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We have recently completed a detailed review of boiler
feed water controls, for a client with some new boilers who was
experiencing occasional problems of high and low-level steam-drum trips in
response to major upsets on the steam header. The feed-water control
system was implemented in a general-purpose Distributed Control System (DCS)
using a combination of the DCS manufacturers standard control-system
functions.
The review included three major elements:
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A comparison of the existing control-system
design strategy with conventional high-performance 3-element boiler
feed water control system design.
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A detailed, function-by-function, review of the
actual implementation in the DCS system, based on listings of the DCS
configuration data, and including checks on instrument ranges, flow
meter compensations and internal calculations.
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A review of the feed-water supply process,
including pump and control valve characteristics.
The required inherent characteristics of the
feed-water control valves and potential for interaction between feed water
flow control and feed-water control valve differential-pressure regulation
were quickly checked using one of our steam-boiler
and feed-water models.
We were able to recommend some corrections to the
detail of the DCS implementation and changes to the implementation of the
3-element control-system strategy within the DCS, to bring its
functionality closer to the conventional 3-element design. These
recommendations included replacing some incremental control functions with
whole-value functions - to make definite the necessary flow mass-balance
relationships and avoid the potential for drift due to the accumulation of
transient inaccuracies.
Background
Boiler feed water controls are responsible for regulating the level of
water in the boiler steam drum, by ensuring that the supply of feed water
to the boiler keeps in step with the changes in boiler steam production.
Simple manipulation of the feed water supply under level control is not
adequate for boiler feed water control: the transient phenomena of
shrinkage and swell of the bulk water volume means that level measurement
is a poor indicator of water inventory in in the short-term. Three-element
boiler feed-water controls combine drum level control, steam-flow
measurement and feed-water regulation in a strategy that combines fast
tracking of changes in boiler load with a slow trim from the level control
to maintain the overall mass balance in the longer term.
Despite a long-history of successful implementation of 3-element boiler
feed-water controls in the boiler industry, there can still be some
confusion of their specific functional and implementation requirements in
the general process control arena.
Further Information
21 June 2009
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