Chemical Processing - December 2007 - (Page 34)

>> PLANT INSITES A single degree can count rounding temperatures doesn’t insulate you from problems We typically set design temperatures 25°F to 50°F above the maximum “normal” operating temperature. then most of us round the temperature to make a nice, neat number. Vessel and piping specifications use 275°F or 450°F, not 278°F or 454°F. after all, the natural tendency is to use a round number with a minimum of significant digits. in one recent case, though, rounding could have led to a significant economic penalty. required insulation thickness for personnel protection and heat conservation was specified based on the design temperature (see table 1 on the following page) — in this instance, given in round 100° intervals, this winds up bumping many of the insulation thicknesses up one size. close examination of table 1 shows four types of discontinuities: 1. uneconomic changes in thickness, but with the same number of layers of insulation, because of the use of design temperature instead of operating temperature. examples include 1½-in. to 3-in. pipe from 199°F to 200°F and 10-in. and larger pipe from 799°F to 800°F. 2. uneconomic changes in thickness and going from one layer to two layers of insulation, resulting from use of design temperature instead of operating temperature. look, for instance, at 4-in. to 8-in. pipe from 599°F to 600°F. 3. thickness jumping two sizes of insulation but requiring the same number of layers — e.g., 4-in. to 8-in. pipe from 199°F to 200°F (one layer) and 6-in. and 8-in. pipe from 799°F to 800°F (two layers). 4. thickness jumping two sizes of insulation and increasing the number of layers. examples include 10-in. and 12-in. pipe from 599°F (one layer) to 600°F (two layers). the root cause of these problems was the cobbling together of project standards from a variety of sources that used different assumptions without an understanding of the consequences of doing this. the best solution combines 1) redefining the temperature basis for insulation selection as operating temperature; 2) accepting normal human round-off practice and raising the thickness break points by 1°; and 3) checking the insulation tables for inconsistencies and figuring out why they exist. CP By Andrew Sloley, contributing editor Asloley@putman.net Don’t use design temperature to select insulation thickness. in contrast, most companies select thickness of insulation based on the expected operating temperature of the piping, which may be much lower than the design temperature. the design temperature determines the type of insulation to use and piping support and flexibility requirements resulting from stress analysis. in some cases, the design temperature may be pushed up to flange limits for the materials involved — to set the piping class to the highest future-use temperature without extra capital expense. as a result, normal operating temperatures proper for selecting the economic insulation thickness may be much, much lower than design temperatures. an extra ½ in. or 1 in. of insulation doesn’t seem too extravagant until you look at a project of $50 million or more. the extra capital now piles up. We also must keep in mind not just total insulation thickness but the number of layers of insulation necessary. calcium silicate insulation may require multiple layers. typically 3-in. and thinner insulation gets installed in one layer while 4-in. and thicker insulation needs two layers. 34 • december 2007 Get troubleshooting insights online Full-text archives of previous Plant InSites columns are available at ChemicalProcessing.com. You also can find numerous answers to questions related to plant operations and design in the “Ask the Experts” feature there. It covers pumps — as well as compressors, heat exchangers, instrumentation, solids processing and more than twenty other topics. Not only can you see what others have asked, but you can pose your own questions. www.chemicalprocessing.com http://www.chemicalprocessing.com http://www.chemicalprocessing.com

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Chemical Processing - December 2007 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor 2007 Vaaler Award - The Quest Ends Take Equipment Diagnostics to a New Level Better Bellows Boosts Blending Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - December 2007

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