The tag is the plant's own label, and it carries the one thing a nameplate cannot: what the valve was actually doing.
The tag is a location, not a specification
A valve tag number is usually a plant asset identifier built from a unit code, a line number, a sequence and sometimes a size or service code. The exact convention varies from site to site, which is why a tag number on its own is not a specification, but it is a thread that leads to one.
With the tag, a plant can pull the data sheet, the loop drawing, the maintenance history and the relief valve record. Without it, the valve is an anonymous object with a nameplate.
What a tag usually encodes
Most tagging systems combine a unit or area identifier, an equipment type letter, a sequence number and sometimes a nominal size. A tag beginning with a unit code tells you which part of the plant the valve served, which in turn tells you what chemistry and what pressure it saw.
Relief valve tags often carry the set pressure as well, either on the tag itself or on a separate test label. That single number can be the difference between a straightforward purchase and an unidentifiable item.
Why the tag matters more than the plate in a crib
In a warehouse, a valve's nameplate tells you what it is. The tag tells you what it was for, and often why it was removed. A valve tagged to a unit that was decommissioned is surplus with a story; a valve with no tag is surplus with a question.
For a buyer, tags also make matching possible. If a plant has a body without a bonnet and a bonnet without a body, tags are how the two get reunited before the lot is sold.
Keep the tag on the valve
The single most common mistake in a stores cleanout is removing tags to make the valves look tidier. It destroys information that cannot be recreated, and it reduces the value of the lot.
If tags have to come off, photograph them in place first, then tape each tag to its valve. A strip of masking tape and a marker costs nothing and preserves the whole identification chain.
When there is no tag
Plenty of surplus has no tag because it was bought as a spare and never installed. In that case the nameplate, the packaging and the purchase record carry the information instead. Send whatever exists: an old purchase order, a stores label, a box with a part number on it.
If nothing exists, the valve is priced on the nameplate and the physical condition. That is still a purchase, just a more cautious one.
Questions sellers ask
Can you trace a valve from a tag number alone?
No. The tag is a plant identifier, and only the plant's records can resolve it. Send the tag with the nameplate and we have both halves.
Do you need the data sheet?
It helps enormously on control valves and relief valves. On standard gate and check valves the nameplate is usually sufficient.
What if the tags are in a spreadsheet but not on the valves?
Send the spreadsheet with a shelf or row reference. That is enough to match most items.