A Primer on Protecting Idle Boilers By Howard Benisvy, Member ASHRAE

Transcription

1 The following article was published in ASHRAE Journal, December Copyright 2002 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. A Primer on Protecting Idle Boilers By Howard Benisvy, Member ASHRAE M any boiler plants have excess boiler capacity. Typical boiler plants maintain backup capacity that must be instantly ready to provide steam should a mechanical problem occur on the primary boilers. Some boiler plants may have had a greater steam demand at one time, and process changes have reduced present steam requirements. The challenge to many managers is to protect the integrity of this equipment while the boilers are not being used. A significant amount of damage that occurs to boilers during their lifetime can be associated with idle periods and extended downtime. The typical processes in place that provide protection for boilers from scale and corrosion during operational times may not be available when the boiler is not running. This article focuses on boiler systems operating below 300 psig (2.07 MPa) and discusses: extended layup (typically greater than 30 days idle time); short-term layup; implementation of wet and dry layup options; and proper startup of idle boilers. A typical boiler system is shown in Figure 1. Your system may have some or all of the equipment shown. Water quality guidelines for boilers are provided by the American Society of Mechanical Engineers (ASME) (Table 1). In addition, the ASME guidelines for feedwater and boiler water quality should be followed for idle boilers. 1 It is critical to provide oxygen levels below 7 ppb to minimize the potential for oxygen-related corrosion. This is accomplished by mechanical deaeration of the feedwater plus proper chemical treatment to eliminate oxygen ingress from various sources. An understanding of the various types of corrosion that can occur during idle periods helps ensure you develop a successful plan. A combination of both chemical changes to your water treatment program and mechanical changes to your system are necessary to provide proper system protection. Waterside Corrosion Two types of corrosion are typically seen in idle boilers: acid attack and oxygen pitting. Both high- and low-pressure systems can be adversely affected by a low ph-acid attack from acid contaminant sources. These acid sources could include condensate system contaminants, a demineralizer system anion exchanger, and leaking feedwater and boiler non-return valves. By far, the most common corrosion in idle boilers is oxygen pitting. If oxygen is present in the water, oxygen pitting can About the Author Howard Benisvy is a senior consultant for Ondeo Nalco Co., Naperville, Ill. 30 ASHRAE Journal December 2002

2 Boilers Oxygen pitting in an idle steam boiler. occur on carbon steel. The rate of corrosion is dependent on the concentration of oxygen and temperature of the water. An example of waterside corrosion on an idle boiler due to oxygen pitting is shown at top right. 2 Fireside Corrosion Fuels such as coal and oil can generate deposits on the fireside including iron, vanadium, sodium and sulfur. Condensation and humidity must be controlled in the furnace so these compounds do not react to form corrosive, low ph deposits. Idle Boiler Layup When deciding on proper layup procedures for your boiler plant, several factors must be considered, including: What is the duration of downtime? Does this boiler need to provide immediate backup steam capacity? Does the boiler have a superheater? Is the superheater drainable? Can the boiler be drained and left dry without affecting plant operations? What types of fireside deposits are present? Can the boiler be sealed? Do the valves hold? The two boiler layup methods used in the industry today are dry and wet layups. 3 Dry layup is recommended for boilers stored more than 30 days, and wet layup usually is used for boilers kept idle less than one month. Under certain circumstances, it may be necessary to store a boiler wet more than 30 days. However, special attention and planning must be used to protect these boilers. Corrosion can begin within one week and can escalate to cause serious damage within one month. By far, the most effective way to protect the waterside of an idle boiler involves instituting a dry layup procedure. Many of the factors that cause severe corrosion, occur when the boiler is wet. Both acid attack and oxygen corrosion occur in a wet boiler. However, with a proper dry layup procedure, you can easily prevent these types of corrosion. Negligible corrosion occurs if the relative humidity remains below 55%. A dry layup program must be controlled to below 55% relative humidity. Dry Layup Guidelines: More Than 30 days Waterside Thoroughly rinse out all boiler internals with clean water. Dry the watersides with warm air (might take several days). Blank all valves to ensure water cannot leak into the boiler. Install a desiccant to maintain low humidity levels in boiler. (Desiccants include commercial grade silica gel [see manufacturer guidelines for proper amount]; quick lime [not hydrated lime], use at least 7 lbs per 1,000 pph boiler steam capacity and use at least 3.2 kg per kg/s steam capacity.) Instead of desiccants, a nitrogen blanket can be used to prevent air contact on the waterside. The boiler must still be dried completely with warm air and pressurized with nitrogen to prevent air intrusion. For this procedure to be effective, a constant nitrogen pressure of at least 5 psig (34.5 kpa) must be maintained. December 2002 ASHRAE Journal 31

