In the case of the 3D rule the length L of the dead leg is measured from the pipe wall of the main pipe and put in relation to the diameter of the parting pipe. One has to know that the 3D and the 6D rule cannot be compared directly because they have a different reference point.

A rule of thumb became the industry standard.īut how does it work in practice? Is one rule more binding or better than the others?.

The rule has been described for hot storage water systems but it is also used for all cleaning in place systems (CIP).

There is a 1.5D, a 2D, a 3D as well as a 6D rule.

This is a standard of the American Society of Mechanical Engineers for BioProcessing Equipment. Further statements can be found in WHO TRS 929 (1,5 D), WHO TRS 970 (3D) and in ASME BPE-2009 (2D). The ISPE Baseline Guide Water and Steam now talks about the 3D rule. In 2001 the dead leg rule reappeared in an ISPE guide. This document is still used by FDA inspectors as guidance for GMP inspections. This requirement was taken up in the FDA Guide to Inspections of high purity water systems in 1993 - only now it was called 6D rule. The rule for the prevention of dead legs (in a WFI system) is mentioned for the first time in the draft of the FDA Guides for Large Volume Parenterals (LVP), 21 CFR 212.49 in 1972. In order to further explain this, please read following the history of this rule.

In calls for tender and tests the 3D/6D rule is often used for the specification, but not always in the completely correct way. Hence, it is harder to clean them and during thermal sanitisation it takes longer until these 'branches' have also reached the required temperature.

The installation of pipework leads to recurrent discussions about how dead legs can be prevented and about the maximum length outgoing pipes/pipe tees may have for the sensor.

GMP Courses & Conferences on Site (in hotels).

Hot to avoid dead legs in pipe