Whenever a line carries more than one fluid, what remains in it after a transfer — the heel — becomes the next system's contamination. Clearing lines by flushing and pigging is how a plant uses shared runs without cross-contaminating, and it is one of the most important and least visible disciplines on the site. This page covers how lines are cleared.
Why the heel matters
A line is never fully empty after a transfer; a heel of the previous fluid remains in the pipe, low points and dead legs. On a dedicated line that heel is the same fluid and harmless, but on a shared line it joins the next fluid — and because incompatible systems like OBM and brine fail on tiny contamination, an uncleared heel is exactly how cross-contamination happens.
Flushing
The simplest clearing is a flush — pushing the next fluid, or a compatible base fluid, through the line to displace the heel ahead of it. The leading, contaminated volume is diverted to slops rather than allowed into the receiving tank, and flushing continues until the line runs clean. It is quick and effective but uses fluid and produces slops, so it is sized to the contamination risk.
Pigging
On longer or larger-diameter runs, a pig is driven through the line by fluid or air pressure to physically scrape and push the heel out, recovering far more product and leaving the line cleaner than a flush alone. Lines designed for pigging have launchers and receivers and avoid features that would trap a pig, and pigging is the method of choice where the recovered volume justifies it.
Where the displaced fluid goes
Whatever is displaced — heel plus flush, or the volume a pig pushes ahead — is captured to slops or reconditioning, where oily and water-based streams are kept segregated. The entire point of line clearing is that the contaminant ends up in a controlled stream, not in the next clean system.
Designing lines to be clearable
Clearability is a design property: minimising dead legs and low points, providing flush and pig connections, and dedicating lines wherever the contamination risk or fluid value justifies it. A line that is hard to clear is a standing contamination risk, so good plants design the clearing in rather than relying on heroics later.
Clear the heel, protect the system
Flushing and pigging exist to push the previous fluid to slops so a shared line can serve the next fluid safely. Line cleaning is the key defence that makes shared runs usable, and combined with dedication where it matters, it is how a plant moves many fluids through limited pipework without contaminating any of them.
Key takeaways
Transfer lines retain a heel of the previous fluid that contaminates the next system unless cleared. Plants clear lines by flushing — displacing the heel with the next or a base fluid to slops — and by pigging larger runs to recover product and leave the line cleaner, with displaced fluid captured to segregated slops. Clearability is designed in through minimal dead legs and pig connections. Line cleaning is the defence that lets shared runs be used without cross-contamination.