Steam Trap Selection Bringing Energy Down to Earth Say energy. Think environment. And vice versa. Any company that is energy conscious is also environmentally conscious. Less energy consumed means less waste, fewer emissions and a healthier environment. In short, bringing energy and environment together lowers the cost industry must pay for both. By helping companies manage energy, Armstrong products and services are also helping to protect the environment. Armstrong has been sharing know-how since we invented the energy-efficient inverted bucket steam trap in 1911. In the years since, customers’ savings have proven again and again that knowledge not shared is energy wasted. Armstrong’s developments and improvements in steam trap design and function have led to countless savings in energy, time and money. This section has grown out of our decades of sharing and expanding what we’ve learned. It deals with the operating principles of steam traps and outlines their specific applications to a wide variety of products and industries. You’ll find it a useful complement to other Armstrong literature and the Armstrong Steam-A-ware™ software program for sizing and selecting steam traps, pressure reducing valves and water heaters, which can be requested through Armstrong’s Web site, armstronginternational.com. This section also includes Recommendation Charts that summarize our findings on which type of trap will give optimum performance in a given situation and why. IMPORTANT: This section is intended to summarize general principles of installation and operation of steam traps, as outlined above. Actual installation and operation of steam trapping equipment should be performed only by experienced personnel. Selection or installation should always be accompanied by competent technical assistance or advice. This data should never be used as a substitute for such technical advice or assistance. We encourage you to contact Armstrong or its local representative for further details. 2 Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com Instructions for Using the Recommendation Charts A quick reference Recommendation Chart appears throughout the “HOW TO TRAP” sections of this catalog, pages CG-17 to CG-43. A feature code system (ranging from A to Q) supplies you with “at-a-glance” information. The chart covers the type of steam traps and the major advantages that Armstrong feels are superior for each particular application. 3. Now refer to Chart 3-2 below, titled “How Various Types of Steam Traps Meet Specific Operating Requirements” and read down the extreme left-hand column to each of the letters B, C, E, K, N. The letter “B,” for example, refers to the trap’s ability to provide energy-conserving operation. 4. Follow the line for “B” to the right until you reach the column that corresponds to our first choice, in this case the inverted bucket. Based on tests and actual operating conditions, the energy-conserving performance of the inverted bucket steam trap has been rated “Excellent.” Follow this same procedure for the remaining letters. For example, assume you are looking for information concerning the proper trap to use on a gravity drained jacketed kettle. You would: Abbreviations 1. Turn to the “How to Trap Jacketed Kettles” section, pages CG-35 to CG-36, and look in the lower right-hand corner of page CG-35. The Recommendation Chart located there is reprinted below for your convenience. (Each section has a Recommendation Chart.) IB IBLV BM F&T CD DC Inverted Bucket Trap Inverted Bucket Large Vent Bimetallic Trap Float and Thermostatic Trap Controlled Disc Trap Automatic Differential Condensate Controller CV Check Valve T Thermic Bucket PRV Pressure Reducing Valve 2. Find “Jacketed Kettles, Gravity Drain” in the first column under “Equipment Being Trapped” and read to the right for Armstrong’s “1st Choice and Feature Code.” In this case, the first choice is an IBLV and the feature code letters B, C, E, K, N are listed. Chart 3-1. Recommendation Chart (See chart below for “Feature Code” References.) 