A Boiler System is comprised of two main sub-systems: The furnace, which provides heat to create steam from the boiler water & the boiler vessels, tubing & transfer devices, which control & deliver the steam to process users. Water quality control is of primary importance to minimize expensive equipment damage & loss of efficiency. Critical attributes of boiler feedwater, condensate return & make-up water are summarized below & in reference documentation. Actual operating pressure of the steam has a major influence on the water quality required to minimize damage. Detailed system analysis is needed on each application, in order to determine an optimum analytical system.
Boiler feedwater is normally obtained from treated natural waters, which contain TOC, carbon dioxide, dissolved oxygen, oily matter, suspended solids, dissolved solids, iron, silica, sodium, salts & a variety of other impurities. Organics, Hydrogen Sulfide & Dissolved Oxygen are major contributors to corrosion. Organics form organic acids, cause foaming & clogging. Sulfates, Chlorides, Bicarbonates & Nitrates are formed & result in unacceptable scaling if feedwater hardness is not strictly controlled. Make-up water is obtained from condensate return & treated natural waters. Liquid water on the boiler bottom collects all the impurities “flashed out” of the steam in droplets formed in the process, & must be periodically “Blown-Down” (released to drain).
Cooling Water is needed in many industrial processes, typically using Cooling Towers as a means for providing heat transfer from various process plants heat exchangers. Two basic approaches are used: “Once Through” & “Recirculating”. Once through Cooling Towers use large volumes of surface water, such as a river, which discharges the heated water back to the surface water after it has been used to cool the processes. Recirculating Cooling Towers conserve water resources & eliminate some potential harm to aquatic life & the environment.
Used water is typically pumped to the top of the Cooling Tower & flows down through diverter passages. Large fans located at the top of the tower exhaust air with an upward draft through the falling water, adding additional cooling effect. Evaporation of some of the water carries away a large portion of heat. A collection basin at the base of the Cooling Tower collects water to be reused in Recirculation Cooling Towers.
As with Boiler Feedwater, Cooling Towers are plagued with corrosion of heat exchangers & piping, scaling & algae & bacteria growth. Water quality monitoring to control these effects are required. Cooling Tower water evaporation causes deleterious salt buildup, which precipitate after saturation is reached. Major contributors to scaling are caused by magnesium & calcium & can be monitored for their ionic strength, providing the Operator with blow-down & water refreshing control parameters. Feedwater should be monitored for salts (carbonates, sulfates, chlorine), CO2 & Oxygen. Corrosion control can be maintained by inhibitors & pH adjustments or ozone treatments. Microbial growth can be controlled by chemical disinfectant additives.
The primary indicator of water quality is TOC (Total Organic Carbon) analysis.
Cooling Tower regulations are a subject of EPA regulations, due to the following considerations: A.A. Above 24oC temperatures, fish are stressed & above 28oC lethal effects occur. B. If the system is open to atmosphere, as the cooling water evaporates, air pollutants, including microorganisms are vented to the environment.
Treatment: Public Water Treatment methods vary, depending on characteristics of the raw source water used. Ground water may require little or no treatment to meet Federal & Local regulations. Surface waters (rivers, lakes, reservoirs) require added treatment because they are exposed to the environment, eg. rain runoff through fertilized fields. Surface water contamination influences ground water. Coagulation/Flocculation of the raw water particulates is used by adding Alum, Polymers & Salts to join smaller particles together to create larger particles for their sedimentation & removal. Basic filtration to remove remaining particles, organics & micro-organics is often augmented by Ion Exchange & Activated Carbon Beds. Disinfection to kill microbes prior to distribution may be accomplished by chlorine, chlorine dioxide, chloramines or ozone. Total Organic Carbon Analysis is the primary screening & monitoring method for water quality. Federal Regulations require monitoring for more than 83 contaminants, consisting primarily of volatile organic compounds (VOCs) synthetic organic compounds (SOCs) & inorganic compounds (IOCs), in addition to microbial/bacterial organisms.
Distribution Systems: Distribution Systems consist of piping & pumping networks to deliver drinking water to homes, businesses & industrial users. Contamination caused by such occurrences of pressure failures causing leakage, piping breaks, seal failures & micro-organism growth continue as normal events, however, National Security interests have heightened concerns for the safety of public drinking water.
WHAT NEEDS TO BE MONITORED TO MAINTAIN DRINKING WATER SAFETY? Various studies by Homeland Security & Others have tried to define a strategy of a network of water analyzers & develop algorithms which could trigger an alert that the drinking water system was under attack. While these studies & analyses have been on-going, initial consensus appears to be narrowing to the following sensors/analyzers:
Total Organic Carbon (TOC), pH, Turbidity, Electrolytic Conductivity, Chlorine
High purity water is required for boiler feedwater to extend the useful life of expensive turbines, boilers, pumps and other associated components. Heated boiler feedwater is converted to steam, which is used for those processes requiring heat & also to drive a turbine/generator system for electrical distribution. The system shown above is commonly used for “Cogen” (Co-Generation) joint ventures, where process steam & electrical users provide a site location to a power generating concern which, in turn, uses condensate & make-up water from the process users as their boiler feedwater. Process plants obtain electricity from the power generating concern & have to assure purity of TOC & control of other impurities in their condensate. Both sides of the joint venture interface are monitored for water quality.
Amines & other organic boiler chemical treatments to inhibit corrosion & scaling are used by the Power Generating concern which show up in the condensate return provided by the Process Plant. StarTOC has developed a method to compensate for these, so that the Process plant is responsible for only its TOC contribution & is not mistakenly charged for the organic additives dispensed by the Power Generating entity.
Wastewaters normally contain large floating debris, which must be screened out and other suspended solid materials and oils to be eliminated by chemical additives in the Primary Treatment phase. Aeration (bubbling air) through the water releases dissolved gases to the atmosphere. After primary treatment, the wastewater is directed to the Secondary Treatment phase. Secondary Treatment is used to lower the organic load of soluted organic compounds, either in an aerobic (aerated) or anaerobic (oxygen-free) condition. Organic load is normally measured by BOD (Biochemical Oxygen Demand) or TOC (Total Organic Carbon). Settling of small suspended solids, not eliminated in the Primary Treatment phase, occurs, as will the formation of Sludge, caused by biological/biochemical digestion of the organics in the wastewater. Sludge & Scum (floating matter) are removed prior to the wastewater being directed to a Tertiary System, if used. Tertiary Treatment may consist of a variety of means to finely filter the water prior to the water Disinfection stage to kill the bacteria and release of the treated Final Effluent to the environment.
During the past number of years, wastewater treatment has become sophisticated, employing various efficient treatment strategies, which require advanced instrumentation to monitor & control such processes as Nitrification/Denitrification, Phosphate removal, Alkalinity, Sludge Treatment & Analysis prior to disposal, & other analytes required by the application.