4 replies to this topic

[Field Engineer]

How to define dead legs ? Which standard can be used as a reference ?

[Subject_Name_Here]

A dead leg is just a section of piping that's filled but has no flow. Dead legs become an issue especially in pharma plants because the static fluid may support the growth of some micro organisms. However, if the water is chlorinated or later heated to at least 55 C micro should not be an issue.

[Field Engineer]

I want to know some information about the length criteria of dead legs. How many cm or meters of piping can cause dead leg etc.

[Field Engineer]

dears. Please share some information about dead legs. Appreciate your support

[Steve Hall]

Dead legs in the pharamaceutical industry are defined by ASME in their Bioprocessing Standard (BPE). The latest version was published in 2012. I only have the 2009 edition so what I'm reporting here might possibly be updated.

 

Definition of "dead leg" is "an area of entrapment in a vessel or piping run that could lead to contamination of the product."

 

The implication is that just because a piping branch may appear to be a dead leg (blocked flow path, long branch), it might not be defined as a dead leg. There are ways to prevent branches from leading to contamination of the product, but eliminating microbial growth in the stagnant section.

 

Now, to directly answer the question:

 

"Dead legs will be measured by the term L/D, where L is the leg extension from the Inside Diameter (ID) wall normal to the flow pattern or direction, and D is the ID of the extension or leg of a tubing fitting or the nominal dimension of a valve or instrument. For valves, L shall be measured to the seal point of the valve. If a branch from a primary pipeline has demonstrated flow during cleaning and SIP (steam-in-place), it does not constitute a dead leg.

 

"For high-purity water and clean steam systems, an L/D ratio of 2:1 is attainable with today's manufacturing and design technology. For other bioprocessing system, such as purification, filtration, and fermentating having cluster, block, and multiport valves, an L/D of 2:1 is achievable. However, it may not be achievable with certain equipment and process configurations as they are currently manufactured. For this part, an L/D of 2:1 or less shall be considered a target ratio, and shall not be construed to be an absolute requirement. The system designer and manufacturer shall make every attempt to eliminate system dead legs. It will be the responsibility of the system manufacturer or designer to identify where exceptions exist or shere the target ratio of 2:1 cannot be met.

 

Note the last sentence in the first paragraph above. In our plant, certain system drain ports in the water system exceed the 2:1 L/D ratio. To comply with BPE, the plant manually flushes through the ports every time the system is sanitized, thus satisfying this last sentence and removing the port from the definition of dead leg.

 

BPE has more to say about dead legs, including diagrams and discussions about instruments, equipment and tanks. It also discusses parallel design concerns such  as sloping, draining, pooling, and fabrication methods.