Document 8RX9ej5qMvqX8DBN8YmoE93zk

--4 STEAM MAINS being covered with higfr-femperoture insulation EXPANSION JOINT on a h-t breeching-ar duct. Space between sections is pocked with fibrous insulation Double-layer high-temperature insulation means Jess maintenance, comfortable working temperatures and no fire hazard. Here's ... By MARSHALL F AtlEN, Manager The Magnesia insulation Manufactured 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 1000 F But selecting insulation by heat tol erance alone oversimplifies the problem. No one insulation is best for all appli cations. Conductivity or It 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 liot metal in a thickness calculated to reduce outer-surface temperature belo* 600 F. Then 85% magnesia is appl*e" in thicknesses needed to reduce heat loss to economical level. High heat re sistance of diatomaceous silica and low conductivity of 85% magnesia combine o give an excellent insulation. - One vs two layers. Some engine** eel that two-iayer insulation is _u' lecessary with materials like calciu10 ilicate and expanded silica. Both illations combine high heat resistance ,nd low conductivity in a single Iayer* n addition, installation cost is But single-layer construction does n flow for thermal expansion of etpl,l,j nent and piping. In every 100 ft 0 tee! diping at 820 F there is about n. of ; expanded hare metal betwee^ isolation joints. Multiply that by V. i . m rl . A ..r,kf 1 AlOf in to* JUNE 1?5? t 14 PLANT OPERATION AND MAINTENANCE SECTION POWER DOUBLE-LAYER sectioned pipe insulation showing staggered joint construction to allow for thermal expansion of the pipe CYLINDRICAL SURFACE of steam drum using double-layer insulation. Blocks are held by wire laced to anchored cable h-t insulation pierage plant and you begin to realize ' bat a lot of heat is wasted. Insulation, jackets or other finishes ore scorched and damaged at open joints calling for constant maintenance. Exposed hot pipe is a considerable fire tszard, particularly in plants handling Sunmable materials. In many cases, rerking temperature is raised to an incomfortable level, jfxpcrnsion joints, like the one on page il4, can solve some of these thermal apansion problems. But the great numw of joints needed for most piping are npensive to install and maintain. Best station is double-layer construction ifaerever there is appreciable thermal apansion of piping and equipment. ^Expansion joints are more often wnd at elevated temperatures on long its of duets or breeching and large pressure vessels. General practice is to leave an inch or two between every sec ond or third insulation block or section for thermal expansion. This space is packed with fibrous insulation and cov ered by a sliding collar or shield. Standard thickness is a holdover from the past that continues to plague engi neers. It implies that there is a stand ard thickness for a given size of pipe. Use of this term was common before the days of the superheater when steam was only saturated. Avoid use of speci fications that call for standard 'hr double - standard thicknesses. Aside from the fact that they are probably inadequate, they no longer exist. Simplified thickness. MIMA has adopted new thickness schedules that list four insulation types by nominal half inches for ease of reference. For 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 give 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. 11OWER JUNE 1957 PLANT OPERATION AND MAINTENANCE SECTION 115