the steam flow inside boiler components with specialized boiler circulation software which can accurately predict the total circulating flow within the boiler under a variety of operating conditions.
Case study problem. The steam turbine in a food processing factory was fouling every 6 to 12 months requiring the boiler and turbine to be shut down to descale the turbine blades. We were engaged to determine the cause of this problem which some thought to be due to poor treatment of boiler make up water.
A site visit was carried out to collect boiler data, drawings relevant specifications and background information fom site engineers and operating personnel.
A boiler performance and circulation model was created to simulate the operation of the boiler. This confirmed that for some operating conditions downcomer instability was occurring
Further computer simulations were carried out to establish the optimum number of downcomers in order to maintain a stable circulation regime.
Internal steam drum baffling was reconfigured to route steam flow to the required number of downcomers as stipulated by the computer simulations.
The internal baffling modifications were implemented within the steam drum and the boiler returned to service. The boiler has operated for many years without any further signs of turbine scaling.