Hot tapping is the technique of attaching mechanical or welded branch fitting to pipeline, piping or equipment in service while while it is under pressure or has been depressurized but has not been cleared for conventional construction method.  , and creating an opening in that pipeline, piping or equipment by drilling or cutting a portion of the pipeline, piping or equipment within the attached fitting.

Hot tapping is usually performed when it is not feasible, or it is impractical, to take the equipment, Pipeline or Piping out of service, or to purge or clean it by conventional methods.

The decision should only be made when 

(1) continuity of service is essential, 

(2) shutdown of the system is impractical, 

(3) documented procedures are followed,

(4) special equipment is used which will provide effective protection for employees.

A hot tap connection can often be safely made without interfering with the process operation.

Hot-tapping is potentially hazardous and therefore should only by undertaken when shown to be technically feasible and to offer a clear advantage over alternatives.

This advantage may be economic and, in some situations, the safety and environmental risks may be less than those associated with more conventional methods.