SIP for WFI nearly always means Sanitize In Place since bulk WFI is not required to be sterile per USP. Sanitization means control of microbial contamination. The best way to do this is to sanitize the distribution loop on a regular basis by heating it to a temperature and for a duration that will kill nearly all the resident microbial organisms. This time and temperature regimen can vary but is often around 80-95 C for 1-4 hours. Many (most?) companies make the mistake of flushing the drops with hot WFI during this sanitization cycle intending to sanitize the drops. If instead you flush the drops with clean air after use and keep the drops dry between uses then microbial growth is minimized. Microbes will not grow on dry surfaces. There is no need to flush drops with hot WFI between uses or on a regular basis at all if the drops are kept dry with air. You flush with WFI just before use to prevent stray particles and microbes that may be present on the dry pipe in the drop from entering the process.
Amazingly, some companies still use steam on WFI distribution systems to sanitize them, a particularly outdated practice that leads to more problems than it solves. Gaskets and diaphragms suffer shorter lifetimes under this sort of treatment. Even more amazing is what I would have thought was surely an archaic practice of using up to 120C liquid water under pressure to sanitize such systems. This is seriously misguided and presents operational safety issues due to increased risk of leakage and burns to nearby personnel and is to be discouraged.
As Nissan is fond of saying, others may have different opinions