SDV is an emergency shutdown valve in process control that work as safety device when there is emergency.
BDV is also used as a safety integrity function (SIF) that allows continuous flow of process. in a process line.
Difference between BDV and SDV
SDV stand for shut down valve and BDV stands for blow down valve and as their names imply that is exactly what they do.
On a offshore platform safety is always looked at first so there must be a way to isolate the incoming risers from the platform or FPSO during a emergency. These are your shut down valves.
So as an example you will install a shut down valve on a main incoming line from another platform or on the risers on a FPSO or platform to isolate your process plant from any other incoming lines in order to isolate a emergency by shutting down the process and isolate it from any other incoming sources. It is called a shut down valve since it will only active via the ESD system in a situation where you want a complete shutdown of the whole plant part of which will be the shutting off of the main incoming lines.
The blow down valve is also used during an emergency shut down of a plant down but it’s job is to de-pressurize the plant that is, as you have rightly stated, why the flare will increase ten fold during a emergency shut down of a plant since that is how the BDV’s de-pressurises the plant. It is pretty scary to be outside during a emergency blowdown of a offshore platform but it is just the SDV’s and BDV’s doing thier jobs.