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Mar-2019

Factors affecting Claus waste heat boiler design and operation

The Claus Waste Heat Boiler (WHB) is a critical piece of equipment in a Sulphur Recovery Unit (SRU); its fundamental purpose is as a heat transfer device used in energy recovery.

Elmo Nasaso, Nasato Consulting Ltd
Nathan A Hatcher, G Simon A Weiland, Steven M Fulk
Optimized Gas Treating, Inc.
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Article Summary
Sulphur plant operators have recently been experiencing higher than normal WHB failure rates, perhaps from oxygen enrichment and pushing rates. The WHB has become the weak link in the SRU; consequently, increased attention is now being given to its design and operation. New performance standards are being considered to limit the mass flux of future designs; however, this may be an unwarranted oversimplification.

In this work, we demonstrate that although mass flux is certainly a parameter for design consideration, there are a host of additional factors particularly on the water side also having vital consequence. In some cases, limiting mass flux may lead to other unintended complications. Unless the boiler is treated as a heat transfer device with simultaneous chemical reactions and radiation, these complications may go unnoticed. Several important parameters beyond mass flux are evaluated and shown to have notable effects on WHB economics, reliability, safety, and on overall Claus unit performance.

Sulphur plant whb background
Sulphur recovery units essentially generate two products, sulphur and steam. In many cases the steam is more valuable than the sulphur. Steam is generated by recovering the significant amount of energy generated by the Claus process in both the thermal and catalytic stages. WHB High Pressure (HP) steam is typically generated at a pressure of 31 to 45 barg (450 to 650 psig) in the WHB downstream of the Claus Reaction Furnace. Low Pressure (LP) steam is usually produced around 3.5 barg (50 psig) in the condensers downstream of the catalytic converters. The HP steam is a valuable utility that is generated in the WHB that can be utilised for SRU catalytic indirect reheat, feed stream preheat and to drive a turbine on the combustion air blower. In most applications, the SRU is a net exporter of steam and the HP and LP steam generated in an SRU are utilised outside the SRU, e.g. LP heat source for amine reboiler, sour water stripper reboiler, steam tracing etc. or HP to spin a turbine.

In the past, the WHB more often generated low pressure steam. Many modern designs generate much higher pressure steam, thus presenting mechanical design and operating challenges. It is recommended that the design of SRUs, and in particular the WHB, be done by experienced sulphur technology licensors and EPC contractors with proper design expertise.

Modern WHB designs of the last 20 years are typically designed as follows:
• Pressure range: 31 to 45 barg (450 to 650 psig)
• Steam temperature: 236 to 258°C (457 to 496°F).

Excessive temperature, rapid process temperature changes and thermal cycling associated with start-ups and shutdowns affect the reliability of the WHB by degrading the tube sheet system (refractory/ferrules/tube sheet/tube-to-tube sheet joint/tubes). Thermal cycling is detrimental to the WHB tube sheet system longevity and reliability. Industry experience indicates that the Reaction Furnace/WHB could have a thermal-cycle life expectancy (a limited number of cycles) of as long as 20 years, in well-designed, operated and maintained systems, and as short as two to three years for inadequately designed, poorly operated and maintained systems. Damage to the tube sheet protection system due to poor operation results in unscheduled outages and impacts SRU reliability.

Heat exchanger fundamentals

The heat exchanger design principles are based on the following requirements:
1.    Heat transfer requirements (surface area)
2.    Cost
3.    Physical size
4.    Pressure drop allowance.

In the process industry many heat exchangers are purchased as off-the shelf items and the selection is made on the basis of cost and specifications furnished by the various manufacturers. The Sulphur Recovery Unit WHB using water as the cooling medium is a more specialised application.

For the WHB, there are several factors particularly on the water side that are of vital importance. It has been well documented1,2,3 that the following design and operations considerations need to be addressed during the design development and operation of the WHB to provide the most robust design that can handle the desired longer operating runs:
• WHB tube diameter
• WHB tube pitch
• WHB tube wall thickness
• Tubesheet thickness
• Materials of Construction
• Welding techniques for the tube-to-tubesheet welds
• Kettle vs Thermosiphon design considerations
• Ferrule design/installation considerations
• Blowdown design/operating practices
• Proper operator and maintenance personnel training.

WHB failure mechanisms
There are a multitude of design options (tube ID and tube length) to satisfy the design principles listed above, but experienced designers understand that two primary considerations must be met for a reliable design; a maximum metal temperature and a maximum heat flux through the tube wall.

A recent industry survey of Waste Heat Boiler design and operation in existing SRUs was performed to explore the determining factors in good WHB design and operation practices.4 This survey has indicated that neither mass flux nor tube diameter is an indicator of risk of failure. The survey confirms that the failure mechanisms can be attributed to a combination of factors which may be design or operation related.

Maximum Metal Temperature – High temperature sulphidation corrosion becomes increasingly significant at temperatures above 343°C (650°F). Therefore, a successful WHB design will have design conditions predicted to be well below this temperature. This takes into account such design features as tube sheet thickness and water side considerations for adequate distribution of boiler feed water and adequate removal for generated steam.
 
Maximum Heat Flux – Heat flux is determined by the radiant contribution plus tube side (process gas) convective heat transfer coefficient, tube wall conductance and water side heat transfer coefficient. The primary variable driving heat flux is the process-to-water temperature difference, which varies by a factor of ten over the length of the tube. The radiation component can add approximately 20% to heat flux but is of no real significance below approximately 538°C (1000°F). In the areas where failures are more prevalent (front-end), radiative heat transfer cannot be ignored or the heat flux will be under-predicted.

Avoiding conditions of steam blanketing on the boiler tubes is important. The heat flux rates over the entire tube length of the WHB should be evaluated, especially the turbulent entrance area near the end of the ferrule. The heat flux should not exceed 50% of maximum nucleate flux at design, and 65% for maximum service conditions. These limits will keep the tube wall within 10°C (18°F) of the water boiling temperature at design, well below the 40°C (72°F) break-over to Leidenfrost film boiling, which occurs when the critical maximum nucleate flux is exceeded. The in-service, fouled condition will be a substantially lower heat flux than maximum clean condition although tube wall metal temperatures are substantial higher.
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