Background
One U.S.-based semiconductor fab faced long-term challenges in the operation and maintenance of their liquid-ring process vacuum (PVAC) system, primarily in the form of microbial fouling. Specifically, the fab's biofilm contained high levels of iron, a corrosion byproduct.
This fouling impacted heat transfer efficiency and system performance, and required frequent, unplanned maintenance of the heat exchangers for cleaning. Maintenance frequency ranged from semi-annually to quarterly or, in some cases, monthly. The customer brought the issue to the attention of their Nalco Water team.
Root-Cause Analysis
Working on-site with the customer, the Nalco Water team found substantial levels of slime-forming bacteria in the heat exchangers, caused by mineral oil within the units, which was added at the point of manufacture for in-transit corrosion protection.
Due to the fab's production demands, new PVAC units were started up quickly, with no time to remove the oil or clean internal surfaces. Once an oil-contaminated unit was up and running, the compression step of each operating stage moved the oil throughout the unit. This embedded the oil into the surfaces of the cast iron housing. The oil would settle throughout the unit and cause bacteria to grow, as a water/oil interface is an excellent starting point for bacterial growth.
Another contributing factor was the lack of rotation among the PVAC units. Facilities often have more than one PVAC unit and may only require one unit to run at any given time. This period of downtime can accelerate bacterial build-up in the exchanger due to the absence of flow; microbio growth favors stagnant water environments.
Lastly, the team confirmed a comprehensive understanding of the systems' metallurgy. Although the biofilm contained high amounts of iron, any chemical corrosion protection program must ensure all wetted metals were protected. Iron levels in the system were sometimes as high as 30 ppm, with water in the system periodically turning red as a result.
Solution
Nalco Water often uses an integrated mechanical/operational/chemical approach in its solution-development process. Here are the specific steps implemented to resolve these problems and prevent future occurrences.
Mechanical / Operational
- Cleaning of new PVAC units prior to being put online, to remove all traces of mineral oil
- Customer staff received a four-hour training on proper maintenance of the PVAC units
- Maximum PVAC unit run time was set to one week, to support proper equipment rotation
- Operating performance of all PVAC units was monitored via Service Intelligence, powered by ECOLAB3D™. System data helped fab personnel identify when units were performing out of spec, requiring additional corrective action.
Chemical: Microbio Control
Resolving the microbial fouling was the team's first priority. Nalco Water's Selecticide™ test enabled the team to choose a biocide with the highest potential effectiveness and minimal operational risk. The test compares the performance of various biocides across samples of system water in a particular application and assesses the microorganism count reduction in each sample. As a result, the test helps eliminate guesswork by indicating the appropriate biocide and optimum dosage range for a particular system.
Chemical: Corrosion Control
A corrosion-control chemistry was added to the program for the purpose of minimizing iron corrosion.
Results
Exchangers were examined after each change in the program. Over a two-year period, heat exchangers were pulled eight times, each time showing progressively less biofilm.
Before the Nalco Water solution, cleaning frequency ranged from monthly to quarterly to semi-annually. Today, the fab has maintained a record of 21 years and counting with no unplanned shutdowns to clean heat exchangers. This reduction in cleaning frequency has yielded $42,850 in annual labor savings plus an additional $42,850 in annual chemical cleaning savings.
In addition, iron levels in the system water were reduced by 97%, from the previous high of as much as 30 ppm (indicating active corrosion) to 0.02-0.8 ppm. Enhanced corrosion protection supports the customer's ability to maximize uptime and meet the manufacturer's projected asset life for each PVAC unit.
While this story details the annual cost savings, it's important to note the enhanced value achieved by maintaining these gains: Through continued support and no unplanned cleanings in 21 years, the customer's cost savings have been compounded to reach $1.8 million in total.
