Would it be possible to copy off a bit of the datasheet so that we can see it. The same TLA* can mean different things to different people, depending on the industry.
Without any other information, I'd normally interpret "TST" as "test", but that doesn't seem to fit with what you were thinking.
*TLA: three letter acromyn
Patricia Lougheed
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Is is a P-3 or P-4 form (Mfr's Data Report form)? What Line of the form does it appear on? I've signed both P-3 and P-4 (and P-4A) data report forms and have not seen TST acronym used. Is there a Tubesheet on the boiler or Part?