1st Choice and Feature Code Alternate Choice Jacketed Kettles Gravity Drain IBLV B, C, E, K, N F&T or Thermostatic Jacketed Kettles Syphon Drain DC B, C, E, G, H, K, N, P IBLV Equipment Being Trapped Chart 3-2. How Various Types of Steam Traps Meet Specific Operating Requirements Feature Characteristic IB BM Code (1) (2) (3) (4) (5) (1) Intermittent (2) Intermittent F&T Disc Thermostatic Wafer DC Orifice A Method of Operation Continuous Intermittent (2) Intermittent Continuous Continuous B Energy Conservation (Time in Service) Excellent Excellent Good Poor Fair (3) Excellent Poor C Resistance to Wear Excellent Excellent Good Poor Fair Excellent Poor D Corrosion Resistance Excellent Excellent Good Excellent Good Excellent Good E Resistance to Hydraulic Shock Excellent Excellent Poor Excellent (4) Poor Excellent Good F Vents Air and CO2 at Steam Temperature Yes No No No No Yes Poor G Ability to Vent Air at Very Low Pressure (1/4 psig) Poor (5) NR Excellent (5) NR Good Excellent Poor H Ability to Handle Start-Up Air Loads Fair Excellent Excellent Poor Excellent Excellent Poor I Operation Against Back Pressure Excellent Excellent Excellent Poor Excellent Excellent Poor J Resistance to Damage From Freezing (6) Good Good Poor Good Good Good Excellent K Ability to Purge System Excellent Good Fair Excellent Good Excellent Poor L Performance on Very Light Loads Excellent Excellent Excellent Poor Excellent Excellent Poor M Responsiveness to Slugs of Condensate Immediate Delayed Immediate Delayed Delayed Immediate Poor N Ability to Handle Dirt Excellent Fair Poor Poor Fair Excellent Poor O Comparative Physical Size (7) Large Small Large Small Small Large Small P Ability to Handle“Flash Steam” Q Mechanical Failure (Open or Closed) Fair Poor Poor Poor Poor Excellent Poor Open Open Closed (8) Open (9) Open NA Drainage of condensate is continuous. Discharge is intermittent. Can be continuous on low load. Excellent when “secondary steam” is utilized. Bimetallic and wafer traps – good. Not recommended for low pressure operations. (6) (7) (8) (9) Cast iron traps not recommended. In welded stainless steel construction – medium. Can fail closed due to dirt. Can fail either open or closed, depending upon the design of the bellows. Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com 3 Steam Tables What They Are…How to Use Them How the Table Is Used The heat quantities and temperature/ pressure relationships referred to in this section are taken from the Properties of Saturated Steam table. In addition to determining pressure/ temperature relationships, you can compute the amount of steam that will be condensed by any heating unit of known Btu output. Conversely, the Saturated Steam is pure steam at the temperature that corresponds to the boiling temperature of water at the existing pressure. Absolute and Gauge Pressures Absolute pressure is pressure in pounds per square inch (psia) above a perfect vacuum. Gauge pressure is pressure in pounds per square inch above atmospheric pressure, which is 14.7 pounds per square inch absolute. Gauge pressure (psig) plus 14.7 equals absolute pressure. Or, absolute pressure minus 14.7 equals gauge pressure. Table 4-1. Properties of Saturated Steam (Abstracted from Keenan and Keyes, THERMODYNAMIC PROPERTIES OF STEAM, by permission of John Wiley & Sons, Inc.) Col. 1 Gauge Pressure Inches of Vacuum Definitions of Terms Used Pressure/Temperature Relationship (Columns 1, 2 and 3). For every pressure of pure steam there is a corresponding temperature. Example: The temperature of 250 psig pure steam is always 406°F. Total Heat of Steam (Column 6). The sum of the Heat of the Liquid (Column 4) and Latent Heat (Column 5) in Btu. It is the total heat in steam above 32°F. Specific Volume of Liquid (Column 7). The volume per unit of mass in cubic feet per pound. Specific Volume of Steam (Column 8). The volume per unit of mass in cubic feet per pound. 4 PSIG Heat of Saturated Liquid (Column 4). This is the amount of heat required to raise the temperature of a pound of water from 32°F to the boiling point at the pressure and temperature shown. It is expressed in British thermal units (Btu). Latent Heat or Heat of Vaporization (Column 5). The amount of heat (expressed in Btu) required to change a pound of boiling water to a pound of steam. This same amount of heat is released when a pound of steam is condensed back into a pound of water. This heat quantity is different for every pressure/temperature combination, as shown in the steam table. table can be used to determine Btu output if steam condensing rate is known. In the application portion of this section, there are several references to the use of the steam table. 