Improved Marine Boiler Reliability — Phase II

Carl F. Horlitz and Arthur L. Plumley*

Recommendations Are Presented Which Will Enable The Boiler Purchaser Or Designer To Specify Materials, Design Features, And Operating Guidelines Which Should Assist In Establishing Boiler Reliability In an attempt to investigate the phenomena which affect marine boiler reliability, the Maritime Administration (MarAd) in early 1975 funded the first phase of a program entitled "Improved Marine Boiler Reliability." The objective of this initial effort was to investigate the main problem areas encountered in the design and operation of marine boilers and, to identify and research certain basic concepts and parameters and to explore their effect upon boiler reliability.

As a result of the investigations carried out in the initial phase, several items relating to specific areas of marine boiler reliability were chosen for additional analytical investigations as well as laboratory-type testing, both onboard ship and in land-based facilities. This second phase, also funded by MarAd, began in July 1976 and will continue into 1980.

This second phase consisted of six task areas. A comparison of wastage potential of several superheater materials over a selected temperature range, approximating metal tem- peratures encountered in the hot end of a 950°F (510°C) marine superheater, was accomplished in Task 1. In Task 2, the wastage potential of various economizer metals operating at normal temperature and below the sulfuric acid dewpoint was determined.

In Task 3, by means of a three-dimensional cold-flow model, flow distribution in marine superheaters was determined as a function of physical characteristics and load conditions.

Results of the fourth task on shipboard stack gas analyses allow characterization of the conversion of SOj to SO3 as it relates to acid dewpoint.

The fifth task outlines thermocouple development and current design in boiler applications based on Combustion Engineering (C-E) experience. In the final task of the initial second-phase effort, a report was prepared with the cooperation of shipowners, fleet operators, boiler manufacturers, and b o i l e r w a t e r chemical vendor-consultants which reviews the current boiler water treatment practice of the U.S. merchant marine and describes the fundamental treatment programs needed for boiler water and evaporator control.

Task 1: Superheater Corrosion Rates Through the cooperation of Waterman Steamship Corporation and the outstanding assistance of the vessel's crew, the port boiler of the LASH vessel Stonewall Jackson was made available. The boiler is a C-E V2M8 welded-wall boiler equipped with two downward- firing C-E HX550 steam atomizing marine oil burners. The two-boiler propulsion plant is rated at 32,000 shp.

While the absolute values of the param- eters monitored were specifically applicable to the type of boiler used for the test, the general trends exhibited over a range of boiler loads should be characteristic of other types and sizes of marine boilers.

Corrosion potential in the superheater area of the boiler was evaluated by controlled temperature probes. The location of these test probes was fixed by the design of the boiler. The high-temperature test probes were installed in the access port of the superheater section of the boiler, as shown in Figure 1.

Probes were removed after 4,080 and 8,390 hours (six months and 12 months) of sailing time. These probes generated data that established the resistance of commercially available materials of fabrication.

Recommendations: 1. On the basis of this test work it is suggested that, in a marine boiler superheater designed for an outlet steam temperature of 950°F, the use of T-9 in the higher-temperature passes could provide significantly extended superheater life over the currently used T - l l or T-22 material at a reasonable cost increase.

2. Continued evaluation of Tp-347-H stainless steel, T-9, and chromium diffusion-coated T-22 material at elevated temperatures is recommended.

3. Other materials which have the potential to resist corrosion, due to chromium content, should be tested. These materials would include Incoloy 800, a material comparable to Inconel 600, and a clad material such as Inconel 671, which has a 50 percent chromium/50 percent nickel composition of the clad layer.

Task 2: Economizer Corrosion Rates This study was aimed at quantifying corrosion potential in marine boiler economizers with regard to acid dewpoint. The test plan called for operation of one probe below the acid dewpoint for the flue gas of the tested boiler, while the second probe was operated at about 100°F (55°C) above the acid dewpoint to provide an indication of gas phase oxidation or erosion or both.