3 Feedwater Dissolved Oxygen ppm (mg/l) O2 (measured before chemical oxygen scavenger addition) Total Iron ppm (mg/l) Fe Total Copper ppm (mg/l) Cu Total Hardness ppm (mg/l) CaCO3 ph range at 25 C Chemicals for Preboiler System Protection Nonvolatile TOC ppm (mg/l) C Oily Matter ppm (mg/l) < < 0.1 < 0.05 < NS < 1 < 1 Boiler Water Silica ppm (mg/l) SiO2 < 150 Total Alkalinity ppm (mg/l) CaCO3 < 700 Free Hydroxide ppm (mg/l) CaCO3 NS Unneutralized Conductivity mhos/cm ( S/cm) 25 C Total Dissolved Solids in TDS (maximum) ppm (mg/l) Boiler Type: industrial watertube, high-duty, primary fuel-fired, drum-type. Makeup Water Percentage: up to 100% of feedwater. Conditions: includes superheater, turbine drives, or process restriction on steam purity. : superheated and/or turbine. Saturated Purity Target: see tabulated values. NS: not specified. Table 1: ASME table of suggested water chemistry limits. 1 Fireside Clean/remove deposits from firesides. If the fireside cannot be completely cleaned and sealed, apply a barrier coating to provide a moisture barrier. Special precautions must be made to ensure the fireside does not cool down and allow moisture to form prior to application of barrier coating. Completely dry fireside by applying warm air throughout internals. Close all air dampers and blank stack to ensure no air/moisture entry. Add desiccants on storage trays to absorb moisture. Install humidity indication cards in easily accessible locations. Inspection Inspections of both the waterside and fireside should be performed weekly. Placing humidity indicator cards near inspection ports allows easy evaluation. A thorough inspection should be performed at least monthly to ensure there is no condensation, water, leaks or active corrosion. Once humidity increases above 55%, change out desiccants. Wet Layups: Less than 30 days 3 Chemistry guidelines for wet layups of less than 30 days are sulfite: 200 to 400 ppm; Hydroxide alkalinity: 600 to 800 ppm; and scale inhibitor that is in normal operating range (avoid a precipitating program to minimize sludge formation). This chemistry can vary, but the goal of the program is to maintain a boiler ph of 11 and an oxygen-free environment. Also, this chemistry cannot be used for layup of superheaters. Demineralized water and all volatile chemistry must be used. To achieve the proper wet layup chemical levels in the boiler, Economizer Feedwater Heater Condensate Treatment (Polisher, EMF, etc.) Supply Deaerator Makeup Boiler Feedwater Pump Figure 1: Typical boiler system. Any Suitable Boiler Connection begin increasing chemical dosages to the boiler at least one week prior to shutdown. Keep in mind that high alkalinity levels can cause foaming and carryover during on-line operations on some boilers. To minimize the potential for foaming, adjust alkalinity just prior to shutdown. Once the boiler is shut down, it is difficult to achieve adequate mixing of chemicals unless the boiler is placed on-line to increase circulation. Initially, maintain 400-ppm sulfite because air intrusion causes sulfite levels to drop. To prevent oxygen pitting, do not let sulfite levels drop below 200 ppm. Short-Term Wet Layup Process Primary Secondary Boiler Superheater Valve Boiler Condensate Receiver WaterLevel Condensate Pump Small Steel Figure 2: Filling boiler drum to the top and changing the location of air water interface. Wet layup, regardless of length of time, needs to maintain the same chemical parameters to minimize the waterside corrosion. Plants that have several boilers sometimes choose to cycle all their boilers on a regular basis. This routine sometimes causes short two to three day run times. The number of startups and shutdowns influences boiler life. The heating and cooling of the boilers due to short run times increases mechanical stress on the boiler. Ultimately, frequent expansion and contraction along various joints and welds can lead to leaks. These guidelines do not refer to boilers kept in hot standby. However, plants that have several boilers may choose to keep them in a hot standby mode at normal operating chemistry levels. Boilers operating under these conditions are very susceptible to significant corrosion. It is important to evaluate how many boilers need to be kept in hot standby vs. using proper layup procedures. When possible minimize short run times. Instead consider the following wet layup options: 32 ASHRAE Journal December 2002

