custody transfer metering requirement for double block and bleed valve

[AntonTRN]

I am working on projects for ship loading facilities, custody tranfer.
All systems I have seen so far has double block and bleed valves up and down stream of the meters. Most experts explain that it is in OIML, API or MID -> nobody can point it out.
the only phrase I found is regarding bypassing the meter.
Does anybody knows why these expensive valves are applied?

 

[roydm]

Probably so you can take the meter out easily for periodic testing. The double block and bleed arrangement is often for safety reasons.
Regards
Roy

 

[zdas04]

I've seen a DBB requirement for meter station bypass lines (i.e., you lock both valves shut and the bleed open to have certain proof that all the product is being measured). DBB on the meter station inlet/outlet is not required by any code or safety regulation. If you are doing hot work on the meter run (never allowed by measurement practices) then DBB might be needed, but for fiscal isolation it is definitely not required.

David