Water Efficiency

Transcript

Water Efficiency for Commercial, Industrial and Institutional Facilities Water Efficiency Water Management Options N.C. Division of Pollution Prevention and Environmental Assistance 1639 Mail Service Center Raleigh, NC 27699-1639 (919) 715-6500 (800) 763-0136 Boilers Boiler Water Impurities All boiler make-up water contains impurities. As clean steam is released from the boiler, impurities build up. The increasing concen- tration of these impurities, such as dissolved solids, can lead to carryover into the steam, causing damage to piping, steam traps and even process equipment. The increasing con- centration of suspended solids impurities in the boiler can form sludge, which impairs boiler efficiency and heat transfer capability. 2 Water Efficiency Fact Sheet May 2009 Blowdown To maintain solids at an acceptable level, wa- ter is removed from the boiler system. This water bleed-off, termed “blowdown,” from industrial boilers is an important part of boiler operations. Achieving the right amount of blowdown is critical. As with cooling tow- ers, insufficient blowdown can lead to exces- sive buildup of impurities. Too much blowdown can lead to wasted water, treatment chemicals and energy. Blowdown is released from beneath the wa- ter surface in the boiler’s steam drum, mud drum, bottom header or from the bottom of the boiler. Surface water blowdown is often done continuously to reduce the level of dis- solved solids, and bottom blowdown is per- formed periodically to remove sludge from the bottom of the boiler. Additionally, the blowdown heat can be used to increase the overall efficiency of the system. The optimum amount of blowdown required is a function of boiler type, steam pressure, chemical treatment program and feedwater quality. Because supply water quality varies from place to place, no hard and fast rules exist as to the exact volume of blowdown re- quired. Blowdown rates can very from one percent (of feedwater flow) to as much as 20 percent, with the typical range of four to eight percent. Maximum Recommended Concentration Limits Boiler Operating Total Dissolved Total Total Pressure Solids Alkalinity Suspended Solids (psig) (ppm) (ppm) (ppm) 0 - 50 2,500 500 -- 50 - 300 3,500 700 15 300 - 450 3,000 600 10 Blowdown amount is typically calculated and controlled by measuring the conductivity of the boiler feed and blowdown water. Conduc- tivity is a viable indicator of the overall total dissolved solid concentration. Blowdown for boilers is usually expressed in percentage: TDS or (μmhos) of Blowdown Percent Blowdown = = Quality of Makeup Water Quality of Blowdown TDS or (μmhos) of Makeup Boiler water quality also is commonly ex- pressed as cycles of concentration, which is simply the inverse of percent blowdown. Optimizing Blowdown Facility managers should know the optimum operating parameters for their boiler water quality. While optimizing boiler water treat- ment and control procedures can conserve water, more importantly, they will maintain proper boiler performance, extend life and save energy. The American Boiler Manufac- turers Association and American Society of Mechanical Engineers have developed guide- lines for water purity controls in boilers. These can be used as a starting point for determin- 3 Water Efficiency Fact Sheet May 2009 ing boiler blowdown needs. The maximum recommended concentration limits according to the ABMA is listed in the table below. Operating the boiler below these levels re- quires more blowdown, wasting water and energy, thus increasing the cost of operation. The total dissolved solids are the sum of all naturally occurring minerals dissolved in sup- ply water and any treatment chemicals added to the system. Recommended boiler blowdown practices also are described in Sections VI and VII of the ASME Boiler and Pressure Vessel Code. Facility managers can identify water- and energy-sav- ing opportunities by comparing the blowdown and makeup water treatment practices with the ASME practices. The ASME Boiler and Pressure Vessel Code can be ordered through the ASME Web site at http://www.asme.org/bpvc/. Automatic Blowdown Controls There are two types of boiler blowdown: manual and automatic. Plants using manual blowdown must check samples many times a day or according to a set schedule, and adjust blowdown accordingly. With manual boiler blowdown control, operators are delayed in knowing when to conduct blowdown or for how long. They cannot immediately respond to the changes in feedwater conditions or variations in steam demand. An automatic blowdown control constantly monitors boiler water conductivity and adjusts the blowdown rate accordingly to maintain the desired water chemistry. A probe measures the conductivity and provides feedback to the controller driving a modulating blowdown valve. An automatic blowdown control can keep the blowdown rate uniformly close to the maximum allowable dissolved solids level, while minimizing blowdown and reducing energy losses. Action Plan for Optimizing Boiler Blowdown Monitor blowdown rates, feedwater quality and blowdown water quality. Work with experienced vendors and boiler service providers to determine best water treatment program to complement water efficiency