Background & Introduction
A steam generating unit (SGU), also called a boiler, generally refers to fired heat transfer equipment installed in the
refinery's utility plant. The unit produces low, medium or high pressure steam for use in on-site and offsite facilities.
Some on-site steam production units may also be designated as a SGU but are generally unfired units.
The utility plant SGU's are typically fired by oil, treated & untreated fuel gas and CO gas streams.
In the case of a new plant, the supplier must demonstrate that the unit meets its MCR ratings (maximum continuous rating)
which are stipulated over full and partial MCR ratings e.g., 100%, 75% and also 110% all to prove the system can perform
to guarranteed contract conditions. Invariably, the Supplier has the experience to achieve these conditions; but, sometimes
the unit does not meet these targets.
This was the case for a number of SG units installed in the utility plant of a multi-national refiner. The test unit was plagued
by an inability to demonstrate it could reach 100% firing and flame impingement occurred during ramp up of the dual fired
burners. After several vexing weeks of attempting to resolve the technical difficulties, the refiner threatened the Supplier with
rejection of the entire SGU equipment battery amounting to a lawsuit well into eight (8) figures. Production loss, delay of
commissioning and consequential damages would increase that total.
Sorting out the Problems; Identifying the Root Cause
In Figure 1, the SGU was jeopardized by failure to reach 100% MCR and flame impingement. Impingement against the water walls of
the unit may cause tube rupture due to excessive, highly localized heat flux.
Figure 1 Flame Impingement at Water Wall Surface
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Flame impingement occurred well prior to reaching 100% MCR and a number of potential, contributing causes
were identified, confirmed via engineering checks, equipment inspection and measurement. These deficiencies
occurred in sub-systems and individual components.
These upgrades were systemically substantiated by field proving. The upgrades covered air delivery prime movers
& distribution, fuel delivery and burner components. Engineering theory was used to rationalize some component
changes; incremental improvements were observed but the flame impingement issue prevailed.
Field experience was brought to bear as well but also only provided incremental improvement. External subject matter
expertise also proved to be ineffective. In summary, the deficiency appeared to be a first time occurrence and
was proving to be enigmatic.
The preceeding deficiencies fortunately, however, resolved firing capacity targets i.e., 100% and 110% MCR; the flame
impingement issue continued and, as expected, the Client was not satisfied and threatened to move forward with legal action.
This prior work all provided incremental insights and suggested that fluid dynamic modeling was the next step in
understanding the observed flame evolution as firing rate was increased.
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The Client begrudgingly agreed to the proposal to allow time for the appropriate study.
Fortunately, the fluid dynamic study revealed the cause of the observed flame behaviour. However, the solution fell
back to the expertise of both engineering & field know-how; wholesale reconfiguration identified by
flow modeling was not a practical or economic solution. The team identified specific design alterations
to the burners themselves providing a practical, reliable work-around.
The alterations were made to the burners, the performance and acceptance testing resumed and was completed to the
satisfaction of the Client. The threat of litigation was averted; the units have operated successfully since.
Other challenging instances where engineering first principles and experience resolved difficult issues and
avoided potential litigation have been encountered.
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