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Dead Legs

Dead Legs

Quality, Consequences and the Construction Industrial Complex (part 98) – All IMHO:

Right or wrong, if something appears enough times, it becomes accepted as normal. I guess it is the law of numbers and repetition. 

Lately, pipework dead legs are appearing on multiple projects. They exist, not because of bad pipework fitting, but because there are drawings instructing fitter welders to build them (engineers, please stop drawing this). As more dead legs appear “in the wild”, they start to become normal via visual pattern recognition.

How is this a problem? 

It is a problem for systems operators and IMHO, violates good engineering practice. 

What are the issues? Dead legs are;

  1. a cause of localized corrosion;
  2. prone to freezing (in certain climates) due to lack of flow;
  3. a major risk for bacterial, Legionella and Pseudonymous growth.

What is a dead leg?
It is a section of capped pipework with a length of ~1 pipe diameter or more and therefore zero fluid flow or turbulence. Treat any length of pipe with a welded cap as a dead leg.

What is best practice?

  1. Systems should be designed to eliminate dead legs.
  2. In the few situations dead legs are unavoidable, suitably sized drain valves should be incorporated to facilitate draining and flushing.
  3. Dead legs that have a length of more than three pipe diameters should be looped to allow effective cleaning in full bore up to and including 50mm diameter pipework, and 50% bore above 50mm diameter.
  4. These loops should incorporate a valve so that they can be shut off to allow increased flushing velocities in other parts of the system.
  5. Ensure a dynamic flush of the system, taking particular care to ensure that no dead legs are left unflushed.
  6. Ensure corrosion inhibitors are used and flushed though the dead legs periodically. 
  7. To address freezing, anything longer than a tee connection to a blind flange or valve is too long without mitigation measures such as trace heating.
  8. On small/medium bore pipework ensure any future connections tee off via the top of the pipe to prevent dirt pockets. 
  9. Consider bolted, flanged end caps for any unavoidable dead legs to provide access for emergency maintenance.

BTW, items 1 thru 6 above are as recommended by BSRIA. 

This all comes down to good engineering practice, which to the building owners cost, is optional for engineers and constructors. 

Twitter: @BLDWhisperer

Related posts & links:

#43 – This Is Not OK ( )

#75 MEP Engineering is a Weak Link Game ( )

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