Liquid heaters and vaporizers – Cleaning – Blow off
Patent
1989-05-25
1990-07-03
Wayner, William E.
Liquid heaters and vaporizers
Cleaning
Blow off
210696, 210746, F22B 3754
Patent
active
049381741
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a steam boiler system, and in particular to an improved apparatus and method for the blowdown control of steam boilers.
All steam boilers need to be "blown down", to avoid or limit the problems caused by impurities introduced by way of the boiler feed (make-up) water; these impurities become increasingly concentrated in the boiler water as steam is produced and used.
A typical problem arising from the build up of impurities, conventionally measured as total dissolved solids (TDS), is that these impurities form scale on the heat transfer surfaces, reducing efficiency and leading to possible component e.g. boiler tube, failure. On occasions there may be foaming within the boiler, with carry-over of boiler water with the steam, and then the contamination downstream of the boiler can result in electrolytic corrosion of the steam pipes, blockage of the pipes and/or slower heat exchange rates.
Traditional steam boiler practice is to specify a maximum permitted concentration of impurities in the boiler water; generally operators seek to maintain the boiler water near to this concentration by a blowdown procedure which consists of the replacement (either continuously or intermittently) of the boiler water by less-contaminated boiler feed water. The usual acceptable boiler water TDS is within the range 1500-3500 parts per million (ppm), but an actual specified value for a particular boiler will typically depend on the type of boiler, its scheduled operating pressure and the type of process plant requiring the steam output. It is a key operating procedure that the smallest quantity possible of boiler water should be blown down, since such water is at boiler temperature, and to discharge it to drain (even if heat recovery is attempted) is wasteful.
Of the existing blowdown procedures "intermittent blowdown" though still widely practised, is least favoured; the boiler water TDS decreases suddenly at each blowdown and then (following blowdown) slowly recovers towards the specified maximum for that boiler, the procedure being that an excess volume of boiler water is discharged at each blowdown which is wasteful of heated water--and this waste may be aggravated if the timing and duration of the blowdowns have been pre-set to cater for a different boiler operating condition or different process plant to be supplied. Thus many boiler operators nowadays use "continuous blowdown, manually set", with the manual setting being made by the operator based upon his experience with various boilers but though the worst excesses of intermittent blowdown are avoided, often too high and wasteful a rate of blowdown (kg/minute) is set by the operator in order that the blowdown will cater for the assumed worst operational boiler condition i.e. that leading to the specified maximum TDS most quickly. It is therefore also known to effect "automatic blowdown" in response to the current difference between the measured TDS in the boiler water and the maximum TDS specified for that boiler water, and to vary the continuous blowdown rate in dependence upon this difference; in known automatic blowdown systems an impurity sensor measuring boiler water conductivity is used to check directly the boiler water TDS, but the sensor is necessarily positioned either in a heated water boiler output line e.g. to a flash steam heat recovery system, or within the boiler itself, and so is subject to the above-mentioned problems of impurity deposition (scale) and accelerated component failure from being required to operate continuously in a hostile high temperature environment.
We propose an alternative procedure, and apparatus and method suitable therefor. Thus we provide a steam boiler system which includes a steam boiler, a feed water inlet to the boiler for boiler feed water supply, a steam outlet from the boiler, a blowdown outlet from the boiler, and valve means which can be adjusted to provide a blowdown rate which is a proportion of the feed rate of the boiler feed water characterised by means comprising an impurity sensor to det
REFERENCES:
patent: 3428557 (1969-02-01), Rivers
patent: 4347430 (1982-08-01), Leicester
patent: 4827959 (1989-05-01), Muccitelli
Spirax Sarco Limited
Wayner William E.
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