What is Blow Off valves at a gas turbine?

blow off valve used at compressor section at gas turbine as it is one method to control the surge problem at compressor and open to atmosphere when the discharge increase

BOV it is a valve used in the start up and in the event of load is lost " tripp" 

it is used to minimize the inirtial force of start up and to prevent compressor surge 

These valves relieve combustion compressor air to the exhaust ducting either to provide DLE turndown or divert the compressor air quickly under a load rejection condition.

A blow off valve opens and dumps hot air in the event the load is lost. Say you run a turbogenerator...the hot gas impinges the turbine wheels, and this turning wheels drive the generator, which feeds am energy load. In the event the load is abruptly shut (like when a transmission tower falls down and destroys), there no instant load and the turbine feels "lighter" and this condition could lead to overspeed in turn. When this happens, the blow off valves opens in 20 miliseconds and dumps the hot air, the turbine wheels loosses energy and no overspeed is produced.

This valve is not to be confused with the "bleed valve" which bleed air from the GT compressor stages according to combustion air demands, protecting it from "stall" or ultimately "surge" condition. 

This is a pressure release system present in most turbocharged engines. Its main purpose is to take the load off the turbocharger when the throttle is suddenly closed.

Bleed Valve, it is used to minimize the inirtial force of start up and to prevent compressor surge 

pressure relief valave when regulating the pressure