Conductivity Measurements for Boiler Blowdown Application Note

Industry: Refining, Food & Beverage, Power, Oil & Gas, Pulp & Paper, Chemical Products: Contacting Conductivity Process Liquid Analyzer

Introduction they build up in the boiler water. To defray energy costs, many industrial plants have their own boilers to generate steam in Boilers operating on soft water will require more order to produce a portion of their energy needs. top or skimmer blowdown to remove dissolved In addition to generating power, the steam may solids, whereas boilers operating on hard water also be used directly in plant processes or will require more bottom blowdown to remove indirectly via heat exchangers or steam jacketed the settled solids. While control of suspended vessels. 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 Challenges (feed) water, treatment chemicals, and fuel. The raw water used to feed the boilers contains varying levels of impurities that must be removed to protect the boiler and associated Solutions equipment. Pre-treatment processes such as The benefits for proper boiler blowdown control reverse osmosis, ion exchange, filtration, include the following: softening, and demineralization may be used to reduce the level of impurities, but even the best • Reduced operating costs (less feedwater pretreatment processes will not remove them all consumption, chemical treatment, and higher and will continuously carry some dissolved heating efficiency). mineral impurities into the boiler. • Reduced maintenance and repair costs These dissolved impurities accumulate in the (minimized carryover and deposits). boiler water when steam is made; only pure • Cleaner and more efficient steam. water leaves the boiler. The increasing concentration of dissolved solids leads to • Minimized energy loss from boiler blowdown carryover of boiler water into the steam, can save about 2% of a facility’s total energy damaging piping, steam traps, and other use with an average simple payback of less process equipment. The concentration of than one year. dissolved solids increases until the boiler water The most common methodologies used for can no longer hold all of them in solution and a boiler blowdown control include: (1) continuous, saturation point is reached. They then begin to (2) manual and (3) automatic. drop out becoming suspended solids and forming a sludge or scale on the boiler walls and Continuous blowdown utilizes a calibrated valve piping. and a blowdown tap near the boiler water surface. As the name implies, it continuously Boiler problems are avoided by periodically takes water from the top of the boiler at a discharging or “blowing down” water from the predetermined rate to reduce the level of boiler to reduce the concentrations of dissolved solids. The rate is usually set slightly suspended and total dissolved solids. Surface greater than necessary to maintain safety water blowdown is often done continuously to protocols. reduce the level of dissolved solids, and bottom blowdown is performed periodically to remove Manual blowdown is accomplished at most sludge or suspended solids from the bottom of plants by taking boiler water samples once a the boiler. The frequency and level of blowdown shift and adjusting the blowdown accordingly. required each day depends upon the This grab sample approach means that concentration of impurities and the rate at which operators cannot immediately respond to

AN-P-20200521-01 1 Conductivity Measurements for Boiler Blowdown changes in feedwater conditions or variations in configurations will carry with the application and steam demand and scaling conditions can occur the customer’s requirements. and go undetected until the next sample check. Product Recommendations Automatic blowdown control is achieved by Conductivity Measurement System: constantly monitoring the conductivity value of Process Liquid Analyzer: the boiler water, adjusting the blowdown rate, 2-wire FLXA202 and the duration based on a specific conductivity Conductivity Analyzer set point. This provides control of the water 4-wire FLXA402 chemistry. Conductivity Analyzer Manual blowdown control cannot maintain this level of control more than 20% of the time. Sensor Selection: Upgrading from manual blowdown control to Analog and Digital SMART sensors are available. automatic control can reduce a boiler’s energy Analog options allow users to interface with a use by 2 – 5 percent and blowdown water losses system that has been used historically. SENCOM by up to 20 percent. technology, which allows sensors to transmit Feed water usually contains one limiting and receive data when connected to a component such as chloride, sulfate, carbonate, transmitter/analyzer or a PC. The SMART digital or silica. Even if the component is not sensors maintain specific measurement and conductive, as is the case with silica, its calibration data on an integrated chip along that concentration is usually proportional to a is an integral part of the sensor providing easy component that can be measured by plug and play solutions. The data management conductivity. Therefore, conductivity is a viable software optimizes the performance of sensors measurement for monitoring the overall total for enhanced reliability and process safety. dissolved solids present in the boiler. A rise in Option #1: conductivity indicates a rise in the “contamination” of the boiler water. SC42-S*34 Large-bore conductivity sensor Summary (fittings available for The frequency and duration required for boiler flow-thru, insertion, or blowdown is significantly affected by the water immersion installations) quality. Improving feedwater quality through Option #2: make-up water, chemical treatment and proper blowdown control can significantly reduce SC4A Conductivity sensor (fittings available for treatment and operational costs including: insertion, sanitary, or retractable installations.) • Reduced operating costs Another alternative would be Toroidal or Inductive Conductivity Sensor: • Reduced maintenance and repair costs ISC40 Inductive conductivity • Cleaner and more efficient steam sensor (fittings available for • Energy savings flow-thru, insertion, or immersion installations) Increased efficiency and reduced operating costs can be achieved by using the Yokogawa conductivity series line of products. Control

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