29.743 29.515 27.886 19.742 9.562 7.536 5.490 3.454 1.418 0.0 1.3 2.3 5.3 10.3 15.3 20.3 25.3 30.3 40.3 50.3 60.3 70.3 80.3 90.3 100.0 110.3 120.3 125.3 130.3 140.3 150.3 160.3 180.3 200.3 225.3 250.3 Col. 2 Absolute Pressure (psia) Col. 3 Steam Temp. (°F) Col. 4 Heat of Sat. Liquid (Btu/lb) Col. 5 Latent Heat (Btu/ lb) Col. 6 Total Heat of Steam (Btu/lb) Col. 7 Specific Volume of Sat. Liquid (cu ft/lb) 0.08854 0.2 1.0 5.0 10.0 11.0 12.0 13.0 14.0 14.696 16.0 17.0 20.0 25.0 30.0 35.0 40.0 45.0 55.0 65.0 75.0 85.0 95.0 105.0 114.7 125.0 135.0 140.0 145.0 155.0 165.0 175.0 195.0 215.0 240.0 265.0 300.0 400.0 450.0 500.0 600.0 900.0 1200.0 1500.0 1700.0 2000.0 2500.0 2700.0 3206.2 32.00 53.14 101.74 162.24 193.21 197.75 201.96 205.88 209.56 212.00 216.32 219.44 227.96 240.07 250.33 259.28 267.25 274.44 287.07 297.97 307.60 316.25 324.12 331.36 337.90 344.33 350.21 353.02 355.76 360.50 365.99 370.75 379.67 387.89 397.37 406.11 417.33 444.59 456.28 467.01 486.21 531.98 567.22 596.23 613.15 635.82 668.13 679.55 705.40 0.00 21.21 69.70 130.13 161.17 165.73 169.96 173.91 177.61 180.07 184.42 187.56 196.16 208.42 218.82 227.91 236.03 243.36 256.30 267.50 277.43 286.39 294.56 302.10 308.80 315.68 321.85 324.82 327.70 333.24 338.53 343.57 353.10 361.91 372.12 381.60 393.84 424.00 437.20 449.40 471.60 526.60 571.70 611.60 636.30 671.70 730.60 756.20 902.70 1075.8 1063.8 1036.3 1001.0 982.1 979.3 976.6 974.2 971.9 970.3 967.6 965.5 960.1 952.1 945.3 939.2 933.7 928.6 919.6 911.6 904.5 897.8 891.7 886.0 880.0 875.4 870.6 868.2 865.8 861.3 857.1 852.8 844.9 837.4 828.5 820.1 809.0 780.5 767.4 755.0 731.6 668.8 611.7 556.3 519.6 463.4 360.5 312.1 0.0 1075.8 1085.0 1106.0 1131.0 1143.3 1145.0 1146.6 1148.1 1149.5 1150.4 1152.0 1153.1 1156.3 1160.6 1164.1 1167.1 1169.7 1172.0 1175.9 1179.1 1181.9 1184.2 1186.2 1188.1 1188.8 1191.1 1192.4 1193.0 1193.5 1194.6 1195.6 1196.5 1198.0 1199.3 1200.6 1201.7 1202.8 1204.5 1204.6 1204.4 1203.2 1195.4 1183.4 1167.9 1155.9 1135.1 1091.1 1068.3 902.7 0.016022 0.016027 0.016136 0.016407 0.016590 0.016620 0.016647 0.016674 0.016699 0.016715 0.016746 0.016768 0.016830 0.016922 0.017004 0.017078 0.017146 0.017209 0.017325 0.017429 0.017524 0.017613 0.017696 0.017775 0.017850 0.017922 0.017991 0.018024 0.018057 0.018121 0.018183 0.018244 0.018360 0.018470 0.018602 0.018728 0.018896 0.019340 0.019547 0.019748 0.02013 0.02123 0.02232 0.02346 0.02428 0.02565 0.02860 0.03027 0.05053 Col. 8 Specific Volume of Sat. Steam (cu ft/lb) 3306.00 1526.00 333.60 73.52 38.42 35.14 32.40 30.06 28.04 26.80 24.75 23.39 20.09 16.30 13.75 11.90 10.50 9.40 7.79 6.66 5.82 5.17 4.65 4.23 3.88 3.59 3.33 3.22 3.11 2.92 2.75 2.60 2.34 2.13 1.92 1.74 1.54 1.16 1.03 0.93 0.77 0.50 0.36 0.28 0.24 0.19 0.13 0.11 0.05 Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com Steam Tables Flash Steam (Secondary) What is flash steam? When hot condensate or boiler water, under pressure, is released to a lower pressure, part of it is re-evaporated, becoming what is known as flash steam. Why is it important? This flash steam is important because it contains heat units that can be used for economical plant operation—and which are otherwise wasted. How is it formed? When water is heated at atmospheric pressure, its temperature rises until it reaches 212°F, the highest temperature at which water can exist at this pressure. Additional heat does not raise the temperature, but converts the water to steam. The heat absorbed by the water in raising its temperature to boiling point is called “sensible heat” or heat of saturated liquid. The heat required to convert water at boiling point to steam at the same temperature is called “latent heat.” The unit of heat in common use is the Btu, which is the amount of heat required to raise the temperature of one pound of water 1°F at atmospheric pressure. If water is heated under pressure, however, the boiling point is higher than 212°F, so the sensible heat required is greater. The higher the pressure, the higher the boiling temperature and the higher the heat content. If pressure is reduced, a certain amount of sensible heat is released. This excess heat will be absorbed in the form of latent heat, causing part of the water to “flash” into steam. Chart 5-3. Percentage of flash steam formed when discharging condensate to reduced pressure. Condensate at steam temperature and under 100 psig pressure has a heat content of 308.8 Btu per pound. (See Column 4 in Steam Table.) If this condensate is discharged to atmospheric pressure (0 psig), its heat content instantly drops to 180 Btu per pound. The surplus of 128.8 Btu re-evaporates or flashes a portion of the condensate. The percentage that will flash to steam can be computed using the formula: % flash steam = SH - SL x 100 H SH = Sensible heat in the condensate at the higher pressure before discharge. SL = Sensible heat in the condensate at the lower pressure to which discharge takes place. H = Latent heat in the steam at the lower pressure to which the condensate has been discharged. % flash steam = 308.8 - 180 x 100 =13.3% 970.3 Chart 5-3 shows the amount of secondary steam that will be formed when discharging condensate to different pressures. Other useful tables will be found on page CG-53 (Useful Engineering Tables). Chart 5-4. Volume of flash steam formed when one cubic foot of condensate is discharged to atmospheric pressure. 