Wet-back: A type of boiler construction in which the rear combustion chamber is water jacketed-a term generally applied to Scotch type boilers. Page 3. Page 4
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BELL SYSTEM PRACTICES Plant Series SECTION 770-21 0-300 Issue 2, August 1970 AT&TCo Standard LOW PRESSURE HEATING BOILERS BASIC FUNDAMENTALS AND BELL SYSTEM STANDARDS CONTENTS PAGE 1. GENERAL 2. GLOSSARY OF TERMS 2 3. CAST IRON BOILERS 4 4. STEEL BOILERS-FIRETUBE TYPE 4 5. FUEL TYPES 9 6. AUTOMATIC OIL BURNING EQUIPMENT 13 7. AUTOMATIC GAS BURNING EQUIPMENT 16 8. CONTROLS FOR LOW PRESSURE HEATING BOILERS 18 COMBUSTION SAFEGUARD CONTROLS-REQUIREMENTS 19 COMBUSTION SAFEGUARD CONTROLS-ARRANGEMENTS 21 COMBUSTION CONTROLS–PROGRAMMING, SENSING, DRAFT CONTROL, AND OIL PREHEATING 27 OPERATING CONTROLS 29 LIMIT CONTROLS 33 SAFETY CONTROLS 36 9. ACCESSORIES 36 10. REFERENCES 39 1. GENERAL 1.01 Thi!’ section CO\’ers general descriptions, terminology and basic fundamentals of low pressure heating boilers, automatic fuel burning apparatus, and associated equipment. Also included are the recommended standards for combustion, operating, and limit and safety controls for heating systems installed in Telephone Company buildings as covered in the engineering letters and memoranda E.L. 72, E.M. 485, P.E.L. 7130, P.E.L 7131, and P.E.L. 7211. 1.02 Since this is a general revJsJOn, arrows ordinarily used to indicate changes have been omitted. 1.03 The illustrations used in this section have been selected to show typical examples of the equipment referred to in the text and are not intended to endorse or recommend any one manufacturer’s product; nor are the illustrations intended to be used as design criteria or as examples of preferred configuration of equipment. 1.04 For detailed information on specific unit, the manufacturer’s literature should be consulted. Also suggested as additional reading is the reference material listed in Part 10. 1.05 More detailed information on maintenance and operation is to be found in Section 770-210-301 (Hot-Water Boilers), and Section 770-210-302 (Steam Boilers). Section 770-2:30-303 covers water treatment for low-pressure heating boilers. 1.06 The boilers discussed in this section will be generally divided into two groups, Cast-Iron Boilers and Steel Boilers. 1.07 The ASME Boiler and Pressure Code, Section IV defines low-pressure boilers as follows: (a) Steam Boilers designed for operation at pressures not exceeding 15 psi. (b) Hot-Water Boilers designed to be operated at pressures not exceeding 160 psi and temperatures not exceeding 250°F. c Anwrican TlŁlephom· and Telegraph Company. 1970 Printed in LS.A. Page 1
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SECTION 770-21 0-300 :\-lost hot-water boilers in telephone buildings are designed for a working pressure of :30 psi. As stated prt>viously, hot-water boilers are available for higher working pressures but they must be designed. tested, and stamped for the higher pressure. 2. GLOSSARY OF TERMS Boiler Heating Surface: The area of the heat transmitting surfaces in contact with the water (or steam) in the boiler on one side and the fire or hot gases on the other. Boiler Horsepower: The equivalent evaporation of 34.5 lbs of water per hour at 212oF to steam at 212°F. This is equal to a heat output of 33,475 Btu per hour, which is equal to approximately 140 sq ft of steam radiation (EDR). British Thermal Unit (Btu): The quantity of heat required to raise the temperature of 1 lb of water 1°F. Combustion Chamber: That portion of a plant within which combustion occurs. In some cases this is also referred to as the firebox. Condensate: In steam heating, the water formed by cooling and condensing steam. Conduction (Thermal): The transmission of heat through and by means of matter. Conduct01· (Thermal): A material capable of readily transmitting heat by means of conduction. Control, Limit: A control responsive to changes in pressure, temperature, or liquid level; to be used for limiting the operation of the controlled equipment. Control, Operating, Modulating: A device used to initiate operation of the burner or to control modulation of fuel and water on demands of variatiOns in temperature, pressure, water level, time, or other influence. Controls, Combustion: A system of controls that assure safe starting and operation of the burner. Control, Safety: An automatic device which is intended to prevent unsafe operation of the controlled equipment. Page 2 Convection: The transmission of heat the circulation (either natural or forced) of a liquid or a gas such as air. If natural, it is caused by the difference in density of hotter and colder fluid. Cooling Leg: A length of uninsulated pipe through which the condensate flows to a trap and which has sufficient cooling surface to permit the condensate to dissipate enough heat to prevent flashing when the trap opens. In the case of a thermostatic trap a cooling leg may be necessary to permit the condensate to drop a sufficient amount in temperature to permit the trap to open. Dry-back: A type of boiler construction in which the rear combustion chamber is refractory lined. A term generally applied