Document a4kqdb1a9wx7GEYYpzLge9wMR

STEAM MAINS being covered with high-temperature insulation -- ~Steei shell ^16 gage wire ties Insulation s- Wire mesh or metal backing -Loose wool //<" hardware doth ' Rosin-sized paper 1/4"plastic asphalt . cement -- Galvanized bands 1/4 plastic insulation + Portland cement "~t/4 "plastic insulation ^^^NSION JOINT on a h-f breechingror duct. Space between sections is packed with fibrous insulation Double-layer high-temperature insulation means less maintenance, comfortable working temperatures and no fire hazard. Here's... By marshall f alien, Manager The Magnesia Insulation Manufacturers Assn What you should know about Steam pressures and temperatures are rising constantly in order to pro duce more economical steam-generation equipment. Because of this, more plant engineers are coming to grips with hightemperature insulation problems. Temperature limits. Insulation de velopment has more than kept pace with increased temperature. Limits recom mended by MIMA for normal service are: Material 85% magnesia Calcium silicate Expanded silica Diatomaceous silica Temperature limit up to 600 F up to 1200 F up to 1600 F 1500 to 1900 F But selecting insulation by heat tol erance alone oversimplifies the problem. No one insulation is best for all appli cations. Conductivity or k factor, struc tural strength, resistance to mechanical damage, water resistance, compressi bility, ease of application and work ability must all be considered for each job. Thermal expansion. One byproduct of high-operating temperature is expan sion of equipment and piping. This problem will be with us until someone develops a metal that does not expand at high temperatures. Insulation problem is generally solved by using double-layer construction with staggered joints, illustration, p 115. Through-joint openings from metal to air are eliminated, since joint separa tions occur in one layer only. Typical combination is diatomaceous silica and 85% magnesia insulation. Diatomaceous silica is applied to the hot metal in a thickness calculated to reduce outer-surface temperature below 600 F. Then 85% magnesia is applied in thicknesses needed to reduce heat loss to economical level. High heat re sistance of diatomaceous silica and lo* conductivity of 85% magnesia combine to give an excellent insulation. One vs two layers. Some engineer* feel that two-layer insulation is u* necessary with materials like calciut silicate and expanded silica. Both sulations combine high heat resistant and low conductivity in a single Iayef* In addition, installation cost is lower- But single-layer construction does ^ allow for thermal expansion of equip* meat and piping. In every 100 ft 0 steel diping at 820 F there is about in. of - expanded bare metal between insulation joints. Multiply that by u*e many hundreds of feet of pipe i *** 11 a PLANT OPERATION AND MAINTENANCE SECTION POWER * 1957 DOUBLE-LAYER sectional pipe insulation showing staggered joint construction to allow for thermal expansion of the pipe CYLINDRICAL SURFACE of steam drum using double-loyer insulation. Blocks are held by wire laced to anchored cable h-t insulation >t Uerage plant and you begin to realize pressure vessels. General practice is to jiat a lot of heat is 'wasted. leave an inch or two between every sec Insulation jackets or other finishes ond or third insulation block or section ire scorched and damaged at open for thermal expansion. This space is joints calling for constant maintenance. packed with fibrous insulation and cov Exposed hot pipe is a considerable fire ered by a sliding collar or shield. iazard, particularly in plants handling Standard thickness is a holdover from hmmable materials. In many cases, the past that continues to plague engi rorking temperature is raised to an neers. It implies that there is a stand mcomfortafale level. ard thickness for a given size of pipe. .Expansion joints, like the one on page Use of this term was common before il4, can solve some of these thermal the days of the superheater when steam spansion problems. But the great num- was only saturated. Avoid use of speci nr of joints needed for most piping are fications that call for standard "or ipensive to install and maintain. Best double - standard thicknesses. Aside slution is double-layer construction from the fact that they are probably ierever there is appreciable thermal inadequate, they no longer exist. spansion of piping and equipment. Simplified thickness. MIMA has Expansion joints are more often adopted new thickness schedules that md at elevated temperatures on long list four insulation types by nominal sis of ducts or breeching and large half inches for ease of reference. For I OWER JUNE 1957 PLANT OPERATION AND MAINTENANCE SECTION U example, 2^-in. nominal thickness in sulation on a 1-in. pipe is 2 21/32-in. thick. Actual thickness is used only for critical engineering calculations. One advantage is that outside diame ter of pipe insulation is then about the same as outside diameter of iron pipe sizes. It can be applied directly to pipe or as a second layer on smaller size in sulation. Another advantage is simplified in ventory records for the plant engineer who has to keep a stock of insulation for maintenance. Odd fractional-inch sizes do not have to be stocked. Outdoor plants. Number of outdoor applications is increasing. Plastic as phaltic compounds with 1-in. wire mesh reinforcement and binder give good protection on smooth insulation sur faces. Flashing is needed to prevent seepage at conical tips of vessels and similar points. Good insulation will withstand repeated wetting and drying. But wet insulation will not effectively prevent heat loss. Aluminum jackets giye good protec tion and improve plant appearance. But corrosion inside the jacket presents a problem. One theory is that alkaline nature of most insulation has a catalytic effect on the aluminum. Another at tributes corrosion to the use of scrap aluminum in producing sheet and/or improper alloy composition. Extensive tests by both insulation and aluminum manufacturers prove that corrosion is generally independent of insulation type and aluminum quality. All you need is moisture. Since it is almost impossible to keep all moisture out of the jacket, the inner surface must be given a protective coating. What's ahead? Insulation is avail able to handle most of today's industrial temperatures. But the trend to higher temperatures in nuclear and criticalpressure power plants spotlights the need for continued research. Since or ganic materials or binders cannot be used as basic ingredients, research is presently confined to known inorganic substances. Much has already been learned about basic theory of heat flow in insulation and basic materials are constantly examined for their effective ness in formulating new insulations. Most of this research is based on the fact that the most effective material for barring heat passage is one with count less millions of microscopic air spaces. Efforts are directed at developing prod ucts with greater heat resistance; lower conductivity and greater durability. 115