400 PERCENTAGE OF FLASH STEAM 25 A 20 B C 15 D E F G 10 CURVE BACK PRESS. LBS/SQ IN A B C D E F G – 10 –5 0 10 20 30 40 5 0 – 20 0 50 100 150 200 250 CU FT FLASH STEAM PER CU FT OF CONDENSATE 30 300 200 100 0 100 200 300 400 PRESSURE AT WHICH CONDENSATE IS FORMED—LBS/SQ IN 300 PSI FROM WHICH CONDENSATE IS DISCHARGED Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com 5 Steam…Basic Concepts Steam is an invisible gas generated by adding heat energy to water in a boiler. Enough energy must be added to raise the temperature of the water to the boiling point. Then additional energy—without any further increase in temperature—changes the water to steam. When steam reaches the heat exchangers in the system, the story is different. Here the transfer of heat from the steam is desirable. Heat flows to the air in an air heater, to the water in a water heater or to food in a cooking kettle. Nothing should interfere with this heat transfer. Steam is a very efficient and easily controlled heat transfer medium. It is most often used for transporting energy from a central location (the boiler) to any number of locations in the plant where it is used to heat air, water or process applications. Condensate Drainage… Why It’s Necessary As noted, additional Btu are required to make boiling water change to steam. These Btu are not lost but stored in the steam ready to be released to heat air, cook tomatoes, press pants or dry a roll of paper. The heat required to change boiling water into steam is called the heat of vaporization or latent heat. The quantity is different for every pressure/temperature combination, as shown in the steam tables. Steam at Work… How the Heat of Steam Is Utilized Heat flows from a higher temperature level to a lower temperature level in a process known as heat transfer. Starting in the combustion chamber of the boiler, heat flows through the boiler tubes to the water. When the higher pressure in the boiler pushes steam out, it heats the pipes of the distribution system. Heat flows from the steam through the walls of the pipes into the cooler surrounding air. This heat transfer changes some of the steam back into water. That’s why distribution lines are usually insulated to minimize this wasteful and undesirable heat transfer. Condensate is the by-product of heat transfer in a steam system. It forms in the distribution system due to unavoidable radiation. It also forms in heating and process equipment as a result of desirable heat transfer from the steam to the substance heated. Once the steam has condensed and given up its valuable latent heat, the hot condensate must be removed immediately. Although the available heat in a pound of condensate is negligible as compared to a pound of steam, condensate is still valuable hot water and should be returned to the boiler. Definitions n The Btu. A Btu—British thermal unit—is the amount of heat energy required to raise the temperature of one pound of cold water by 1°F. Or, a Btu is the amount of heat energy given off by one pound of water in cooling, say, from 70°F to 69°F. n Temperature. The degree of hotness with no implication of the amount of heat energy available. n Heat. A measure of energy available with no implication of temperature. To illustrate, the one Btu that raises one pound of water from 39°F to 40°F could come from the surrounding air at a temperature of 70°F or from a flame at a temperature of 1,000°F. OEZDWHU DW) %WX &RQGHQVDWH 6WHDP OEZDWHU DW) %WX OEVWHDP DW) Figure 6-1. These drawings show how much heat is required to generate one pound of steam at atmospheric pressure. Note that it takes 1 Btu for every 1°F increase in temperature up to the boiling point, but that it takes more Btu to change water at 212°F to steam at 212°F. 6 OEZDWHU DW) %WX SVLJ OEZDWHU DW) SVLJ %WX OEVWHDP DW) SVLJ Figure 6-2. These drawings show how much heat is required to generate one pound of steam at 100 pounds per square inch pressure. Note the extra heat and higher temperature required to make water boil at 100 pounds pressure than at atmospheric pressure. Note, too, the lesser amount of heat required to change water to steam at the higher temperature. Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com Steam…Basic Concepts The need to drain the distribution system. Condensate lying in the bottom of steam lines can be the cause of one kind of water hammer. Steam traveling at up to 100 miles per hour makes “waves” as it passes over this condensate (Fig. 7-4). If enough condensate forms, high-speed steam pushes it along, creating a dangerous slug that grows larger and larger as it picks up liquid in front of it. Anything that changes the direction—pipe fittings, regulating valves, tees, elbows, blind flanges—can be destroyed. In addition to damage from this “battering ram,” high-velocity water may erode fittings by chipping away at metal surfaces. The need to drain the heat transfer unit. When steam comes in contact with condensate cooled below the temperature of steam, it can produce another kind of water hammer known as thermal shock. Steam occupies a much greater volume than condensate, and when it collapses suddenly, it can send shock waves throughout the system. This form of water hammer can damage equipment, and it signals that condensate is not being drained from the system. Obviously, condensate in the heat transfer unit takes up space and reduces the physical size and capacity of the equipment. Removing it quickly keeps the unit full of steam (Fig. 7-5). As steam condenses, it forms a film of water on the inside of the heat exchanger. Non-condensable gases do not change into liquid and flow away by gravity. Instead, they accumulate as a thin film on the surface of the heat exchanger—along with dirt and scale. All are potential barriers to heat transfer (Fig. 7-3). $ The need to remove air and CO2. Air is always present during equipment start-up and in the boiler feedwater. Feedwater may also contain dissolved carbonates, which release carbon dioxide gas. The steam velocity pushes the gases to the walls of the heat exchangers, where they may block heat transfer. This compounds the condensate drainage problem, because these gases must be removed along with the condensate. Figure 7-3. Potential barriers to heat transfer: steam heat and temperature must penetrate these potential barriers to do their work. % Figure 7-4. Condensate allowed to collect in pipes or tubes is blown into waves by steam passing over it until it blocks steam flow at point A. Condensate in area B causes a pressure differential that allows steam pressure to push the slug of condensate along like a battering ram. Figure 7-5. Coil half full of condensate can’t work at full capacity. &RQGHQVDWH 6WHDP 9DSRU Figure 7-6. Note that heat radiation from the distribution system causes condensate to form and, therefore, requires steam traps at natural low points or ahead of control valves. In the heat exchangers, traps perform the vital function of removing the condensate before it becomes a barrier to heat transfer. Hot condensate is returned through the traps to the boiler for reuse. Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com 7 Steam…Basic Concepts Effect of Air on Steam Temperature When non-condensable gases (primarily air) continue to accumulate and are not removed, they may gradually fill the heat exchanger with gases and stop the flow of steam altogether. The unit is then “air bound.” When air and other gases enter the steam system, they consume part of the volume that steam would otherwise occupy. The temperature of the air/steam mixture falls below that of pure steam. Figure 8-7 explains the effect of air in steam lines. Table 8-2 and Chart 8-5 show the various temperature reductions caused by air at various percentages and pressures. Corrosion Two primary causes of scale and corrosion are carbon dioxide (CO2) and oxygen. CO2 enters the system as carbonates dissolved in feedwater and, when mixed with cooled condensate, creates carbonic acid. Extremely corrosive, carbonic acid can eat through piping and heat exchangers (Fig. 9-9). Oxygen enters the system as gas dissolved in the cold feedwater. It aggravates the action of carbonic acid, speeding corrosion and pitting iron and steel surfaces (Fig. 9-10). Effect of Air on Heat Transfer The normal flow of steam toward the heat exchanger surface carries air and other gases with it. Since they do not condense and drain by gravity, these non-condensable gases set up a barrier between the steam and the heat exchanger surface. The excellent insulating properties of air reduce heat transfer. In fact, under certain conditions as little as 1/2 of 1% by volume of air in steam can reduce heat transfer efficiency by 50% (Fig. 9-8). Eliminating the Undesirables To summarize, traps must drain condensate because it can reduce heat transfer and cause water hammer. Traps should evacuate air and other non-condensable gases because they can reduce heat transfer by reducing steam temperature