As a general rule, routine tube sampling is employed to assess the condition and cleanliness of tubes which are filled with steam/water mixtures during service. The primary objectives of routine tube sample collection are to measure the internal deposit weight per unit area and determine the deposit composition. This information is used to determine if a cleaning is required and can be used to assess the performance of the feedwater and boiler water treatment programs.

Lay-up and start-up of a combined cycle power plant are particularly important as the plants are often cycled regularly to meet peak demands. Start-up is not gradual, as in a conventional fossil-fired boiler, but very sudden because the combustion turbine exhaust gases produce very rapid heating of the tubes. Combustion turbine purge cycles that occur just prior to ignition exacerbate the situation by pushing cold air through the HRSG followed by the engine ignition and sudden temperature rise. These factors can produce condensate in superheaters that can result in thermal low-cycle fatigue. This result can be minimized with proper design and operational procedures. Start-up practice should ensure that oxygen is less than or equal to 10 ppb in the feedwater and any other water that will be fed to the HRSG before the deaerator is operational. This stipulation applies to water in the HRSG drums and the condenser hotwell and may even extend to the condensate storage tanks in some configurations. The purity of steam supplied by auxiliary boilers to a HRSG should be the same as that of steam produced by the HRSG.

Section I applies to several types of boilers and components of boilers, such as economizers, superheaters, reheaters, and, in some circumstances, feedwater heaters. Although its title is Rules for Construction of Power Boilers, its scope is somewhat broader. The Preamble to Section I explains that it covers electric boilers, miniature boilers, high-temperature water boilers, heat recovery steam generators, certain fired pressure vessels, and organic fluid vaporizers. Since the precise definitions of these various types of boilers are not generally known, the following definitions, found in footnotes to the Preamble, are helpful: Power boiler —a boiler in which steam or other vapor is generated at a pressure of more than 15 psi (100 kPa) for use external to itself. Electric boiler —a power boiler or a high-temperature water boiler in which the source of heat is electricity. Miniature boiler —a power boiler or high-temperature water boiler in which the limits in PMB-2 are not exceeded. (PMB-2 establishes limits on shell diameter, heating surface, gross volume, and maximum allowable working pressure. See further discussion under Part PMB later in this chapter.) High-temperature water boiler —a water boiler intended for operation at either pressures in excess of 160 psi (1.1 MPa) or temperatures in excess of 250° F (120°C), or both. Heat recovery steam generator (HRSG) —a boiler that has as its principal source of thermal energy a hot gas stream having high-ramp rates and temperatures such as the exhaust of a gas turbine. Fired pressure vessel —reheater, isolable superheaters, and nonintegral separately fired superheaters.

The designer of a boiler must provide adequate cooling flow of water or steam through the various pressure parts so that they will not exceed their intended design temperatures. Water-cooled furnaces must contain high-temperature combustion gases and cool them sufficiently to pass through closely spaced rows of heating surface without overheating that surface or (for coal-fired boilers) causing slagging problems in the furnace or elsewhere. As discussed in Chapter 3 under Special Concerns, Ferritic alloys (carbon and low alloy steels) are used for water-cooled furnace tubes because of their resistance to chloride-induced stress corrosion cracking. Boiler circulation must be sufficient to cool the furnace walls to temperatures at which ferritic steels have adequate strength and oxidation resistance. Section I has never provided rules for boiler circulation, as the responsibility for functional design has traditionally remained with the boiler manufacturer. However, Section I does have rules that implicitly recognize the serious consequences resulting from the loss of adequate water supply to heat-absorbing circuits critical to the safe operation of the boiler. Firing an inadequately cooled or partially dry boiler can lead to overheating of the pressure parts, with consequent loss of strength and potentially catastrophic failures due to internal pressure. Section I addresses this problem with rules intended to assure the availability of feedwater and the presence of water in the drum.

An advanced method for assessing dynamic safety of nuclear power plants is introduced and applied. The commercial software, VENtana SIMulation environment, VENSIM, is used to develop a dynamics model for an example system. In this study the 18-month refuel cycle is simulated for the dynamic analysis. The failure rate when the plant is a zero power like maintenance, test, and refueling processes, which are not properly modeled in conventional method using event/fault trees, is higher than that of the full power. This also means the human failure rate during both standby and shutdown operation is higher than that of normal operations. Various time steps are applied for the different failure cases. The simulation results show that the common cause failure is much affected by the time step process. The results also include the dynamic simulation for the standby-running and shutdown-running cases. The graphical presentation has been easily modeled by a unique graphic designed method incorporated in the VENSIM. The diagrams well understood by operators or system analysts are constructed and evaluated quantitatively using system dynamics.