Economizer cold-end corrosion is caused principally by condensed sulfuric acid from the flue gas, and the quantity of condensible vapor formed (as well as the dewpoint) is a function of type of fuel burned, burner design, furnace design, amount of excess air, etc.

The economizer probes generated corrosion data that corroborated the effect of acid dewpoint corrosion. The wastage rate demonstrated on the economizer surface operating below the acid dewpoint, while firing approximately three percent sulfur oil, is three times that found at the ambient temperature of the economizer.

Recommendations: 1. All test materials can be recommended for service in both the hot and cold ends of a marine economizer. A prolonged service life may be expected provided significant changes in fuel type and excess air levels do not occur.

2. The use of Corten tubing and fins for marine economizers is recommended where extended tube life, particularly in the lowtemperature cold end, is desirable. In this test program the Corten was somewhat supperior to carbon steel and cast iron, and no advantage was gained by use of aluminized surface on the carbon steel.

3. Additional testing at lower metal temperature in the range of 240° to 250°F (116° to 121 °C) is desirable since this represents the economizer cold-end temperature on older vessels employing lower-pressure (25 psia) deaerators. This would allow f o r a quantitative assessment of the life of replacement economizer elements on older ships.

Task 3: Superheater Airflow Model Tests A three-dimensional cold-flow model was built and tested to determine the flow distribution in a marine boiler superheater as a function of physical characteristics and load conditions. An analytical model also was developed to determine flow distribution in marine superheaters f o r simple one-dimensional flow. A heat-transfer program was developed and used to predict superheater tube metal temperatures as a function of steam flow distribution. Superheater arrangements having low flow and high temperature in certain tubes were identified. A design procedure was developed to take into account the effect of flow distribution on tube metal temperature.

Recommendations: 1. The results of this study and the design procedure developed should be used to predict tube metal temperatures in proposed marine superheater designs. Those designs which have predicted tube metal temperatures above desired limits can then be modified to achieve lower temperatures.

2. General methods which should be used to control tube metal temperatures are as follows: (a) The number of tube columns in a pass should be kept to a minimum, especially in downstream passes, to maintain an optimum balance of pressure drop and tube metal temperatures; (b) If it is necessary to omit or remove any tubes in a pass for pressure-drop or heat-transfer considerations, the tubes immediately after the partition plate should be the ones omitted or removed, and (c) If it is desired to determine flow distribution for geometries other than those already modeled, work should be done to develop an analytical model which will predict flow distribution with two- and three-dimensional header flows. This will eliminate the need for physically modeling every geometry of interest.

Task 4: Boiler Stack Gas Testing In order to define more clearly a typical corrosion environment for cold-end heat exchangers in modern marine boilers, a program of stack-gas analysis was undertaken on an operating LASH vessel. The major thrust of the investigation was to obtain sufficient information to permit the calcu- lation of sulfuric acid dewpoints at various boiler loadings and excess air levels.

Over the course of five days, extensive testing and shipboard analyses were conducted at a variety of boiler loads and excess air levels, including a series of tests during minimum-load conditions in port.

The results of this testing effort have been extensively presented to the Society in 1977 and are not repeated in this paper.

Task 5: Thermocouple Modernization The application and use of thermocouples in modern utility, industrial, or marine boiler technology classify into three broad areas: Operating data readout and recording; equipment and safety monitoring and alarms, and research and development. The first two areas make use of rugged instrumentation well engineered to offer maximum life, accuracy, and protection. This equipment, except for subtle modification to adapt it to boiler or operating-room installations, will be no different than applications in other industries. It may be expected that a long, useful life is designed into these thermocouples.

The third area usually requires unique special applications to produce the desired results. Techniques have been evolved to install the thermocouples in relatively inaccessable areas and hostile environments. These designs assure that the thermocouples will have a useful life.