and insulating the system. They can also foster destructive corrosion. It’s essential to remove condensate, air and CO2 as quickly and completely as possible. A steam trap, which is simply an automatic valve that opens for condensate, air and CO2 and closes for steam, does this job. For economic reasons, the steam trap should do its work for long periods with minimum attention. Table 8-2. Temperature Reduction Caused by Air Temp. of Steam, No Air Present (°F) 10.3 25.3 50.3 75.3 100.3 240.1 267.3 298.0 320.3 338.1 Temp. of Steam Mixed With Various Percentages of Air (by Volume) (°F) 10% 234.3 261.0 291.0 312.9 330.3 20% 228.0 254.1 283.5 304.8 321.8 30% 220.9 246.4 275.1 295.9 312.4 E–% LUM VO R BY T AI CEN Figure 8-7. Chamber containing air and steam delivers only the heat of the partial pressure of the steam, not the total pressure. PER 100 90 80 70 60 50 40 30 20 PRE 10 0 300 2 50 SSU 200 RE– 150 PSIG 100 75 5 0 25 0 450 425 400 450 375 425 400 350 375 325 350 300 275 250 325 300 225 275 200 250 225 150 200 Steam chamber 100% steam Total pressure 100 psia Steam pressure 100 psia Steam temperature 327.8°F 100 150 100 0 0 300 Steam chamber 90% steam and 10% air Total pressure 100 psia Steam pressure 90 psia Steam temperature 320.3°F 8 TEMPERATURE F Pressure (psig) 250 200 150 100 75 5 0 0 25 0 100 9 80 70 60 50 40 30 20 10 0 Chart 8-5. Air Steam Mixture Temperature reduction caused by various percentages of air at differing pressures. This chart determines the percentage of air with known pressure and temperature by determining the point of intersection between pressure, temperature and percentage of air by volume. As an example, assume system pressure of 250 psig with a temperature at the heat exchanger of 375°F. From the chart, it is determined that there is 30% air by volume in the steam. Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com Steam…Basic Concepts What the Steam Trap Must Do 1. Minimal steam loss. Table 9-3 shows how costly unattended steam leaks can be. 7. Freedom from dirt problems. Dirt is an ever-present concern since traps are located at low points in the steam system. Condensate picks up dirt and scale in the piping, and solids may carry over from the boiler. Even particles passing through strainer screens are erosive and, therefore, the steam trap must be able to operate in the presence of dirt. 2. Long life and dependable service. Rapid wear of parts quickly brings a trap to the point of undependability. An efficient trap saves money by minimizing trap testing, repair, cleaning, downtime and associated losses. A trap delivering anything less than all these desirable operating/design features will reduce the efficiency of the system and increase costs. When a trap delivers all these features the system can achieve: 3. Corrosion resistance. Working trap parts should be corrosion-resistant in order to combat the damaging effects of acidic or oxygen-laden condensate. 1. Fast heat-up of heat transfer equipment 2. Maximum equipment temperature for enhanced steam heat transfer 3. Maximum equipment capacity 4. Maximum fuel economy 5. Reduced labor per unit of output 6. Minimum maintenance and a long trouble-free service life The job of the steam trap is to get condensate, air and CO2 out of the system as quickly as they accumulate. In addition, for overall efficiency and economy, the trap must also provide: 4. Air venting. Air can be present in steam at any time and especially on start-up. Air must be vented for efficient heat transfer and to prevent system binding. 5. CO2 venting. Venting CO2 at steam temperature will prevent the formation of carbonic acid. Therefore, the steam trap must function at or near steam temperature since CO2 dissolves in condensate that has cooled below steam temperature. Sometimes an application may demand a trap without these design features, but in the vast majority of applications the trap which meets all the requirements will deliver the best results. 