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Frequently Asked Questions
When is a plug better than a cap?
When the feature needing protection is internal.
Female threads, machined bores and internal sealing lands are the cases.
It also suits ends where a cap will not stay on.
A plug grips inside the bore rather than over the outside.
That frequently survives handling better.
Male threads and bevels are still better served by caps.
Why do internal threads matter particularly?
Because a damaged female thread cannot easily be cleaned up in the field.
It will not accept its fitting once damaged.
Impact and debris are both causes.
A plug engaging or filling the threaded portion protects against both.
External threads can sometimes be chased or recut.
Internal ones generally cannot without proper equipment.
How well do plugs stay in?
Better than friction-fit caps stay on, generally.
A plug wedged or expanded into the bore grips firmly.
That survives dragging, stacking and slinging well.
Simple push-in plugs rely on an interference fit.
Expanding designs grip considerably harder still.
On pipe handled repeatedly, retention is what decides whether protection survives.
What are expanding plugs for?
Firm retention, and sometimes light pressure testing.
Tightening expands the plug against the bore.
That grips well enough to survive transport.
Some seal well enough to hold a light pressure.
Those are a different product from a simple push-in plug.
They are rated accordingly and should not be assumed interchangeable.
How is a plug removed?
By a tab, ring or head - and that should be decided before fitting.
A plug pushed deep with none of those is awkward to remove.
Tools used to extract one can damage the bore.
Expanding plugs are released by loosening rather than pulling.
Bright colours make any remaining plug visible.
Thinking about retrieval first avoids creating a problem to solve later.
What if a plug is left inside a line?
It is a genuine problem rather than an inconvenience.
It can block flow entirely.
It can damage a pump or a valve.
It will fail a cleanliness inspection on a specified system.
Counting plugs out is used on critical pipework.
Tabs and bright colours are the practical defences against it.
Do they degrade outdoors?
Yes, exactly as caps do.
Pipe sits outdoors for months at a time.
Ultraviolet exposure makes many plastics brittle.
A brittle plug no longer seals or grips.
UV-stabilised grades are specified for extended storage.
Inspecting stored pipe periodically catches protection that has already failed.