to Scotch type boilers. Equipment Ground: A separate metallic connection to ground electrical equipment enclosures and conductor enclosures. The grounding connection is made at the main central office ground connection. The equipment ground “Green Wire ‘ concept is covered in detail in Section 802-001-180. Feed Water: Condensate returned to the boiler plus make-up water. Flash Point (Oil): The lowest temperature at which the fuel oil gives off vapors that will ignite when a small flame is passed over the surface of the oil. Furnace: That portion of a heating plant which includes the combustion chamber and flue up to the first plane of entry into the tubes or secondary heat exchange area. Gas Appliance Pressure Regulator: A pressure balancing device which will maintain the gas pressure to a burner within ± 10 percent of the operating pressure at any given rate from maximum to minimum firing rates, with variations in inlet pressure not exceeding +40 percent of the rated inlet pressure. Gauge Pressure: The pressure above that of the atmosphere. It is the pressure indicated on an ordinary pressure gauge. It is expressed as a unit pressure such as lbs per sq in. gauge. Hartford Loop: An arrangement of the return pipe in the form of a loop so that the water in each boiler cannot be forced out below the safe
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water level. Fig. 1 shows the arrangement and is intended to indicate that the horizontal piping on the right side of the figure is to be located at an elevation that will place the bottom of the inside of the piping at the same relative height as the Lowest Safe Water Level in the boiler. NORMAL \lATER LEVEL LOWEST SAFE \lATER LEVEL D RETURN fig. 1-Hartford Loop Heat: That form of energy into which all other forms may be changed. Heat always flows from a body of higher temperature to a body of lower temperature. Heating Value: Amount of heat energy (Btu) released when a pound of fuel is burned. Lay-up Period: A period when the boiler has been shut down for seasonal or routine maintenance, repairs. or alterations or for other similar reasons. Make-up Water: Water added from outside the boiler water system to the condensate. MBh: 1000 Btu per hour. Modulation: Changing the firing rate in small increments to balance the firing rate with the load to maintain operation between set limits. Mud Ring (Mud Leg): The bottom of the water leg or other a_rea of a boiler designed to collect dirt or other foreign matter for removal by hlowdown. ISS 2, SECTION 770-210-300 Pilot, Constant (Standing): A pilot that burns continuously regardless of whether the main burner is off or on. Pilot, Intermittent: A pilot that is normally electric spark ignited each time there is a call for heat, and burns without turndown the entire time the main burner is firing. Pilot, Interrupted: A pilot that is electrically ignited automatically each time there is a call for heat and is cut off automatically at the end of the trial-for-ignition period of the main burner. Pilot, Proved: A pilot that has been proved by a flame detection device. Pour Point: The minimum temperature at which fuel oil can be pumped or readily flows. Radiation (Thermal): The transmission of heat in a straight line through space. SSU: Saybolt Seconds Cniversai-A measure of oil viscosity at 100°F. The higher the number, the greater the resistance to flow. Square Foot of Heating Surface (Equivalent), or Equivalent Direct Radiation (EDRJ: By definition, that amount of heating surface which will give off 240 Btu per hour when filled with a heating medium at 215oF and surrounded air at 70°F. The equivalent square foot of heating surface may have no direct relation to the actual surface area. Thermostat: An instrument which responds to changes in temperature and which directly or indirectly controls the temperature of a medium. Viscosity: Measure of flowability at a definite temperature. It is measured in the number of seconds it takes a given amount of the oil to pass through a certain opening (orifice) at 100°F. High viscosity oil requires special treatment (preheating). Wet-back: A type of boiler construction in which the rear combustion chamber is water jacketed-a term generally applied to Scotch type boilers. Page 3
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SECTION 770-210-300 3. CAST -IRON BOILERS 3.01 Cast-iron boilers are divided into two general classifications: ( 1) Round boilers with horizontal or pancake sections, normally limited to very small buildings or locations requiring a maximum heating capacity of approximately 500 MBh gross output. (2) Square or rectangular boilers with vertical sections, used for any application up to capacities of approximately 13,000 MBh gross output. Most of these boilers are of the internal push-nipple type, but some are constructed with external headers and screw nipples. Round Cast-Iron Boilers 3.02 Round cast-iron boilers are made up of horizontal sections stacked like pancakes. These boilers have a good reputation for economy and dependability. The trend in smaller telephone buildings, however, has been toward heating systems of a type requiring less maintenance, eg, hot-air systems and electric heating. As a result, the round boiler is not often used in Bell System buildings. Sectional Cast-iron Boilers 3.03 Sectional cast-iron boilers are square or rectangular in shape and have vertical sections stacked like slices in a loaf of bread. The sectional boiler can be expanded or enlarged by adding sections. The heating surface of this type boiler is large in comparison to its water volume, and it has excellent steaming, internal water circulation and efficiency characteristics. Cast-iron boilers require less maintenance than steel boilers and · can be expected to give good service for many years. 