why in loop system provided for recirculation of water the dead end should of 6 inches i.e the distance betwwen the outlet and the loop should be of 6 inches
if I understood your question well, your concern is the length between the loop and the branch at the points of use. If that is the case, you have the wrong figure: In a loop system for WFI distribution, the dead end should not be more than 1,5 times the branch diameter, measured from the wall of the loop pipe to the closing element of the valve (Source:
, page 54). 6 inches is far too long. The reason for this distance is that to avoid “anchoring” of microorganisms on the pipe walls a certain minimum flow speed is necessary, and in the branches the water is essentially stagnant if not for the turbulence generated in the branch by the water flow in the loop. The greater the distance from the loop, the smaller the turbulence becomes, and at the recommended flow speed of 2 m/s in the loop, at more the 1,5 times the branch diameter the water flow in the branch falls below the flow speed that prevents the microorganisms to stick to the walls. In practice, this distance means the use of “zero-deadleg” valves installed in the loop.