Hello.

Why do we need to show maximum and minimum liquid level in a vessel fabrication drawing?

Regards,
Sharjeel

Due to following main reasons;

1. To give an idea about the operating conditions to operation and maintenance personnels (few people may argue that P & ID and/ or datasheet shall be available for the same but personally I have seen many times that when you need, the required document is not readily available and every one take reference from the first document they get). This is a preference not a mandatory requirement.

2. In case you wish to use actual liquid levels for static head calculation and not the equipment with full of liquid.

3. This becomes a single document where all the informations are available. Good specially for process/ instrumentation to check the elevations for their instrument nozzles. No need to refer various docs to verify the data

Regards

Sanjeev

Sharjeel,
If you are a fabricator, then LLs on a drawing are of very little use to you.
If you are the vessel owner, then see reasons provided by Sanjeev. Regarding reason (2), I would add some design margin to account for density variation and/or some problem occurring that causes actual level to be much higher than design LLs.
PVer