All units and sub-systems in the power train of industrial, utility and heat recovery steam generation facilities must be adequately sampled and monitored to maintain operation as desired and to prevent damage to individual pieces of equipment. Besides the obvious effects of failure to maintain design production of treated water and steam on plant production costs, failure to adequately monitor operating conditions can cause failure of major system components and cause unplanned outages. This document has been prepared by the Water Technology Subcommittee of the ASME Research and Technology Committee on Water and Steam in Thermal Systems. This publication provides recommendations for water chemistry monitoring and is a companion to the ASME “Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers,” “A Practical Guide to Avoiding Steam Purity Problems in the Industrial Plant,” and “Consensus on Operating Practices for Control of Water and Steam Chemistry in Combined Cycle and Cogeneration Power Plants”. These documents provide guidance for avoiding the penalties of severe corrosion or deposition in steam generation systems and their auxiliary steam user equipment.

This document provides recommendations for the sampling and analytical testing required to maintain the suggested water chemistry limits outlined in the “Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers” and the “Consensus on Operating Practices for Control of Water and Steam Chemistry in Combined Cycle and Cogeneration Power Plants.” The boilers covered by these recommendations are: • industrial water tube, high duty, primary fuel fired, drum type with superheaters and∕or process restrictions on steam purity • industrial water tube, high duty, primary fuel fired, drum type without superheaters • industrial fire tube, high duty, primary fuel fired • industrial coil type, water tube, high duty, primary fuel fired • marine propulsion, water tube, oil fired, drum type • electrode type, high voltage, recirculating jet type • single pressure industrial heat recovery steam generators

Effective monitoring of makeup water, condensate, feedwater and boiler water qualities and steam purity is necessary to control deposition and corrosion within the boiler, steam, and condensate systems. The absence of adequate monitoring and control can lead to operational upsets or unscheduled outages and is ill advised from the point of view of safety, economy and reliability. A key component of effective monitoring is sampling. The sample must be truly representative of the stream being monitored. This requires a proper sampling system, adequate system flushing, the required sample velocity, and appropriate sample handling during the time between sample collection and analysis. If proper procedures are not followed, analytical results may not be representative of the stream being monitored and the results will be meaningless and potentially harmful if used as the basis of operating decisions.

The steam purity required for any given boiler system is dictated by the use of the steam. Steam of inadequate purity can cause deposition and corrosion in superheaters, steam lines, control valves and turbines. Steam contaminants can also adversely affect catalyst performance and product purity. The ASME “Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers” and “Consensus on Operating Practices for Control of Water and Steam Chemistry in Combined Cycle and Cogeneration Power Plants” provide steam purity targets for typical industrial uses of steam. The purity of steam is usually monitored by measuring the cation conductivity or sodium content of condensed steam samples. In special situations, as noted in the Combined Cycle and Cogeneration document, silica and∕or cation conductivity will be measured.

Successful control of boiler system water chemistry requires that meaningful and representative samples be acquired at various points throughout the boiler system. Samples and∕or sample points must meet the specific requirements (e.g. flow, temperature, velocity) of the individual tests or the continuous analyzers that utilize them. They must also allow the user to acquire samples in a safe manner; e.g. provide cooling as necessary, route the sample to a protected area, etc. Recommended sample points for make-up, feedwater, blowdown, steam, and condensate are shown in Figures 1 and 2. Locations are included for both grab samples and continuous analyzers. Systems operating with high purity make-up typically have additional and more stringent sampling requirements.

The tube side of feedwater heaters should be treated the same as the boiler during lay-up periods. The shell side normally is steam blanketed or dried and pressurized with nitrogen during lay-up. However, the shell side may be flooded with treated condensate or condensate-quality water. All-steel systems should be laid up with condensate containing 200 mg/L (ppm) volatile oxygen scavenger or equivalent, and ammonia or suitable amine such as cyclohexylamine as needed to maintain the pH at 10.0 min. For copper or copper alloy systems, use 50 to 100 mg/L (ppm) of volatile oxygen scavenger and adjust the pH to 9.5.

It is essential that proper procedures be followed for restarting equipment after lay-up. The effectiveness of the lay-up may become apparent during start-up, if the start-up is adequately monitored. Indicators may be very high or very low crud concentrations entering the boiler from the condensate / feedwater system. If satisfactory care was not taken, equipment recleaning or even replacement may be necessary before returning the equipment to service.

This document has been prepared by the Industrial Subcommittee of the ASME Research and Technology Committee on Steam and Water in Thermal Power Systems as a consensus of proper current operating practices for the control of feedwater and boiler water chemistry in the operation of modern industrial, high duty, primary fuel fired boilers. These practices are aimed at minimizing the penalties of severe corrosion or deposition, frequent cleaning requirements, or unscheduled outages in the steam generator systems and their auxiliary steam users.

Proper treatment of makeup and feedwater is necessary to prevent scale, other deposits, and corrosion in preboiler, boiler, steam and condensate systems, and to provide required steam purity.

The maintenance of specified feedwater and boiler water chemistry must be well regulated and documented by frequent analysis and record keeping. Either manual or instrumental water chemistry measurement is necessary to ensure continuous satisfactory equipment operation, and it is indispensable as an aid to follow up troubleshooting.