spectacle blind close/open selection

[delf]

Dear professional

I notice that almost line drain in P&ID (operating / non-operation drain) always
has isolation valves or isolation valve before spectacle blinds (usually closed)
at these drain line from equipmennts or process main lines.

Howver, in hazardous service, such as hydrogen, chlorine, drain connections shall be provided only when
it is not possible to drain through flanges in the system to ensure that sources of potential leaks
to keep a minimum.

A valve and blinded atmospheric vent also be provided at overhead piping high point to enalbe hydrostatic testing
piping system.

But,sometimes I saw on the p&ID, there is drain lines provided near the end oof the overhead piping
(N2 or other Air gas) with spectacle blind (close) after the isolation vlave to discharge the remaining liquid
in the overhead piping to the sewage system.

It seems don't make any sense to me cause this air/N2 line seems not required any isolation valve except
the drain lines(with blind/isolation valve) is located at the pump or vessel?

Is my point right?

Thank you.

 

[Duwe6]

Concur with your thinking. The P&ID markings may just be a CAD 'artifact', where the draftsman copies the same symbol over and over. I know I have made that mistake, inserting an item that was not intended.