Equivalent Length of Pipe : The resistance of valves, controls, and fittings to flow, expressed as an equivalent length of straight pipe for convenience in calculating pipe sizes
NFPA 97 Standard Glossary of Terms Relating to Chimneys, Vents, and Heat-Producing Appliances, 2003 ed. http://travelwidpinx.info/nfpa-97-standard-glossar y-of-terms-relating-to-chimneys-vents-and-heat-pro ducing-appliances-2003-edition.html, (Std: Glossary of Terms Relating to Chimneys, Vent, and Heat-Producing Appl - Amer) {241 NFC}