6. Operation against back pressure. Pressurized return lines can occur both by design and unintentionally. A steam trap should be able to operate against the actual back pressure in its return system. Figure 9-8. Steam condensing in a heat transfer unit moves air to the heat transfer surface, where it collects or “plates out” to form effective insulation. Figure 9-9. CO2 gas combines with condensate allowed to cool below steam temperature to form carbonic acid, which corrodes pipes and heat transfer units. Note groove eaten away in the pipe illustrated. Figure 9-10. Oxygen in the system speeds corrosion (oxidation) of pipes, causing pitting such as shown here. Figs. 9-9 and 9-10 courtesy of Dearborn Chemical Company. Table 9-3. Cost of Various Sized Steam Leaks at 100 psi (Assuming steam costs $10.00/1,000 lbs) Size of Orifice 1/2" 7/16" 3/8" 5/16" 1/4" 3/16" 1/8" Condensate Steam 12, 7 mm 11, 2 mm 9, 5 mm 7, 9 mm 6, 4 mm 4, 8 mm 3, 2 mm Lbs Steam Wasted Per Month Total Cost Per Month (USD) Total Cost Per Year (USD 553,000 423,500 311,000 216,000 138,000 78,000 34,500 $5,530.00 4,235.00 3,110.00 2,160.00 1,380.00 780.00 345.00 $66,360.00 50,820.00 37,320.00 25,920.00 16,560.00 9,360.00 4,140.00 The steam loss values assume typical condensate load for drip trap applications. Armstrong methodology for steam trap management and condensate return is sanctioned by the Clean Development Mechanism of the United Nations Framework Convention on Climate Change. Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com 9 Trap Selection To obtain the full benefits from the traps described in the preceding section, it is essential to select traps of the correct size and pressure for a given job and to install and maintain them properly. One of the purposes of this section is to supply the information to make that possible. Actual installation and operation of steam trapping equipment should be performed only by experienced personnel. Selection or installation should always be accompanied by competent technical assistance or advice. This section should never be used as a substitute for such technical advice or assistance. We encourage you to contact Armstrong or its local representative for further details. Basic Considerations Unit trapping is the use of a separate steam trap on each steam-condensing unit including, whenever possible, each separate chest or coil of a single machine. The discussion under the Short Circuiting heading explains the “why” of unit trapping versus group trapping. Rely on experience. Select traps with the aid of experience– either yours, the know-how of your Armstrong Representative or what others have learned in trapping similar equipment. Even without this computer program, you can easily size steam traps when you know or can calculate: 1. Condensate loads in lbs/hr 2. The safety factor to use 3. Pressure differential 4. Maximum allowable pressure 1. Condensate load. Each “How To” portion of this section contains formulas and useful information on steam condensing rates and proper sizing procedures. 2. Safety factor or experience factor to use. Users have found that they must generally use a safety factor in sizing steam traps. For example, a coil condensing 500 lbs/hr might require a trap that could handle up to 1,500 lbs/hr for best overall performance. This 3:1 safety factor takes care of varying condensate rates, occasional drops in pressure differential and system design factors. Safety factors will vary from a low of 1.5:1 to a high of 10:1. The safety factors in this book are based on years of user experience. Configuration affects safety factor. More important than ordinary load and pressure changes is the design of the steam-heated unit itself. Refer to Figs. 10-21, 10-22 and 10-23 showing three condensing units each producing 500 pounds of condensate per hour, but with safety factors of 2:1, 3:1 and 8:1. Do-it-yourself sizing. Do-it-yourself sizing is simple with the aid of Steam-A-ware™, Armstrong’s sizing and selection software program, which can be downloaded at armstronginternational.com. Short Circuiting 5LJKW :URQJ Figure 10-19. Two steam-consuming units drained by a single trap, referred to as group trapping, may result in short circuiting. Figure 10-20. Short circuiting is impossible when each unit is drained by its own trap. Higher efficiency is assured. If a single trap connects more than one drain point, condensate and air from one or more of the units may fail to reach the trap. Any difference in condensing rates will result in a difference in the steam pressure drop. A pressure drop difference too small to register on a pressure gauge is enough to let steam from the higher pressure unit block the flow of air or condensate from the lower pressure unit. The net result is reduced heating, output and fuel waste (see Figs. 10-19 and 10-20). Identical Condensing Rates, Identical Pressures With Differing Safety Factors Condensate Steam 10"-12" 6" Figure 10-21. Continuous coil, constant pressure gravity flow to trap. 500 lbs/ hr of condensate from a single copper coil at 30 psig. Gravity drainage to trap. Volume of steam space very small. 2:1 safety factor. 