May 2023 Volume 5

MAINTENANCE

Nozzle Clogging, A Symptom of Underlying Issues By Frank Sobnosky

Once again, AML Industries will be covering a forging lubricant-related troubleshooting topic. This time, we want to elaborate on one particular topic that we see frequently in the field. This is spray nozzles clogging because of graphite based lubricant buildup. While this may seem like a ‘cut and dried’ issue, there is much more nuance involved in this subject, and we believe that understanding how and why this issue occurs is a step toward solving it. Additionally, we want to provide some common solutions to this issue. As always, if further questions arise after reading this article, feel free to reach out. We love using our expertise to help forgers solve practical problems. Without further ado, let’s talk nozzle clogging.

Clogged spray nozzles can be an absolute nightmare in the forging industry. Being down during valuable production time and devoting manpower to a non-value added action such as this can be a headache at every level of production. Also, cleaning out nozzle orifices can be difficult and time consuming for shop employees. However, what operators and engineers do not often realize is that clogged nozzles are not so much a problem in themselves, but rather they are a symptom of greater problems with lubricant handling. Usually, there is a factor or multiple factors upstream of the nozzle that is not adhering to lubricant handling best practices. Additionally, having a good basic understanding of dispersion chemistry is important so that those involved in using lubricants at a forging facility understand why this occurs. These dispersions are primarily held in place using electrostatic repulsion principles. Slightly negatively charged particles interact with anionic (negatively charged) dispersants and binders that keep individual graphite particles separated. This works based on the chemical principle that similar charges repel one another. However, these binders and dispersants also make the graphite particles stickier which can cause them to compact with one another.This can cause an issue when coupled with a lack of best practices in several areas upstream of the nozzles. Graphite particle size is also a factor in this discussion. If graphite particles are large and this phenomenon occurs, they have a higher tendency to agglomerate and for those agglomerates to be larger than if the graphite was fine when it was dispersed. These factors coupled lead to nozzle clogging. A common example of this is having lubricant flow through small diameter channels that may contain sharp turns that create high tension areas. Graphite particles like to build up in these high tension areas, and once there are a few particles latched onto the surface, agglomeration will begin rapidly. Think of it like plaque on your teeth, once a certain amount builds, it is difficult for the build up to stop naturally.This may cause problems, but more importantly there may be an event in which this agglomerate detaches from that point and flows downstream to the nozzle which causes clogging. Further, improper mixing practices can lead to nozzle clogging downstream as well. When a propeller style (pushes fluid downward) mixing blade rather than an impeller mixing blade

(pulls fluid up into a vortex)is used, dispersed particles are pushed to the bottom of the mixing tank which can also lead to agglomerates and a hard solid settlement at the bottom of that tank. This allows for even dispersion of particles throughout the tank. When the tank is flushed, this agglomerated graphite can move downstream into the spray system and work its way to the nozzles. Additionally, this is more likely to occur in a square bottom tank than a round bottom tank as those tension points exist with sharp corners in a square tank. Another common practice in lubricant handling that can lead to nozzle clogging downstream is using an improperly sized impeller blade relative to the size of the tank in which it operates. As a rule of thumb, the impeller diameter should be one third of the tank diameter. Additionally, this impeller should be positioned one third of the way from the bottom of the tank and one third of the distance from one wall of the tank. These rules of thumb ensure proper mixing so that graphite does not settle at the bottom of the tank or on the walls. Further, mixing by adding pressurized air to a fluid tank is not recommended. This adds potential for air entrainment in the lubricant which should always be avoided for forging lubricants. It can affect dispersion stability as well as functionality. Finally, mixing speed should be chosen so that there is a slight turning in the fluid, but not so fast that there is a vortex. This can also lead to air entrainment as well as shearing binder and dispersion agents in the product. The bottom line with each of these examples is that if you give a graphite lubricant the opportunity to dry, it will do so, and that is bound to cause issues downstream which terminates at the nozzle.

Figure 1: A visual representation of why round bottom tanks resist settling better than square bottom tanks.

FIA MAGAZINE | MAY 2023 20