By: Aubrey Green, Lead Analyst, Allied Reliability
Case Study Information:
While working as a Predictive Maintenance Technician in a paper mill, an analyst received a telephone call from the Pulping area. Their samples of white liquor were showing particles (turbidity). These particles were affecting the quality of pulp that was being produced. When these sand-like particles were processed in the paper machine, they produced a large mark. This mark lowered the quality of the paper. The department wanted to know if the analyst had any information on the process that could be causing the problem. They were particularly interested in pump vibration that would show the origins of the particles.
A quick look through the vibration data showed no issues that could be noted. The analyst asked for a simple piping diagram of the route of the white liquor from the clarifier to the digester. Armed with this information, the analyst decided to start at the beginning of the system and work to the end.
The system began with white liquor clarifiers started the system. Samples of liquor at the clarifier stage showed a few expected particles in the system. There was not enough turbidity to cause the problem the Pulping Department was experiencing. The analyst continued to study the diagram, looking for potential problem areas.
A holding tank was located before the digester. Between the clarifier and the tank there were only pipes and pumps, and the analyst decided to focus his attention on the tank. He knew he needed to find an accurate way to see what was in the tank. White liquor is extremely caustic, so dropping a weight into the tank would be dangerous. In addition, there were only one or two places to access the tank from the top.
Based on past experience, the analyst knew that the tank level could be determined with an Infrared (IR) camera. The temperature differential between the tank contents and the gas above the contents can be enough to show the tank level. The analyst had used this procedure many times in the past to confirm actual tank levels versus instrument readings. In this situation, he thought that if there was material in the bottom of the tank, it may have a temperature differential also.
Since the tank was not insulated, the image was not difficult to acquire. Using a thermal imager, the analyst took an image of the tank (Figure 1). The level of the tank was quickly noticeable. In addition, there was a wedge of material in the bottom of the tank, which was evident because the tank was known to have a flat bottom. This deposit was the result of years of particles from the clarifier settling in this tank. The inlet piping shown on the left of the image was actually acting as an agitator, mixing the particles with the white liquor before sending it to the digester.
Figure 1: Image of White Liquor Tank Showing Deposit
The analyst showed the image to the operator, who remarked, “Now we have an x-ray machine to see inside our tanks.” In this case, his comment was true.
An immediate resolution to the problem was needed, but the tank could not be taken out of service. The concentration of particles increased as the tank level lowered. Therefore, the tactical plan was to keep the tank level high enough to keep the concentration of particles at an acceptable level. The strategic plan was to clean the tank when it could be taken out of service.
Conclusion:
The deposits were removed from the White Liquor Tank during the next mill outage. Following that, the issues with turbidity were determined to be eliminated.
Dealing with hazardous chemicals in tanks and discovering problems within a containment may seem to be a daunting issue. Using the right technology in the right manner can give you insight to internal conditions and help you solve the problems in an efficient and effective manner.