4 Boilers Nitrogen Blanket A nitrogen blanket is formed by filling the boiler completely with water and pressurizing with nitrogen gas. The nitrogen gas forms an inert barrier, and minimizes corrosive oxygen intrusion into the water phase. Generally, nitrogen blankets are difficult to maintain and only are used for short intervals. The nitrogen blanket can prevent air introduction into the idle boiler. It only can be effective on boilers that are capable of being completely sealed. Add treated water to the boiler in the recommended layup ranges. All valves should be blanked to avoid leakage. Fill the water level of the boiler to the top of the drum. Apply nitrogen at 5 psig (34.5 kpa) to the steam drum. Slowly lower drum level while maintaining at least 5 psig (34.5 kpa). Nitrogen must be continuously maintained with a minimum of 5 psig (34.5 kpa) to maintain the blanket. A nitrogen atmosphere is fatal, and oxygen testing is required to ensure safe breathable air is available prior to any boiler inspection. Flooded Wet Layup Flood the boiler with treated water and maintain a 55-gallon drum (208 L)(with treated water) above the height of boiler with a sight glass (Figure 2). Install a cover. The drum allows for expansion and minimizes air intrusion into boiler. Cascading Blowdown Cascading blowdown involves feeding blowdown water from the on-line boiler to the idle boiler. The idle boiler receives treated water, remaining heated and pressurized. Cascading blowdown has been successful where the boiler is on a low suspended solids treatment program such as one provided by Mud Operating Boiler Surface Blowdown all-polymer chemistry. This is not recommended for superheaters. To successfully implement a cascade system, you must have sufficient continuous blowdown to keep the standby boiler treated and hot. Basically, surface blowdown from the on-line boiler is supplied to the mud drum of the standby boiler. The surface blowdown of the standby boiler is wasted to a blowdown flash tank (Figure 3). If there are corrosive fireside deposits, it is critical that the boiler be maintained at a pressure and temperature to prevent condensation and potential corrosion in the furnace. All dampers on the furnace should be closed to minimize any heat loss. Extended Wet Layup: More than 30 days Wet layup periods exceeding 30 days have the greatest potential for significant corrosion. There are cases where it has been done very successfully. However, meticulous implementation of guidelines and continuous evaluation and monitoring were critical to the success of the layup program. Plant personnel must evaluate idle boiler conditions as much as those of on-line boilers to minimize the potential for failures and extend equipment longevity. I have seen many more failures than successes with the extended wet layup option. By far, dry layup provides the best boiler protection for extended periods of downtime. Water Sampling & Inspection Water sampling should be performed at least once per week. Since the water is stagnant, it is difficult to obtain accurate samples. Test from several locations. If the chemistry falls below any of the recommended levels, add more chemicals. Inspect the boiler prior to bringing back on-line. Drainable and Non-Drainable Superheaters Protecting an idle superheater is extremely challenging. If you have drainable superheaters, dry layup is the best option. Ensure the superheater is completely dry. A nitrogen cap in combination with draining can provide the best protection for a drainable superheater. Superheaters require the highest purity water and all-volatile layup chemistry. Demineralized Mud Standby Boiler To Blowdown Flash Tank Figure 3: Cascade system with operating and standby boilers. water quality or better is recommended. Back filling the superheater with treated water with an all-volatile chemistry will allow the introduction of treatment. Fill the superheater completely to minimize air. A nitrogen blanket also can be applied to minimize air intrusion. Chemistry for idle superheaters includes a morpholine-type chemistry to raise the ph to 10 and a volatile oxygen scavenger. Test weekly to ensure chemistry is in range and that residual oxygen scavenger is present. Proper Startup of Idle Boilers Proper startup of boilers is critical to minimize scale and corrosion, and to ensure good steam quality. It is not unusual for iron levels to be slightly elevated from layup. Every effort needs to be made to ensure the boiler water quality is returned to ASME guidelines as soon as possible. Prior to returning an idle boiler to operation, a full inspection should be performed to evaluate and record the condition of the waterside and fireside. The following procedure for startup is recommended: 1. Perform a boiler inspection and record condition (strongly recommended). 2. If boiler is in dry layup, remove all desiccants and humidity cards. December 2002 ASHRAE Journal 33

5 3. Open and clear all valves on the waterside. 4. Open all dampers to the fireside. Remove any desiccants/humidity cards. 5. Fill boiler with high quality feedwater per ASME guidelines. 6. Increase surface blowdown to reduce chemistry to normal operating ranges (if boiler was not drained and inspected prior to startup). 7. Increase frequency of bottom blowdown for first week of operation to remove suspended solids: maintain short 3 to 5 second blowdowns. 8. Monitor chemistry and adjust to get in range. 9. Monitor iron levels in feedwater and boiler to ensure they are within ASME guidelines. In my experience, I have seen each of these guidelines work well in different boiler plants. Where these techniques have failed, it has been due to poor implementation practices. Dry layup has the best success rate, providing you follow the guidelines and ensure the boiler is kept completely dry. You must ensure the fireside is free of corrosive deposits. Some of the issues that prevent a successful layup include: Valves not completely sealed or blanked, allowing corrosive untreated water to enter the boiler. Inadequate sampling locations. Insufficient logging and evaluation of data. Lack of weekly testing and adjustment of chemistry. Inadequate cleaning of the fireside. Infrequent inspection. Conclusion As much as practical, this article provides guidance for protecting idle boilers. ASME is working on a detailed document on this subject titled, Consensus for the Layup of Boilers, Turbines, Turbine Condensers, and Auxiliary Equipment. It should be available by the end of Hopefully, these guidelines have provided a good foundation for you to develop a successful layup system for your boilers. Evaluating the mechanical limitations of your equipment is important to selecting the best layup approach for your facility. By selecting the approach that can best be followed properly, you help ensure good results and extended boiler equipment life. References 1. Holloway, R.T., et al Consensus on operating practices for the control of feedwater and boiler water chemistry in modern industrial boilers. American Society of Mechanical Engineers. 2. Port, R.D. and H.M. Herro The NALCO guide to boiler failure analysis. McGraw-Hill, New York, pp Kemmer, F.N The NALCO Water Handbook. Second Edition. McGraw-Hill, New York, pp Advertisement in the print edition formerly in this space. 34 ASHRAE Journal December 2002