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To defray energy costs, many industrial plants have their own boilers to generate steam in order to produce a portion of their energy needs. In addition to generating power, the steam may also be used directly in plant processes or indirectly via heat exchangers or steam jacketed vessels.
The raw water used to feed the boilers contains varying levels of impurities that must be removed to protect the boiler and associated equipment. Pre-treatment processes such as reverse osmosis, ion exchange, filtration, softening, and demineralization may be used to reduce the level of impurities, but even the best pretreatment processes will not remove them all and will continuously carry some dissolved mineral impurities into the boiler.
These dissolved impurities accumulate in the boiler water when steam is made; only pure water leaves the boiler. The increasing concentration of dissolved solids leads to carryover of boiler water into the steam, damaging piping, steam traps, and other process equipment. The concentration of dissolved solids increases until the boiler water can no longer hold all of them in solution and a saturation point is reached. They then begin to drop out becoming suspended solids and forming a sludge or scale on the boiler walls and piping.
Boiler problems are avoided by periodically discharging or “blowing down” water from the boiler to reduce the concentrations of suspended and total dissolved solids. Surface water blowdown is often done continuously to reduce the level of dissolved solids, and bottom blowdown is performed periodically to remove sludge or suspended solids from the bottom of the boiler. The frequency and level of blowdown required each day depends upon the concentration of impurities and the rate at which they build up in the boiler water.
Boilers operating on soft water will require more top or skimmer blowdown to remove dissolved solids, whereas boilers operating on hard water will require more bottom blowdown to remove the settled solids. While control of suspended and dissolved solids in the boiler is critical, care must be taken to avoid excessive blowdown, as this would increase the demand for make-up (feed) water, treatment chemicals, and fuel.
The benefits for proper boiler blowdown control include the following:
The most common methodologies used for boiler blowdown control include: (1) continuous, (2) manual and (3) automatic.
Continuous blowdown utilizes a calibrated valve and a blowdown tap near the boiler water surface. As the name implies, it continuously takes water from the top of the boiler at a predetermined rate to reduce the level of dissolved solids. The rate is usually set slightly greater than necessary to maintain safety protocols.
Manual blowdown is accomplished at most plants by taking boiler water samples once a shift and adjusting the blowdown accordingly. This grab sample approach means that operators cannot immediately respond to changes in feedwater conditions or variations in steam demand and scaling conditions can occur and go undetected until the next sample check.
Automatic blowdown control is achieved by constantly monitoring the conductivity value of the boiler water, adjusting the blowdown rate, and the duration based on a specific conductivity set point. This provides control of the water chemistry.
Manual blowdown control cannot maintain this level of control more than 20% of the time. Upgrading from manual blowdown control to automatic control can reduce a boiler’s energy use by 2 – 5 percent and blowdown water losses by up to 20 percent.
Feed water usually contains one limiting component such as chloride, sulfate, carbonate, or silica. Even if the component is not conductive, as is the case with silica, its concentration is usually proportional to a component that can be measured by conductivity. Therefore, conductivity is a viable measurement for monitoring the overall total dissolved solids present in the boiler. A rise in conductivity indicates a rise in the “contamination” of the boiler water.
The frequency and duration required for boiler blowdown is significantly affected by the water quality. Improving feedwater quality through makeup water, chemical treatment and proper blowdown control can significantly reduce treatment and operational costs including:
Increased efficiency and reduced operating costs can be achieved by using the Yokogawa Conductivity Series line of products. Control configurations will carry with the application and the customer’s requirements.
Process Liquid Analyzer:
2-wire FLXA202 Conductivity Analyzer
4-wire FLXA402 Conductivity Analyzer
Analog and Digital SMART sensors are available. Analog options allow users to interface with a system that has been used historically. SENCOM™ technology, which allows sensors to transmit and receive data when connected to a transmitter/analyzer or a PC. The SMART digital sensors maintain specific measurement and calibration data on an integrated chip along that is an integral part of the sensor providing easy plug and play solutions. The data management software optimizes the performance of sensors for enhanced reliability and process safety.
SC42-S*34 Large-Bore Conductivity Sensor
(fittings available for Flow-Thru, Insertion, or Immersion installations)
SC4A Conductivity Sensor
(fittings available for Insertion, Sanitary, or Retractable installations.)
Another alternative would be Toroidal or Inductive Conductivity Sensor:
ISC40 Inductive Conductivity Sensor (fittings available for Flow-thru, Insertion, or Immersion installations)
The FLEXA™ series analyzers are used for continuous on-line measurements in industrial installations. With an option for single or dual sensor measurement, they are the most flexible two-wire analyzer available.
The measurement of specific conductivity in aqueous solutions is becoming increasingly important for the determination of impurities in water. Yokogawa has designed a full range of precision sensors and instruments to cope with these measurements, even under extreme conditions.
Reusable SMART adapter, requiring only the analog sensor to be disposed of when it reaches the end of its lifetime. With the SENCOM 4.0 platform, Yokogawa delivers reduced costs and waste while contributing to its long-term business goals of a sustainable future for all.
The model ISC40 sensors are designed for use with the FLEXA ISC analyzers. This combination exceeds all expectations for conductivity measurement in terms of reliability, accuracy, rangeability and price performance.
The FLEXA™ series analyzers are modular-designed analyzers used for continuous online measurements in industrial installations. They offer single or multi-sensor measurement.
Intended for the low conductivity applications found in the semi-conductor, power, water and pharmaceutical industries, these sensors are in a convenient compact style.
Conductivity meters, analyzers and transmitters are used for continuous process measurement and monitoring of conductivity, resistivity, WIFI, demineralizer water, RO water, percent concentration, boiler blowdown and TDS.
Conductivity sensors and electrodes are used to measure process conductivity, resistivity, WIFI, demineralizer water, RO water, percent concentration, boiler blowdown and TDS. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper electrode/sensor selection is critical for optimal measurement results.
Yokogawa’s SENCOM SMART Sensor Platform is an innovative analyzer platform that optimizes maintenance, reduces configuration time, and simplifies in-field maintenance and calibration.
Contact a Yokogawa Expert to learn how we can help you solve your challenges.