View non-flash version
Maintaining Design Performance In Marine Boilers Thomas P. Mastronarde* The responsibility to maintain design fuel rates over the service life of a marine steam powerplant rests with the operator. With good plant management, this task is usually accomplished by a com- bination of frequent periodic ad- justment of the equipment con- trolling powerplant efficiency, and by regular maintenance of major components. Of the numerous monitoring points associated with main-pro- pulsion boilers, the following are considered to be the most import- ant parameters affecting power- plant efficiency as measured by the specific fuel consumption: excess-air fired, superheater out- let temperature, superheater out- let pressure, and stack temper- ature. Other parameters, such as feed temperature j.-nd ambient air tem- perature, affect the boiler output and the cycle efficiency, but are not controlled by the boiler sys- tem. Deviations from design excess air and boiler outlet pressure are governed by control systems which may be adjusted by the operating personnel. The super- heater outlet temperature may also deviate from design through improper control or from deteri- oration of superheater surface, in which case the design tempera- ture cannot be restored without cleaning or restoration of the com- ponent itself. The stack temper- ature reflects the overall cleanli- ness of the entire boiler, includ- ing heat-recovery surface, and cannot be adjusted with controls (assuming the design excess-air level is maintained). Excess-Air Deviation—The achievable excess-air level for a given marine boiler is a function of furnace configuration, the amount of fuel fired per unit fur- nace volume, and the oil-burner characteristics. In general, the de- sign value, specified by the pur- chaser, includes some margin for operation with slightly worn burner tips or with fuel-oil vis- cosities and atomizing steam con- ditions which are not quite opti- mum. The design excess-air level maintained by the combustion- control system must also include margins for the following condi- tions : 1. Under steady steaming con- ditions, the combustion-control system may hunt cyclicly above and below the set point value by one or two percent excess air. Al- ternating torque loadings from the propeller, especially during pitching of the vessel, can also cause significant cyclic variations in the excess air maintained by the control system. 2. Under changing load condi- tions, such as maneuvering, suf- ficient margin on excess air must be provided to prevent smoking during the transient. Flue-gas testing carried out aboard ship with the traditional ORSAT type of analyzer empha- sizes the measurement of carbon- dioxide content of the flue gas, with oxygen and carbon monoxide determined to a lesser degree of accuracy because of the propor- tionally smaller amounts present. It is most desirable to measure the oxygen content of flue gas ac- curately since this parameter is not dependent on fuel composi- tion, as is carbon-dioxide content. Excess-air values can be accu- rately determined from the oxy- gen content of the flue gas with- out any knowledge of the fuel composition, a definite advantage for shipboard monitoring. The following procedure used by the author's company on sev- eral shipboard tests appears to be readily adaptable to periodic mon- itoring of the combustion-control system and burners by operating personnel: 1. Obtain an oxygen analyzer which can give continuous read- ings of oxygen content and can be readily calibrated with air. 2. Fabricate a portable gas- sampling probe from ^-inch di- ameter stainless-steel tubing by drilling small holes along the por- tion of the probe to be inserted into the gas stream. 3. Install a sampling penetra- tion in the uptake ducting below the economizer or regenerative air heater. This may consist of a i/o-inch pipe nipple threaded into the duct and fitted with a Vi-inch O.D. compression adapter that is normally plugged. 