Dead legs are not just a problem for zombies; they can be a problem for your bioprocessing system, too. Why? A dead leg in your system can cause:
- Fluid pooling
- Reduced cleanability
- Difficult sterilization
These issues can potentially lead to your worst nightmare, product contamination. By minimizing or eliminating dead legs, you improve the cleanability and sterility of your system, reducing the possibility of contamination.
What is a dead leg? The term is defined in the American Society of Mechanical Engineers BioProcessing Equipment Standard (ASME BPE) as:
An area of entrapment in a vessel or piping run that could lead to contamination of the product.
– ASME BPE 2012 Edition
This definition gives you an understanding of what a dead leg is. However, to understand how to minimize dead legs, you also need to know how to measure a potential dead leg. This is also well defined:
Dead legs will be measured by the term L/D, where L is the leg extension from the I.D. wall normal to the flow pattern or direction, and D is the I.D. 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….
There is evidence that an L/D of 2 or less may prevent the branch from being a dead leg; however, the size and shape of the branch are also important in determining if the branch could lead to contamination. With sufficient flow through a primary pipeline, a branch may not constitute a dead leg.
– ASME BPE 2012 Edition
To give you an example of a dead leg, take a look at Figure 1, which shows the typical full-size tee that you might find in your system.
Figure 1. Dead leg in a tee structure with size comparison table
|Full Size Tee Nominal Size, Inches||Wall Thickness||I.D. (D)||Branch L||L/D (Branch)|
As you can see in the table, the L/D varies, and for tees 1-inch and smaller, the L/D increases significantly. The rule of thumb for many years was that a branch of 6:1 or less was cleanable. However, current evidence suggests that whenever a branch exceeds a 2:1 ratio, it is considered a dead leg. There are many points, not just tees, where you could potentially have a dead leg. You could have a tank valve that creates a dead leg at the bottom of your vessel, a line valve that leaves a dead leg like the tee in Figure 1, and so on. Eliminating these dead legs can be challenging, but when it comes to valves, it is possible in almost every case to completely eliminate dead legs from your system.
Today, there are infinite valve configurations available. Depending on your needs, you can almost always find a configuration with the performance you require and the cleanability you desire. From the simplest tank valve (Figure 2) to complex block bodies (Figure 3) and almost anything in between, you can configure a valve/valve assembly that minimizes or eliminates dead legs in your system.
Figure 2. Tank valve
By using valve configurations like these and working with your valve suppliers to customize valves that suit your needs and eliminate dead legs, you can avoid the problems of fluid pooling, reduced cleanability, and difficult sterilization, giving yourself a system that minimizes the potential for contamination.
Figure 3. Complex block bodies