10 Figure 10-22. Multiple pipes, modulated pressure gravity flow to trap. 500 lbs/ hr of condensate from unit heater at 80 psig. Multiple tubes create minor shortcircuiting hazard. Use 3:1 safety factor at 40 psig. Figure 10-23. Large cylinder, syphon drained. 500 lbs/hr from a 4’ diameter, 10’ long cylinder dryer with 115 cu ft of space at 30 psig. The safety factor is 3:1 with a DC and 8:1 with an IB. Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com Trap Selection Economical steam trap/orifice selection. While an adequate safety factor is needed for best performance, too large a factor causes problems. In addition to higher costs for the trap and its installation, a needlessly oversized trap wears out more quickly. And in the event of a trap failure, an oversized trap loses more steam, which can cause water hammer and high back pressure in the return system. 3. Pressure differential. Maximum differential is the difference between boiler or steam main pressure or the downstream pressure of a PRV and return line pressure. See Fig. 11-24. The trap must be able to open against this pressure differential. NOTE: Because of flashing condensate in the return lines, don’t assume a decrease in pressure differential due to static head when elevating. 4. Maximum allowable pressure. The trap must be able to withstand the maximum allowable pressure of the system or design pressure. It may not have to operate at this pressure, but it must be able to contain it. As an example, the maximum inlet pressure is 350 psig and the return line pressure is 150 psig. This results in a differential pressure of 200 psi; however, the trap must be able to withstand 350 psig maximum allowable pressure. See Fig. 11-24. Factors Affecting Pressure Differential Except for failures of pressure control valves, differential pressure usually varies on the low side of the normal or design value. Variations in either the inlet or discharge pressure can cause this. Inlet pressure can be reduced below its normal value by: Operating differential. When the plant is operating at capacity, the steam pressure at the trap inlet may be lower than steam main pressure. And the pressure in the condensate return header may go above atmospheric. 1. A modulating control valve or temperature regulator. 2. “Syphon drainage.” Every 2’ of lift between the drainage point and the trap reduces the inlet pressure (and the differential) by one psi. See Fig. 11-25. If the operating differential is at least 80% of the maximum differential, it is safe to use maximum differential in selecting traps. Discharge pressure can be increased above its normal value by: Modulated control of the steam supply causes wide changes in pressure differential. The pressure in the unit drained may fall to atmospheric or even lower (vacuum). This does not prevent condensate drainage if the installation practices in this handbook are followed. IMPORTANT: Be sure to read the discussion to the right, which deals with less common but important reductions in pressure differential. 4psi 1. Pipe friction. 2. Other traps discharging into a return system of limited capacity. 3. Elevating condensate. Every 2’ of lift increases the discharge pressure (and the differential) by one psi when the discharge is only condensate. However, with flash present, the extra back pressure could be reduced to zero. See Fig. 11-26, noting the external check valve. 7UDS Condensate 3psi Steam 2psi 'LIIHUHQWLDO3UHVVXUHRU 0D[LPXP2SHUDWLQJ3UHVVXUH023 6WHDP0DLQ $ ,QOHW3UHVVXUH RU0D[LPXP $OORZDEOH3UHVVXUH 0$3 1psi % 7UDS %DFN3UHVVXUH RU9DFXXP 3UHVVXUH GURSRYHU ZDWHUVHDO WROLIWFROG FRQGHQVDWH ([WHUQDO &KHFN 9DOYH 7UDS :DWHU6HDO /LIWLQIHHW Figure 11-24. “A” minus “B” is Pressure Differential: If “B” is back pressure, subtract it from “A”. If “B” is vacuum, add it to “A”. Figure 11-25. Condensate from gravity drain point is lifted to trap by a syphon. Every 2’ of lift reduces pressure differential by 1 psi. Note seal at low point and the trap’s internal check valve to prevent backflow. Figure 11-26. When trap valve opens, steam pressure will elevate condensate. Every 2’ of lift reduces pressure differential by 1 psi. Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information. North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com 11 Armstrong provides intelligent system solutions that improve utility performance, lower energy consumption, and reduce environmental emissions while providing an “enjoyable experience.” Armstrong International North America • Latin America • India • Europe / Middle East / Africa • China • Pacific Rim armstronginternational.com 856-EN Printed in U.S.A. - 4/15 © 2015 Armstrong International, Inc.
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