3.04 There are two basic types of sectional boilers, the internal push-nipple type, which is the type most commonly used for telephone buildings, and the external header or drum type, which utilizes screw nipples. 3.05 The sections of internal push-nipple boilers are joined with tapered nipples and pulled tightly together with tie rods or bolts (see Fig. 2). These tie rods are rrot necessary to hold the boiler together. The tapered design and the ground Page 4 surfaces of the nipples and the openings into which they fit are such that when the sections are drawn together with the tie rods the joints will remain tight even though the rods are removed. They are left in the boiler merely for convenience in reassembling the sections in case repairs at a later date require their use. 3.06 Internal push-nipple boilers designed exclusively for gas fuel (Fig. 3) are constructed with minimum sized combustion chambers. These, along with studs cast into the sections, provide for a maximum of heat absorbing surface. 3.07 External header or drum (manifold) type boilers (Fig. 4) have the sections individually connected to the header drums (manifolds) with screw nipples. A common arrangement is a steam drum at the top, with two return drums at the bottom, one on each side. Individual sections may be disconnected and replaced without disturbing the rest of the assembly. 4. STEEL BOILERS-FIRETUBE TYPE 4.01 Steel boilers can be generally divided into two types: firetube and watertube. In the firetube boiler the gases of combustion pass through the tubes and the water circulates around them. In a watertube boiler the gases circulate around the tubes and water is contained in the tubes. Most low-pressure, steel, heating boilers are of the firetube type; therefore, the discussion here will be limited to that type. 4.02 Capacities of steel firetube boilers range from those required for small buildings up to about 26,000 MBh output. This type boiler has a large water storage capacity giving it the ability to dampen the effect of wide and sudden fluctuations in steam demand. It offers favorable manufacturing cost and is well adapted to standardization. As a result, the firetube boiler is widely used for heating systems. It is limited to applications where the demands for steam are relatively small as compared to those of large power station installations that use watertube boilers. Firetube steel boilers can be further classified as external furnace and internal furnace. EXTERNAL FURNACE BOILERS 4.03 In this type boiler the external furnace, which may be brick set or steel jacketed,
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SECTION 770-210-300 is essentially surrounded by a refactory rather than water. The best example of an external furnace boiler is the horizontal return tubular (HRTJ boiler (Fig. ;)). These boilers are primarily designed for use as high-pressure boilers and are used for industrial process steam applications, although many are used for heating large buildings. INTERNAL FURNACE BOILERS 4.04 Internal furnace boilers may have the furnace in the form of a straight flue completely surrounded by water as in the Scotch type, or may have the furnace substantially surrounded as in a firebox boiler where the water cooled surface IS brought down to grate level on each side but is omitted on the bottom. See Fig. 6 and 7. 4.05 The portable firebox boiler, also called a compact or a smokeless boiler, is very popular for commerical and industrial installations and is one of the types commonly used in telephone buildings. It is a water leg boiler with two lengths of tube nests providing three passes for gas flow. It can be furnished for use with either natural gas, oil, or coal. This type boiler is available for field assembly, or it can be purchased as a package, with burner, controls and fittings installed at the factory. 4.06 The Scotch boilers used in modern heating systems are similar to those originally designed for shipboard installation and are sometimes called Scotch Marine Boilers. This boiler is available both for field assembly or as a package assembly and in some cases test fired and adjusted for maximum efficiency at the factory. This type boiler has a relatively low water capacity and, therefore, can be brought up to operating pressure quickly. It is economical in first cost, simple to install, and requires little headroom. 4.07 Scotch boilers are manufactured in two designs, the wet-back (Fig. 8) and the dry-back (Fig. 9) plus a modification of the former known as a wet-top or partial wet-back. The gas flow patterns vary from two pass to four pass and while these boilers can be adapted to any fuel they are ideally suited for natural gas or oil. Fig. 10 illustrates these. Fig. 5-Horizontal Return Tubular Boiler Showing Cut-Away Section and End View (Erie City Iron Works) Page 8
