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In Chapter 7 of Modern Engineering for Design of Liquid propellant Rocket Engines by Huzel and Huang, mentions the following engine cut off procedure.

The cutoff sequence usually consists of shutoff of subsystems ....

... and, in the case of test firings, postfiring securing (purges, flushes).

Why is it important to purge? And how is the system purged- only the fuel line or both the propellant feed lines?

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Purging is done to remove residual fuel from the lines. This is normally done using nitrogen gas.

https://www.grc.nasa.gov/www/retf/textFiles/lessonsRocketEngineTesting.html

Residual fuel and oxidiser represents a combustion hazard - it could leak out or evaporate and start a fire. Also, if your oxidiser is something like nitric acid or dinitrogen tetroxide or (god forbid) some kind of fluorine-based oxidiser, that can create a severe safety hazard to the engineers working on or cleaning up the rocket engine after a test.

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  • $\begingroup$ Thanks for the informative link. What I don’t understand is the requirement of a prestart purge mentioned? $\endgroup$ Nov 5, 2018 at 8:17
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    $\begingroup$ I guess that's to remove anything that accumulated in the time since it was last fired. Dust and mice and stuff. I may not remember this correctly, but in the book Ignition!, a rocket test firing disturbed a family of bats that had taken residence in a rocket chamber. $\endgroup$
    – Ingolifs
    Nov 5, 2018 at 8:23
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    $\begingroup$ You'd also want to purge all the oxygen from the fuel system before you put any fuel in. $\endgroup$
    – Hobbes
    Nov 5, 2018 at 9:13
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    $\begingroup$ and anything organic from the oxidiser system $\endgroup$ Nov 5, 2018 at 9:26
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    $\begingroup$ Purging also removes any on-going leakage through seals, etc from the engine before starting. Some seals don't fully "seal" until the system is pressurized. $\endgroup$ Nov 5, 2018 at 13:35

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