The sheathed thermocouple, Figure 2, or the marine sheathed thermocouple, Figure 3, because of the environment they are exposed to, will have a short life expectancy. The sheathed thermocouple is recommended over the marine sheathed thermocouple for general boiler use.

A combination of good features in both the sheathed and marine sheathed thermocouple should provide, in marine applications, both accuracy of measurement and increased service life. By covering the exposed length of the sheathed thermocouple with a properly installed cover plate as used with the two-element marine sheathed thermocouple, vastly improved operating life of such thermocouples may be expected.

Task 6: Boiler Water And Feedwater As a result of this study, several items have been identified which require particular attention in order to maintain the plant such that routine feedwater maintenance procedures will be effective. These areas are as follows: Makeup-water preparation: If dissolved salts were permitted in boiler water, the internal surfaces would quickly scale up and the boiler would suffer severe overheating damage due to the scale interfering with normal heat transfer. Heavy scale can f o rm in evaporators, adversely affecting heat-transfer capacity and purity of the distillate. Various chemical additives are available which permit higher saline concentrations. These include antisealants, dispersants, and foaming agents which are effective in controlling scale and foaming. By permitting higher concentrations of seawater, heat losses are reduced, while at the same time limiting scaling and carryover.

Thermal deaerators: A properly maintained deaerating feed heater will lower dissolved oxygen levels to the five to seven parts per billion range, thereby allowing final oxygen removal to be accomplished by use of normal amounts of sodium sulfite or hydrazine feed.

It is recommended that deaerators be inspected regularly (not less than once every two years) for proper mechanical condition and that instructions for their proper use be strictly adhered to.

It should be further noted that, while virtually all steam-propelled vessels are fitted with deaerators, many nonsteam vessels which contain auxiliary steam systems are not so equipped. A thermal deaerator is the simplest and perhaps least expensive means of effective oxygen removal and its use in all marine steam systems, main or auxiliary, is highly recommended.

Condenser tube leakage: This is a widely encountered problem within the marine industry.

In general, normal chemical feed additives and boiler water treatment can handle saline concentrations below 1.0 grains per gallon (17.1 ppm) and hence most salinity alarms are set at this value. Once excessive salinity is detected, it is imperative that corrective action be taken as soon as possible.

Condensate cycle materials: Although the majority of vessels have been equipped with only 90-10 copper-nickel, occasional reports indicated that there is some aluminum-bronze and aluminum-brass remaining on the older vessels. This should be removed as soon as economically feasible. In the case of condenser tubing, 90-10 and 70-30 copper-nickel will give substantially longer life, particularly if a vessel is forced to spend a large portion of its time in polluted coastal waters.

A good economic choice is 90-10, which several recent studies have shown to be more resistant than 70-30 in polluted, brackish waters.

The higher cost of the 70-30 has been a significant deterrent in the past, but the changing economics with regard to fuel ver- sus downtime should justify a review of these factors.

Use of hydrazine: The use of hydrazine in the preboiler system f o r oxygen control imposes certain restrictions on the proper handling of the components at the low-pressure end of the system. Hydrazine is completely broken down in passing through the superheater, the reaction products being nitrogen, hydrogen, and ammonia.

When it is realized that analysis of a boiler deposit specimen often reveals a content of 25 to 50 percent copper, it is apparent that corrosion in the low-pressure end of the system is a significant contributor to the total amount of foreign material which interferes with heat transfer. If it is found that both oxygen and ammonia are in high concentrations, it may be desirable to modify the amount of hydrazine injected, as well as where it is added to the cycle.

Carryover: If deposits of sodium salts are found in the superheater, boiler water carryover in the steam is indicated. If only an oxide buildup is found, an iron-steam reaction is the probable cause. Pitting of tubes occurs during outages due to the ingress of oxygen. Strict attention must be given, therefore, to proper layup procedures during outages.

Other stories from April 15, 1980 issue


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