Bacteria Prevention in High Purity Water System, Part 2

SYSTEM DESIGN & BACTERIA PREVENTION
Identifying and treating potential problems of bacteria formation once a system is operational can be much more difficult than addressing them during the system design. Bacteria ultimately grows by feeding off of nutrients in a system and water is a great medium for bacteria to grow since it can be a significant source of nutrients. Therefore, to prevent growth of bacteria it’s important to remove nutrient sources and prevent any introduction of nutrients into a system.

When nutrients are available, bacteria growth is more commonly found in dead zones or areas of low flow velocity, such as sample ports and weld points in piping. Bacteria can also form on the surfaces of piping, which is why the piping design is an important aspect of a high purity water system.

Depending on the piping material, special welding techniques may be required to prevent surfaces that allow bacteria to grow. Bacteria can grow as a result of leaks due to improper welding or incorrect instrumentation connections, both of which may lead to the introduction of air into a system.

Some common pipe materials used in high purity systems are 316L stainless steel, polypropylene and polyethylene piping. For stainless steel, the proper care needs to be taken to ensure a consistent weld. Proper welding processes need to be followed (reference AWS2 and ASME welding procedures). As part of quality control, all welds should be tested and examined to ensure processes are being followed.

Bacteria buildup on heat exchanger used for high purity water coolingWhen using polypropylene or polyethylene piping, there are many aspects to consider for proper installation. The most important design aspect is the welding procedure used to install the piping of a system. When smaller diameter poly tubing is used (2″ and below), it is common to use butt-fusion welding where the joint surfaces of pipe lengths or fittings are each heated using a hot plate then pressed together until cool to form a sealed surface.

Certain processes, more commonly found in the semiconductor industry, may be more critical of oxygen levels in the water. While this may not be such a consideration when using stainless steel piping or tubing, it is an important aspect of dealing with poly piping as poly piping has inherent oxygen permeability. The oxygen permeability of piping depends on the type and quality of the poly material used. If the permeability is too high such that the rated level of introduction of oxygen into a system exceeds tolerance levels, then the use of degassing membranes or a nitrogen sweep may be required.

Piping systems should be designed to utilize the correct connection types depending on equipment and materials. Tri-clamp or sanitary connections are typically accepted for piping connections at equipment. It is important to call this out in the design phase and to communicate this to any equipment suppliers so that the correct equipment and components are supplied for the process. Problems with bacterial growth on systems in operation can be corrected with replacing or rerouting piping and heat tracing. During operation, if a leak or improper connection is located, manufacturers may choose to use heat tracing as a way to sanitize the connection and prevent the growth of bacteria. This is not common or typically preferred as a final solution due to the ongoing expense associated with it.

It is important to identify certain criteria that a system must adhere to when designing the instrumentation to be included in a high purity water system. Instruments such as pressure gauges should be installed using sanitary connection points and should be isolated from the process using gauge guards to prevent any contamination due to failure of the instrument.
When bacteria exist in high purity water systems, it can cause interference in water quality and potentially damage products. Also, additional sanitation and testing must take place – both of which will be discussed in later blog postings.

If you have any questions on high purity water systems, or regarding the prevention and control of bacteria, please don’t hesitate to contact us, or your U.S. Water representative.

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