Inadequate treatment of boiler feedwater can have a high cost to manufacturing and process businesses, as Mark Bosley of SUEZ Water Purification Systems explains.
Despite their vital importance to so many operations, industrial boilers and plant heating systems are often given a low priority in terms of maintenance and overhaul. This can be a costly mistake that leads to growing levels of scaling and corrosion; these will increase energy consumption and reduce heating efficiency, shorten the operating life of tubes, condensors and pipework and, in the worst case, lead to unplanned shutdown.
Water contains dissolved gases, including Oxygen and Carbon Dioxide, and minerals. When these are heated and pressurised they can attack metal surfaces, causing corrosion, and be deposited as scale; corrosion tends to produce localised pitting, can also cause systems to fail without warning while scale lowers the efficiency of heat transfer and liquid flow.
For modern boilers, with high heat transfer rates and efficiencies, feedwater quality specification is crucial. Water must be carefully treated, monitored and managed. Both the raw feedwater and the condensate returning from steam circulation need to be appropriately purified using a combination of equipment and chemicals.
The chief scaling elements, calcium and magnesium, should be removed before reaching the boiler. One approach is water softening. This ‘base-exchange’ process uses brine to regenerate a resin which replaces calcium and magnesium with sodium. As no single treatment is totally efficient, chemicals are also added to the boiler.
A side effect of softening is an increase in total dissolved solids. As water in the boiler evaporates, they become further concentrated and begin to precipitate. They can produce a form of scaling, as well as foaming and loss of boiler water level control. In such a situation, boiling water may be ejected into the steam main and damage pipework designed for gaseous flow. To avoid this, water at the bottom is periodically discarded or ‘blown down’ to remove excess solids.
Alternative pre-treatment methods include demineralisation, otherwise known as deionisation (DI) and reverse osmosis (RO). By preventing scaling and foaming, and reducing the need for blowdown, they improve energy efficiency.
Deionisation uses ion-exchange resins to swap hydrogen and hydroxide ions for dissolved minerals. The ions then- combine to form water. Another technology is electro-deionisation (EDI), in which ion capture resins are regenerated using electrical potential. In reverse osmosis, water passes under pressure through a semi-permeable membrane which can remove 96-98 per cent of the dissolved ions. While tackling the scale problem, all these treatments make water more corrosive and require additional chemicals to lessen that effect.
Dissolved oxygen and incorrect pH are the main corrosion agents. Heating and stripping with steam in a hotwell or deaerator will remove much of the oxygen, but chemicals are still needed. Sulphite is a good oxygen scavenger, although it adds to the total dissolved solids. Oxygen-reducing organic chemicals do not increase solids but are unsuitable for some steam uses.
Maintaining an alkaline pH minimises corrosion, but this should not involve excessive use of sodium hydroxide (caustic soda) which can corrode iron and increase solid levels. Caustic is not needed in demineralised water, where alkalinity can be maintained by chemicals including phosphates and amines.
With awareness of scaling and corrosion issues, and proactive investment in water treatment, monitoring and management, the boiler’s efficiency, uptime, longevity and cost control are all improved.