A US Ethanol plant had been crippled by a serious infection. Lactics, glycerols and sugars were high while ethanol yield was low. For weeks, the plant team had been diligently working with no success of bringing the infection under control. The entire production team was frustrated. Increased antibiotics, acid applications, longer CIP cycles, and equipment tear downs were all part of the all out attack on this troublesome infection.
When the plant brought in a team of experts, it took two days to find a small valve that was open. The valve fed a 2" diameter pipe that went 8 feet in length.
Originally going to an old piece of equipment, the pipe had long been abandoned. But somehow the manual valve had been left open allowing the pipe to fill with mash.......mash that would ultimately fester into an infectious destructive compound. In this situation, the open valve would probably not hold more than 15 lbs of mash, yet it had a crippling impact on the entire ethanol plant. When the valve was closed the problem, almost immediately, went away.
While this plant's problem may be an extreme example of a dead leg, these situations and associated problems exist in many ethanol operations.
The goal of this article is to increase awareness of dead legs and create an urgency to reduce or minimize their infectious existence. The best way to start our discussion is a definition .......
A dead leg is any area where mash can become stagnant and materials are not exchanged during process flow or CIP cycles. Further, bacteria in dead leg pipes are protected from process flow and sanitization procedures and can re-contaminate the mash train and most likely develop biofilms.
A text book approach says that dead legs can be minimized by adhering to the 6D rule. The 6D rule suggest a dead leg should not be longer than 6X the primary pipe's diameter. In general, plant designers will tell you they are able to keep designs to under 3D, yet plants still experience sanitation problems.
Some types of dead legs include:
- Dead Ts - At one point it may have made sense to have a T in your design, but requirements changed and one of the outlets is plugged or shuttered with a valve.
- Sample Ports - Sample ports are necessary, but the length of the port should be limited and, if possible, positioned on the top of the pipe.
- Process Instrumentation and Indicators - These useful process monitoring and control devices often introduce dead legs due to their design or the need to make them replaceable without shutting down the process.
- Monkey Trees - These are the Rube Goldberg designs (defined as a very complicated way of solving a simple issue) of the Ethanol industry that start with someone saying one or all of the following......
- If we tie these together.....
- I know how we can save money . . . .
- Let me show you some artwork
- Long Neck Valves - Exist when personnel use longer nipples than actually required.
The bacterium that forms in the dead leg will almost always result in a biofilm. Think of a biofilm as a protective housing that will accumulate and propagate bacteria; even when the strongest of acids or biocides are used. As a result, dead legs can have a very accumulating negative impact on plant output.
What can be done? Many plant managers wear the number of dead legs they removed or decreased like Medals of Honor. It is a good start to remove or shorten these dead legs. But you can not remove them all.....kind of a necessary evil.
PureMash brings you the following approaches to improve your facility's performance and reduce the impact of dead legs......
- Pay most attention to the dead legs that are in environments below 135 degrees Fahrenheit.
- Have two protocols addressing dead legs.....one for normal operations and one for shut downs. Bring awareness to dead legs and focus on their possible impact.
- Use PureMash's chlorine dioxide to disinfect and clean dead legs.....nothing in your plant is a better biocide than PureMash's chlorine dioxide.
- In October 2010, PureMash is introducing a new simple chlorine dioxide chemical generator for small dead leg applications. The PureVista product line is capable of making up to 200 gallons of solution to treat dead leg infections.....portable, manual and can be applied at the point of infection.
While some dead legs are necessary or at least unavoidable, plant operators can improve output through cleaning, attention to dead legs and use of PureMash's chlorine dioxide.....ask your PureMash rep about PureVista.