4. When a periodic check of combustion-system performance is desired, the plug can be re- moved from the compression fit- ting, the probe inserted into the gas stream, and a continuous read- out of oxygen content can be ob- tained from the portable analyzer. Excess air for a given oxygen con- tent can be determined from a chart. The amount of excess air fired in a boiler has an impact on the long-term material condition of boiler components related to ther- mal performance. In the super- heater area, excess air is related to the rate of high-temperature corrosion of superheater tubing materials exposed to vanadium compounds. In the economizer or regener- ative air heater, the amount of excess air is related to the cor- rosion of heat-recovery surface exposed to sulfuric acid in the flue gas. Metal surfaces at tem- peratures below the sulfuric-acid dewpoint will condense concen- trated acid onto the surface. In general, the lower the metal tem- perature is oelow the dewpoint, the greater the rate of corrosion. A reduction of excess air from 15 percent to 5 percent causes a re- duction in sulfuric-acid dewpoint of about 10°F. Regenerative air- heater corrosion appears to be more definitely influenced by excess-air level so that failure to maintain design excess-air could produce a significant deterioration of air-heater performance, and specific fuel consumption, over a long period of time. Aside from the long-term as- pects of boiler component replace- ment, the amount of excess air has an immediate effect on both the boiler efficiency and the power- plant cycle fuel rate. Increasing the amount of excess air above the design value causes an in- crease in stack temperature, an increase in stack losses from the additional mass of heated gas leaving the boiler, and an increase in power consumption for the forced-draft fans. For an increase in excess air of 10 percent above design value (e.g., firing 20 per- cent excess air in a cycle with de- sign excess air of 10 percent), the increase in specific fuel rate is approximately 0.65 percent. For a vessel with an annual fuel con- sumption cost of $3,000,000, op- eration with excess air 5 percent above design level throughout the year would cost up to $9,100 per year. This figure is based on the simplifying assumption that 95 percent of the fuel used annually is consumed at the full-power rating. Superheater Outlet Tempera- ture Deviation — Excursions in superheater outlet temperature can be caused by improper adjust- ment of the control desuperheater, if fitted, or by severe slag accu- mulation over a period of time. Although the operator observes superheater outlet temperature on a continuous basis, the accuracy of the ship's instruments can rea- sonably be questioned. Remote temperature gages can easily be 30°F to 50°F (17°C to 28°C) out of calibration after months of op- eration. The technique used by the author's company during ship- board tests on vessels in service is recommended for use by the vessel's operators for checking and informal calibration over a wide range of temperatures. Sheathed Chromel-Alumel (Hoskins Manufacturing Com- pany) (Type K) thermocouple probes are connected by a short length of wire to a portable digi- tal indicator (potentiometer). Sev- eral pocket-size indicators on the market are compatible with Type K thermocouples, have built-in reference junction circuitry, am- bient compensation, rechargeable batteries, and cover a range of temperatures from -60°F to 2,000°F (— 51°C to 1,094°C). These devices require no adjust- ment and give a reasonably accu- rate readout of the probe tem- perature at the flick of a button. A small thermocouple probe, %-inch diameter by 0.12-inch long (3 mm by 305 mm), responds readily to temperature transients and can be inserted into existing thermowells in the piping by tem- porarily disconnecting the ship's instrument probe. In this way, the control desuperheater set- point may be adjusted periodically to maintain the proper design val- ue of superheater outlet temper- ature at full load. It has been observed that su- perheater outlet temperatures on some relatively new vessels are maintained 10°F to 20°F (6°C to 11 °C) below design solely because of instrumentation error. A re- duction of 20°F (11°C) from de- sign value increases the specific fuel rate by almost 0.5 percent. For a vessel with annual fuel con- sumption of $3,000,000, operating continuously with superheater outlet temperature depressed by 10°F (6°C) would cost about $7,100 per year. It should be noted that other conditions, such as severe slag- ging or burnout of refractory baffles, can cause reductions in steam temperature of 30°F and 60°F (17°C and 33°C). An addi- tional technique for controlling severe slagging has been recently tested aboard ship. The addition of between 6 percent and 10 per- cent water to the fuel in a ho- mogenizing process appears to *Mr. Mastronarde, senior engineer, C-E Marine Power Systems, Combus- tion Engineering, Inc., Windsor, Conn., presented the paper condensed here at the Symposium on Sustaining De- sign Thermal Performance of Ship Propulsion Machinery held at The United States Merchant Marine Acad- emy, Kings Point, N.Y. 34 Maritime Reporter/Engineering News