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ISS 2, SECTION 770-210-300 Fig. 6-Short Firebox Water-Leg Boiler (Industrial Division, American Radiator and Standard Sanitary Corp) 4.08 Most package Scotch boilers are of the or partial wet-back type and are arranged for multiple gas passes. This design allow::. for a minimum use of refractory lining material and provides a maximum capacity per ::.quare foot of heating surface. 5. FUEL TYPES 5.01 The principal fuels used to heat telephone buildings are oil and gas. The choice of fuel should be based on dependability, cleanliness, availability, economy, maintenance, operating requirements, and control. GAS 5.02 :\atural gas is a popular fuel for heating telephone buildings. It burns clean without smoke. soot. or ash. and is very flexible from a combustion standpoint. High boiler efficiency can be achieved since gas is burned with relatively little excess air, and burner flexibility permits easy automatic control under varying load conditions. This fuel has a calorific value of approximately 1000 Btu/cubic foot. As this gas is distributed through underground piping, it requires no storage facilities. OIL 5.03 Fuel oils are graded in accordance with specifications of the American Society of Testing Materials. Oils are classified their \·iscosities. Other characteristics of fuel oils which determine their grade classification and suitability for given uses are the flash point, pour point, water and sediment content, carbon residue. ash and distillation characteristics. Fuel oils are prepared for combustion in most low-pressure boiler burners by atomization (spraying) and vaporization (heating). Atomization is further divided into three Page 9
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SECTION 770-210-300 Fig. 7-Portable Firebox Boiler (Industrial Division American Radiator and Standard Sanitary Corp) namely, high-pressure atomization, low-pressure atomization and centrifugal atomization. Fuel oil prices decrease as the grade number of the oil increases; eg, No. 6 is usually cheaper than No. 5. Fuel Oil Grade Characteristics 5.04 Grade No. i-A distillate oil intended for vaporizing pot type burners. The usual Page 10 calorific value is 135,000 Btu/ gal and the oil weighs 6.8 lbs/ gal. This is not diesel fuel. S.OS Grade No. 2–A distillate oil used for general purpose domestic heating and for use in high-and low-pressure atomizing burners not requiring No. 1. Before going to a heavier grade fuel oil careful consideration should be given to competitive prices, cost of fuel burning equipment,
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ISS 2, SECTION 770-210-300 Fig. 8-Scotch Boiler, Wet-Back Type (Industrial Division, American Radiator and Standard Sanitary Corporation) maintenance, and availability of qualified service. Burners using this fuel are available up to capacities of 20 gph without firing rate modulation. The usual caloric value is 138,000 Btu/ gal and the oil weighs 7.1 lbs/gal. 5.06 Grade No. 4-An oil for burner installations that are not normally equipped with preheating facilities. This oil is generally used in burners employing centrifugal atomization (rotary cup burners) and modulating low-pressure gun type burners. Because the oil is no longer available in many locations as a straight run distillate, but is a mix of No. 2 and No. 6, it may be necessary in northern climates to provide tank heaters or small preheaters to ensure delivery of the fuel to the burner at a minimum temperature of 80°F. If the fuel is not blended properly under heat and pressure, the No. 2 and the No. 6 may separate in time. Be sure the fuel supplier has these blending facilities. Many dealers blend the two grades of oil in the tank truck while delivering to the location. This results in physical separation of the two grades, if they stand in the tank for any length of time. The usual calorific value 1s 147,000 Btu/ gal and the oil weighs 7. 7 lbs/ gal. 5.07 Grade No. 5-A residual type oil for burner installations equipped with preheating facilities. The fuel is normally used in centrifugal atomizing burners. In the past this grade has been divided into hot No. 5 and cold No. 5. The hot grade had to be preheated and the cold could be burned as is from the tank, but because of the increased demand for distillate products the residual oils are lower in quality and must be preheated for good results. Sometimes Grade No. 5 is a mix of No. 2 and No. 6. The usual calorific value for this grade is 152,000 Btu/ gal and the oil weighs 8.1 lbs/ gal. 5.08 Grade No. 6-An oil for use in burners equipped with preheaters which permit the use of a high viscosity fuel. No. 6 fuel oil is commonly referred to as Bunker C. The usual calorific value for this grade is 153,000 Btu/ gal and the oil weighs 8.2 lbs/ gal. Page 11
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