goals. Establish maximum boiler water contaminant levels. Estimate cost and operation savings in water use, heat loss and chemical loss that can be accomplished by modifying concentration ratios. Evaluate implementing systems to continu- ously monitor and blowdown boiler water. Purchasing and installing an automatic blowdown control system can cost from $2,500 to $6,000 with generally a one- to three-year payback period on the investment. A complete system should consist of a low- or high-pressure conductivity probe, temperature compensation and signal condition equip- ment, and a blowdown-modulating valve. Changing from manual blowdown control to automatic control can reduce a boiler’s en- ergy use by two to five percent and reduce blowdown water losses by up to 20 percent. Maximizing Condensate Return Improving condensate return is another way to minimize blowdown water and maximize cycles of concentration at which a boiler operates. By increasing condensate return, operators will in- crease the concentration cycles, decrease chemi- cal usage, decrease blowdown and conserve the 4 Water Efficiency Fact Sheet May 2009 heat value of the high-temperature condensate. A well-functioning steam trap inspection pro- gram is essential to maximizing condensate re- turn. When steam traps exceed condensate tem- perature, the trap is leaking steam. Use infrared temperature gun/device to check this. Steam lines and traps should be checked for leaks peri- odically and repairs should be scheduled. Such repairs are typically very cost effective because of the potential for energy savings. Condensate return systems and automatic shut-off controls should be considered for boiler systems not uti- lizing them. Consultation can be conducted with boiler vendors, service providers and other tech- nical assistance providers. Improving External and Internal Water Treatment External or feedwater pre-treatment systems remove impurities form the boiler feedwater. Treatment systems address three areas: Removal of suspended solids Removal of hardness and other soluble impurities Oxygen removal 1. 2. 3. There are several technologies available to pre- treat boiler feed water. These include soften- ers, reverse osmosis and demineralization. In- creasing feedwater quality will increase the cycles of concentration at which a boiler can operate. Internal water treatment regimes for boilers seek to manage corrosion and deposits. Choices for internal and external water treat- ment approaches are interdependent. While seeking to optimize boiler water systems, the importance of using knowledgeable people to ensure proper evaluation of water treatment needs cannot be overemphasized. It is best to utilize someone familiar with boiler system op- eration as well. Blowdown Heat Recovery Units The evaluation of reclaiming heat from blowdown is a wise consideration. Systems with continuous blowdown rates exceeding five percent of the steam generation rate are often good candidates for a blowdown waste heat recovery system. The blowdown water has the same temperature and pressure as the boiler water. Before this high-energy waste is discharged, the residual heat in blowdown can be recovered with a flash tank, a heat ex- changer or the combination of the two. A boiler blowdown heat recovery project at Au- gusta Newsprint Mill in Georgia saved the company $31,000 in fuel costs and 14,000 MMBtu in energy annually. CASE STUDY Clean Cooling Water Reuse Safelite Glass Company in Enfield, N.C., utilizes water from air compressors and hydraulic fluid cooling water for boiler makeup. Clean once-through cooling water is a good candidate for boiler water make up. The reuse practice saved 8.5 million gallons of city water per year and was implemented for $3,000. Simple payback was two months. 5 Water Efficiency Fact Sheet May 2009 The North Carolina Division of Pollution Prevention and Environmental Assistance provides free, non-regulatory technical assistance and education on methods to eliminate, reduce, or recycle wastes before they become pollutants or require disposal. Telephone DPPEA at (919) 715-6500 or (800) 763-0136 for assistance with issues in this fact sheet or any of your waste reduction concerns. CASE STUDY Chemical Free Boiler Water Treatment Vanir Solar Construction in Fletcher op- erates a 150-hp boiler around the clock during the heating season. The boiler has a high condensate return and very high- quality make-up water. The water treat- ment system for the boiler utilized a conventional approach of chemical treat- ment using phosphate, hydroxide alka- linity (caustic) and sulfite. Beginning in the 2007-08 heating season, a non- chemical treatment water system was installed by Fluidyne International. The new treatment system reduced boiler corrosion and deposits while significantly reducing boiler blowdown water. With visual inspection, rusty deposits in the boiler and condensate return lines were disappearing from the walls of the wet- ted areas. The annual (heating season) savings related just to blowdown, includ- ing water, sewer and energy costs, was $4,070. The new system is saving 189,000 gallons of water annually. Sig- nificant additional savings were achieved in eliminated chemical costs and chemi- cal servicing.