When I read P&IDs I see isolation valves and block valves located throughout? What is the purpose of each?

Question

When does each apply?

Thanks.

Answer 1

The valves are generally of the same type.
Isolation valves are placed either side of a piece of flowline or a control valve, a piece of equipment ..etc and, as its name implies, to 'Isolate' the section from the process for maintenance.
The isolated section must be drained and depressured and may need purging with air or nitrogen before being handed over to maintenance to be opened up for inspection, repair or renewal of a defective part or section.

A block valve is simply used to temporarily shut off a process until needed again or to shut off a process stream for it to be diverted through another block valve to say, a different pipeline or storage tank...etc.
(The diverting valve must be opened first).

If the closing of any valve results in a section becoming isolated, a safety relief system must be incorporated into the shut-off section particularly a liquid line due to the possibility of heat (even from the sun) causing expansion of the liquid. This can cause a tremendous hydraulic pressure build up for only a few degrees increase. The danger of a line rupture and other damage is great.

Answer 2

Maintenance.