Tanks, reactors and other vessels can be cleaned in many ways. However, use of automated clean-in-place systems has increased rapidly. That's because automated devices clean more thoroughly than other methods, dramatically reducing or eliminating risk of cross-contamination caused by product or cleaning-chemical residue.
Automated cleaning provides other benefits as well:
• faster return to service of vessels -- downtime can be reduced by as much as 90%;
• decreased water and chemical use;
• lowered wastewater disposal costs;
• improved safety because workers no longer have to enter tanks; and
• better staff productivity because people can be deployed to other tasks.
The decision to automate is easy -- a plant usually can recoup the cost of an automated system in the first few months of operation through reduced chemical and water costs and increased production. However, determining the best cleaning equipment can be hard. If vessels are large, have obstructions such as mixing paddles, and contain sticky, flammable or toxic residues, selection can get challenging. So, this article offers some guidelines to help you choose the most appropriate equipment for your operation.
TANK CLEANING: THE STARTING POINT
Before you begin evaluating cleaning equipment, you must understand your cleaning requirements.
Residue. First, assess the residue to determine what's required to remove it. Is the substance sticky or easily cleaned? Can a cleaning liquid dissolve it? If not, what level of impact is needed to break it up and wash it away?
(If you're not sure how to remove the residue, a cleaning equipment vendor can use computational fluid dynamics modeling to determine the flow rate, operating pressure, coverage and the position of the spray head for complete cleaning of the vessel and any permanently installed equipment.)
Cleaning agents. Once you understand the residue's characteristics, you can sort out which cleaning agents to use. Chemical additives typically are employed to remove contaminants, improve tank wetability and reduce foam. Heat can boost the cleaning action of many water-based detergent chemicals.
Vessel size. Interior surface area and distance between the walls substantially affect selection. Evaluate the spray distance, usually measured in terms of the vessel diameter, but also consider vessel length and height. For example, for a 20-ft.-dia., 40-ft.-long vessel, use two vessel cleaners that each can handle up to 20 ft. or a single vessel cleaner that can handle up to 40 ft. You many need multiple nozzles if the spray can't reach a part of the vessel due to internal obstructions such as an agitator (Figure 1).
[Avoid Costly Design Mistakes]
Impact. The level of impact needed to thoroughly clean vessels depends on the residue, cleaning chemicals and water temperature. Hard-to-clean residues require greater impact. The theoretical spray impact, I, equals K Q P½ where K is a constant, Q is flow rate and P is liquid pressure.