The techniques of boiloff calorimetry, following the guidance give in C1774 Standard Guide for Thermal Performance Testing of Cryogenic Insulation Systems, will be applied to thermal performance testing of pipe insulation and systems operating at below-ambient temperatures. Applications include chilled water, refrigeration, cryogenics, and most any use where the process fluid inside the pipe is at a temperature lower than ambient. Both guarded (absolute) and calibrated (comparative) techniques will be covered. For direct measure of heat flow, the boiloff cryogen chiefly used is liquid nitrogen (LN2) but the use of other cryogens is possible for specific applications. The boundary temperatures for testing using the LN2 boiloff method are in the range of 80 degC to -196 degC (353 K to 77 K). Pipe insulation systems can be mechanical, double-walled, or vacuum-jacketed.
KeywordsPipes And Fittings - Piping Systems - Guarded-End Apparatus - Heat Transfer Properties - Test Specimens - Testing Instruments - Thermal And Thermodynamic Properties - Thermal Insulation- Boiloff Calorimetry - Cryogenic Systems - Thermal Analysis - Thermal Test - Heat Transmission Vacuum-Jacketed Piping Mechanical Insulation Single Wall piping Double-Wall Piping Flexible Hose Flange Connections Bayonet Connections
No testing standard exists for thermal performance testing of pipe insulation operating below ambient temperatures. Boiloff calorimetry provides both the necessary refrigeration cold boundary temperature AND a direct measurement of heat flow rate (not electrical heater-based). The thermal transmission properties of pipe insulation generally have to be determined using pipe test apparatus rather than flat specimen apparatus such as the guarded hot plate or the heat flow meter apparatus, if results are to be representative of end-use performance. Insulation material formed into flat sheets often has different internal geometry from that of the same material formed into cylindrical shapes. Furthermore, properties often depend significantly upon the direction of heat flow in relation to inherent characteristics of the material: thus flat specimen one-dimensional heat flow measurements may not necessarily be representative of the two-dimensional radial heat flow encountered in pipe insulation. Another consideration is that commercial insulations for pipes are often made with the inside diameter slightly larger than the outside diameter of the pipe, otherwise manufacturing tolerances may result in an imperfect fit on the pipe, thus creating an air gap of variable thickness. In those cases where end-use performance data rather than material properties are to be determined, the insulation is mounted on the test pipe in the same loose manner so that the effect of the air gap will be included in the measurements. This would not be the case if properties were determined in a flat plate apparatus where good plate contact is required. Still another consideration is that natural convection currents around insulation installed on a pipe will cause non-uniform surface temperatures. Such conditions will not be duplicated in a flat plate apparatus with uniform plate temperatures. Industries of interest include commercial buildings, city infrastructure, transportation, electrical power cables, liquefied natural gas (LNG) piping and distribution, commercial space launch systems, cryogenic systems, and vacuum-insulated piping. The Cryogenics Test Laboratory at NASA Kennedy Space Center has operated a number of cold pipe insulation test apparatuses for the last 16 years. This include the absolute Cryostat-P100 apparatus with guarded ends and two 40-foot-long pipe test specimens in parallel.
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