Q. I am cleaning storage equipment and dry material hoppers sizing from 40–300 cu ft with alcohol wipes. The equipment is intended to hold dry powders that require minimal to no contaminants. What methods would you recommend for me to verify that the containers are 100 percent clean and meet my specifications? S.G.
A. Without knowing your cleanliness specifications, it’s difficult to determine exactly how you should go about cleanliness verification. However, typically cleanliness testing is performed to evaluate particulate residue for size and weight. Often you can flush and/or rinse down the vessel with an appropriate fluid that should be chosen for maximum solubility of any organic residue, if applicable, compatibility with the materials of construction and ease of removal after test (typically by drying). Another consideration is post-test compatibility in case there is residual fluid remaining in the hopper.
Once the fluid is chosen, in your case the test would be done by rinsing the sides of the hopper with the fluid into a collection dish or beaker. Then take the collected fluid with any residual debris and filter it. The filter needs to be dried and weighed before and after the test so you can determine a particulate weight. Additionally, you can evaluate the filter under magnification to document particle dimensions and classification, such as fabrication debris, cloth fibers, etc. You could benchmark known good process samples in order to develop your own internal specifications in lieu of a customer specification.
Considering the size of your hoppers, this rinsing type of test may be prohibitive. In that case, you may just need to have a clean set of alcohol wipes and set up some known “good” and “bad” wipes as boundary samples. In this case, post photographs at a workstation that indicate that, following the cleaning process, workers need to be able to wipe the inside of the hopper and have it visually appear similar to the previous “good” specimen.